INTRODUCTION

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CURRENT RESEARCH (14, 20, 23, 25) indicates that the tonic release of nitric oxide (NO) from neuronal and endothelial cells mediates vascular resistance directly via vasodilator action on smooth muscles and indirectly via modulation of sympathetic neurotransmission, but the precise regulatory mechanisms involved are not clearly understood (5). The NO-forming enzyme NO synthase (NOS) has been found not only in vascular endothelium and neurons of the sympathetic nervous system but also in nerve fibers innervating blood vessels. It has been demonstrated that NO is not only a mediator of neurotransmitter release (17) but also an inhibitory transmitter of nonadrenergic and noncholinergic (NANC) nerves (3). NO appears to have a dual function in the sympathetic nervous system in vivo: reduction of central sympathetic nerve activity (21) and inhibition of peripheral sympathetic vasoconstriction (29).

Addicks et al. (1) reported that blood flow regulation via neuronal NO inhibition of norepinephrine (NE) release becomes more important with the decrease in the size of blood vessels and in the expression of endothelial NOS. Neuronally produced NO has been shown to play a more important role than endothelial NO in some regional vascular beds (3, 16). However, pre- and postjunctional effects remain controversial, and the origin of adventitial NO is yet to be ascertained (6, 28). It is also unknown in which parts and at what level of the vascular system the interaction between NO and NE takes place.

Recent research has shown NE to induce endothelium-dependent relaxation via stimulation of endothelial ₁- and ₂-adrenoceptors (19) and to counteract its vasoconstrictor action (14, 20, 25). The inhibitory mechanism of NO in sympathetic neurotransmission is presumably adjusted locally by a complex feedback mechanism (26) similar to the intercellular mechanism, i.e., the guanylate cyclase-cGMP system of endothelial NO in vascular smooth muscle relaxation (4).

The side effects of anesthesia may lead to unique alterations in the peripheral circulation of experimental animals, as shown in the effects of anesthesia on the contributions of endothelium-derived vasodilators to circulatory control (15). For this reason, the present study examined the role of NO in blood flow regulation in the conscious resting state. Comprehensive whole body regional and systemic hemodynamic data can provide a profound understanding of flow regulation mechanisms.

The aims of this study were to investigate the level of the sympathetic nervous system in which NO mediates regional sympathetic vasoconstriction and to determine whether neural mechanisms are involved in the vasoconstriction after NO inhibition.

	MATERIALS AND METHODS

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The present study was conducted in accordance with the guidelines of the Committee on Animal Care and Use of the University of Hiroshima School of Medicine and with the Guide for Care and Use of Laboratory Animals in the Field of Physiological Science of the Physiological Society of Japan (1988).

Implantation of flow probes and catheters. Male normotensive control Wistar rats (n = 93), 10-23 wk of age and 270-450 g body wt, were used in this study. The superior mesenteric artery, renal artery, or terminal aorta was reached retroperitoneally by a left flank incision, and an electromagnetic flow probe (type FC, Nihon Koden; 1, 1.5, or 2 mm ID) was implanted around the vessel under anesthesia with thiamylal sodium (50 mg/kg ip). A polyethylene catheter (PE-10 fused to PE-20) for arterial pressure measurement was inserted into the right femoral artery so that the tip was placed in the terminal aorta or the right common carotid artery in rats with a probe on the terminal aorta. In all rats another catheter for injection of drugs was inserted into the right external jugular vein (10).

Spinal cord transection. After implantation, each rat was isolated in a polyethylene cage containing wood chips. Three to four days later, blood flow and arterial pressure were measured in the conscious resting state in the home cage. To determine at what level of the sympathetic nervous system NO suppresses sympathetic vasoconstriction in regional vascular beds, 27 rats underwent spinal transection. Rats were anesthetized with ether and placed ventrally, and a midline skin incision was made in the dorsal neck. The spinous process of the vertebra prominens was cut and removed. Next the spinal cord was transected between the first and second dorsal vertebra with an ophthalmological scalpel under visual control. Immediately after spinal transection, administration of ether was stopped. Bleeding was minimal and usually ceased within 10-15 s. A local anesthetic, xylocaine jelly, was applied only around the incision made for transection, and the skin was sutured (11). Arterial pressure and regional blood flows stabilized at a new plateau level several hours after transection. Throughout these procedures and thereafter during the infusion of drugs, blood flow and arterial pressure were recorded continuously.

