INTRODUCTION

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THE MYOGENIC RESPONSE has been investigated extensively at the arteriolar level and is thought to play an important role in local blood flow regulation (27). Segmental differences in myogenic responsiveness exist within some vascular beds (13, 22, 38) such that responsiveness increases with decreasing arteriolar size down to vessels 15-20 µm internal diameter (13, 38). An intact endothelium is not required for the generation of pressure-induced myogenic tone in most vascular beds (16, 28, 45), but studies on isolated arterioles suggest that myogenic responses can be attenuated by endothelium-derived factors released by luminal shear stress (29, 44). The extent to which shear stress may modulate arteriolar myogenic activity in vivo is unknown.

Enhanced myogenic responsiveness has been postulated to underlie increased arteriolar tone in hypertension (17, 23). Coronary arterioles from patients with essential hypertension exhibit greater myogenic activity than those from normotensive patients (39). However, more extensive studies on the spontaneously hypertensive rat (SHR) indicate that this is not characteristic of all vascular beds. Increased myogenic responsiveness has been observed in arterioles from some SHR vascular beds (17, 20, 23, 25) but not others (20, 26, 40). As with some forms of human hypertension, arterial pressure in the SHR is augmented by high dietary salt intake (2). Increased salt intake can also influence microvascular structure and function independent of any effect on arterial pressure (3-5, 21, 35, 46). For example, ingestion of a high-salt diet leads to remodeling of proximal arterioles in the spinotrapezius muscle of normotensive rats (5), and these vessels also exhibit a blunted responsiveness to acetylcholine and increased wall shear stress due to a selective loss of local nitric oxide (NO) activity (3, 4).

We undertook the current study to determine whether hypertension and/or high salt intake alters the myogenic responsiveness of arterioles in the rat spinotrapezius muscle and to determine whether this responsiveness is modulated by luminal shear stress under either of these conditions. Arteriolar responses to acute increases in transmural pressure were evaluated in SHR and Wistar-Kyoto (WKY) rats fed low (0.45%)- or high (7%)-salt diets. Large- (arcade bridge) and intermediate-sized (arcade) arterioles were studied to also evaluate the possibility of longitudinal differences in myogenic behavior.

	METHODS

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All surgical and experimental procedures were approved by the West Virginia University Animal Care and Use Committee.

Animals. Male WKY and SHR (Harlan Sprague Dawley, Indianapolis, IN) were received at 4 wk of age and immediately placed on a whole-grain, low-salt diet containing 0.45% NaCl by weight (Teklad TD 88311, Madison, WI). After 1 wk, half of the rats from each strain were randomly selected and placed on a high-salt diet containing 7% NaCl by weight (Teklad TD 92100), with the remaining rats continued on the low-salt diet. All rats were studied 4-5 wk after assignment to "low-salt" (LS) or "high-salt" (HS) groups.

Surgical preparation of spinotrapezius muscle. Each rat was anesthetized with thiopental sodium (100 mg/kg ip) and placed on a heating pad to maintain a 37°C rectal temperature. The trachea was intubated to ensure a patent airway, and the right carotid artery was cannulated to measure arterial pressure. In some experiments, the right femoral vein was also cannulated to simultaneously measure central venous pressure. The right spinotrapezius muscle was exteriorized for microscopic observation as previously described (30). This method of preparation does not interrupt any feed vessels or neural input to the muscle. Throughout the surgery and subsequent experimental period, the muscle was continuously superfused with an electrolyte solution (in mM: 119 NaCl, 25 NaHCO₃, 6 KCl, and 3.6 CaCl₂) warmed to 35°C and equilibrated with 95% N₂-5% CO₂ (pH = 7.35-7.40). Superfusate flow rate was maintained at 4-6 ml/min to minimize equilibration with atmospheric oxygen (6).

