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.     

AMPLIFICATION BY SEROTONIN OF RESPONSES TO OTHER VASOCONSTRICTOR AGENTS IN THE RAT TAIL ARTERY

1. A low concentration of serotonin (3 nmol/L), which did not exert a direct vasoconstrictor action, amplified the responses to certain other vasoconstrictor agents (alpha 1-adrenoceptor agonists, KCl, ATP and vasopressin) in isolated perfused segments of the rat tail artery. 2. Low concentrations of serotonin (0.3 and 1 nmol/L) amplified vasoconstrictor responses to sympathetic nerve stimulation, but higher concentrations of serotonin (10 and 30 nmol/L) produced vasoconstriction and reduced responses to sympathetic nerve stimulation. 3. The calcium channel blocking drug diltiazem (1 and 10 mumol/L) produced concentration-dependent reductions of vasoconstrictor responses to phenylephrine. The amplifying effect of serotonin on responses to phenylephrine was attenuated by 1 mumol/L and abolished by 10 mumol/L diltiazem, and was also abolished in a Ca2+-free medium. 4. Ketanserin (10 nmol/L) antagonized the vasoconstrictor action of serotonin and, to a lesser extent, the vasoconstrictor actions of phenylephrine and noradrenaline. It abolished the amplifying effect of a low concentration of serotonin on responses to noradrenaline and phenylephrine. 5. The amplification of vasoconstrictor response in the rat tail artery by serotonin appears to be due to activation of receptors of the 5-HT2 subtype which are coupled to an increase in Ca2+ influx into the vascular smooth muscle cells.     

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.(ABSTRACT TRUNCATED AT 250 WORDS)     

Baroreceptor reflexes and vascular reactivity during inhibition of nitric oxide synthesis in conscious rabbits.

The effect of the nitric oxide (NO) synthase inhibitor N-nitro-L-arginine (NOLA) on vascular reactivity and the baroreceptor heart rate reflex was examined in chronically instrumented conscious rabbits. NOLA (15 mg/kg i.v.) significantly increased mean arterial pressure and hindlimb vascular resistance and decreased heart rate. Increases and decreases in arterial pressure were produced by the intravenous injection of phenylephrine and sodium nitroprusside respectively and the values obtained relating mean arterial blood pressure to heart rate were fitted to a sigmoid curve. NOLA significantly reduced the lower plateau of the arterial pressure--heart rate curve but did not significantly affect baroreceptor sensitivity. Depressor and hindlimb vasodilator responses to acetylcholine were significantly impaired by NOLA whereas responses to sodium nitroprusside were significantly enhanced. The pressor and hindlimb vasoconstrictor responses to phenylephrine were significantly enhanced in the presence of NOLA. We conclude that the bradycardia produced by NOLA does not result from a change in baroreceptor sensitivity. The continuous generation of NO appears to be important in regulating basal vascular resistance and in modulating vascular reactivity to both vasodilator and vasoconstrictor agents.     

Attenuation by bradykinin of adrenergically-induced vasoconstriction in the isolated perfused kidney of the rabbit: relationship to prostaglandin synthesis.

1 In the isolated kidney of the rabbit perfused with oxygenated Tyrode solution, we studied the effect of bradykinin on the vasoconstriction evoked by sympathetic nerve stimulation (3Hz, 1 ms) and by injections of noradrenaline (50 to 75 ng) in the presence and in the absence of indomethacin (1 microgram/ml), an inhibitor of prostaglandin biosynthesis. Prostaglandin E(PGE)-like material in the renal effluent was measured by bioassay after extraction with organic solvents and separation by thin layer chromatography. 2 Bradykinin in concentrations of 10 to 100 ng/ml reduced the vasoconstrictor response to sympathetic nerve stimulation and to injected noradrenaline. Also, the peptide (1 to 10 ng/ml) increased the basal release of PGE-like material and the release induced by sympathetic nerve stimulation. 3 Indomethacin, 1 microgram/ml, diminished the inhibitory effect of bradykinin on the vasoconstrictor response to nerve stimulation, minimized the reduction of the noradrenaline-induced vasoconstriction caused by bradykinin (100 ng/ml), and abolished the release of PGE-like material. 4 This study indicates that bradykinin reduces the renal vascular reactivity to adrenergic stimuli and suggests that part of the action of the kinin at the vascular adrenergic neuroeffector junction in the rabbit kidney depends upon the biosynthesis of renal prostaglandins.     

