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Am J Physiol Heart Circ Physiol 294: H2344-H2351, 2008. First published March 28, 2008; doi:10.1152/ajpheart.00053.2008
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Increased myogenic responsiveness of skeletal muscle arterioles with juvenile growth

Julie Balch Samora, Jefferson C. Frisbee, and Matthew A. Boegehold

Department of Physiology and Pharmacology and Center for Interdisciplinary Research in Cardiovascular Sciences, West Virginia University School of Medicine, Morgantown, West Virginia

Submitted 15 January 2008 ; accepted in final form 20 March 2008


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Previous studies from this laboratory suggest that during juvenile growth, structural changes in the arteriolar network are accompanied by changes in some of the mechanisms responsible for regulation of tissue blood flow. To test the hypothesis that arteriolar myogenic behavior is altered with growth, we studied gracilis muscle arterioles isolated from Sprague-Dawley rats at two ages: 21–28 and 42–49 days. When studied at their respective in vivo pressures, the myogenic index (instantaneous slope of the active pressure-diameter curve) of arterioles from 42–49-day-old rats was more negative than that of arterioles from 21–28-day-old rats, indicating greater myogenic responsiveness. Endothelial denudation, or prostaglandin H2 (PGH2)/thromboxane A2 (TxA2) receptor antagonism without denudation, significantly reduced the myogenic responsiveness of arterioles from the older rats over a wide range of pressures but had no consistent effects on the myogenic responsiveness of arterioles from the younger rats. The heme oxygenase inhibitor chromium (III) mesoporphyrin IX chloride had no effect on the myogenic activity of arterioles from either age group. These findings indicate that microvascular growth in young animals is accompanied by an increase in the myogenic behavior of arterioles, possibly because PGH2 or TxA2 assumes a role in reinforcing myogenic activity over this period. As a result, the relative contribution of myogenic activity to blood flow regulation in skeletal muscle may increase during rapid juvenile growth.

skeletal muscle microcirculation; endothelium; postnatal growth; myogenic response

POSTNATAL GROWTH OF THE MICROVASCULATURE in skeletal muscle is characterized by not only changes in vascular morphology and network structure (3, 33, 40, 42, 50) but also changes in pressure, flow, and shear stress (35, 50, 53) and in other factors that influence arteriolar tone or the capacity for acute adjustments in this tone (4–6, 34, 35, 38). In resting muscle, oxygen delivery and oxygen consumption are tightly linked throughout postnatal growth, with no age-related variations in the steady-state level of tissue oxygenation (41). However, measurements made in contracting muscle indicate that there is a marked change in the dynamic coupling between blood flow and metabolic activity over this rapid growth period (7, 41). Although it is not clear whether this is related to the changing metabolic demands that accompany muscle growth (24, 27, 29), this change in dynamic coupling would most likely involve a corresponding shift in the gain of local blood flow control mechanisms.

The myogenic activity of resistance vessels contributes importantly to local blood flow regulation (11, 12, 15). In most vascular beds, this behavior reflects a direct influence of pressure or stretch on smooth muscle contractile activity. Although the signaling pathways involved in myogenic activation are complex and not fully understood, many elements of these pathways have been identified. As reviewed by Davis and Hill (12), the depolarization of the smooth muscle membrane (due in part to the influx of Na+ through stretch-activated cation channels) triggers an influx of extracellular Ca2+ through voltage-gated membrane channels. The rise in cytosolic [Ca2+], which may be augmented by Ca2+ released from intracellular stores, increases the activity of myosin light chain kinase to promote actin-myosin interaction. Numerous enzymes and second messenger systems (e.g., protein kinase C, phospholipase C, inositol 1,4,5-trisphosphate, and cytochrome P-450 metabolites) also play important but incompletely understood roles in this pathway (12).

Although not necessary for the genesis of myogenic activity in most vascular beds, factors released from the vascular endothelium can modulate the magnitude of myogenic responses (23, 26, 30, 36, 37, 46). We have consistently found that the specific factors mediating endothelium-dependent control of arteriolar tone can change during juvenile growth (4–6, 38), but the impact of such changes on arteriolar myogenic behavior has not been investigated. Furthermore, there may also be changes in the intrinsic responsiveness of vascular smooth muscle to stretch during rapid network growth. In a previous study, we found that when pressurized to their respective in vivo pressures, isolated gracilis muscle arterioles from 21–28-day-old rats develop a higher level of spontaneous tone than arterioles from 42–49-day-old rats (4), suggesting that vascular smooth muscle responsiveness to myogenic stimuli could be greater in the younger rats. We undertook the current study to more directly investigate the possibility that alterations in myogenic responsiveness, or in the ability of the endothelium to modulate this responsiveness, may occur during microvascular network growth in skeletal muscle.


   MATERIALS AND METHODS
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 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
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Animals. All surgical and experimental procedures were approved by the West Virginia University Animal Care and Use Committee. Experiments were performed on isolated gracilis muscle arterioles from male Sprague-Dawley rats (Harlan Sprague-Dawley, Indianapolis, IN) at two ages: 21–28 and 42–49 days old.