After the spinotrapezius muscle was surgically prepared, the rat was placed in an airtight, ventilated Plexiglas box (Fig. 1), and the muscle was exteriorized from the box through a small slot. The muscle was then secured ventral side up over an optical pedestal and enclosed in a superfusate bath chamber. Fresh air was continuously circulated through the box containing the rat, and box pressure was increased by raising the air inflow rate. These pressure increases were monitored with a mercury manometer and a Gould P23 ID pressure transducer. The box pressurization technique has been widely used to study the myogenic behavior of arterioles in numerous vascular beds (9, 38, 49). This procedure causes simultaneous and equivalent increases in systemic arterial and venous pressure, thereby maintaining a normal arterial-venous pressure gradient across the exteriorized vascular bed. However, because the exteriorized tissue is subjected only to atmospheric pressure, transmural pressure is increased in all segments of its vascular network. This increased transmural pressure elicits a pronounced arteriolar myogenic constriction without changing heart rate, respiration rate, neurogenic vascular tone, or renin-angiotensin system activity (38).

View larger version (21K): [in this window] [in a new window]   Fig. 1.   Plexiglas box preparation used to study myogenic responses. Animal is enclosed in an airtight box except for the spinotrapezius muscle. Box pressure is raised by increasing air flow through the box.

Intravital microscopy and measurement of microvascular variables. The animal preparation was transferred to the stage of an Olympus BHMJ intravital microscope (Hyde Park, NY) coupled to a charge-coupled device video camera (Dage-MTI, Michigan City, IN). Video images were displayed on a Sony high-resolution video monitor and videotaped for off-line analysis. Observations were made with an Olympus ×20 water immersion objective (final video image magnification = ×1,460). Arteriolar centerline red blood cell velocities were measured on-line with an optical Doppler velocimeter (Microcirculation Research Institute, Texas A & M University), and arteriolar inner diameters were measured off-line with a video image-shearing monitor (IPM, San Diego, CA).

Branches of three different arteries enter the spinotrapezius muscle, where they connect to form an arteriolar structure known as the arcade bridge (43). The arcade bridge gives rise to numerous interconnected arcade arterioles that form an extensive anastomosing network. In the experiments described below, both arcade bridge and arcade arterioles were studied to identify any longitudinal response gradient that may exist in this portion of the network.

Experimental protocol 1. One to three arcade bridge or arcade arteriole segments were studied per animal. After a 2-min control period, box pressure was raised to 10, 20, or 30 mmHg above atmospheric pressure for 2 min. Pressurization was followed by a minimum 2-min recovery period to allow vessel diameter and red blood cell velocity to return to control levels. This sequence was repeated a total of three times for each arteriole to assess responsiveness to each of the three pressure increases delivered in random order. At the end of every experiment, adenosine was added to the superfusate at a final concentration of 10⁴ M to determine the passive diameter of each vessel studied.

Experimental protocol 2. These experiments were conducted in additional rats from each experimental group. To verify that systemic pressure increases were fully transmitted to the arteriolar network, luminal pressure was measured in arcade bridge arterioles during the control, pressurization, and recovery periods described above. The microvascular pressures obtained in these experiments were also used to estimate pressure changes in the arcade bridge arterioles studied under protocol 1.

Microvascular pressures were measured by the servo-null technique (24) (IPM model 5A). Pressure pipettes were beveled at a 23° to 25° angle to an outer tip diameter of 3-5 µm and filled with 2 M NaCl. Because of transient tissue movement during box pressurization, the pipette was briefly removed from the vessel immediately before pressurization and depressurization in some experiments. Once the tissue had moved, the pipette was immediately reinserted into the vessel. In other experiments, tissue movement was minimal, and it was possible to pressurize the box without removing the pipette tip from the vessel lumen.