Estimation of sympathetic vasoconstriction. After stable plateau values were reached, N-nitro-L-arginine methyl ester (L-NAME, 3-5 mg/kg iv) and hexamethonium bromide (C6, 25 mg/kg iv) were infused successively at intervals of 10-15 min. After arterial pressure and blood flow stabilized, the NO precursor L-arginine (L-Arg, 70 mg/kg iv) was injected by bolus.

Drugs. L-NAME and prazosin were purchased from Sigma Chemical (St. Louis, MO) and C6 and L-Arg from Nakarai Tesque (Kyoto, Japan). In a preliminary test we checked how long each agent was effective in the conscious rat. The maximal effects of 3-5 mg/kg L-NAME on pressure and flow lasted >90 min, and recovery toward normal levels did not occur for >2 h with 25 mg/kg C6.

Statistical analysis. Hemodynamic data are expressed as means ± SD. Significant difference from the preceding point in the mean value after drug (L-NAME, C6, L-Arg, or prazosin) infusion was compared using two-way ANOVA for repeated measurements. Significant difference between intact and spinal-sectioned rats within each drug was analyzed by using one-way ANOVA. P < 0.05 was used as the criterion for statistical significance in all experiments.

	RESULTS

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Figure 1 shows simultaneous recordings of arterial pressure and superior mesenteric, renal, or hindquarter flow under conscious and resting states in an intact rat. The recordings show the changes in mean arterial pressure and regional blood flows during the successive infusions of L-NAME and C6 and the bolus injection of L-Arg. Regional blood flows decreased with elevation of arterial pressure during L-NAME infusion for 10-15 min. C6 was injected 10-20 min after arterial pressure and blood flow had plateaued at a new stable level. In succession, L-Arg was injected by bolus after infusion of C6 in the same way. Superior mesenteric and renal flows increased, with reduction of arterial pressure after C6 and L-Arg infusions. However, hindquarter flow decreased in parallel with the reduction of arterial pressure.

View larger version (27K): [in this window] [in a new window]   Fig. 1.   Simultaneous recordings of arterial pressure (AP) and superior mesenteric (MF), renal (RF), and hindquarter (HQF) flows in an intact rat. N-nitro-L-arginine methyl ester (L-NAME) and hexamethonium bromide (C6) were infused successively for periods shown as horizontal bars to total doses of 3.5, 4.3, or 5.0 mg/kg iv and 25 mg/kg iv, respectively. At arrow, L-arginine (L-Arg) was injected at 70 mg/kg iv by bolus.

Figure 2 shows the simultaneous recordings of arterial pressure and regional blood flow in an acute spinal-sectioned rat. Several hours after spinal transection, arterial pressure and regional blood flow stabilized at a new plateau level, and they were measured again in the conscious resting state. Even in spinal-sectioned rats, regional blood flow decreased with elevation of arterial pressure after infusion of L-NAME.

View larger version (24K): [in this window] [in a new window]   Fig. 2.   Simultaneous recordings of arterial pressure and superior mesenteric, renal, and hindquarter flows in a spinal-sectioned rat. L-NAME and C6 were infused successively for periods shown as horizontal bars to total doses of 5.0 mg/kg iv and 25 mg/kg iv, respectively. At arrow, L-Arg was injected at 70 mg/kg iv by bolus. CS, cord section.

In Figs. 1 and 2, mesenteric, renal, and hindquarter hemodynamics were not measured simultaneously in the same animal but in separate animals under control conditions or after spinal cord section. The entire measurement with drug infusions in an intact or spinal-sectioned rat was completed within 90 min. The duration of the experiment was not a factor in the vascular responses.

Figure 3 shows arterial pressure, heart rate, and regional blood flow responses to L-NAME, C6, and L-Arg from 36 intact and 27 spinal-sectioned rats. Mean arterial pressure significantly increased and mean superior mesenteric, renal, and hindquarter flows, as well as heart rate, significantly decreased during infusion of L-NAME in intact and spinal-sectioned rats. Further injections of C6 and L-Arg did not change blood flow in any of the beds in intact rats, although arterial pressure did fall and heart rate returned to the control level. This suggests that blood flow autoregulation was maintained even after inhibition of NO synthesis in the mesentery, kidney, and hindquarter in intact rats. On the other hand, mesenteric and renal flows in spinal-sectioned rats increased with decrease of arterial pressure by infusions of C6 and L-Arg, but hindquarter flow did decrease by C6, and L-Arg had no additional effect. Heart rate still did not recover to the control level. These findings suggest that hindquarter flow autoregulation was disrupted by the depression of heart rate caused by spinal cord section and that local neurohumoral factors had a great influence on hindquarter blood flow.