Rats were anesthetized by intraperitoneal injection of pentobarbital sodium (50 mg/kg body wt), with a supplemental anesthetic (10% of original dose) administered if needed. The right carotid artery was cannulated for measurement of mean arterial pressure, which was assessed immediately before the removal of the gracilis muscle arteriole.

Preparation of isolated vessels. An arteriolar branch of the femoral artery supplying the gracilis muscle was removed, handling only the surrounding connective tissue to minimize vessel stretching or damage. The rat was then euthanized by intracardiac injection of pentobarbital sodium. The vessel was placed in warmed physiological salt solution (PSS) equilibrated with 21% O2-5% CO2-74% N2 and having the following composition: (in mM) 119 NaCl, 4.7 KCl, 1.17 MgSO4, 1.6 CaCl2, 1.18 NaH2PO4, 24 NaHCO3, 0.026 EDTA, and 5.5 glucose. After isolation, each vessel was prepared for in vitro video microscopy as previously described (16). Briefly, the vessel was mounted in a heated (37°C) chamber that allowed its lumen and exterior surface to be perfused and superfused, respectively, with PSS from separate reservoirs. The vessel was cannulated at both ends with glass micropipettes (50- or 70-µm-tip diameters for arterioles from the younger or older rats, respectively) and secured to the inflow and outflow pipettes using 9-0 nylon suture. Any side branches were ligated with a single strand teased from 6-0 silk suture. The inflow pipette was connected to a reservoir perfusion system for control of intralumenal pressure and flow. The vessel was then extended to its in situ length and equilibrated at 80% of the animal's mean arterial pressure to approximate its in vivo perfusion pressure (14).

Experimental protocol. Vessel diameter was measured with a video micrometer. All vessels developed spontaneous tone during equilibration. Any vessel that did not demonstrate endothelial viability, as judged by pronounced dilation in response to 10–7 M acetylcholine (ACh, Sigma Chemical, St. Louis, MO), was not used in this study. Changes in vessel diameter following step changes in pressure were measured under no-flow conditions after the vessel had been continuously perfused for 30 min.

To evaluate the myogenic responsiveness of each arteriole, the perfusate outflow line was clamped and the height of the perfusion reservoir was increased or decreased to vary intralumenal pressure between 40 and 140 mmHg. Steady-state vessel diameter was measured before and then 3–5 min after each pressure change. The order in which the different pressure steps were applied was as follows: 80->100->120->140->60->40 mmHg. After active arteriolar diameter responses to each pressure change were measured, the superfusate was replaced with Ca2+-free PSS, and, after full vessel relaxation, passive arteriolar diameter was determined at each of the pressure steps used.

Endothelial denudation. To investigate the role of the vascular endothelium in modulating myogenic activity, diameter responses to each step change in intralumenal pressure were assessed before and then after the removal of the endothelium by mechanical abrasion (47). To accomplish this, the pipette tip at each end of the vessel was gently advanced and then withdrawn through the vessel lumen three times. We have previously found that this method successfully denudes the endothelium of gracilis muscle arterioles without any functional change in the underlying smooth muscle (4). In the current study, we verified that endothelial function had been selectively abolished by assessing arteriolar responses to the endothelium-dependent dilator ACh (10–7 and 10–5 M), the endothelium-independent dilator sodium nitroprusside (SNP, 10–5 M, Sigma) and the endothelium-independent constrictor phenylephrine (PE, 10–5 M, Sigma) before and then after the denudation procedure. Only those vessels showing a loss of responsiveness to ACh with unchanged responses to SNP and PE were included in the final data set.

Inhibitors and agonists. All agents were purchased from Sigma Chemical unless otherwise specified. Because some studies have documented a role for constrictor prostanoids in reinforcing the myogenic activity of skeletal muscle arterioles (21, 22, 48), the pressure-diameter relationship for some vessels was investigated under control conditions and then in the presence of the prostaglandin H2/thromboxane A2 (PGH2/TxA2) receptor antagonist SQ-29548 (10–6 M, 20-min equilibration period). In previous studies on isolated rat skeletal muscle arterioles of the same size as those studied here, 10–6 M SQ-29548 was found to completely abolish arteriolar responses to the TxA2/PGH2 receptor agonist U-46619, thereby demonstrating its efficacy (21, 22).

Since there is some evidence that endogenous carbon monoxide (CO) can also modulate the myogenic activity of gracilis muscle arterioles (52), the pressure-diameter relationships were assessed in other vessels before and then, after a 20-min equilibration period, during the exposure to the competitive heme oxygenase (HO) inhibitor chromium (III) mesoporphyrin IX chloride (CrMP, Frontier Scientific, Logan, UT). Although there are many commercially available metalloporphyrins that inhibit HO, we chose CrMP because it is the most selective inhibitor of HO activity (2) and because it is less sensitive to photodegradation than other metalloporphyrins (49). For these experiments, a 10–2 M stock solution of CrMP in 0.1 N NaOH was diluted in the bath to produce a final concentration of 10–5 M (1). Findings from a previous study on isolated rat gracilis muscle arterioles suggest that CrMP at this concentration produces maximal or near-maximal HO inhibition (28), with this conclusion being further supported by a subsequent study on rat gracilis muscle arterioles in which CO production was measured in the absence or presence of CrMP (52).

