Effect of endothelium removal on the vasoconstrictor response to neuronally released 5-hydroxytryptamine and noradrenaline in the rat isolated mesenteric and femoral arteries

Sympathetic and Sensory-Motor Nerves in Peripheral Small Arteries

Small arteries, which play important roles in controlling blood flow, blood pressure, and capillary pressure, are under nervous influence. Their innervation is predominantly sympathetic and sensory motor in nature, and while some arteries are densely innervated, others are only sparsely so. Innervation of small arteries is a key mechanism in regulating vascular resistance. In the second half of the previous century, the physiology and pharmacology of this innervation were very actively investigated. In the past 10-20 yr, the activity in this field was more limited. With this review we highlight what has been learned during recent years with respect to development of small arteries and their innervation, some aspects of excitation-release coupling, interaction between sympathetic and sensory-motor nerves, cross talk between endothelium and vascular nerves, and some aspects of their role in vascular inflammation and hypertension. We also highlight what remains to be investigated to further increase our understanding of this fundamental aspect of vascular physiology.

Neural regulation of the contractility of nutrient artery in the guinea pig tibia

Nutrient arteries provide the endosteal blood supply to maintain bone remodelling and energy metabolism. Here, we investigated the distribution and function of perivascular nerves in regulating the contractility of the tibial nutrient artery. Changes in artery diameter were measured using a video tracking system, while the perivascular innervation was investigated using fluorescence immunohistochemistry. Nerve-evoked phasic constrictions of nutrient arteries were suppressed by phentolamine (1 μM), an α-adrenoceptor antagonist, guanethidine (10 μM), a blocker of sympathetic transmission, or fluoxetine (10 μM), a serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor. In arteries pretreated with guanethidine, residual nerve-evoked constrictions were abolished by a high concentration of propranolol (10 μM) that is known to inhibit 5-HT receptors, or ketanserin (100 nM), a 5-HT₂ receptor antagonist, but not SB207216 (1 μM), an antagonist of 5-HT₃ and 5-HT₄ receptors. Bath-applied 5-HT (100 nM) induced arterial constriction that was suppressed by propranolol (10 μM) or ketanserin (100 nM). Nerve-evoked arterial constrictions were enhanced by spantide (1 μM), a substance P (SP) receptor antagonist, or L-nitro arginine (L-NA; 100 μM), an inhibitor of nitric oxide synthase (NOS). Immunohistochemistry revealed 5-HT-positive nerves running along the arteries that are distinct from perivascular sympathetic or substance P-positive primary afferent nerves. For the first time, functional serotonergic nerves are identified in the tibial nutrient artery of the guinea pig. Thus, it appears that tibial nutrient arterial calibre is regulated by the balance between sympathetic and serotonergic vasoconstrictor nerves and vasodilator afferent nerves that release substance P-stimulating endothelial nitric oxide (NO) release.

Intrinsic regulation of microvascular tone by myoendothelial feedback circuits

The endothelium is an important regulator of arterial vascular tone, acting to release nitric oxide (NO) and open Ca2+-activated K+ (KCa) channels to relax vascular smooth muscle cells (VSMCs). While agonists acting at endothelial cell (EC) receptors are widely used to assess the ability of the endothelium to reduce vascular tone, the intrinsic EC-dependent mechanisms are less well characterized. In small resistance arteries and arterioles, the presence of heterocellular gap junctions termed myoendothelial gap junctions (MEGJs) allows the passage of not only current, but small molecules including Ca2+ and inositol trisphosphate (IP3). When stimulated to contract, the increase in VSM Ca2+ and IP3 can therefore potentially pass through MEGJs to activate adjacent ECs. This activation releases NO and opens KCa channels, which act to limit contraction. This myoendothelial feedback (MEF) is amplified by EC Ca2+ influx and release pathways, and is dynamically modulated by processes regulating gap junction conductance. There is a remarkable localization of key signaling and regulatory proteins within the EC projection toward VSM, and the intrinsic EC-dependent signaling pathways occurring with this highly specialized microdomain are reviewed.

Adrenergic stimulation-released 5-HT stored in adrenergic nerves inhibits CGRPergic nerve-mediated vasodilatation in rat mesenteric resistance arteries

BACKGROUND AND PURPOSE

5-HT is taken up by and stored in adrenergic nerves and periarterial nerve stimulation (PNS) releases 5-HT to cause vasoconstriction in rat mesenteric arteries. The present study investigated whether PNS-released 5-HT stored in adrenergic nerves affects the function of perivascular calcitonin gene-related peptide-containing (CGRPergic) nerves.

EXPERIMENTAL APPROACH

Rat mesenteric vascular beds without endothelium and with active tone were perfused with Krebs solution. Changes in perfusion pressure in response to PNS and CGRP injection were measured before (control) and after perfusion of Krebs solution containing 5-HT (10 µM) for 20 min. Distributions of 5-HT- and TH-immunopositive fibres in mesenteric arteries were studied using immunohistochemical methods.