Data and statistical analysis. Arteriolar diameter (D, µm) and centerline red blood cell velocity (V_cl, mm/s) were sampled at 10-s intervals during all control, pressurization, and recovery periods. Resting vascular tone (T) was calculated for each vessel as follows: T = [(D_pass  D_c)/D_pass] × 100, where D_pass is passive diameter under adenosine, and D_c is the diameter measured during the control period. A tone value of 100% represents complete vessel closure, whereas 0% represents the passive state. Total wall tension (in N/m) for arcade bridge arterioles was also estimated as follows: total wall tension = r × P_m, where r is vessel radius and P_m is estimated microvascular pressure (based on the fraction of arterial pressure transmitted to the arcade bridges in each group studied in protocol 2). Arteriolar myogenic responsiveness was evaluated by comparing diameter values (absolute or normalized to passive diameter) before and after box pressurization. Normalized diameters and absolute changes in diameter (D) were calculated as follows: D (% of passive) = (D_ss/D_pass) × 100, and D = D_min  D_c, where D_ss is steady-state arteriolar diameter during the period of interest, and D_min is minimum arteriolar diameter recorded during box pressurization. To quantify any secondary change in myogenic constriction (possibly due to changes in arteriolar flow or shear stress), an "escape index" (EI_D) was calculated as follows: EI_D = (D_end  D_min)/(D_c  D_min), where D_end is mean arteriolar diameter during the final 60 s of box pressurization.

Mean red blood cell velocity (V_mean) was calculated as V_cl/1.6, where 1.6 represents the ratio of centerline red blood cell velocity to mean velocity for vessels down to 10 µm in diameter (51). Paired values for D and V_mean were used to calculate arteriolar volume flow (, nl/s) and wall shear rate (WSR, s¹) as follows:  = V_mean × ( × D²/4), and WSR = 8 × (V_mean/D). WSR was used as an index of wall shear stress and its calculation assumes a parabolic velocity profile.

Control values for all microvascular variables (including arteriolar pressure) were calculated as the mean of 12 samples obtained during the 2-min control period, and minimum values were those measured during maximum arteriolar constriction in response to box pressurization. Because of transient tissue movement associated with the onset of box pressurization, no microvascular variables could be measured during the first 10 s of pressurization. The effect of box pressurization on arteriolar and WSR was quantified as percent change from control value {[(minimum value/control value)1] × 100}.

All data are reported as means ± SE. Statistical analysis was performed by commercially available software (Sigmastat, Jandel Scientific; Prism, Graphpad Software). Two-way ANOVA was used to determine the effects of strain, diet, and strain-diet interactions on the measured variables. One-way ANOVA was used to determine differences within a group subjected to repeated measures. For all ANOVA procedures, the Student-Newman-Keuls method for post hoc analysis was used to isolate pairwise differences among specific groups. Analysis of covariance was used to determine differences in regression line slopes. Significance was assessed at the 95% confidence level (P < 0.05) for all tests.

	RESULTS

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General characteristics of all rats used in this study are reported in Table 1. In both the WKY and SHR strains, rats fed a high-salt diet were slightly but significantly older at the time of use than those fed a low-salt diet, but there were no significant differences in body weight among the four groups. MAP was significantly higher in SHR fed a low-salt diet than in WKY fed either a low- or high-salt diet. The high-salt diet significantly increased arterial pressure in SHR but not in WKY.

Table 2 displays the control variables for arcade bridge arterioles studied in protocol 1. A total of eight WKY rats (4 LS, 4 HS) and eight SHR (4 LS, 4 HS) were used in these experiments. High salt intake had no effect on resting arteriolar diameters in either strain, but when data from LS and HS rats of each strain were combined, resting arteriolar diameters were significantly smaller in SHR than in WKY. In contrast, passive arteriolar diameters were not different between strains. Arterioles in SHR also displayed a significantly higher resting tone than those in WKY rats, with dietary salt having no effect on resting tone in either strain. In addition, arterioles in SHR displayed a lower resting volume flow than those in WKY, again with no effect of salt in either strain. There were no differences among groups in resting arteriolar wall shear rate. Total arteriolar wall tension at rest averaged 0.089 ± 0.006 N/m for WKY-LS, 0.085 ± 0.006 N/m for WKY-HS, 0.108 ± 0.008 N/m for SHR-LS, and 0.124 ± 0.006 N/m for SHR-HS. The wall tension values for SHR were significantly greater than those for WKY.