Data and statistical analysis. Vascular tone at each pressure was calculated as follows: Tone = [(Dmax – Drest)/Dmax] x 100, where Dmax is passive diameter and Drest is resting diameter with tone present. A tone of 100% would represent complete vessel closure, whereas 0% would represent a passive vessel. The myogenic index (MI), an indicator of the slope of the active pressure-diameter relationship for an arteriole at a given pressure, was determined using the following equation: MI = 100 x [(rf – ri)/ri]/(Pf – Pi), where rf is the final radius, ri is the initial radius, Pf is the final intralumenal pressure, and Pi is initial intralumenal pressure. The greater the myogenic responsiveness, the more negative the MI.

All variables were verified to have a normal distribution using the Kolmogorov-Smirnov Normality Test, with Lilliefors' correction (P = 0.05). All data are presented as means ± SE. For all analyses, a probability value of P < 0.05 was considered to be statistically significant. Dilation in response to Ca2+-free PSS is expressed as the percent increase from the control diameter. Differences between the means of individual experimental groups were determined by ANOVA/Newman-Keuls test or by an unpaired Student's t-test when two means were compared.


   RESULTS
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DISCUSSION
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General characteristics of all rats from which vessels were removed and studied are reported in Table 1. Body weight and mean arterial pressure were significantly greater for 42–49-day-old rats than for 21–28-day-old rats. Table 1 also summarizes the general characteristics of all vessels studied, measured at equilibration pressures of 61 ± 1 mmHg for arterioles from 21–28-day-old rats and 77 ± 2 mmHg for arterioles from 42–49-day-old rats (80% of mean arterial pressure, which approximates steady-state in vivo pressures for these vessels). Resting and passive diameters of arterioles from the older group were significantly greater than those of arterioles from the younger group, but at these equilibration pressures, resting vascular tone was significantly less in arterioles from the older group.


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  		Table 1. General characteristics of all rats and vessels used in this study

 
Arterioles from both age groups exhibited clear myogenic activity, as judged by the maintenance of a constant diameter at pressures above 80 mmHg in arterioles from 21–28-day-old rats and active constriction following pressure increases above 80 mmHg in arterioles from 42–49-day-old rats (Fig. 1). At pressures between 40 and 80 mmHg, the steady-state (resting) tone of arterioles from the older rats was significantly less than that of arterioles from the younger rats (Fig. 2). In contrast, calculated myogenic indexes were similar in the two age groups over this pressure range and significantly more negative for the older rats at pressures of 100 and 120 mmHg, indicating greater myogenic responsiveness (Fig. 3A). When intralumenal pressures were normalized to each age group's estimated in vivo arteriolar pressure, these differences in myogenic responsiveness between the two age groups were even more profound, especially at pressures closest to their respective in vivo values (Fig. 3B).


Figure 1
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  		Fig. 1. Active and passive diameters as a function of intralumenal pressure for gracilis muscle arterioles from 21–28- (A) and 42–49-day-old rats (B). Values in this and all subsequent figures are given as means ± SE; n = number of vessels. *P < 0.05 vs. passive diameter.

 

Figure 2
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  		Fig. 2. Resting vascular tone as a function of intralumenal pressure for arterioles from both age groups (n = number of vessels). *P < 0.05 vs. arterioles from 21–28-day-old rats.

 

Figure 3
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  		Fig. 3. Calculated myogenic indexes of arterioles from each age group as a function of absolute intralumenal pressure (A) and intralumenal pressure normalized to estimated in vivo pressure (0.8 x mean arterial pressure) (B) (n = number of vessels). *P < 0.05 vs. arterioles from 21–28-day-old animals.

 
In arterioles from both age groups, mechanical abrasion to disrupt endothelial function completely abolished arteriolar dilator responses to ACh but had no significant effect on arteriolar dilation to SNP or constriction to PE (Table 2). As shown in Fig. 4, endothelial disruption had a significant effect on the steady-state diameter of arterioles from 21–28-day-old rats only at 140 mmHg (a modest increase) and had no consistent effect on the myogenic indexes of these vessels (Fig. 4, left). In contrast, endothelial disruption significantly reduced the steady-state diameter of arterioles from 42–49-day-old rats at pressures of 60 and 80 mmHg, whereas the myogenic indexes of these vessels were shifted to less negative values (indicating reduced myogenic responsiveness) at 80, 100, and 120 mmHg (Fig. 4, right).


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  		Table 2. Effect of lumenal abrasion on arteriolar responses to endothelium-dependent and -independent agonists

 

Figure 4
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  		Fig. 4. Effect of endothelial removal on steady-state diameters and myogenic indexes over entire pressure range for arterioles from each age group (n = number of vessels). *P < 0.05 vs. intact vessel.

 
The PGH2/TxA2 receptor antagonist SQ-29548 tended to increase the steady-state diameter of arterioles from the younger rats (a significant effect at 40, 60, and 140 mmHg), but it had no significant effect on the myogenic index for these vessels at any pressure (Fig. 5, left). In contrast, SQ-29548 did not change the steady-state diameter of arterioles from the older rats at any pressure, but it did have an effect on the myogenic index of these vessels that was similar to that of endothelial removal, i.e., a shift to less negative values at pressures of 80, 100, and 120 mmHg (Fig. 5, right).