KEY RESULTS

PNS (1-4 Hz) frequency dependently caused adrenergic nerve-mediated vasoconstriction followed by CGRPergic nerve-mediated vasodilatation. 5-HT treatment inhibited PNS-induced vasodilatation without affecting exogenous CGRP-induced vasodilatation, while it augmented PNS-induced vasoconstriction. Guanethidine (adrenergic neuron blocker), methysergide (non-selective 5-HT receptor antagonist) and BRL15572 (selective 5-HT1D receptor antagonist) abolished inhibition of PNS-induced vasodilatation in 5-HT-treated preparations. Combined treatment with 5-HT and desipramine (catecholamine transporter inhibitor), but not fluoxetine (selective 5-HT reuptake inhibitor), did not inhibit PNS-induced vasodilatation. Exogenous 5-HT inhibited PNS-induced vasodilatation, which was antagonized by methysergide. In immunohistochemical experiments, 5-HT-immunopositive nerves, colocalized with adrenergic TH-immunopositive nerves, were observed only in 5-HT-treated mesenteric arteries, but not in control preparations or arteries co-treated with desipramine.

CONCLUSIONS AND IMPLICATIONS

These results suggest that 5-HT can be taken up by and released from adrenergic nerves in vitro by PNS to inhibit CGRPergic nerve transmission in rat mesenteric arteries.

Endothelium removal augments endothelium-independent vasodilatation in rat mesenteric vascular bed

BACKGROUND AND PURPOSE

The vascular endothelium regulates vascular tone by releasing various endothelium-derived vasoactive substances to counteract excess vascular response. We investigated whether the vascular endothelium regulates vasodilatation via released endothelium-derived contracting factors (EDCFs), by examining the effect of endothelium removal on responses to periarterial nerve stimulation (PNS) and various vasodilator agents.

EXPERIMENTAL APPROACH

The rat mesenteric vascular bed was perfused with Krebs solution. Vasodilator responses to PNS and 5 min perfusion of vasodilator agents in preparations with endothelium were compared with those in the same preparations without endothelium. The endothelium was removed by 30 s perfusion with sodium deoxycholate.

KEY RESULTS

Endothelium removal significantly augmented vasodilator responses to PNS and calcitonin gene-related peptide (CGRP), isoprenaline (beta-adrenoceptor agonist), SNP and 8-bromo-cGMP (8-Br-cGMP; cGMP analogue) but not BAY41-2272 (soluble guanylate cyclase activator). The augmentation of SNP-induced vasodilatation after denudation was much greater than that of CGRP- or isoprenaline-induced vasodilatation. In the preparations with an intact endothelium, L-NAME (nitric oxide synthase inhibitor) significantly augmented vasodilator responses to PNS and CGRP, isoprenaline, SNP and 8-Br-cGMP, but not BAY41-2272. Indomethacin (cyclooxygenase inhibitor) and seratrodast (thromboxane A(2) receptor antagonist), but not phosphoramidon (endothelin-1-converting enzyme inhibitor) or BQ-123 (selective endothelin type A receptor antagonists), significantly augmented vasodilator responses to PNS and CGRP, isoprenaline, SNP and BAY41-2272.

CONCLUSION AND IMPLICATION

These results suggest that the endothelium in rat mesenteric arteries regulates and maintains vascular tone via counteracting not only vasoconstriction through releasing endothelium-derived relaxing factors, but also vasodilatation, in part by releasing an EDCF, thromboxane A(2).

Effect of 5-HT(2) receptor blockade on cadmium-induced acute toxicity

The protective effect of 5-HT(2) receptor blockade with ketanserin or ritanserin against cadmium liver injury was investigated. Male Wistar rats were injected intraperitoneally with a sublethal dose of cadmium (3.5 mg/kg body weight). Rats were treated with normal saline (group I), ketanserin (3 mg/kg body weight; group II), or ritanserin (3 mg/kg body weight; group III) 2 hr prior and 4 hr after cadmium administration and killed at different time points. Hematoxylin/eosin-stained liver sections were assessed for necrosis, apoptosis, peliosis, mitoses, and inflammatory infiltration. Apoptosis was also quantified by the TUNEL assay. Nonparenchymal liver cells and activated Kupffer cells were identified histochemically. Necrosis, hepatocyte apoptosis, nonparenchymal cell apoptosis, and macroscopic and microscopic peliosis were markedly reduced or minimized in ketanserin- or ritanserin-treated rats. The observed protective effect was almost identical for both ketanserin and ritanserin administration. 5-HT(2) receptor blockade exerts a protective effect against acute cadmium-induced hepatotoxicity.