Table 2 also displays the control variables for arcade arterioles studied in protocol 1. A total of 15 WKY rats (8 LS, 7 HS) and 16 SHR (9 LS, 7 HS) were used in these experiments. In contrast to the more proximal arcade bridge arterioles, the resting diameter of these arterioles was similar in all four groups. Although there were also no strain-related differences in passive arcade arteriole diameter, there was a significant diet effect in each strain, such that passive diameters in HS rats were significantly smaller than those in LS rats. No differences in resting tone were found among the four groups, but there was a diet effect on volume flow and wall shear rate in each strain, with these variables being significantly lower in HS rats than in LS rats.

In all groups, box pressurization produced immediate and equal increases in arterial and venous pressure (venous pressure data not shown). In WKY rats, these systemic pressure increases were identical to the box pressure increase at all steps. In SHR, systemic pressure increases were also identical to the box pressure increase at +10 mmHg, and 70-80% of the box pressure increase at +20 and +30 mmHg (Table 3). In protocol 2, pressure in arcade bridge arterioles was measured in 11 WKY rats (5 LS, 6 HS) and 10 SHR (6 LS, 4 HS). Resting arteriolar pressures averaged 33 ± 2 mmHg (45 ± 3% of arterial pressure) in WKY-LS, 27 ± 2 mmHg (39 ± 3% of arterial pressure) in WKY-HS, 37 ± 4 mmHg (40 ± 4% of arterial pressure) in SHR-LS, and 46 ± 5 mmHg (42 ± 5% of arterial pressure) in SHR-HS. Resting arteriolar pressures were significantly higher in SHR than in WKY, but high salt intake had no effect on arteriolar pressure in either strain. In each experimental group, each level of box pressurization increased arteriolar pressure by the same amount as arterial pressure. Box pressurization had no effect on heart rate or respiration rate in any experimental group (data not shown).

After the increases in arterial and arteriolar pressure, arcade bridge arterioles in each group either maintained their control diameter or constricted to a new steady-state diameter within 10-30 s. Compared with the other three groups, arterioles in the WKY-HS group displayed attenuated myogenic responses (Fig. 2A). Arteriolar diameters in WKY-HS were significantly greater than those in the other three groups at each level of increased pressure, except when compared with WKY-LS at +20 mmHg box pressurization. The slope of the first-order regression line fit to the WKY-HS data is significantly less than that for the WKY-LS data and, in contrast to all other groups, is not significantly different from zero. In contrast to WKY, the slope of the regression line for the SHR-HS data is modestly but significantly greater than that for the SHR-LS data, indicating an opposite effect of high salt intake in this strain. There were no differences in arcade bridge myogenic responsiveness (as assessed by absolute diameters or pressure-diameter line slopes) between the WKY-LS and SHR-LS groups. The regression line equations for each group are given in the legend for Fig. 2.

View larger version (24K): [in this window] [in a new window]   Fig. 2.   Arcade bridge (A) and arcade arteriole (B) myogenic responsiveness in each experimental group. WKY-LS, Wistar-Kyoto rats fed 0.45% NaCl; WKY-HS, Wistar-Kyoto rats fed 7% NaCl; SHR-LS, spontaneously hypertensive rats fed 0.45% NaCl; SHR-HS, spontaneously hypertensive rats fed 7% NaCl. Equations of 1st-order regression lines in A: WKY-LS, y = 0.3x + 52.5, r2 = 0.90; WKY-HS, y = 0.08x + 42.4, r2 = 0.17; SHR-LS, y = 0.2x + 47.8, r2=0.90; SHR-HS, y = 0.5x + 61.1, r2 = 0.98. Equations of 1st-order regression lines in B: WKY-LS, y = 0.2x + 38.2, r2 = 0.94; WKY-HS, y = 0.2x + 36.9, r2 = 0.95; SHR-LS, y = 0.2x + 45.6, r2 = 0.89; SHR-HS, y = 0.2x + 44.2, r2 = 0.90. * P < 0.05 vs. WKY-LS, SHR-LS, and SHR-HS.  P < 0.05 vs. SHR-LS and SHR-HS. In A, pressure-diameter line slope for WKY-HS is significantly less than those for other 3 groups (P < 0.05) and pressure-diameter line slope for SHR-HS is significantly greater than that for SHR-LS (P < 0.05).

