Relaxation Of Arterial Smooth Muscle Research Articles

Acute and long-term effects of angiotensin converting enzyme inhibition on larger arteries and cardiac hypertrophy

AIM OF STUDIES: The extent of cardiovascular alterations in hypertension is related quantitatively to the degree of hypertension. However, there is increasing evidence that humoral processes, which are not dependent on blood pressure, are of major importance in these alterations. In particular, angiotensin converting enzyme (ACE) activity seems to be involved. We therefore compared the effects of blood pressure reduction and of acute and long-term ACE inhibition on cardiac hypertrophy, arterial function and morphologic alterations in hypertension. To examine the acute effect of ACE inhibition on arterial compliance, we used an experimental model allowing a pressure-volume determination in the carotid artery in situ to assess arterial compliance in normotensive Wistar-Kyoto and spontaneously hypertensive rats. The pressure-volume relationship was determined after an acute, single, oral administration of either placebo or the ACE inhibitor quinapril in doses of 0.3 and 3 mg/kg. Both doses of quinapril produced a similar relaxation of arterial smooth muscle, approximately 80-90% of the maximum relaxation produced by potassium cyanide. LONG-TERM STUDY: We then studied the long-term effects of ACE inhibition compared with those of hydralazine. Four groups of 4-week-old spontaneously hypertensive rats were treated for 4 months with quinapril at 1 or 10 mg/kg per day, hydralazine at 15 mg/kg per day or placebo. Blood pressure, plasma and aortic ACE activity, left ventricular weight and histomorphometric parameters of the thoracic aorta, the renal and the mesenteric arteries were evaluated. We found a marked dissociation between the effects on blood pressure and cardiovascular structural parameters. Whereas both hydralazine and 10 mg/kg quinapril prevented the development of hypertension and aortic hypertrophy in a pressure-dependent manner, only the ACE inhibitor prevented left ventricular hypertrophy and aortic collagen accumulation. These quinapril effects were observed even with a small non-antihypertensive dose (1 mg/kg); the effect on aortic collagen was related to a reduction in aortic but not plasma ACE. These results indicate that the acute and long-term effects of ACE inhibitors on arterial function and structure go well beyond their antihypertensive actions and seem to be related to inhibition of tissue ACE. However, while clear effects were observed in the particular arterial segment explored, it may not be valid to extrapolate these findings to other arterial segments.

Pharmacological characterization and autoradiographic localization of dopamine receptors in the portal vein

1. Dopamine (DA) and DA receptor agonists exert a variety of effects on the cardiovascular system through interaction with specific DA receptors, including decreases in blood pressure and heart rate. 2. The decrease in blood pressure is due primarily to arterial vasodilation. This phenomenon is due to the stimulation of both postjunctional (D1-like or DA1) and prejunctional (D2-like or DA2) receptors causing respectively relaxation of arterial smooth muscle and decrease of the sympathetic vasoconstriction tone. 3. In view of the lack of detailed information on the existence of DA receptors in venous tissue, we have analysed D1-like and D2-like receptors in the rat portal vein using radioligand binding techniques associated with light microscope autoradiography. 4. No D1-like receptors were demonstrated in sections of the rat portal vein, whereas the D2-like receptor ligand, [3H]-spiroperidol, was bound to sections of the vein in a manner consistent with the labelling of D2-like sites. Anatomically, D2-like sites were located within the tunica adventitia, including the adventitia-media border, and in the endothelium. 5. These findings suggest the existence of D2-like but not D1-like receptor sites in the rat portal vein. D2-like sites of the tunica adventitia are probably prejunctional and involved in the modulation of sympathetic outflow. The functional significance of endothelial D2-like sites, if any, should be clarified in future studies.