Figure 5
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  		Fig. 5. Effect of SQ-29548 treatment on steady-state diameters and myogenic indexes over entire pressure range for arterioles from each age group (n = number of vessels). *P < 0.05 vs. control.

 
The HO inhibitor CrMP had no effect on either steady-state diameters or the myogenic indexes of arterioles from either age group at any pressure (Fig. 6).


Figure 6
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  		Fig. 6. Steady-state diameters and myogenic indexes over entire pressure range for arterioles from each age group before and then after treatment with the heme oxygenase inhibitor chromium (III) mesoporphyrin IX chloride (CrMP) (n = number of vessels).

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
Smooth muscle myogenic activity assists in the establishment of resting microvascular tone and in the autoregulation of tissue blood flow and capillary hydrostatic pressure (12). During rapid postnatal growth, autoregulatory adjustments in arteriolar diameter play a crucial role in maintaining sufficient blood flow to growing tissues, but the contribution of myogenic activity to these adjustments may change over time. Evidence of such temporal changes in myogenic behavior comes mostly from studies on vessels upstream from the arterioles. For example, the myogenic responsiveness of mesenteric arteries from 10-day-old piglets is significantly less than that of arteries from 1-day-old piglets (44), and middle cerebral arteries from 6-mo-old rats exhibit weaker myogenic responses than those from 14-day-old rats (9). In contrast, there is no difference between neonatal mice (4–8 days old) and adult mice (42–56 days old) in the intrinsic responsiveness of middle cerebral artery smooth muscle to myogenic stimuli, although the adult arteries develop less tone over a wide pressure range due to greater opposition of myogenic behavior by endothelium-derived vasodilators (17).

In a previous study, we found that when equilibrated at their respective estimated in vivo pressures, gracilis muscle arterioles from 21–28-day-old rats develop a higher level of resting tone than those from 42–49-day-old rats (4). Because a large fraction of resting arteriolar tone is attributable to myogenic activity (12), we undertook the current study to specifically test the hypothesis that responsiveness of skeletal muscle arterioles to myogenic stimuli is greater in the younger rats. In addition, because factors released from the endothelium can modulate smooth muscle responses to myogenic stimuli (13, 15, 17, 20, 30, 46) and growth-related changes in arteriolar endothelial function have been widely reported (4, 35, 38, 51), we also investigated the effect of endothelial denudation on pressure-diameter relationships in arterioles from both age groups.

In the current study, we found that resting vascular tone is greater in arterioles from the younger rats than in those from the older rats at lumenal pressures of 40, 60, and 80 mmHg (Fig. 2), whereas there were no age-related differences in myogenic index over this same absolute pressure range (Fig. 3A). However, when the values for myogenic index are normalized to each arteriole's estimated in vivo pressure, a direct comparison of the corresponding values for each age group reveals that the myogenic gain for older vessels is much greater than that for younger vessels over the range extending from ~90% to 130% of their in vivo pressures (Fig. 3B). Therefore, arterioles in the two age groups may have quite different capacities for myogenic behavior when in their normal in vivo environments. This raises the possibility that the contribution of myogenic activity to blood flow regulation in skeletal muscle becomes progressively greater during juvenile growth. However, it is important to recognize that the vessels studied here were not perfused during pressure changes and therefore not exposed to lumenal shear stress as they would be in vivo. As mentioned earlier, shear stress can induce the release of endothelial factors that modify myogenic responses (26, 30, 46), so that the age-related differences in myogenic behavior that we report here could be either minimized or enhanced in vivo.

In the evaluation of these data, it is also important to keep in mind that arterial pressure was measured in each animal while under pentobarbital sodium anesthesia. In adult rats, anesthetization with pentobarbital sodium at the dose used here typically reduces arterial pressure by 15–20% (8, 32). Consequently, our estimated in vivo arteriolar pressures (80% of mean arterial pressure) are to some extent underestimated. Our comparison of myogenic indexes after normalization of intralumenal pressures to "in vivo" pressure (Fig. 3B) will remain valid if the anesthesia reduced arterial pressure by the same proportion in both age groups. However, if this is not the case, then the differences we found could be either underestimated or overestimated, depending on which age group is more susceptible to the pressure-lowering effect of pentobarbital sodium.

The fact that arterioles from younger rats developed greater steady-state tone than those from older rats (at 40–80 mmHg) without having a higher myogenic gain indicates that there must be an age-related difference in the influence of some other factor on resting arteriolar tone. Nonmyogenic factors that can also be important determinants of resting tone include the level of ambient oxygen, which promotes vascular smooth muscle contraction (16, 19, 25), and basally released endothelial relaxing factors, which promote vascular smooth muscle relaxation (13, 22, 34, 35, 38). The higher tone of arterioles from the younger rats may reflect a greater responsiveness to oxygen in the vessel bath, but further studies are needed to critically test this hypothesis. In contrast, the current study provides some evidence that the higher tone of arterioles from the younger rats may be due at least in part to a lesser influence of the endothelial relaxing factors that normally limit myogenic constriction. As shown in Fig. 4, top, endothelial removal increased the resting tone of arterioles from 42–49-day-old rats but had no effect on arterioles from 21–28-day-old rats, which is consistent with an absence of basally released relaxing factors in these younger vessels.