Arcade arterioles in each group always constricted after box pressurization, reaching a new steady-state diameter within 10-30 s. In contrast to the arcade bridge arterioles, there were no significant intergroup differences in arcade arteriole responses to any pressure increase or in the slopes of the regression lines fit to these data (Fig. 2B). The slope of each regression line was significantly different from zero, and line equations for each group are given in the legend for Fig. 2.

The top panels of Fig. 3 illustrate changes in total arcade bridge wall tension during box pressurization. In WKY rats fed either diet, total wall tension increased with arteriolar pressure (Fig. 3, top left). However, this increase was greater in WKY-HS than WKY-LS, as indicated by the significantly greater slope of the regression line fit to the WKY-HS data. Total wall tension also increased with arteriolar pressure in SHR-LS but not in SHR-HS (Fig. 3, top right). The slope of the regression line fit to the SHR-HS data is significantly less than that for the SHR-LS data. There is no difference in the regression line slope between WKY-LS and SHR-LS. The regression line equations for each group are given in the legend for Fig. 3.

View larger version (25K): [in this window] [in a new window]   Fig. 3.   Arcade bridge arteriole total wall tension during myogenic constriction in each experimental group. Equations of 1st-order regression lines are top left: WKY-LS, y = 0.001x + 0.04, r2 = 0.99 and WKY-HS, y = 0.003x + 0.002, r2 = 0.98; top right: SHR-LS, y = 0.001x + 0.05, r2 = 0.95 and SHR-HS, y = 0.0002x + 0.11, r2 = 0.33; bottom left: WKY-LS, y = 0.006x + 0.35, r2 = 0.90 and WKY-HS, y = 0.03x  1.31, r2 = 0.26; bottom right: SHR-LS, y = 0.005x + 0.29, r2 = 0.73 and SHR-HS, y = 0.0003x + 0.14, r2 = 0.20. In all panels, total wall tension-pressure and -diameter line slopes for HS groups are significantly different from those for LS groups (P < 0.05).

The bottom panels of Fig. 3 illustrate arcade bridge wall tension values during pressurization, plotted as a function of vessel diameter. In WKY-LS rats, total wall tension increased as arteriolar diameter decreased, whereas in WKY-HS rats, total wall tension increased to a higher level as arteriolar diameter remained constant (Fig. 3, bottom left). Total wall tension also increased as arteriolar diameter decreased in SHR-LS but not in SHR-HS (Fig. 3, bottom right). The slope of the regression line fit to the SHR-HS data is significantly less than that for the SHR-LS data. There is no difference in regression line slope between WKY-LS and SHR-LS. The regression line equations for each group are given in the legend for Fig. 3.

The myogenic responsiveness of arcade bridge versus arcade arterioles in each group is shown in Fig. 4. In WKY-HS, the diminished myogenic responses of arcade bridge arterioles produce a difference in regression line slopes between the two vessel types, revealing slightly greater myogenic activity in the arcade arterioles. There were no such differences between these two vessel types in any other group. Similar results are obtained when the data for each group are compared on the basis of absolute diameter values (not shown).

View larger version (25K): [in this window] [in a new window]   Fig. 4.   Myogenic responsiveness of arcade bridge arterioles vs. arcade arterioles in each group. Diameters are normalized to passive diameter for direct comparison.