View larger version (27K): [in this window] [in a new window]   Fig. 3.   Arterial pressure, heart rate (HR), and superior mesenteric, renal, and hindquarter flows in response to L-NAME, C6, and L-Arg from 36 intact () and 27 spinal-sectioned rats (). C and CS, resting control state and cord section, respectively. Statistical comparisons were made by 2-way ANOVA for repeated measurements: * P < 0.05 for significant difference from preceding point in arterial pressure, heart rate, and regional blood flow caused by infusions of L-NAME, C6, and L-Arg and by spinal cord section.

With spinal cord section, mesenteric resistance significantly increased [change in resistance (R) = 50.14 ± 22.82% of control] and hindquarter resistance significantly decreased (R = 7.95 ± 7.49% of control), whereas the change in renal resistance was not significant (R = 5.61 ± 16.01% of control). The regional local mechanism appeared to be partially maintained by higher central nervous system control in the conscious resting state.

Figure 4 shows the percent changes in mesenteric, renal, and hindquarter resistances caused by additional infusions of L-NAME, C6, and L-Arg. The changes in regional vascular resistances are given as percentages of stable plateau values in the conscious resting state before L-NAME infusion in intact rats (n = 36) and after spinal cord section in spinal-sectioned rats (n = 27). Mesenteric and renal resistances were increased markedly by L-NAME infusion, and the percent increments in mesenteric and renal resistances were significantly decreased by infusions of C6 and L-Arg in intact and spinal-sectioned rats. Mesenteric resistances in both groups of rats returned to control level before L-NAME infusion. There was no significant difference between intact and spinal-sectioned rats in the percent change of mesenteric resistance after infusion of L-NAME, C6, or L-Arg. However, the percent increments in renal resistances after L-NAME, C6, and L-Arg infusions were greater in spinal-sectioned than in intact rats, presumably because of increased resistance produced by spinal cord section. On the other hand, the percent increment in hindquarter resistance after L-NAME infusion was not significantly different between intact and spinal-sectioned rats. Intact and spinal-sectioned rats differed in terms of hindquarter resistance after C6 infusion. The elevated hindquarter resistance after L-NAME was significantly decreased by infusion of C6 in intact but not in spinal-sectioned rats. Furthermore, hindquarter resistance (8) in intact and spinal-sectioned rats was unchanged by injection of L-Arg. The influences of L-NAME on vascular resistances were different among regional vascular beds.

View larger version (36K): [in this window] [in a new window]   Fig. 4.   Effects of L-NAME, C6, and L-Arg infusions on mesenteric, renal, and hindquarter resistances in intact (n = 36) and spinal-sectioned rats (n = 27). Values are changes (R) caused by L-NAME, C6, and L-Arg expressed as percentage of control value before L-NAME infusion in intact rats and stable plateau value after spinal cord section in spinal-sectioned rats. * P < 0.05 vs. L-NAME infusion; ** P < 0.05 vs. L-NAME + C6 infusion (2-way ANOVA for repeated measurements); # P < 0.05 vs. corresponding intact rats within each drug infusion (1-way ANOVA). Statistical significance at P < 0.05.

To elucidate whether L-NAME-induced hindquarter tone is mediated by peripheral sympathetic nerves at the synaptic level, we examined the effect of prazosin on hindquarter resistance after infusion of L-NAME in ganglion-blocked rats. Arterial pressure was increased and regional blood flows were decreased by infusion of L-NAME after ganglionic blockade. The reduced superior mesenteric and renal flows after L-NAME were partially restored by injection of L-Arg, but hindquarter flow was unchanged. Hindquarter flow was increased by supplement of prazosin given before or after L-Arg, whereas superior mesenteric and renal flows were unchanged.