Inhibition of vasoconstrictor mechanisms by dazoxiben in the rat mesenteric vasculature

Sympathetic neurotransmission can be modulated by prostaglandins in a number of tissues, but it is not known whether thromboxane A₂ also influences neurotransmission. In this study, vasoconstrictor responses to electrical stimulation of the sympathetic nerves and to injection of noradrenaline were examined in the blood perfused mesentry of the rat in situ. The thromboxane synthetase inhibitor dazoxiben, infused into the perfusion circuit at 10–100 μmol/l, significantly inhibited constrictor responses to nerve stimulation and to injected noradrenaline and vasopressin. The cyclo-oxygenase inhibitor indomethacin (28 μmol/kg intravenously) had no effect on responses to nerve stimulation or noradrenaline, but pretreatment with indomethacin abolished the inhibitory effect of dazoxiben vasoconstrictor responses. The thromboxane-mimetic (U46619, 10 nmol/l) slightly reduced responses to nerve timulation (but not to noradrenaline), whereas prostacyclin (3–10 nmol/l) and PGE₂ (3 nmol/l) markedly reduced responses both to nerve stimulation and to injections of noradrenaline. These prostanoids did not alter perfusion pressure at these concentrations. The data suggest that the inhibitory effect of dazoxiben on sympathetic neurotransmission is unlikely to be due directly to inhibition of thromboxane synthesis. Inhibition might result from diversion of endoperoxide metabolism to endogenous prostanoids that, in turn, inhibit activation of vasoconstrictor mechanisms.     

The effect of ischaemia on endothelium-dependent vasodilatation and adrenoceptor-mediated vasoconstriction in rat isolated hearts.

1. The aim of this study was to investigate whether global ischaemia and reperfusion in rat isolated hearts affects endothelium-dependent vasodilatation and adrenoceptor-mediated vasoconstriction. In addition, it was first determined whether inhibition of the actions of nitric oxide (NO) influenced the responses to alpha-adrenoceptor agonists in the rat coronary vasculature. 2. In rat isolated, Langendorff perfused hearts, inhibition of NO with haemoglobin (Hb, 6 microM) significantly inhibited the vasodilator responses to the endothelium-dependent vasodilators, acetylcholine (ACh, 3-100 pmol), carbachol (CCh, 10-300 pmol), bradykinin (Bk, 1-30 pmol) and histamine (0.3-10 nmol) but did not affect responses to the endothelium-independent vasodilator, sodium nitroprusside (SNP, 0.01-1 nmol). 3. Inhibition of the action of NO by Hb significantly enhanced the vasoconstrictor response to the non-selective alpha-adrenoceptor agonist, noradrenaline (NA, 0.1-10 nmol) and the alpha 2-adrenoceptor agonist, B-HT 920 (0.001-1 mumol) but had no effect on the vascular response to the alpha 1-adrenoceptor agonist, methoxamine (MTX, 10-300 nmol). 4. In the perfused hearts ischaemia, induced by 30 min perfusion at 5% of the normal rate of flow, followed by 15 min of reperfusion (ischaemia/reperfusion) selectively impaired the vasodilator responses to ACh and CCh which act by muscarinic receptor stimulation but did not affect responses to the other endothelium-dependent vasodilators Bk and histamine or to the endothelium-independent dilator SNP. 5. After ischaemia/reperfusion the coronary vasoconstrictor responses to B-HT 920 were slightly but significantly enhanced whereas the responses to NA and MTX were unaffected. 6. Thus, in the rat isolated heart, low flow induced-ischaemia and reperfusion causes a selective impairment of muscarinic receptor-mediated vasodilatation but does not impair responses to all endothelium-dependent vasodilators. Enhanced constrictor responses to noradrenaline and B-HT 920 in the presence of Hb indicates that endogenous NO modulates the constriction of coronary resistance vessels in response to stimulation of alpha 2-adrenoceptors. Ischaemia and reperfusion in this isolated vascular bed caused only a small increase in the coronary vasoconstrictor response to alpha 2-adrenoceptor stimulation. It appears that in the rat isolated heart the degree of endothelial dysfunction caused by ischaemia/reperfusion is insufficient to cause a functionally significant change in alpha-adrenoceptor-mediated constriction.     