[Mechanisms underlying enhanced vasodilator responses to various vasodilator agents following endothelium removal in rat mesenteric resistance arteries]

We reported that vasodilator responses to various vasodilator agents were augmented by endothelium removal. To explain this mechanism, we hypothesized that endothelium removal eliminates the release of endothelium-derived contracting factor EDCF, which counteracts the vasodilation. However, the underlying mechanism is unknown. Therefore the present study investigated the second messenger system further to investigate the mechanisms underlying enhanced vasodilator response after endothelium removal in rat mesenteric resistance arteries. Mesenteric vascular beds isolated from Wistar rats were perfused and perfusion pressure was measured. The vascular endothelium was removed by 30-s perfusion of sodium deoxycholate. Vasodilator responses to sodium nitroprusside (SNP) perfusion were markedly augmented and prolonged by endothelium removal. In preparations with intact endothelium and active tone, 5-min perfusion of sodium azide (non-specific guanylate cyclase (GC) activator), ANP (membrane-linked GC activator), and 8-Br-cGMP (cGMP analogue) caused a concentration-dependent vasodilation that was markedly augmented by endothelium removal. However, vasodilation induced by YC-1 and BAY41-2272 (selective soluble GC activator) was not augmented by endothelium removal. When methylene blue (soluble GC inhibitor) was present in the medium, SNP caused a concentration-dependent vasodilation in the preparation with intact endothelium, which was less augmented by endothelium removal compared with control (preparation without methylene blue). These findings suggest that endothelium removal affects intracellular cGMP-mediated signal transduction system in vascular smooth muscle cells.

Peliosis hepatis: microscopic and macroscopic type, time pattern, and correlation with liver cell apoptosis in a model of toxic liver injury

Macroscopic and microscopic types of peliosis hepatis, time pattern, and correlation with hepatocyte and sinusoidal cell apoptosis were investigated. Male Wistar rats were injected with a dose of cadmium (6.5 mg CdCl(2)/kg body weight, intraperitoneally; group I). Putrescine (300 micromol/kg body weight, intraperitoneally; group II) was injected at 2, 5, and 8 hours and vascular endothelial growth factor (VEGF; 400 ng/animal, intravenously; group III) at 2 hours. Animals from each group were humanely killed 0, 6, 12, 24, 48, or 60 hours after cadmium intoxication. Liver tissue was histologically assessed for necrosis, apoptosis, and peliosis. Apoptosis was also quantified by the TUNEL assay for hepatocytes and nonparenchymal liver cells. The discrimination between hepatic cell subpopulations was done histochemically. Sinusoidal cell apoptosis and macroscopic peliosis hepatis evolved in a monophasic pattern and correlated closely. Putrescine or VEGF administration totally reversed macroscopic peliosis. Putrescine exerted a major protective effect on hepatocytes, whereas the protective effect of VEGF was more pronounced for nonparenchymal liver cells. Microscopic peliosis also evolved in a monophasic pattern preceding macroscopic type. The extent of the lesion was reduced by putrescine and almost totally reversed by VEGF. Macroscopic peliosis progresses as a compound lesion closely correlating with nonparenchymal cell apoptosis. Both hepatocyte and nonparenchymal cell injury are prerequisites for the genesis of the lesion. Microscopic peliosis precedes macroscopic peliosis and up to a degree seems to be independent of initial hepatocyte injury, but it seems to depend on nonparenchymal cell injury.

Effects of sevoflurane on sympathetic neurotransmission in human omental arteries and veins

BACKGROUND

Sevoflurane reduces blood pressure, the regulation of which requires an intact sympathetic neurotransmission. This study was designed to evaluate the effect of sevoflurane on the coupling between peripheral sympathetic neurones and vascular smooth muscle in isolated human omental vessels.

METHODS

Segments of arteries and veins were exposed to sevoflurane 1%, 2% and 4% (corresponding to approximately 0.5, 1 and 2 MAC in humans, respectively). The vessels were studied in vitro to determine the effects on (i) isometric contraction during electrical field stimulation (EFS) or in the presence of exogenous norepinephrine (NE); (ii) electrical field stimulated release of [(3)H]-NE from vessel segments previously incubated with [(3)H]-NE; (iii) uptake of [(3)H]-NE.

RESULTS

In artery segments, sevoflurane 4% attenuated the contraction induced by both EFS and exogenous NE. In vein segments, sevoflurane 4% attenuated only the EFS-induced contractions. Sevoflurane 1% and 2% had no effect. The release of [(3)H]-NE was inhibited by sevoflurane 2% and 4% in arteries and by sevoflurane 1%, 2% and 4% in veins. Sevoflurane had no effect on the uptake of [(3)H]-NE in either vessel.

CONCLUSIONS

Sevoflurane depresses sympathetic neuromuscular transmission in human omental vessels by reducing neuronal NE release and NE sensitivity in arteries and by reducing NE release in veins. This could contribute to the hypotension seen during sevoflurane anaesthesia, at least at concentrations above 1 MAC.