Figure 5 shows percent changes in mesenteric, renal, and hindquarter resistances caused by intravenous infusions of L-NAME, L-Arg, and prazosin in ganglion-blocked rats with C6 (n = 30). Here, the percent changes in regional vascular resistances after L-NAME, L-Arg, and prazosin infusions are given as the percentage of the stable plateau value in the conscious resting state after ganglionic blockade, but the effects of C6 on regional vascular resistances are given as the percentage of the control value before C6 infusion. Regional vascular resistances also increased significantly by infusion of L-NAME after ganglionic blockade. NO-mediated perivascular nerves and/or endothelial NO production appeared to be involved in regional vasoconstrictor responses to L-NAME. The elevated mesenteric and renal resistances by L-NAME decreased significantly after bolus injection of L-Arg in both groups of rats, whereas the hindquarter resistance did not decrease significantly. The elevated hindquarter resistance decreased significantly in conjunction with prazosin given before or after infusion of L-Arg (data for prazosin given before L-Arg infusion are not shown here). ₁-Receptor-mediated peripheral sympathetic vasoconstriction was indicated to be implicated in elevated hindquarter resistance by L-NAME (19, 20).

View larger version (31K): [in this window] [in a new window]   Fig. 5.   Percent changes in superior mesenteric, renal, and hindquarter resistances caused by infusions of L-NAME (3.5 mg/kg iv), L-Arg (70 mg/kg iv), and prazosin (200 µg/kg iv) after ganglionic blockade with C6 (25 mg/kg iv). Values are expressed as percent changes from stable plateau values in conscious resting state after ganglionic blockade; percent change in regional vascular resistances by C6 is given as percentage of control value in conscious resting state before C6 infusion. * P < 0.05 vs. L-NAME infusion; ** P < 0.05 vs. L-NAME + L-Arg infusion within each vascular group (2-way ANOVA for repeated measurements).

Furthermore, mesenteric resistance in resting control rats was significantly increased (R = 7.14 ± 8.55% of control) by infusion of C6, suggesting that mesenteric tone was suppressed by central sympathetic nerve activity in the conscious resting state, whereas renal and hindquarter resistances were significantly decreased (R = 15.83 ± 9.05 and 12.41 ± 9.16% of control, respectively). In comparison with L-NAME-infused intact rats (Fig. 4), the percent decreases in mesenteric, renal, and hindquarter resistances after C6 infusion were 25.02 ± 13.17, 25.58 ± 10.81, and 20.08 ± 13.17% of L-NAME, respectively. The enhanced C6 responses after L-NAME are possibly due to sympathetic nerve activity augmented by NO inhibition but not to increased resistance produced by L-NAME.

	DISCUSSION

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The aim of this study was to investigate whether spinal or supraspinal mechanisms control NO-mediated sympathetic vasoconstriction in the mesentery, kidney, and hindquarter. Our criterion for the level of the sympathetic nervous system was whether there was a significant difference in vascular response to C6 in spinal-sectioned compared with intact rats. The criterion for differentiating between supraspinal and synaptic mechanisms was also whether there was a significant difference in vascular response to ₁-receptor blockade with prazosin in ganglion-blocked rats compared with spinal-sectioned rats.

The percent increments of renal resistances produced by L-NAME were significantly greater in spinal-sectioned than in intact rats, presumably because of some vasoconstrictor metabolites (ANG II and vasopressin) produced by spinal cord section or release of tonic depression of the NO system in the intact state (i.e., greater contribution of NO to regional resistance when the sympathetic system is disrupted), whereas there was no significant difference between intact and spinal-sectioned rats in the percent increments of mesenteric and hindquarter resistances. The elevated mesenteric and renal resistances by L-NAME were significantly reduced by infusion of C6 in intact and spinal-sectioned rats. However, C6 did not significantly decrease the hindquarter resistance in spinal-sectioned rats but significantly decreased the hindquarter resistance in intact rats. The elevated hindquarter resistances were also decreased significantly by bolus injection of the ₁-blocker prazosin after ganglionic blockade. NO-mediated sympathetic vasoconstrictions in the mesentery and kidney were spinal in origin, but hindquarter tone was supraspinal in origin. Mesenteric and renal tones were potentiated more at the spinal level by spinal cord section, whereas hindquarter tone was at the synaptic level (22, 23).