ACTIVATION OF NORADRENERGIC AND NITRERGIC MECHANISMS IN THE RAT ANOCOCCYGEUS MUSCLE BY NICOTINE

1. Nicotine (10 mumol/L) produced rapidly developing but transient contractions of anococcygeus muscle isolated from rats. The magnitude of the response varied considerably between preparations. Tachyphylaxis occurred, such that no response was elicited by the same or a larger concentration in the continued presence of 10 mumol/L nicotine. 2. Contractions produced by nicotine were not affected by atropine, but were abolished by Hexamethonium and the alpha-adrenoceptor antagonists prazosin and phentolamine. Contractions were absent in the anococcygeus muscles of rats pretreated with reserpine. 3. The alpha 2-adrenoceptor agonist UK14304, or guanethidine, raised the tone of the anococcygeus muscle, and converted responses to field stimulation and nicotine to relaxations. Nicotine-induced relaxations were more pronounced in the presence of UK14304 than guanethidine. 4. Relaxations produced by nicotine (1-18 mumol/L) were transient, and tachyphylaxis occurred. When precautions were taken to avoid tachyphylaxis, concentration-response curves could be constructed. The relaxations elicited by nicotine were abolished or greatly reduced by hexamethonium, tetrodotoxin or omega-conotoxin GVIA. 5. The nitric oxide synthase inhibitor L-NG-nitroarginine methyl ester (90 mumol/L) enhanced contractile responses to field stimulation and nicotine, and markedly reduced relaxations elicited by field stimulation and nicotine in the presence of UK14304. These relaxations were restored by L-arginine (270 mumol/L). 6. The results suggest that nicotine acts on nicotinic receptors of noradrenergic and nitrergic nerve terminals in the rat anococcygeus muscle, resulting in the release of noradrenaline and nitric oxide respectively.     

Effects of adenosine on adrenergic neurotransmission; Prejunctional inhibition and postjunctional enhancement

Summary The action of adenosine on adrenergic neuroeffector transmission was studied in the rabbit kidney in vitro and in situ, in the canine subcutaneous adipose tissue in situ and in the guinea-pig vas deferens in vitro. In the kidney, adenosine (0.1–10 M) caused a concentration-dependent increase in vascular resistance and in vasoconstrictor responses to nerve stimulation and administered noradrenaline. In the adipose tissue, adenosine also increased the vasoconstrictor responses but it decreased vascular resistance.In all three tissues studied adenosine significantly and reversibly depressed noradrenaline release evoked by nerve stimulation in a concentration-dependent manner. This effect of adenosine was not altered by phenoxybenzamine which blocked all vasoconstrictor responses and diminished the rise in vascular resistance by adenosine in the kidney. It is concluded that adenosine affects adrenergic neuroeffector transmission by two discrete mechanisms, prejunctional inhibition and postjunctional enhancement.     

Effects of reduced calcium ion concentration and of diltiazem on vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation in rat isolated tail artery.

In isolated, perfused proximal segments of Sprague-Dawley rat tail artery, idazoxan (100 nmol l-1) displaced the concentration-response curve to noradrenaline (NA) to the right. The log shift of the NA concentration-response curve was greater at lower concentrations than at higher concentrations of NA. Idazoxan (100 nmol l-1) had no effect on responses to electrical stimulation. Prazosin (10 nmol l-1) displaced the concentration-response curve to NA to the right as well as markedly reducing responses to sympathetic nerve stimulation. The concentration-response curve to NA, obtained after reducing the concentration of calcium ions in the Krebs solution from 2.5 to 0.6 mmol l-1, was significantly displaced to the right. Responses to sympathetic nerve stimulation were not affected by this reduction in the concentration of calcium ions. Diltiazem (1 and 10 mumol l-1) significantly displaced the concentration-response curve to NA to the right but had no effect on sympathetic nerve stimulation. These in vitro results in peripheral arterial smooth muscle confirm the findings of previous in vivo studies which suggest that alpha2-adrenoceptors contribute to the vasoconstrictor responses elicited by alpha-adrenoceptor agonists and that these responses but not those mediated by alpha1-adrenoceptors are dependent on extracellular calcium.     