Because resting blood flow or luminal shear stress could have attenuated the magnitude of myogenic constriction in the vessels that we studied (29, 44), the possibility of a relationship among these variables was investigated in each group. In Fig. 5, the initial myogenic constriction with each pressurization step is plotted as a function of the immediately preceding wall shear rate for each arcade bridge arteriole. If myogenic constriction is influenced by the prevailing wall shear stress, then the constriction should be less in vessels with high prevailing shear stress. In each group, the clear absence of any correlation between these variables argues against a shear-dependent attenuation of the initial myogenic constriction. Similar results were found for the arcade arterioles in each group, and there was also no correlation between resting arteriolar volume flow and initial myogenic constriction for either arcade bridge or arcade arterioles in any group (not shown).

View larger version (21K): [in this window] [in a new window]   Fig. 5.   Initial myogenic constriction plotted as a function of preconstriction (resting) wall shear rate for arcade bridge arterioles in each group. , Data for +10 mmHg box pressure. , Data for +20 mmHg box pressure. , Data for +30 mmHg box pressure. D, change in diameter.

After box pressurization was increased to +30 mmHg, myogenic constriction was accompanied by a significant flow reduction in arcade bridges of SHR-HS and in arcade arterioles of all four groups (Table 4). Box pressurization to this level also led to increased wall shear rates in arcade bridges of WKY-LS, SHR-LS, and SHR-HS and in arcade arterioles of all four groups. We also evaluated the possibility that these flow or shear stress changes could have secondarily influenced the magnitude of myogenic constriction. In Fig. 6, the escape index for diameter is plotted as a function of the initial change in wall shear rate for each arcade bridge arteriole. In each group, there was clearly no correlation between these variables, arguing against a secondary shear-dependent lessening of myogenic constriction. Similar results were found for the arcade arterioles in each group, and there was also no correlation between initial flow changes and any secondary diameter changes for either the arcade bridge or arcade arterioles in any group (not shown).

View larger version (19K): [in this window] [in a new window]   Fig. 6.   Diameter escape index plotted as a function of initial change in wall shear rate with myogenic constriction for arcade bridge arterioles in each group. , Data for +10 mmHg box pressure. , Data for +20 mmHg box pressure. , Data for +30 mmHg box pressure.

	DISCUSSION

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There are four major findings in this study. First, a high-salt diet in the absence of hypertension attenuates the myogenic responsiveness of large arterioles, but not intermediate-sized arterioles, in rat spinotrapezius muscle. Second, a high-salt diet combined with hypertension modestly augments the myogenic responsiveness of large but not intermediate-sized arterioles. Third, a change in the myogenic responsiveness of either vessel type is not associated with hypertension in the absence of high salt intake. Fourth, the myogenic behavior of either vessel type is not modulated by arteriolar blood flow or its attendant shear stress within the range of these variables that we observed in vivo.

Resting microvascular characteristics in hypertensive and salt-fed rats. Despite elevated input pressure to the SHR spinotrapezius muscle, blood flow in its arcade bridge vessels is actually lower than that in WKY (Table 2). This indicates that the hemodynamic resistance of this vascular bed is markedly increased in the SHR. Increased vascular resistance in spontaneous hypertension may be due to structural or active reductions in arteriolar diameter (1, 31) and/or arteriolar density (arteriolar rarefaction) (12, 41). Because arteriolar rarefaction does not occur in spinotrapezius muscle of the SHR (6, 15), we anticipated that arteriolar diameters would be reduced in these animals. We found the resting diameter of arcade bridge arterioles to be significantly smaller in SHR than in WKY, with no difference between strains in passive arcade bridge diameters (Table 2). As a result, the calculated resting tone of arcade bridge arterioles is higher in SHR than WKY. We found no difference between SHR and WKY in either resting or passive diameter of the smaller arcade arterioles, indicating that active diameter reductions are limited to the most proximal arterioles in SHR spinotrapezius muscle. This confirms previous observations in this vascular bed (31) and in the adjacent cutaneous maximus muscle of conscious SHR (32). The reduction in resting arcade bridge diameters undoubtedly contributes to increased spinotrapezius muscle vascular resistance in the SHR, helping to prevent tissue overperfusion and the transmission of high luminal pressures to the terminal arterioles and capillaries (14, 50).