The denudation-induced constriction of arterioles from the older rats at pressures of 60 and 80 mmHg is probably not due to increased myogenic activity because endothelial denudation reduces the myogenic gain of these arterioles at pressures from 80–120 mmHg (Fig. 4, bottom, right). This contrasts with an earlier report that endothelial removal has no effect on the myogenic index of gracilis muscle arterioles isolated from 42–49-day-old Wistar rats (45). Also in that study, the myogenic index of endothelium-intact arterioles was found to be constant between pressures of 60 and 120 mmHg, whereas we found that the myogenic index of arterioles from Sprague-Dawley rats of the same age becomes progressively more negative as pressure is increased from 40–100 mmHg (Fig. 3A). These differences suggest that there may be strain-related differences in the relative influence of myogenic activity versus that of other factors on steady-state arteriolar tone.

Our denudation procedure did not change responses to the endothelium-independent agonists SNP and PE in arterioles from either age group (Table 2), suggesting that smooth muscle function was generally not altered by this procedure. Furthermore, since PE elicits arteriolar constriction through a signaling pathway that has many elements in common with that for myogenic activation (12), it seems reasonable to conclude that this procedure did not more specifically alter smooth muscle responsiveness to myogenic stimuli. We cannot exclude the possibility that our abrasion procedure may have selectively disrupted some cellular or biochemical process that is more specific to the myogenic signaling pathway, but this seems unlikely because in that instance we would expect to see blunted myogenic responses in arterioles from the younger rats as well, which did not occur (Fig. 4).

The reduction in myogenic gain after endothelial removal in arterioles from 42–49-day-old rats suggests that an endothelium-derived constrictor normally augments myogenic activity over a wide pressure range in these vessels. TxA2, a metabolite of PGH2, can be formed in the endothelium and is one of the most potent endogenous vasoconstrictors known (18). Treatment of endothelium-intact arterioles from this older group with the PGH2/TxA2 receptor antagonist SQ-29548 reduced their myogenic indexes by approximately the same amount, and over the same pressure range, as that seen with endothelial removal (Fig. 5, bottom, left). This is consistent with PGH2 or TxA2 being the endothelial factor that reinforces myogenic activity in these vessels. However, we cannot rule out the alternate possibility that this PGH2/TxA2 is released from the smooth muscle, another well-documented source of these prostanoids (10, 39), with a different endothelial factor also acting to reinforce myogenic activity. Other investigators have found an enhancement of myogenic behavior by endothelium-derived constrictors, but this is more typically found in hypertensive rats (21, 48). For example, Ungvari and Koller (48) have reported that the combined actions of endothelium-derived endothelin and PGH2/TxA2 enhance the myogenic reactivity of gracilis muscle arterioles isolated from spontaneously hypertensive rats.

In contrast to our findings in arterioles from the older rats, endothelial removal had no significant effect on the steady-state diameter of arterioles from the younger rats, except for a modest increase at 140 mmHg, which is clearly above the normal pressure range for these animals (Fig. 4, top, left). Endothelial removal also had no consistent effect on the myogenic index of these vessels (Fig. 4, bottom, left). These findings, when viewed in the context of our findings in the older arterioles, suggest that an influence of the endothelium on resting smooth muscle tone arises in these skeletal muscle arterioles sometime between 28 and 42 days of age.

PGH2/TxA2 receptor antagonism tended to increase the steady-state diameter of arterioles from the younger rats (Fig. 5, top, left), suggesting that basally released PGH2 or TxA2 can contribute to the resting tone of these vessels. The ability of this basally released PGH2/TxA2 to enhance arteriolar tone is apparently not related to any change in myogenic activity, since SQ-29548 has no effect on the myogenic index of these vessels at any pressure (Fig. 5, bottom, left).

Because we recently identified a role for endogenous CO in mediating endothelium-dependent dilations of arterioles from both 21–28 and 42–49-day-old rats, with exogenous CO at higher concentrations causing endothelium-independent constriction (5), we reasoned that CO could also be modulating the myogenic reactivity of arterioles from rats of either age in the current study. Based on their use of CrMP, Zhang et al. (52) concluded that endogenous CO does modulate the myogenic constriction of isolated gracilis muscle arterioles. However, in the present study, treatment with CrMP had no significant effect on myogenic responses in either age group (Fig. 6). It may be germane to point out that judging from their body weights (250–300 g), the rats used in the study by Zhang et al. were probably between 55 and 65 days old, which is older than the rats we studied. This raises the possibility that a modulating role for CO in these responses may not arise until after adolescence.