INCREASED SENSITIVITY TO ENDOTHELIN-1 IN ISOLATED KREBS-PERFUSED KIDNEYS OF STREPTOZOTOCIN-DIABETIC RATS

1. Vascular responses to endothelin-1 (ET-1) and noradrenaline (NA) were measured in isolated Krebs'-perfused kidneys of 2 week old streptozotocin-diabetic and non-diabetic rats. 2. Bolus injections of either ET-1 or NA caused dose-dependent increases in perfusion pressure. Responses to ET-1 (10-60 ng/g kidney), but not to NA (0.001-10 micrograms/g kidney), were significantly potentiated in kidneys of diabetic rats compared with non-diabetics. 3. Indomethacin significantly attenuated responses to NA (0.3-10 micrograms/g kidney) in kidneys of both diabetic and non-diabetic rats. 4. Neither indomethacin (1 mumol/L) nor the cyclo-oxygenase/lipoxygenase inhibitor BW755C (1 mumol/L) had any significant effect on the log dose-response curve to ET-1 in either group of kidneys. 5. Perfusion with N-nitro-L-arginine (NOLA; 10 mumol/L) had no effect on basal perfusion pressures, but potentiated responses to ET-1 in both groups of kidneys. However, the difference in responses to ET-1 between kidneys from diabetic and non-diabetic rats remained significant in the presence of NOLA. 6. ET-1 responses were inhibited in Ca(2+)-free Krebs' solution (plus 1 mmol/L EGTA). 7. The results of the present study indicate an increased sensitivity to ET-1 in isolated Krebs'-perfused kidneys of diabetic rats. Responses to ET-1 were unaffected by cyclo-oxygenase and/or lipoxygenase inhibitors, but were potentiated by an endothelium-derived relaxing factor (EDRF) synthesis inhibitor.     

ENDOTHELIN-1 ENHANCES VASOCONSTRICTOR RESPONSES TO SYMPATHETIC NERVE STIMULATION AND NORADRENALINE IN THE RABBIT EAR ARTERY

1. In rabbit isolated perfused ear arteries denuded of endothelium, a low concentration of endothelin-1 (0.1 nmol/L) that had no direct vasoconstrictor action produced slowly developing enhancements of vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation. The enhancements reached maximal levels after 60 min of exposure to endothelin-1. 2. A higher concentration of endothelin-1 (1 nmol/L), which produced a slow-developing increase in perfusion pressure of 70 mmHg over the course of 1 h, significantly enhanced vasoconstrictor responses to sympathetic nerve stimulation for the first 30 min, after which there was no significant enhancement. Responses to noradrenaline were not enhanced by 1 nmol/L endothelin-1. 3. The enhancing effect of low concentrations of endothelin-1 on vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline may play a physiological role in modulating vasomotor function.     

INHIBITION OF ENDOTHELIAL NITRIC OXIDE BIOSYNTHESIS BY N-NITRO-l-ARGININE

1. The actions of N-nitro-L-arginine (NOLA) on the release of nitric oxide (NO) from arterial endothelial cells was studied in rat isolated thoracic aortic rings and by bioassay of NO derived from cultured bovine aortic endothelial cells. 2. NOLA (3-10 mumol/L) caused concentration-dependent inhibition of acetylcholine-induced relaxation of phenylephrine-contracted rat aortic rings, which is dependent on the release of NO from the endothelium. The inhibitory actions of NOLA could be prevented by pre- and co-incubation with L-arginine (1 mmol/L). 3. Endothelium-independent relaxation induced by sodium nitroprusside was not affected by NOLA. 4. The release of NO from bovine aortic endothelial cells, induced by bradykinin (10 nmol/L), was detected by bioassay on pre-contracted rabbit aortic strips. NOLA (1-3 mumol/L, given through the cell column) reduced or abolished the release of NO, but did not affect relaxations of the bioassay tissues induced by glyceryl trinitrate or authentic NO. 5. These data indicate that NOLA potently inhibits the biosynthesis of NO from L-arginine, and thus prevents its release from arterial endothelial cells. It may be a useful pharmacological tool for probing the significance of NO biosynthesis in cardiovascular function.     