Massive diltiazem overdose

Diltiazem is a calcium antagonist used in the treatment of angina, hypertension and some tachyarrhythmias. It has negative chronotropic and dromotropic effects on the sinoatrial and atrioventricular nodes, and is negatively inotropic. It also reduces systemic vascular resistance by arterial smooth muscle relaxation. There is limited experience in the management of a massive diltiazem overdose that can be fatal. 1–3 The present case, in which the patient survived, involves one of the largest diltiazem overdoses described.

Intrinsic effect of antihypertensive treatment with isradipine and metoprolol on large artery geometric and elastic properties

The effects of isradipine and metoprolol were studied on the brachial arteries of two groups of 14 patients with hypertension, 90 minutes after the first dose and after 3 months of treatment. Diameter (pulsed Doppler) and compliance (pulse-wave velocity) were measured and calculated in isobaric conditions by way of a model that allowed discrimination of the active intrinsic drug action. Isradipine increased measured and isobaric diameter during short-term (p < 0.05) and long-term administration (p < 0.05), whereas metoprolol did not change it. Active diameter effects were different between drugs during short-term administration (p < 0.05). Isaradipine increased measured and isobaric compliance during short-term (p < 0.05) and long-term administration (p < 0.05). Short-term administration of metoprolol decreased measured compliance (p < 0.01). Metoprolol decreased isobaric compliance during short-term (p < 0.01) and long-term (p < 0.05) administration. Active compliance effects were different between drugs during short- and long-term administration (p < 0.01). These arterial intrinsic drug effects, independent of the pressure-lowering influence, suggested different mechanisms, consisting of a large artery smooth muscle relaxation for isradipine and an isometric arterial constriction for metoprolol.

Effects of dobutamine and/or nitroprusside on the pulmonary circulation in patients with pulmonary hypertension secondary to end-stage heart failure

Nine NYHA class III-IV patients awaiting heart transplantation (HTx) were studied with a right ventricular ejection fraction (RVEF) catheter. The first aim of the study was to explore the pulmonary and systemic circulatory effects of dobutamine (D) and/or nitroprusside (N) in these patients. The second aim was to search for the parameter(s) among those usually measured that best predicted RVEF. Baseline data were recorded after 30 minutes of stabilization. Then, three drug regimens were administered in a randomized order for 30 minutes each: D, 4 μg/kg/min; N, 0.25 to 1.0 μg/kg/min; and their combination. Significant changes in RV loading and function were observed with all three therapies. The combination of both drugs was superior to either drug alone. The best predictor of RVEF was pulmonary arterial elastance. The second best was pulmonary capillary wedge pressure (PCWP). Pulmonary artery pressures, pulmonary vascular resistances, and transpulmonary gradient (TPG) were of less predictive value, as shown by a multiple regression analysis. None of the drugs showed any selectivity for the pulmonary vasculature, because the ratio PVRI/SVRI was never changed significantly. Selective pulmonary arterial vascular smooth muscle relaxation is probably not the most important mechanism to explain the unloading and improvement in function of the RV with D and/or N. Improved myocardial pump function appears to be the major factor in unloading the RV via reduction of PCWP and mean pulmonary artery pressure with essentially no change in TPG. The RVEF catheter provides valuable additional information in the screening of HTx candidates for pulmonary hypertension.

Mechanical function and histological structure of the arterial wall. The response to antihypertensive treatment.

Alterations in the structure and function of large arteries could be a major factor in the mortality and morbidity of hypertensive patients. This paper summarises the effects of various pharmacological treatments on the mechanical (functional) properties of large arterial walls and on the structure and composition of these vessels. Firstly, during arterial hypertension, increases in arterial stiffness, associated with medial hypertrophy and/or hyperplasia and alterations in extracellular matrix, are always observed. Sodium intake and aging may influence the arterial wall properties, causing an increase in stiffness, independently of their effect on blood pressure. In parallel, administration of low doses of diuretics to hypertensive rats did not cause significant haemodynamic changes, but did cause an increase in arterial distensibility. Nitrates and derivatives have specific effects on the smooth muscle from large arteries in hypertensive patients. Significant increases in the diameter of large arteries and in arterial wall stiffness have been reported in clinical studies, but these effects have a short time constant. In humans and animals, angiotensin converting enzyme (ACE) inhibitors and calcium antagonists cause relaxation of arterial smooth muscle and, therefore, improve the functional component of the reduced distensibility observed in hypertension. Long term treatment with ACE inhibitors and calcium antagonists induces a significant reduction in medial hypertrophy. However, it is difficult to distinguish between the direct effects resulting from pressure reduction and a specific drug effect.