The pressurization sequence that we used to investigate myogenic behavior involved an assessment of arteriolar responses to 60 and 40 mmHg after the vessel had been pressurized to 140 mmHg. To verify that the vessels weren't damaged by this high pressure, we often paused during the experiment to evaluate arteriolar responses to ACh and PE after the pressure had been reduced from 140 to 60 mmHg. In these instances, we found little or no differences between the post-140 responses and those we measured just before starting the experiment, suggesting that no endothelial or smooth muscle damage had occurred. The data in Fig. 1 also indicate that the ability of the vascular smooth muscle to respond to myogenic stimuli is not compromised by exposure to the high pressure. At 140 mmHg, vessels from 21–28-day-old rats maintained the same steady-state diameters that they had at 80, 100, or 120 mmHg, and the diameters of vessels from 42–49-day-old rats were consistent with the trend we saw at pressures above 80 mmHg, i.e., a progressive decrease in diameter with increasing pressure.

In conclusion, the findings of this study suggest that rapid postnatal growth is accompanied by an increase in the myogenic responsiveness of skeletal muscle arterioles, possibly because PGH2 or TxA2 assumes a role in reinforcing myogenic activity over this period. However, other mechanisms may also contribute to this phenomenon. For example, there is growing evidence that in addition to a voltage-gated influx of Ca2+ (and possibly Ca2+ release from intracellular stores), myogenic activation of smooth muscle also depends on an increase in myofilament Ca2+ sensitivity (31, 43). As recently documented in rat cerebral arteries (9), the age-related change in myogenic behavior that we observed may also involve a change in the magnitude of stretch-induced Ca2+ mobilization and/or myofilament Ca+ sensitivity. Further studies will be required to test this hypothesis.


   GRANTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This work was funded by the American Heart Association Grants 0330194N (to J. C. Frisbee) and 0150199N (to M. A. Boegehold) and the National Institutes of Health Grants R01-DK-64668 (to J. C. Frisbee) and RO1-HL-44012 (to M. A. Boegehold).


   FOOTNOTES
 

Address for reprint requests and other correspondence: M. A. Boegehold, Center for Interdisciplinary Research in Cardiovascular Sciences, Robert C. Byrd Health Sciences Ctr., PO Box 9105, West Virginia Univ. School of Medicine, Morgantown, WV 26505-9105 (e-mail: mboegehold{at}hsc.wvu.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