Angiotensin-(1-7) attenuates vasoconstriction evoked by angiotensin II but not by noradrenaline in man

Angiotensin-(1-7) has been suggested to be a novel vasodilating peptide. We investigated the direct vascular effect of angiotensin-(1-7) in human forearm resistant vessels, particularly with regard to the interaction with angiotensin II, in healthy normotensive men by strain-gauge venous occlusion plethysmography with intra-arterial infusions of peptides. Intra-arterial infusion of angiotensin-(1-7) at 0.1 to 2000 pmol/min did not cause vasodilatation but rather reduced forearm blood flow by approximately 10% at the highest dose. A placebo-controlled study showed that angiotensin-(1-7) at 0.5 to 40 nmol/min caused weak but significant vasoconstriction (P=0.0016 by ANOVA). Angiotensin-(1-7) at 100 pmol/min, but not at 10 pmol/min, significantly shifted the angiotensin II dose-response curve toward the right (mean+/-SD of percent changes in forearm blood flow: -19+/-17%, -33+/-22%, -55+/-12%, -63+/-10%, and -68+/-5% at 5, 10, 25, 50, and 100 pmol/min of angiotensin II, respectively, with saline; 5+/-13%, 0. 9+/-18%, -40+/-16%, -54+/-9%, and -61+/-6% with angiotensin-(1-7), P=0.0021 by ANOVA). Angiotensin-(1-7) did not affect the dose-response curve of noradrenaline [3+/-12%, 5+/-16%, -20+/-22%, -31+/-18%, and -40+/-12% at 25, 50, 100, 300, and 600 pmol/min of noradrenaline, respectively, with saline; -4+/-15%, -2+/-23%, -29+/-22%, -34+/-16%, and -42+/-9% with angiotensin-(1-7)]. Our results suggest that angiotensin-(1-7) antagonizes vasoconstriction by angiotensin II in human resistant vessels and might act as an endogenous angiotensin II antagonist.

Differential effect of propofol on sympathetic neurotransmission in isolated human omental arteries and veins

1. The present study was undertaken to elucidate the effect of propofol on sympathetic neurotransmission in isolated human omental vessels. 2. Segments of both arteries and veins were exposed to 0, 10(-7), 10(-6), 10(-5) or 10(-4)M propofol, and studied in vitro to determine effects on: (i) isometric tension after electrical field stimulation (EFS) or after exogenous administration of noradrenaline (NA); (ii) EFS-stimulated release of [3H]-NA from vessel segments preincubated with [3H]-NA; (iii) uptake of [3H]-NA. 3. Propofol at 10(-6) M enhanced EFS-induced contraction in artery segments, 10(-7) and 10(-5) M had no effect, and 10(-4) M propofol depressed EFS-induced contraction in both artery and vein segments. 4. Propofol did not affect the response to exogenous NA in artery and vein segments. 5. EFS-stimulated release of [3H]-NA was depressed by 10(-5) and 10(-4) M propofol in artery segments, and by 10(-4) M in vein segments. 6. Uptake of [3H]-NA was depressed by 10(-6)-10(-4) M propofol in artery but not in vein segments. 7. The results suggest that sympathetic neurotransmission is enhanced at clinical concentrations (10(-6) M) of propofol in human omental arteries, but not veins. This may be due to an increased availability of NA in the neuromuscular junction resulting from a reduced presynaptic reuptake. Propofol at probably supraclinical concentrations (10(-5)-10(-4) M) impairs the sympathetic neurotransmission in both human omental arteries and veins, probably due to an inhibitory effect on the NA release from the sympathetic nerves.

Alpha-adrenoceptors and vascular regulation: molecular, pharmacologic and clinical correlates

This manuscript is intended to provide a comprehensive review of the alpha-adrenoceptors (ARs) and their role in vascular regulation. The historical development of the concept of receptors and the division of the alpha-ARs into alpha 1 and alpha 2 subtypes is traced. Emphasis will be placed on current understanding of the specific contribution of discrete alpha 1- and alpha 2-AR subtypes in the regulation of the vasculature, selective agonists and antagonists for these receptors, the second messengers utilized by these receptors, the myoplasmic calcium pathways activated to initiate smooth muscle contraction, as well as the clinical uses of agonists and antagonists that work at these receptors. New information is presented that deals with the molecular aspects of ligand interactions with specific subdomains of these receptors, as well as mRNA distribution and the regulation of alpha 1- and alpha 2-AR gene transcription and translation.