Purinergic and adrenergic cotransmission in canine isolated and perfused gastroepiploic arteries

1. The vasoconstrictor responses of canine gastroepiploic artery to periarterial electrical nerve stimulation (PNS; 30 s trains of pulses at a frequency of 2, 4 or 8 Hz) were observed in a frequency dependent manner. The PNS-induced vasoconstrictions were abolished by tetrodotoxin (1 micromol/L) and mostly depressed but not completely by guanethidine (10 micromol/L). 2. Vasoconstrictor responses to administered noradrenaline were antagonized significantly by prazosin (0.1 micromol/L), an alpha1-adrenoceptor antagonist, but were not significantly affected by suramin (100 micromol/L), a P2 purinoceptor antagonist, or alpha,beta-methylene ATP (1 micromol/L), a P2X receptor desensitizing agent. Exogenous ATP-induced responses were clearly depressed by suramin or alpha,beta-methylene ATP, but were not significantly affected by prazosin. 3. The vasoconstrictor responses to PNS at a low frequency (2 and 4 Hz) of stimulation were markedly inhibited by suramin (100 micromol/L) and by alpha,beta-methylene ATP (1 micromol/L). The remaining responses after suramin or alpha,beta-methylene ATP were abolished by subsequent application of prazosin (0.1 micromol/L). At a high frequency (8 Hz) of stimulation, the vascular response was not significantly inhibited by suramin or alpha,beta-methylene ATP, but it was abolished by prazosin. 4. Injection of xylazine (0.3-30 nmol/L), an alpha2-adrenoceptor agonist, did not induce any clear vasoconstriction. The exposure of tissues to rauwolscine (0.1-0.3 micromol/L), an alpha2-adrenoceptor antagonist, dose-dependently increased PNS-induced vasoconstrictions at all frequencies tested. 5. The present results indicate that ATP acts as a cotransmitter with noradrenaline and is responsible for post-junctional vasoconstrictor responses at low frequencies of sitmulation, whereas the effect of noradrenaline is dominant at high-frequency stimulation in canine gastroepiploic artery. Prejunctional alpha2-adrenoceptor autoinhibition may modulate the release of either noradrenaline or ATP from sympathetic nerve terminals.     

Alpha 2-adrenoceptor agonists enhance responses to certain other vasoconstrictor agonists in the rat tail artery.

1. The effects of the alpha2-adrenoceptor agonists clonidine, rilmenidine, TL99 and UK14304 on the vasoconstrictor response to sympathetic nerve stimulation and on the concentration-response curves to noradrenaline and phenylephrine were compared in two isolated, perfused vascular tissues: the rat tail artery (which has both postjunctional alpha 1- and alpha 2-adrenoceptors), and the rabbit ear artery (in which only alpha 1-adrenoceptors are present postjunctionally). 2. In the rabbit ear artery, the first observable effect of alpha 2-adrenoceptor agonists was inhibition of vasoconstrictor responses to sympathetic nerve stimulation. This occurred with concentrations of the alpha 2-adrenoceptor agonists which were far below those producing vasoconstriction. Responses to noradrenaline were not affected. 3. In contrast, in the rat isolated perfused tail artery, alpha 2-adrenoceptor agonists, in concentrations that produced no other observable effects, enhanced the vasoconstrictor responses to sympathetic nerve stimulation and to noradrenaline. Much higher concentrations of alpha 2-adrenoceptor agonists produced vasoconstriction in most preparations and only then reduced the response to sympathetic nerve stimulation. The enhancing effect of alpha 2-adrenoceptor agonists was blocked by idazoxan, but not by prazosin. 4. Vasoconstrictor responses in the rat tail artery to the relatively selective alpha 1-adrenoceptor agonist phenylephrine were enhanced by alpha 2-adrenoceptor agonists. The enhancement of the response to phenylephrine was greater than that to the mixed alpha 1- and alpha 2-adrenoceptor agonist noradrenaline. 5. Vasoconstrictor responses in the rat tail artery to vasopressin, ATP and KCl, like those to alpha 1-adrenoceptor agonists, were enhanced by alpha 2-adrenoceptor agonists.2+owever, vasoconstrictor responses to     