Consumption of a high-salt diet for 4-5 wk increased MAP by ~15% in SHR (Table 1), confirming that this strain has evolved to become moderately salt sensitive (2). This augmentation of hypertension was not associated with an additional reduction in arteriolar diameters (Table 2), suggesting that if spinotrapezius muscle vascular resistance is further increased in these rats, the vascular changes responsible for this increase must occur elsewhere. The high-salt diet was associated with decreased passive arcade arteriole diameters in both WKY and SHR (Table 2). Similarly, high dietary salt leads to decreased passive arcade bridge diameters in spinotrapezius muscle of normotensive and hypertensive Dahl rats, most likely due to vascular remodeling (5). Because circulating angiotensin II is required for the maintenance of normal arteriolar wall structure (47), this salt-dependent decrease in passive diameters could be due to the reduction in circulating angiotensin II that occurs with salt loading (18). Additional studies are required to critically test this hypothesis.

Myogenic responsiveness in hypertensive and salt-fed rats. In the current study, systemic pressure increases induced by box pressurization were completely transmitted to the arterioles (Table 3), as has been reported in other microvascular studies using this approach to investigate myogenic behavior (34, 37). The myogenic responsiveness of spinotrapezius muscle arterioles was not enhanced in hypertensive rats fed a normal diet (Fig. 2), which differs from reports that essential or spontaneous hypertension is associated with enhanced myogenic responsiveness of coronary arterioles (39), the intermediate interlobular artery (20), and cremaster muscle arterioles (17, 23). However, myogenic responsiveness remains unchanged in the distal interlobular artery (20), renal afferent arterioles (19), and small mesenteric arteries (26) of the SHR and is reduced in small SHR cerebral arteries (40). The functional basis for such heterogeneity of myogenic behavior within the SHR is unclear. In those vascular beds where myogenic responsiveness is enhanced, this change could contribute to a rightward shift and/or widening of the autoregulatory range (9, 10).

Another major finding of this study is that a high-salt diet in the absence of hypertension attenuates the myogenic responsiveness of arcade bridge arterioles but not arcade arterioles. Dietary salt has previously been shown to attenuate the myogenic behavior of small interlobular arteries and afferent arterioles in normotensive Dahl rats (46). In contrast, high salt intake may not alter the myogenic behavior of small cerebral and gracilis muscle arteries (48), indicating that this effect of dietary salt may be specific to certain vessel types or vascular beds.

Effect of hypertension and myogenic activity on arteriolar wall mechanics. At normal resting tone, vascular smooth muscle activity accounts for ~70% of total wall tension in intestinal arterioles of WKY and SHR (11). Assuming a similar contribution of smooth muscle activity to total arteriolar wall tension in spinotrapezius muscle, the elevated resting wall tensions that we found in arcade bridge arterioles of SHR fed either diet strongly suggest that smooth muscle force generation is increased in these vessels at rest. Their smaller resting diameters (Table 2) despite elevated luminal pressure (Table 3) are also suggestive of increased smooth muscle force generation. Although we did not measure pressure in the arcade arterioles, previous measurements indicate that these pressures are also higher in SHR than in WKY (50). Our finding of normal arcade arteriole diameters in SHR despite elevated luminal pressure suggests that smooth muscle force generation is also increased in these vessels. Because smooth muscle mass is not increased in either arcade bridge or arcade arterioles of spinotrapezius muscle at this stage of hypertension (42), this increase in resting force generation must be due to increased contractile activity of existing smooth muscle. This in turn could reflect an enhanced responsiveness to certain vasoconstrictor influences, such as neurally released norepinephrine (8) or local oxygen (36). Whatever the underlying cause(s), our findings suggest that an enhanced responsiveness to pressure-induced stretch does not contribute to increased smooth muscle contractile activity in arterioles of SHR spinotrapezius muscle.