Attenuation of neurogenic vasoconstriction by nitric oxide in hindlimb microvascular beds of the rat in vivo

There is evidence that sympathetic nerve activity leads to endothelium-derived nitric oxide release, which in turn attenuates neurogenic vasoconstriction. Here we tested in vivo (1) whether the magnitude of the vasoconstriction induced by N(G)-nitro-L-arginine methyl ester given systemically is altered when ongoing sympathetic activity is abolished by sectioning the lumbar sympathetic trunk, and (2) whether hindlimb sympathetic vasoconstriction elicited by electrical stimulation of the lumbar sympathetic trunk is enhanced after inhibition of nitric oxide synthesis. Blood flow in the microvascular beds of hairless skin and skeletal muscle of the rat hindlimb was measured with laser Doppler flowmetry. Sectioning the lumbar sympathetic trunk resulted in an increase of blood flow in both tissues, indicating that tonic neurogenic vasoconstriction was abolished. Inhibition of nitric oxide synthesis resulted in vasoconstriction in both vascular beds. This vasoconstriction was more pronounced after abolition of sympathetic activity than with intact sympathetic supply in skin but was smaller in skeletal muscle. The vasoconstriction elicited by graded electrical stimulation of the centrally sectioned lumbar sympathetic trunk with frequencies less than 5 Hz was significantly enhanced after blockade of nitric oxide in skeletal muscle but not in skin microvasculature. These findings suggest that under physiological conditions, sympathetic nerve impulses directly promote the release of nitric oxide in skeletal muscle but not in cutaneous blood vessels. Therefore, basal nitric oxide release is probably in part dependent on sympathetic activity in skeletal muscle, whereas it appears to be mainly due to flow-dependent shear stress in hairless skin microvasculature.

Nitric oxide-dependent and -independent modulation of sympathetic vasoconstriction in the human saphenous vein

The possible modulation by the endothelium of the contractile responses to sympathetic nerve stimulation was examined in isolated superfused human saphenous vein. Contractile response curves for transmural nerve stimulation and noradrenaline were higher in endothelium-denuded than in intact human saphenous vein rings. In vessels with endothelium, transmural nerve stimulation- and noradrenaline-induced contractions were unaffected by the cyclooxygenase inhibitor, indomethacin (10 microM), but were potentiated by the nitric oxide (NO) synthase inhibitor, L-N omega-nitro-L-arginine (L-NNA, 3 microM) even when combined with D-arginine (0.3 mM), but not with L-arginine (0.3 mM). As in the case of noradrenaline, contractile responses to 5-HT, but not to KCI, were enhanced by endothelium removal, L-NNA or L-NNA plus D-arginine, but were unaffected by L-NNA plus L-arginine. The guanylyl cyclase inhibitor, methylene blue (10 microM), potentiated both transmural nerve stimulation- and noradrenaline-induced contractions in endothelium intact rings, whereas it enhanced, although to a lesser degree, only the neurally evoked contractions in endothelium-denuded human saphenous vein. In the vessels without endothelium L-NNA failed to affect the vasoconstriction induced by both transmural nerve stimulation and noradrenaline. Our results suggest that at least two inhibitory factors are involved in modulating the sympathetic vasoconstriction in the human saphenous vein: (1) at a postjunctional level, NO, the release of which from endothelial cells is probably stimulated by the activation of specific receptors, and (2) at a prejunctional level, an unidentified vasodilator agent, which is unmasked by the removal of the endothelium layer and which is probably co-released along with noradrenaline, and which acts through the guanylyl cyclase pathway.

Differential contribution of endothelial function to vascular reactivity in conduit and resistance arteries from deoxycorticosterone-salt hypertensive rats

The purpose of these studies was to compare changes in conduit and resistance artery function in deoxycorticosterone-salt hypertensive rats. We hypothesized that if there was a common mechanism producing changes in vascular function in hypertension, then there would be similar alterations in reactivity of conduit and resistance arteries. Helically cut strips of common carotid artery were prepared for measurement of isometric force generation, and segments of small mesenteric arteries were pressurized for video dimension analysis. Sensitivity of arteries to phenylephrine and acetylcholine was determined. Carotid arteries from deoxycorticosterone-salt hypertensive rats were more sensitive to phenylephrine than arteries from control rats, whereas mesenteric resistance arteries from hypertensive rats were less sensitive to phenylephrine. In carotid arteries, endothelial denudation or incubation with N psi-nitro-L-arginine increased phenylephrine sensitivity in control rats to the level seen in deoxycorticosterone-salt rats. These manipulations had no effect on phenylephrine sensitivity in arteries from deoxycorticosterone-salt rats. In mesenteric resistance arteries, endothelium denudation normalized the depressed phenylephrine sensitivity in arteries from hypertensive rats but had no effect on arteries from normotensive rats. This depressed phenylephrine sensitivity in deoxycorticosterone-salt mesenteric arteries was not reversed by incubation with Npsi-nitro-L-arginine. Acetylcholine-induced relaxation was depressed in carotid arteries from deoxycorticosterone-salt hypertensive rats, and Npsi-nitro-L-arginine blocked these relaxations. In contrast, acetylcholine relaxation in the mesenteric arteries from normotensive and hypertensive rats did not differ. N psi-nitro-L-arginine slightly but significantly attenuated acetylcholine dilation only in mesenteric resistance arteries from the hypertensive rats. We conclude that qualitatively different changes in vasoconstrictor sensitivity to phenylephrine occur in carotid arteries and mesenteric resistance arteries of deoxycorticosterone-salt hypertensive rats. The increased phenylephrine sensitivity in carotid arteries in this model of hypertension is due to the loss of endothelium-derived nitric oxide production. In contrast, the decreased phenylephrine sensitivity in mesenteric resistance arteries from deoxy-corticosterone-salt rats is due to a non-nitric oxide-mediated influence of the endothelium that is absent in arteries from normotensive rats.