N-NITRO-L-ARGININE AND INDOMETHACIN DO NOT AFFECT ENDOTHELIN-INDUCED CONSTRICTION OF LARGE AND SMALL CORONARY ARTERIES IN THE ANAESTHETIZED GREYHOUND

• 1 The aim of this study was to investigate whether endothelin-l (ET-1)-induced constriction of large and small coronary arteries in the anaesthetized greyhound is modulated by the endogenous release of nitric oxide or prostanoids.
• 2 ET-1 (1–100 ng/kg) and the α₁-adrenoceptor agonist phenylephrine (0.5-2 pg/kg), when injected directly into the circumflex coronary artery, caused dose-dependent decreases in epicardial coronary artery diameter and coronary vascular conductance without affecting systemic arterial pressure or the rate and force of cardiac contraction.
• 3 Inhibition of NO synthesis with N-nitro-L-arginine (NOLA, 5 mg/kg, i.c.) decreased coronary artery diameter, coronary conductance and heart rate and increased arterial pressure. The coronary vasoconstrictor response to ET-1 was unaffected by NOLA. By contrast, NOLA significantly increased the phenylephrine-induced constriction of the epicardial coronary artery but not the resistance vessels.
• 4 Indomethacin (5 mg/kg, i.v.), an inhibitor of cyclo-oxygenase, significantly decreased epicardial coronary artery diameter but did not affect coronary conductance. Indomethacin had no effect on the coronary vascular responses to ET-1 or phenylephrine. Combined treatment with NOLA plus indomethacin also failed to affect the coronary vasoconstrictor effects of ET-1.
• 5 Basal release of NO and vasodilator prostanoids modulated resting coronary vascular tone but did not Influence the vasoconstrictor responses to endothelin in either large or small coronary arteries.

     

MECHANISMS OF THE EFFECTS OF ENDOTHELIN ON RESPONSES TO NORADRENALINE AND SYMPATHETIC NERVE STIMULATION

1. The effects of endothelin (0.1-1 nmol/L) and the phorbol ester, phorbol 12-myristate 13-acetate (PMA, 0.1-100 nmol/L) have been studied on vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline in the rabbit isolated ear artery. 2. Endothelin (0.1 nmol/L) significantly enhanced the biphasic response of the arteries to a prolonged period of sympathetic nerve stimulation (2 Hz for 60 s). Both the component of the response attributable to mobilization of intracellular calcium (phasic response) and the component due to the influx of extracellular calcium (tonic response) were enhanced. 3. PMA (0.1 and 0.3 nmol/L), which activates protein kinase C, enhanced vasoconstrictor responses to noradrenaline but a higher concentration of PMA (100 nmol/L) inhibited responses to noradrenaline. 4. It is concluded that the enhancement of responses to sympathetic nerve stimulation and noradrenaline produced by endothelin may be due to facilitation of the influx of extracellular calcium and to the activation of protein kinase C.     

Effect of endothelium on the actions of sympathetic and sensory nerves in the perfused rat mesentery.

We and others have previously demonstrated that pretreatment with capsaicin produces an augmentation of vasoconstrictor responses to transmural nerve stimulation. In the present study, removal of endothelium by saponin or inhibition of nitric oxide synthesis by N omega-nitro-L-arginine methyl ester produced an augmentation of vasoconstrictor responses to transmural nerve stimulation, responses which were further potentiated after treatment with capsaicin to desensitize sensory nerves. Capsaicin treatment decreased vasodilator responses to acetylcholine, but only at low acetylcholine concentrations. Potentiation by capsaicin of vasoconstrictor responses to transmural nerve stimulation was not affected by indomethacin. In the presence of guanethidine and methoxamine, transmural nerve stimulation caused vasodilator responses in the perfused rat mesentery. These responses were unaffected by removal of endothelium, as were vasodilator responses to exogenous calcitonin gene-related peptide (CGRP). In contrast, substance P did not produce any relaxation in the methoxamine-contracted mesentery. This study suggests that facilitation of vasoconstrictor responses to transmural nerve stimulation after capsaicin treatment primarily reflects inhibition of sensory nerve effects resulting in an increase of sympathetic vasoconstrictor actions. The present results also suggest that vasodilator responses to sensory nerve activation or exogenous CGRP are endothelium-independent and that substance P does not significantly contribute to modulation of vascular tone in the rat mesentery.     