These neuronal mechanisms can possibly be explained by the hypothesis (30) that the supraspinal inhibitory influences on spinal sympathetic generators were released by spinal cord section and the outflow from the spinal generator to the mesentery and kidney was potentiated and also that the supraspinal excitatory input to the hindquarter was replaced by NE releases from sympathetic nerve endings by spinal cord section or ganglionic blockade. NE releases from nerve endings in the hindquarter must be modified at pre- and/or postsynaptic levels. The enhanced NE release from sympathetic nerve endings and/or the prejunctional depression of NO-mediated NANC nerve activity must be a likely cause of the depressed response to L-Arg in the hindquarter. In addition, hindquarter tone must be more preferentially controlled by neuronal than by endothelial NO.

Alternatively, we cannot dismiss the possibility (23) that L-NAME is a muscarinic receptor antagonist and can produce abnormal tone in regional vascular beds by blocking prejunctional M₂ receptors. However, the blocking effect of L-NAME on muscarinic receptors is an unlikely cause of the depressed response to L-Arg in the hindquarter. This is because depressed responses to L-Arg in the hindquarter have also been observed by the infusion of N^G-monomethyl-L-arginine (an inhibitor of NOS) (8), which does not have a blocking effect on muscarinic receptors.

In the present study, higher resting resistance was observed in the kidney and hindquarter after spinal cord section and ganglionic blockade. The increased resting resistance may be partially mediated by other neurohumoral factors such as ANG II and arginine vasopressin (AVP) at regional vascular beds. This is because spinal cord section or ganglionic blockade must enhance the vasoconstrictor actions of a number of neurohumoral systems, as well as inhibition of NO synthesis (16). NO is not only a negative modulator of sympathetic nerve activity but also an important modulator of the vasoconstrictor influence of ANG II and AVP in regional blood circulation. The regulation of regional blood flow is intimately associated with the local interaction between the vasodilator NO and several vasoconstrictor systems such as ANG II, AVP, and the sympathetics. In addition, the diverse regional effects in NO-mediated peripheral and organ perfusions may reflect variation in the sensitivity of different vascular beds to these neurohumoral factors (24). We did not measure plasma levels of ANG II and AVP before and/or after spinal cord section and ganglionic blockade. However, the balance of the vasodilator NO and the vasoconstrictor systems at renal and hindquarter beds will most likely shift toward the vasoconstrictor state when the plasma levels of ANG II and AVP and/or the sensitivity of vascular beds are elevated by spinal cord section or ganglionic blockade. It is conceivable that higher resting resistances in the kidney and hindquarter are due to increased vascular sensitivity to circulating vasoconstrictor factors potentiated by spinal cord section or ganglionic blockade.

Zanzinger et al. (29) reported that intravenous administration of the -adrenergic receptor antagonist prazosin almost completely reversed hypertension caused by L-NAME and that inhibition of peripheral sympathetic vasoconstriction is an important mechanism of in vivo NO vasodilation. In contrast, Huang et al. (9) reported that the vasoconstrictor and pressor responses to L-NAME were not attenuated by pretreatment with the ganglion blocker C6, suggesting that a neurogenic mechanism is not involved in L-NAME-induced abnormal tone and resultant hypertension.

Our experimental results with conscious intact rats demonstrated that increased mesenteric, renal, and hindquarter resistances by L-NAME were significantly decreased by the ganglion blocker C6 and that the percent decreases by C6 were greater in L-NAME-infused intact rats than in resting control rats. A neurogenic mechanism was involved in regional vasoconstrictor responses to L-NAME. On the other hand, the vasoconstrictor responses to L-NAME observed in ganglion-blocked vascular beds might be explained by NO-mediated perivascular nerve activity (27) and endothelial NO production potentiated by ganglionic blockade or spinal cord section, in addition to the vasoconstrictor actions of the neurohumoral system (16). The difference between the findings of Zanzinger et al. (29) and Huang et al. (9) is most likely due to the different species studied (e.g., cat and rat) and different anesthesia (e.g., -chloralose and thiobutabarbital sodium salt) rather than some universal mechanism applicable to all species.

Neuronal NO appears to regulate regional blood flows interdependently at the supraspinal, spinal, and synaptic levels. For a profound understanding of the NO-mediated blood flow regulatory mechanism in the regional vascular beds, further exploration of the cellular mechanisms (4, 7, 12), the membrane K⁺ channel (2, 13, 18), and the neural mechanism of NANC nerve fibers (27) is needed.

In conclusion, the results of the present study show that 1) NO suppresses sympathetic vasoconstriction in the mesentery and kidney at the spinal level and 2) in the hindquarter, sympathetic vasoconstriction is suppressed by NO at the supraspinal and synaptic levels.