Endothelium-dependent relaxation by alpha 2-adrenoceptor agonists in spontaneously hypertensive rat aorta

Differences in alpha(2)-adrenoceptor-induced relaxation of the aorta between stroke-prone spontaneously hypertensive rats (SHRSP) and control normotensive Wistar Kyoto rats (WKY) were studied. Changes in the tension of ring preparations of the aortas were measured isometrically. Relaxation was observed in the preparations precontracted in the presence of ONO-11113, a thromboxane A(2) analogue. The alpha(2)-agonist clonidine and UK-14304 induced dose-dependent relaxation in both the WKY and SHRSP preparations. The relaxation was impaired in the SHRSP preparation. A modified sandwich experiment showed that the relaxing substance from the SHRSP endothelium was decreased. Acetylcholine (ACh) also induced dose-dependent relaxation, and the relaxation was impaired in the SHRSP preparations. alpha(2)-Agonists induced a greater degree of impairment in the relaxation than did ACh. The relaxation induced by alpha(2)-agonists and by ACh was blocked by N G-nitro-L-arginine (L-NNA). Indomethacin improved the relaxation induced by ACh but not that induced by alpha(2)-agonists in the SHRSP aortas. These results suggest that the impairment of relaxation by alpha(2)-agonists in SHRSP is not caused by the increase in the release of endothelium-derived contracting factor (EDCF) but by the reduction in the release of nitric oxide (NO). Alteration of the alpha(2)-adrenoceptors and/or the intracellular mechanism through which NO is synthesized by stimulation of the alpha(2)-adrenoceptors may be the cause of the reduction in relaxation.

The effect of endotoxin on sympathetic responses in the rat isolated perfused mesenteric bed; involvement of nitric oxide and cyclo-oxygenase products

1. The effects of endotoxin on the vasoconstrictor responses to sympathetic nerve stimulation (SNS) were investigated in the rat isolated perfused mesenteric bed. 2. Rats received either saline (0.1 ml h-1) or endotoxin (2.5 mg kg-1 h-1) intravenously for 4 h; the mesenteric beds were then isolated, perfused with Krebs and prepared for SNS (50 V, 3 ms, 7-40 Hz). 3. SNS caused a frequency-dependent vasoconstrictor response which was abolished by either tetrodotoxin (10(-7) M), prazosin (2.4 x 10(-7) M) or guanethidine (2.4 x 10(-7) M). 4. In mesenteric vascular beds removed from rats infused with endotoxin, there were markedly impaired vasoconstrictor responses to SNS, although responses to noradrenaline were not modified. 5. Removal of the endothelium with distilled water prevented endotoxin-induced impairment of vasoconstrictor responses to SNS, without modifying these responses in preparations from control rats. 6. Pretreatment with dexamethasone (3 mg kg-1 i.p. 1h before commencing endotoxin or saline infusions) did not modify responses to SNS in control rats but prevented the effects of endotoxin. 7. Both L-NAME (10(-3) M) and indomethacin (10(-5) M) restored responses to SNS in preparations from endotoxin-treated rats without modifying these responses in control preparations. However, co-administration of L-NAME and indomethacin markedly augmented responses in both control and endotoxin-treated preparations. 8. The effects of L-NAME were reversed by addition of L-arginine (10(-3) M). 9. The data suggest that endotoxin impairs the release of noradrenaline and that this effect is secondary to increased production of nitric oxide and prostanoids, possibly by the endothelium.

Modulation of vasoconstriction by endothelium-derived nitric oxide: the influence of vascular disease

1. The endothelium makes a significant contribution to the regulation of vascular tone through the release of potent vasodilator agents such as nitric oxide (NO) and prostacyclin (PGI2) as well as vasoconstrictor compounds such as endothelin. Recognition of this function of the endothelium has created a new focus for the investigation of vasoconstrictor activity under physiological and pathological conditions. 2. It has been well established that removal of the endothelium enhances responses to a variety of contractile agents in conductance arteries and that such modulation is predominantly due to the release of NO. The use of selective inhibitors of NO synthesis has confirmed that the endothelium-derived nitric oxide also modulates constriction in resistance vessels. 3. In a number of cardiovascular disease states there is impairment of endothelial function. Thus one of the consequences of atherosclerosis, hypertension and ischaemia is a reduction in endothelium-dependent vasodilatation, both at a basal level and in response to endogenous and exogenous stimuli. In addition, enhanced responses to vasoconstrictors have been reported in those disease states. Such observations have led to the attractive hypothesis that enhanced constriction in vascular disease results from attenuate NO-induced dilatation. However, whilst there is some evidence that pathological impairment of endothelial function is accompanied by increased constrictor activity, particularly where serotonergic mechanisms are involved, it is inappropriate to make the general assumption that where disease impairs NO activity there will also be increased sensitivity to all constrictor stimuli.