Ethanol relaxes pulmonary artery by release of prostaglandin and nitric oxide

Acute-intake of ethanol is associated with vasodilation of vascular smooth muscle (VSM). Relaxation of VSM is dependent, in part, on the actions of nitric oxide (NO) and prostaglandin (PG) produced by endothelial cells (EC) lining the VSM. We examined the effects of endothelium rubbing and inhibition of EC synthesis of NO and PG on ethanol-induced relaxation of bovine pulmonary artery (BPA) and pulmonary vein (BPV) in vitro. Rings of isolated BPA and BPV were mounted in muscle chambers for the isometric recording of force development. Blood vessels were precontracted with an EC 50 concentration of the thromboxane receptor mimetic U46619. Ethanol (0.01, 0.02, 0.04, 0.08, 0.16, 0.32, 0.64, and 1.28% (w/v) produced concentration-dependent relaxation of BPA and BPV. Ethanol-induced relaxation was attenuated in BPA with rubbed EC and by the NO synthase inhibitors, l -N G monomethylarginine (LNMMA, 50 μM) and l-nitroarginine (NOLA, 10 μM), and the prostaglandin cyclooxygenase inhibitor, ibuprofen (10 μM). In contrast, ethanol-induced relaxation of BPV was not affected by endothelium rubbing or by NOLA or LNMMA, but was partially attenuated by ibuprofen. Nitric oxide was measured with the chemiluminescence technique. Ethanol increased the content of NO released under basal conditions by the BPA but did not effect basal NO release from BPV. However, ethanol enhanced bradykinin-induced release of NO from BPA and BPV and, at low concentrations, augmented bradykinin-induced relaxation of both BPA and BPV. These results indicate that arterial smooth muscle relaxation following ethanol administration occurs predominantly by an endothelium-dependent mechanism involving release of both NO and PG, whereas relaxation of venous smooth muscle results from direct stimulation of the smooth muscle. However, ethanol facilitates release of NO and endothelium-dependent relaxation to bradykinin.

Impairment by damage of the pterygopalatine ganglion of nitroxidergic vasodilator nerve function in canine cerebral and retinal arteries.

Histochemical study revealed that transcutaneous injection of ethanol into the vicinity of the pterygopalatine ganglion greatly decreased the positive staining for NADPH diaphorase activity after 1 week in the ipsilateral ganglion of a dog and abolished the staining of perivascular nerves in the middle and posterior cerebral arteries. Transmural electrical stimulation or nicotine produced a relaxation in middle and posterior cerebral arteries isolated from the side with the nontreated ganglion (control side), whereas the relaxation was abolished or reversed to a contraction in the arteries from the side with the ethanol-treated ganglion. Nitric oxide-induced relaxations did not differ in the arteries from both sides. The response to nerve stimulation of the control arteries was suppressed by treatment with NG-nitro-L-arginine (L-NA), an inhibitor of nitric oxide synthase, and the inhibition was reversed by L-arginine. Nicotine produced a contraction followed by a relaxation in central retinal arterial strips obtained from the control side; the relaxation was abolished and the contraction was potentiated in the arteries from the treated side. The nicotine-induced relaxation was abolished by L-NA, and the contraction was suppressed by phentolamine. On the other hand, the nicotine-induced relaxation in superficial temporal arteries, susceptible to L-NA, was not attenuated by treatment with ethanol. The findings obtained so far support our hypothesis that nitric oxide released from the vasodilator nerve acts as a transmitter to produce arterial smooth muscle relaxation and suggest that the nerve fibers to the cerebral and retinal arteries arise from the pterygopalatine ganglion.