DIFFERENTIAL BLOCKING EFFECTS OF TETRODOTOXIN ON DOUBLE-PEAKED VASOCONSTRICTOR RESPONSES TO PERIARTERIAL NERVE STIMULATION IN CANINE ISOLATED, PERFUSED SPLENIC ARTERY

1. In the present study, we investigated the effects of progressive inhibition of neuronal sodium channels by increasing concentrations of tetrodotoxin (TTX; 1-30 nmol/L) on the double-peaked vasoconstrictor responses to electrical periarterial nerve stimulation in the canine isolated and perfused splenic artery. 2. Double-peaked vasoconstrictions (biphasic vasoconstrictor responses) were consistently observed in following electrical stimulation with 30 s trains of pulses at 1-10 Hz. At low frequencies of stimulation (1-3 Hz), a submaximal concentration of 3 nmol/L TTX had no effect on the first phase of the contractile response, but almost completely inhibited the second-phase response. At high frequencies (6-10 Hz), the two vasoconstrictor phases were almost equally inhibited by 50% by 3 nmol/L TTX. A three-fold increase in the concentration of TTX used (10 nmol/L) abolished the second-phase vasoconstriction at all stimulation frequencies tested, whereas this concentration of TTX failed to block the first-phase response. Further increasing the concentration of TTX to 30 nmol/L completely blocked the remaining first-phase response. 3. Treatment with 0.1 mumol/L prazosin did not modify the first-phase response to any of the stimulation frequencies in the presence of 3 nmol/L TTX. Moreover, 0.1 mumol/L prazosin had no affect on the second-phase response at low frequencies (1-3 Hz), while at high frequencies (6-10 Hz) it slightly, but significantly inhibited the second-phase response. The vasoconstrictor responses that persisted after 3 nmol/L TTX and 0.1 mumol/L prazosin were completely suppressed by subsequent application of 1 mumol/L alpha, beta-methylene ATP at all stimulation frequencies (1-10 Hz). 4. In conclusion, progressive inhibition of sodium channels by increasing the concentration of TTX may exert a more preferential inhibition on adrenergic rather than purinergic components, suggesting that TTX-sensitive sodium channels may have a more important role in determining the adrenergic rather than purinergic transmission of sympathetic nerves.     

EFFECTS OF 17P-OESTRADIOL ON VASCULAR RESPONSES IN THE IN SITU BLOOD-PERFUSED MESENTERY OF WISTAR-KYOTO RATS

1. To determine whether endogenous oestrogen plays a role in pregnancy induced decreased vascular reactivity we have examined the effects of 173-oestradiol on vasoconstrictor responses to various stimuli using an in situ blood-perfused mesenteric vascular preparation in Wistar-Kyoto (WKY) rats. 2. Daily administration of 17β-oestradiol (500 μg/kg, s.c.) for 15 days significantly enhanced mesenteric vasoconstrictor responses to noradrenaline (NA), without affecting responses to the electrical stimulation of sympathetic nerves (ES) and angiotensin II (AngII). 3. Nitric oxide (NO) synthesis inhibition by nitro-L-arginine methyl ester (L-NAME; 100 mg/kg, i.v.) significantly potentiated mesenteric vasoconstrictor responses to all stimuli in both 173-oestradiol-treated and control animals. The difference in NA responses between groups was diminished following NO synthesis inhibition. 4. These findings do not support the hypothesis that increased endogenous oestrogen plays a role in decreased mesenteric vascular reactivity in pregnancy. However, responses to oestrogen may be dose-dependent and enhancement of vasoconstrictor responses to NA may be relevant to oral contraceptive-induced hypertension.