Propofol differentially attenuates the responses to exogenous and endogenous norepinephrine in the isolated rat femoral artery in vitro

Propofol causes a decrease in vascular resistance mediated in part by a decrease in sympathetic output. To determine whether attenuation of norepinephrine release from sympathetic perivascular nerve terminals could contribute to decreased vascular resistance, we examined the effects of propofol on the contractile responses to exogenous and endogenous norepinephrine in the rat femoral artery. Endogenous norepinephrine was released from sympathetic nerve terminals using electrical field stimulation. The responses to both exogenous norepinephrine and neurally released norepinephrine were attenuated by propofol in concentrations from 1.0 to 10.0 micrograms/mL. At 50% of maximal and at maximal contractile responses to norepinephrine and electrical field stimulation, the response to electrical field stimulation was inhibited to a greater extent than the response to exogenous norepinephrine. This suggests that, in addition to direct postsynaptic vasodilation, propofol has the presynaptic effect of inhibiting norepinephrine release from perivascular nerves.

Enhancement of noradrenergic constriction of large coronary arteries by inhibition of nitric oxide synthesis in anaesthetized dogs

1. Coronary vascular responses to bilateral carotid occlusion (BCO) and the intravenous infusion of tyramine (Tyr, 20 micrograms kg-1 min-1) and noradrenaline (NA, 0.5 microgram kg-1 min-1) were examined after bilateral vagotomy and antagonism of beta-adrenoceptors. BCO, Tyr and NA decreased large coronary artery diameter and increased mean coronary resistance and systemic arterial pressure without affecting heart rate. 2. Inhibition of nitric oxide (NO) synthase with NG-nitro-L-arginine (L-NNA, 5 and 15 mg kg-1) significantly increased mean arterial pressure and decreased heart rate and large coronary artery diameter. Mean coronary resistance was unaffected by either dose of L-NNA. L-NNA significantly reduced depressor and coronary vasodilator responses to the endothelium-dependent vasodilator acetylcholine (ACh, 10 micrograms kg-1, i.v.). Systemic and coronary vasodilator responses to sodium nitroprusside (SNP, 5 micrograms kg-1) were unaffected by L-NNA with the exception that the dilatation of the large coronary artery was significantly enhanced by the higher dose. 3. L-NNA significantly enhanced constriction of the large coronary arteries caused by BCO, Tyr and NA but did not affect the increases in mean coronary resistance or systemic arterial pressure. 4. Inhibition of NO synthesis enhances adrenergic constriction of large coronary arteries caused by both neuronally released and exogenous noradrenaline. In contrast, L-NNA did not affect adrenergic constriction of coronary or systemic resistance vessels. Endothelium-derived NO may play an important role in the modulation of noradrenergic vasoconstriction in coronary conductance arteries.

Effects of N omega-nitro-L-arginine and capsaicin on neurogenic vasomotor responses in isolated mesenteric arteries of the monkey

Monkey isolated mesenteric arterial rings denuded of endothelium constricted upon transmural nerve stimulation (TNS) in the absence of active muscle tone. The constriction was potentiated by N omega-nitro-L-arginine (3 x 10(-5) M), but not by the D-enantiomer (3 x 10(-5) M). The potentiation was reversed by L-arginine (3 x 10(-4) M). The neurogenic vasoconstriction of mesenteric arteries was also augmented by capsaicin, but to a lesser extent than that induced by N omega-nitro-L-arginine. Indomethacin (10(-5) M) did not affect TNS-induced vasoconstriction. These findings suggest that nerve-derived nitric oxide or a related substance may play a greater role than do capsaicin-sensitive vasodilator transmitters in neurogenic regulation of mesenteric arterial tone in the monkey. The transmitter mechanisms for vasodilation in mesenteric circulation vary among species.

Involvement of alpha-adrenoceptors in the endothelium-dependent depression of noradrenaline-induced contraction in rat aorta

The involvement of endothelium-derived nitric oxide (NO) in the depressant action of the endothelium on noradrenaline-induced contractions and characterization of the receptor involved in the release of NO were studied using rat aorta. The noradrenaline-induced contraction was significantly potentiated by endothelium removal and in the presence of NG-nitro-L-arginine (L-NNA) or NG-monomethyl-L-arginine (L-NMMA). The contraction induced by phenylephrine was also potentiated in the presence of L-NNA. Clonidine could induce contraction only in endothelium-denuded preparations or in the presence of L-NNA. The potentiating action of L-NNA on noradrenaline-induced contractions could also be observed in the presence of yohimbine or rauwolscine, although dose-response curves were shifted to the right. The depression of noradrenaline-induced contractions observed in the presence of the endothelium was increased by repeated stimulation. The depression was prevented by L-NNA and this effect was reversed by L-arginine. These results indicate the possibility that NO can be released through stimulation of alpha 1- and alpha 2-adrenoceptors on the endothelium and depresses noradrenaline-induced contractions of smooth muscle, although the contribution of the respective adrenoceptors remains to be investigated. The release of NO was increased when the stimulation was applied repeatedly.