The effect of hypertension and ACE inhibition on arterial structure and compliance.

1. Large artery dilatation may be produced by angiotensin-converting enzyme inhibition in hypertensive subjects independently of blood pressure reduction. The resulting increase in arterial compliance may be due to both blood pressure reduction and to arterial smooth muscle relaxation. 2. In healthy volunteers and in hypertensive subjects, dosages producing large artery dilatation seem to be even higher than those causing arteriole dilatation with the resulting blood pressure reduction. 3. It may be important to consider such findings for the remodelling of the cardiovascular system produced by angiotensin-converting enzyme inhibition.

Cyclic nucleotide-dependent regulation of Mn2+ influx, [Ca2+]i, and arterial smooth muscle relaxation.

Elevations in cyclic nucleotide levels can decrease myoplasmic [Ca2+] ([Ca2+]i) and thereby induce arterial smooth muscle relaxation. We evaluated whether cyclic nucleotide-induced reductions in [Ca2+]i are caused by 1) decreased Ca2+ influx or 2) increased Ca2+ sequestration or efflux. Swine carotid medial tissues were loaded with fura-2, and Ca2+ influx was estimated from the quenching rate of 360-nm fluorescence after addition of extracellular Mn2+. Histamine stimulation or high KCl depolarization increased Mn2+ influx, [Ca2+]i, and contractile force. The Ca2+ channel blocker diltiazem attenuated histamine- or KCl-induced increases in Mn2+ influx, [Ca2+]i, and force. Addition of forskolin (which increases cAMP) or nitroglycerin (which increases cGMP) attenuated histamine-induced increases in Mn2+ influx, [Ca2+]i, and force. Addition of forskolin or nitroglycerin also relaxed KCl depolarized tissues; however, Mn2+ influx and [Ca2+]i remained high. These results suggest that Mn(2+)-induced quenching of 360-nm fluorescence is an estimate of Ca2+ influx in the intact swine carotid artery. These results also suggest that cyclic nucleotides can relax swine arterial smooth muscle by at least two mechanisms: 1) reduction of [Ca2+]i primarily induced by decreases in Ca2+ influx and 2) uncoupling force from [Ca2+]i without changing Ca2+ influx or [Ca2+]i.

Nitrovasodilators relax arterial smooth muscle by decreasing [Ca2+]i and uncoupling stress from myosin phosphorylation.

Elevations in guanosine 3',5'-cyclic monophosphate concentration ([cGMP]) are proposed to induce arterial smooth muscle relaxation by either 1) decreasing myoplasmic [Ca2+] ([Ca2+]i), 2) decreasing the [Ca2+]i sensitivity of phosphorylation, or 3) uncoupling force from myosin phosphorylation. We evaluated the importance of each of these mechanisms by measuring changes in [cGMP], aequorin- and fura-2-estimated [Ca2+]i, myosin light chain phosphorylation, and stress in histamine-stimulated swine carotid arteries. In tissues submaximally stimulated with 3 microM histamine, nitroprusside (NP) induced a proportional decrease in myoplasmic [Ca2+] and myosin phosphorylation, suggesting that the relaxation was at least partially induced by decreases in [Ca2+]i without a change in the [Ca2+]i sensitivity of phosphorylation. In tissues maximally stimulated with 10 microM histamine, NP and nitroglycerin produced significant relaxations that were not associated with significant sustained reductions in [Ca2+]i or myosin phosphorylation. With both submaximal and maximal histamine stimulation, nitrovasodilators produced more substantial relaxation than that expected from the nitrovasodilator-induced reduction in myosin phosphorylation. These results suggest that nitrovasodilators relax histamine-stimulated swine arterial smooth muscle by at least two mechanisms: 1) reducing [Ca2+]i, an effect observed in submaximally stimulated tissues, and 2) uncoupling of stress from myosin phosphorylation.