  1. Andresen JJ, Shafi NI, Durante W, Bryan RM Jr. Effects of carbon monoxide and heme oxygenase inhibitors in cerebral vessels of rats and mice. Am J Physiol Heart Circ Physiol 291: H223–H230, 2006.[Abstract/Free Full Text]
  2. Appleton SD, Chretien ML, McLaughlin BE, Vreman HJ, Stevenson DK, Brien JF, Nakatsu K, Maurice DH, Marks GS. Selective inhibition of heme oxygenase, without inhibition of nitric oxide synthase or soluble guanylyl cyclase, by metalloporphyrins at low concentrations. Drug Metab Dispos 27: 1214–1219, 1999.[Abstract/Free Full Text]
  3. Aquin L, Sillau AH, Lechner AJ, Banchero N. Growth and skeletal muscle microvascularity in the guinea pig. Microvasc Res 20: 41–50, 1980.[CrossRef][Web of Science][Medline]
  4. Balch Samora J, Frisbee JC, Boegehold MA. Growth-dependent changes in endothelial factors regulating arteriolar tone. Am J Physiol Heart Circ Physiol 292: H207–H214, 2007.[Abstract/Free Full Text]
  5. Balch Samora J, Frisbee JC, Boegehold MA. Mechanisms of microvascular control during juvenile growth (Abstract). Microcirculation 14: 439, 2007.[CrossRef][Web of Science][Medline]
  6. Balch Samora J, Frisbee JC, Boegehold MA. Hydrogen peroxide emerges as a regulator of arteriolar tone during juvenile growth of the arteriolar network. Microcirculation 15: 151–161, 2008.[CrossRef][Web of Science][Medline]
  7. Berg BR, Sarelius IH. Erythrocyte flux in capillary networks during maturation: implications for oxygen delivery. Am J Physiol Heart Circ Physiol 271: H2263–H2273, 1996.[Abstract/Free Full Text]
  8. Bouairi E, Neff R, Evans C, Gold A, Andresen MC, Mendelowitz D. Respiratory sinus arrhythmia in freely moving and anesthetized rats. J Appl Physiol 97: 1431–1436, 2004.[Abstract/Free Full Text]
  9. Charles SM, Zhang L, Longo LD, Buchholz JN, Pearce WJ. Postnatal maturation attenuates pressure-evoked myogenic tone and stretch-induced increases in Ca2+ in rat cerebral arteries. Am J Physiol Regul Integr Comp Physiol 293: R737–R744, 2007.[Abstract/Free Full Text]
  10. Cseko C, Bagi Z, Koller A. Biphasic effect of hydrogen peroxide on skeletal muscle arteriolar tone via activation of endothelial and smooth muscle signaling pathways. J Appl Physiol 97: 1130–1137, 2004.[Abstract/Free Full Text]
  11. Davis MJ. Myogenic response gradient in an arteriolar network. Am J Physiol Heart Circ Physiol 264: H2168–H2179, 1993.[Abstract/Free Full Text]
  12. Davis MJ, Hill MA. Signaling mechanisms underlying the vascular myogenic response. Physiol Rev 79: 387–423, 1999.[Abstract/Free Full Text]
  13. De Wit C, Jahrbeck B, Schafer C, Bolz SS, Pohl U. Nitric oxide opposes myogenic pressure responses predominantly in large arterioles in vivo. Hypertension 31: 787–794, 1998.[Abstract/Free Full Text]
  14. DeLano FA, Schmid-Schonbein GW, Skalak TC, Zweifach BW. Penetration of the systemic blood pressure into the microvasculature of rat skeletal muscle. Microvasc Res 41: 92–110, 1991.[CrossRef][Web of Science][Medline]
  15. Ekelund U, Bjornberg J, Grande PO, Albert U, Mellander S. Myogenic vascular regulation in skeletal muscle in vivo is not dependent of endothelium-derived nitric oxide. Acta Physiol Scand 144: 199–207, 1992.[Web of Science][Medline]
  16. Fredricks KT, Liu Y, Lombard JH. Response of extraparenchymal resistance arteries of rat skeletal muscle to reduced PO2. Am J Physiol Heart Circ Physiol 267: H706–H715, 1994.[Abstract/Free Full Text]
  17. Geary GG, Buchholz JN, Pearce WJ. Maturation depresses mouse cerebrovascular tone through endothelium-dependent mechanisms. Am J Physiol Regul Integr Comp Physiol 284: R734–R741, 2003.[Abstract/Free Full Text]
  18. Hamberg M, Svensson J, Samuelsson B. Thromboxanes: a new group of biologically active compounds derived from prostaglandin endoperoxides. Proc Natl Acad Sci USA 72: 2994–2998, 1975.[Abstract/Free Full Text]
  19. Harder DR, Narayanan J, Birks EK, Liard JF, Imig JD, Lombard JH, Lange AR, Roman RJ. Identification of a putative microvascular oxygen sensor. Circ Res 79: 54–61, 1996.[Abstract/Free Full Text]
  20. Hill MA, Davis MJ, Meininger GA. Cyclooxygenase inhibition potentiates myogenic activity in skeletal muscle arterioles. Am J Physiol Heart Circ Physiol 258: H127–H133, 1990.[Abstract/Free Full Text]
  21. Huang A, Koller A. Endothelin and prostaglandin H2 enhance arteriolar myogenic tone in hypertension. Hypertension 30: 1210–1215, 1997.[Abstract/Free Full Text]
  22. Huang A, Sun D, Koller A. Endothelial dysfunction augments myogenic arteriolar constriction in hypertension. Hypertension 22: 913–921, 1993.[Abstract/Free Full Text]
  23. Huang A, Sun D, Koller A, Kaley G. Gender difference in myogenic tone of rat arterioles is due to estrogen-induced, enhanced release of NO. Am J Physiol Heart Circ Physiol 272: H1804–H1809, 1997.[Abstract/Free Full Text]
  24. Hudlicka O. Development and adaptability of microvasculature in skeletal muscle. J Exp Biol 115: 215–228, 1985.[Abstract/Free Full Text]
  25. Jackson WF, Duling BR. The oxygen sensitivity of hamster cheek pouch arterioles. Circ Res 53: 515–525, 1983.[Abstract/Free Full Text]
  26. Juncos LA, Garvin J, Carretero OA, Ito S. Flow modulates myogenic responses in isolated microperfused rabbit afferent arterioles via endothelium-derived nitric oxide. J Clin Invest 95: 2741–2748, 1995.[Web of Science][Medline]
  27. Kelly AM, Rosser BW, Hoffman R, Panettieri RA, Schiaffino S, Rubinstein NA, Nemeth PM. Metabolic and contractile protein expression in developing rat diaphragm muscle. J Neurosci 11: 1231–1242, 1991.[Abstract]