Acute increases in luminal pressure and resultant myogenic activity will lead to changes in arteriolar wall tension. In WKY fed a low-salt diet, total arcade bridge wall tension increases with arteriolar pressure (Fig. 3, top left), indicating that myogenic constriction is not sufficient to maintain a constant wall tension as luminal pressure rises. This increase in wall tension is greater in WKY fed a high-salt diet, reflecting the fact that arteriolar diameter did not significantly decrease as luminal pressure increased in this group (Fig. 2A). In SHR fed a low-salt diet, total arcade bridge wall tension also increases with arteriolar pressure (Fig. 3, top right). However, in SHR fed a high-salt diet, wall tension remains constant despite the pressure increases, reflecting the enhanced myogenic constriction (Fig. 2A), which is apparently sufficient to offset the effect of increased luminal pressure. When plotted as a function of arteriolar pressure (Fig. 3, top panels) or diameter (Fig. 3, bottom panels), the wall tension changes associated with myogenic activity are similar in WKY-LS and SHR-LS. This is consistent with the similar pressure-diameter relationships in these two groups (Fig. 2A) and further supports our conclusion that myogenic responsiveness is not increased in SHR spinotrapezius muscle.

Independence of myogenic responsiveness from hemodynamic influences. The arterioles that we studied were somewhat heterogeneous with respect to resting blood flow and wall shear rate, and their myogenic constriction was often accompanied by a reduction in blood flow and an increase in wall shear rate (Table 4). Although hemodynamic shear stress (proportional to wall shear rate) is an important stimulus for arteriolar NO release (3) and reduced blood flow could lead to local accumulation of vasodilator metabolites (7), we did not find any evidence of flow- or shear-dependent attenuation of the myogenic responses that we observed (Figs. 5 and 6). This is consistent with a previous report that myogenic constriction of arterioles in rat cremaster muscle is fully sustained for up to 30 min despite prolonged decreases in arteriolar blood flow and perivascular PO₂ (38).

Our in vivo findings are not consistent with findings that flow-related shear stress attenuates the myogenic constriction of isolated rat cremaster muscle arterioles (29, 44). One major difference between the two types of studies is that we studied myogenic behavior over a narrower range of pressure and flow changes. We increased transmural pressure randomly from 10 to 30 mmHg, whereas the in vitro studies employed a stepwise range from 0 to 120 mmHg (29, 44). Resting arteriolar flow rates in our study ranged from 4.0 to 7.5 nl/s, with subsequent reductions of up to 41% during myogenic constriction (Tables 2 and 4). In contrast, Sun et al. (44) reported attenuation of myogenic constriction with flow rates between 170 and 1,000 nl/s. However, despite these differences, the arterioles that we studied and those studied in vitro may have been exposed to a similar range of shear stress. The arterioles studied by Sun et al. (44) were exposed to shear stresses of 10-70 dyn/cm², and assuming a microvascular blood viscosity of 3.5 cP (32), we estimate that shear stresses in the arcade bridge arterioles we studied ranged from 20 to 26 dyn/cm² at rest to 50 dyn/cm² with myogenic constriction. We also estimate that arcade arteriole shear stresses ranged from 65 to 125 dyn/cm² at rest to 160 dyn/cm² with myogenic constriction. Myogenic constriction may simply be more intense in spinotrapezius muscle arterioles than in cremaster muscle arterioles and therefore more sustainable during secondary hemodynamic changes. A comparison of our data with those reported for the same myogenic stimuli in rat cremaster muscle (38) suggests this is true for the arcade bridge arterioles but not the arcade arterioles. Alternatively, shear stress may be a more potent stimulus for the release of endothelium-derived relaxing factors in cremaster muscle arterioles than in spinotrapezius muscle arterioles.

This study provides some insight into the extent to which different local microvascular control mechanisms may or may not interact in skeletal muscle and their susceptibility to changes during hypertension and/or high salt intake. Additional studies are required to critically investigate the underlying mechanisms responsible for salt-dependent alterations in the arteriolar myogenic response.