Large artery dilation produced by converting enzyme inhibition in hypertension: Therapeutic aspects

Large artery dilation is produced by angiotensin-converting enzyme inhibition in hypertensive subjects despite a significant blood pressure reduction. The resulting increase in arterial compliance may be due both to blood pressure decrease and to arterial smooth muscle relaxation. In healthy volunteers and in hypertensive subjects, dosages causing large artery dilation seem to be higher than those causing pure arteriolar dilation with resulting blood pressure reduction. Similar findings have been noted to obtain compliance enhancement. Such results may be important in considering antihypertensive therapy, particularly when remodeling of the cardiovascular system is considered in long-term treatment.

Lack of increased cardiac output during hemoglobin hemodilution can be reversed with sodium nitroprusside.

To investigate a possible connection between EDRF or nitric oxide (NO) and the unchanged cardiac output (CO) during hemoglobin-hemodilution we infused nitroprusside (NP) in eight Hb-diluted dogs (Hct approximately 20%). Normal hypotensive doses of NP were not effective and supranormal doses (133.0 micrograms/kg/min) were needed to induce even a modest decrease in mean AoP (approximately 25 mmHg). With these NP doses, cardiac output increased 177%, diastolic AoP (afterload) decreased 30%, while systolic AoP and LVEDP (preload) were unchanged. Heart rate, LV contractility (pressure-volume function) and blood volume were not changed throughout the study. Normally, NP alone decreases both preload and afterload resulting in unchanged CO. In the Hb + NP dogs, CO increased because only afterload decreased suggesting a selective effect of Hb on venous and arterial smooth muscle relaxation. In hemodilution with nonhemoglobin colloids, CO increases primarily because the diluted blood offers less viscous resistance to ventricular ejection. It appears that in order for cardiac output to increases in the presence of Hb, some decrease in arteriolar resistance is needed, presumably to unmask the effects of reduced viscosity. These results suggest the unchanged CO during Hb-dilution is related to a selective effect of Hb on venous and arteriolar nitric oxide (EDRF) function.

Modification of Nitrovasodilator Effects on Vascular Smooth Muscle by Exogenous GTP and Guanosine

The effects of exogenous guanosine 5'-triphosphate (GTP) and guanosine on nitroglycerin-, sodium nitrite- and SIN-1-induced guanosine 3',5'-cyclic monophosphate (cyclic GMP) accumulation and smooth muscle relaxation were studied using endothelium-denuded rat mesenteric artery rings precontracted with noradrenaline. Preincubation of contracted artery rings with GTP (100 microM) or guanosine (100 microM) before eliciting relaxations with nitrovasodilators significantly shifted the dose-response curves of nitrocompounds to the left and augmented the increases in cyclic GMP. GTP and guanosine alone also induced cyclic GMP accumulation in pre-contracted artery rings. These effects of GTP and guanosine on nitrovasodilator responses were not related to the preincubation period (0-30 min). The present results raise the possibility of a cell membrane site of action for GTP and guanosine, which mediates the activation of soluble guanylate cyclase and leads to increased nitrovasodilator-induced cyclic GMP accumulation and arterial smooth muscle relaxation.

Cyclic AMP relaxes swine arterial smooth muscle predominantly by decreasing cell Ca2+ concentration.