  28. Kozma F, Johnson RA, Zhang F, Yu C, Tong X, Nasjletti A. Contribution of endogenous carbon monoxide to regulation of diameter in resistance vessels. Am J Physiol Regul Integr Comp Physiol 276: R1087–R1094, 1999.[Abstract/Free Full Text]
  29. Krijgsveld KL, Olson JM, Ricklefs RE. Catabolic capacity of the muscles of shorebird chicks: maturation of function in relation to body size. Physiol Biochem Zool 74: 250–260, 2001.[CrossRef][Web of Science][Medline]
  30. Kuo L, Chilian WM, Davis MJ. Interaction of pressure- and flow-induced responses in porcine coronary resistance vessels. Am J Physiol Heart Circ Physiol 261: H1706–H1715, 1991.[Abstract/Free Full Text]
  31. Lagaud G, Gaudreault N, Moore ED, Van Breemen C, Laher I. Pressure-dependent myogenic constriction of cerebral arteries occurs independently of voltage-dependent activation. Am J Physiol Heart Circ Physiol 283: H2187–H2195, 2002.[Abstract/Free Full Text]
  32. Lahlou S, Magalhaes PJC, de Siqueira RJB, Figueiredo AF, Interaminense LFL, Maia JGS, Sousa PJC. Cardiovascular effects of the essential oil of aniba canelilla bark in normotensive rats. J Cardiovasc Pharmacol 46: 412–421, 2005.[CrossRef][Web of Science][Medline]
  33. Linderman JR, Boegehold MA. Arteriolar network growth in rat striated muscle during juvenile maturation. Int J Microcirc Clin Exp 16: 232–239, 1996.[Web of Science][Medline]
  34. Linderman JR, Boegehold MA. Modulation of arteriolar sympathetic constriction by local nitric oxide: onset during rapid juvenile growth. Microvasc Res 56: 192–202, 1998.[CrossRef][Web of Science][Medline]
  35. Linderman JR, Boegehold MA. Growth-related changes in the influence of nitric oxide on arteriolar tone. Am J Physiol Heart Circ Physiol 277: H1570–H1578, 1999.[Abstract/Free Full Text]
  36. Nagi MM, Ward ME. Modulation of myogenic responsiveness by CO2 in rat diaphragmatic arterioles: role of endothelium. Am J Physiol Heart Circ Physiol 272: H1419–H1425, 1997.[Abstract/Free Full Text]
  37. Nurkiewicz TR, Boegehold MA. Limitation of arteriolar myogenic activity by local nitric oxide: segment-specific effect of dietary salt. Am J Physiol Heart Circ Physiol 277: H1946–H1955, 1999.[Abstract/Free Full Text]
  38. Nurkiewicz TR, Boegehold MA. Calcium-independent release of endothelial nitric oxide in the arteriolar network: onset during rapid juvenile growth. Microcirculation 11: 453–462, 2004.[CrossRef][Web of Science][Medline]
  39. Peredo HA, Enero MA. Effect of endothelium removal on basal and muscarinic cholinergic stimulated rat mesenteric vascular bed prostanoid synthesis. Prostaglandins Leukot Essent Fatty Acids 48: 373–378, 1993.[CrossRef][Web of Science][Medline]
  40. Price RJ, Skalak TC. Distribution of cellular proliferation in skeletal muscle transverse arterioles during maturation. Microcirculation 5: 39–47, 1998.[CrossRef][Web of Science][Medline]
  41. Proctor KG, Damon DN, Duling BR. Tissue PO2 and arteriolar responses to metabolic stimuli during maturation of striated muscle. Am J Physiol Heart Circ Physiol 241: H325–H331, 1981.[Abstract/Free Full Text]
  42. Sarelius IH, Damon DN, Duling BR. Microvascular adaptations during maturation of striated muscle. Am J Physiol Heart Circ Physiol 241: H317–H324, 1981.[Abstract/Free Full Text]
  43. Schubert R, Lidington D, Bolz SS. The emerging role of Ca2+ sensitivity regulation in promoting myogenic vasoconstriction. Cardiovasc Res 77: 8–18, 2008.[Abstract/Free Full Text]
  44. Su BY, Reber KM, Nankervis CA, Nowicki PT. Development of the myogenic response in postnatal intestine: role of PKC. Am J Physiol Gastrointest Liver Physiol 284: G445–G452, 2003.[Abstract/Free Full Text]
  45. Sun D, Kaley G, Koller A. Characteristics and origin of myogenic response in isolated gracilis muscle arterioles. Am J Physiol Heart Circ Physiol 266: H1177–H1183, 1994.[Abstract/Free Full Text]
  46. Sun D, Huang A, Koller A, Kaley G. Flow-dependent dilation and myogenic constriction interact to establish the resistance of skeletal muscle arterioles. Microcirculation 2: 289–295, 1995.[Medline]
  47. Uluoglu C, Zengil H. Comparison of different de-endothelization procedures in the isolated rat thoracic aorta: a short communication. Res Commun Mol Pathol Pharmacol 113–114: 289–297, 2003.
  48. Ungvari Z, Koller A. Endothelin and prostaglandin H2/thromboxane A2 enhance myogenic constriction in hypertension by increasing Ca2+ sensitivity of arteriolar smooth muscle. Hypertension 36: 856–861, 2000.[Abstract/Free Full Text]
  49. Vreman HJ, Ekstrand BC, Stevenson DK. Selection of metalloporphyrin heme oxygenase inhibitors based on potency and photoreactivity. Pediatr Res 33: 195–200, 1993.[Web of Science][Medline]
  50. Wang DH, Prewitt RL. Microvascular development during normal growth and reduced blood flow: introduction of a new model. Am J Physiol Heart Circ Physiol 260: H1966–H1972, 1991.[Abstract/Free Full Text]
  51. Willis AP, Leffler CW. NO and prostanoids: age dependence of hypercapnia and histamine-induced dilations of pig pial arterioles. Am J Physiol Heart Circ Physiol 277: H299–H307, 1999.[Abstract/Free Full Text]
  52. Zhang F, Kaide J, Wei Y, Jiang H, Yu C, Balazy M, Abraham NG, Wang W, Nasjletti A. Carbon monoxide produced by isolated arterioles attenuates pressure-induced vasoconstriction. Am J Physiol Heart Circ Physiol 281: H350–H358, 2001.[Abstract/Free Full Text]
  53. Zweifach BW, Kovalcheck S, De Lano F, Chen P. Micropressure-flow relationships in a skeletal muscle of spontaneously hypertensive rats. Hypertension 3: 601–614, 1981.[Free Full Text]



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