Amplifying effect of serotonin on contractile responses in rat aorta and depletion of intracellular Ca-stores

1. Serotonin, 1 microM, induces a contractile response in isolated rat aorta in the presence or absence of extracellular Ca. 2. In Ca-free media, the fast phasic contraction is lower in magnitude and further addition of serotonin evokes no response. 3. Recovery of the contractile response in Ca-free medium is obtained by a 40 min incubation in Ca-containing solution. 4. In Ca, Mg-free medium, the response to serotonin is significantly higher than that obtained in the presence of Mg. 5. An amplifying effect of serotonin on the contractile responses induced by serotonin itself or by noradrenaline was observed in Ca-containing but not in Ca-free solution.

Attenuation of vasoconstriction by endogenous nitric oxide in rat caudal artery

1. The effects of NG-nitro-L-arginine (L-NNA), NG-nitro-L-arginine methyl ester (L-NAME), haemoglobin and methylene blue have been examined on vascular reactivity in the rat isolated caudal artery. The effects of L-NNA and sodium nitroprusside were also investigated on the stimulation-induced (S-I) efflux of noradrenaline in the rat caudal artery. 2. L-NNA (10 microM) and L-NAME (10 microM) significantly attenuated the vasodilator responses to acetylcholine (1 nM-1 microM), but had no effect on vasodilator responses to papaverine (1-100 microM). 3. Vasoconstrictor responses to sympathetic nerve stimulation (3 Hz, 10 s), noradrenaline (0.01-1 microM), methoxamine (1-10 microM), 5-hydroxytryptamine (0.01-0.3 microM), phenylephrine (0.1-10 microM), endothelin-1 (10 nM) and KCl (40 mM) were significantly enhanced by 10 microM L-NNA. L-NAME (10 microM) caused a significant enhancement of vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation in endothelium-intact, but not in endothelium-denuded tissues. 4. Haemoglobin and methylene blue (both 10 microM) enhanced the vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline. The enhancements were absent in endothelium-denuded arterial segments. 5. In endothelium-denuded arterial segments precontracted with phenylephrine, the vasodilator responses to the nitric oxide donor, sodium nitroprusside (0.1-300 nM) were decreased by increasing the level of precontraction. 6. L-NNA (10 microM) had no effect on the S-I efflux of radioactivity from arteries in which transmitter stores had been labelled with [3H]-noradrenaline. 7. These results suggest that endothelial nitric oxide attenuates vasoconstrictor responses in the rat caudal artery through activation of soluble guanylate cyclase to decrease smooth muscle contractility. Therefore, the findings provide evidence that nitric oxide acts as a functional antagonist to oppose vasoconstriction.

Inhibition of nitric oxide synthase specifically enhances adrenergic vasoconstriction in rabbits

1. The effect of inhibition of nitric oxide biosynthesis using N-nitro-L-arginine (NOLA) was examined in conscious rabbits and rabbit isolated aortae. 2. In autonomically blocked conscious rabbits intravenous infusion of NOLA (15 mg/kg) significantly increased arterial pressure and hindlimb vascular resistance but did not affect heart rate. Depressor and hindlimb vasodilator responses to acetylcholine (3-12 micrograms/kg per min) were significantly attenuated in the presence of NOLA. In contrast, NOLA significantly enhanced responses to intravenous infusion of glyceryl trinitrate (10-40 micrograms/kg per min) in vivo. 3. Infusion of noradrenaline (1-4 micrograms/kg per min) or the release of neuronal noradrenaline in response to the infusion of tyramine (80-320 micrograms/kg per min) increased arterial pressure and hindlimb vascular resistance in autonomically blocked conscious rabbits. After the administration of NOLA, the vasoconstrictor responses to both noradrenaline and tyramine were significantly enhanced. 4. In isolated rabbit aortae, NOLA (10 mumol/L) significantly impaired relaxant responses to acetylcholine but did not affect responses to glyceryl trinitrate. NOLA enhanced contractile responses to the adrenoceptor agonists noradrenaline and phenylephrine but did not affect the contractile responses to the thromboxane-mimetic U46619. 5. These data indicate that in autonomically blocked conscious rabbits, NOLA causes systemic vasoconstriction, impairs dilator responses to acetylcholine and enhances dilator responses to glyceryl trinitrate. In addition, NOLA enhances constrictor responses to both exogenous and neuronally-released noradrenaline. These results suggest that nitric oxide is important in the regulation of normal vascular tone and in the modulation of vascular responses to vasodilator and vasoconstrictor agents.