1. Our objective was to evaluate the mechanism of cyclic AMP-dependent arterial smooth muscle relaxation. Cyclic AMP-dependent relaxation has been proposed to result from either (a) a decrease in intracellular [Ca2+] or (b) a decrease in [Ca2+] sensitivity of myosin light chain kinase by protein kinase A-dependent phosphorylation of myosin kinase. 2. We evaluated these proposed mechanisms by examining forskolin-induced changes in aequorin-estimated myoplasmic [Ca2+], [cyclic AMP], myosin phosphorylation and stress generation in agonist-stimulated or KCl-depolarized swine common carotid media tissues. 3. Forskolin, an activator of adenylyl cyclase, increased [cyclic AMP] and reduced [Ca2+], myosin phosphorylation and stress in tissues pre-contracted with phenylephrine or histamine. This relaxation was not associated with an alteration of the [Ca2+] sensitivity of phosphorylation, nor the dependence of stress on phosphorylation. 4. Forskolin pre-treatment attenuated, but did not abolish, agonist-induced increases in [Ca2+] and stress. 5. These results suggest that cyclic AMP-induced relaxation of the agonist-stimulated swine carotid media is primarily caused by cyclic AMP-mediated decreases in myoplasmic [Ca2+].

Response of Perfused Lung and Isolated Pulmonary Artery to Adenosine

Adenosine (AD) has been reported to induce both pulmonary arterial constriction and dilation. We investigated the effect of AD using two complementary techniques. The isolated rat lung perfused with Earle's balanced salt solution containing albumin was used to measure pulmonary arterial (Ppa), venous, and double occlusion (microvascular; Pmv) pressure, and resistance changes. AD alone had no effect on Ppa, Pmv, or resistance at any dose tested (5 x 10(-7) through 10(-3) M). However, when Ppa was elevated by pretreatment with 5 x 10(-7) M norepinephrine (NE), then 10(-4) M AD lowered Ppa by 19.5 +/- 3.2% and Pmv by 6.0 +/- 6.1% and attenuated the increase in upstream resistance caused by NE. Higher doses of AD (10(-3) M) resulted in greater relaxation. In isolated segments from rat and guinea pig pulmonary lobar arteries, isometric force production in response to AD was measured as a percentage of the active isometric force produced in response to 10(-5) M NE (% NE contraction). No response was observed in rat pulmonary arterial rings for doses of AD less than 10(-6) M. In vessels with intact endothelium, 10(-6) M AD caused a slight increase in isometric tension (2.3 +/- 1.2% NE contraction; p less than 0.05), but 10(-4) M AD caused relaxation (-17.2 +/- 2.2% NE contraction; p less than 0.05), and 10(-3) M caused further relaxation (-61.5 +/- 5.0% NE contraction; p less than 0.05). In vessels without endothelium, only relaxation was observed. Isolated guinea pig arterial rings responded to AD with vasodilation similar to the results in the rat arterial rings. Results of this study show that AD primarily causes a direct dose-dependent relaxation of pulmonary arterial smooth muscle in both the isolated perfused lung and isolated arterial ring preparation.

Effects of Isosorbide Dinitrate on the Pattern of Arterial Blood Flow in Healthy Human Subjects

1. Blood velocity measurements have been made in the superficial femoral artery, 10 cm downstream of the common femoral artery bifurcation, in healthy human subjects, using a multi-channel Doppler ultrasound device. 2. In a randomized double-blind protocol, the effects of isosorbide dinitrate were examined during a 2 h period. 3. The changes induced by isosorbide dinitrate include: (i) an increase in the width of the artery and a reduction in brachial arterial blood pressure, implying relaxation of arterial smooth muscle; (ii) an increase in reverse flow and a decrease in time-averaged mean velocity associated with a relatively small decrease of the velocity excursion during the cardiac cycle, implying an increase in flow pulsatility; and (iii) an alteration of the flow pattern both in the core and near the vessel walls.

Cytoplasmic calcium and the relaxation of canine coronary arterial smooth muscle produced by cromakalim, pinacidil and nicorandil

1. In order to investigate the vasodilator mechanisms of the K+ channel openers, cromakalim, pinacidil and nicorandil, we measured changes in cytoplasmic Ca2+ concentration [( Ca2+]i) simultaneously with force by a microfluorimetric method using fura-2, a calcium indicator, in canine coronary arterial smooth muscle cells. 2. The three K+ channel openers all produced a concentration-dependent reduction of [Ca2+]i in 5 and 30 mM KCl physiological salt solution (PSS) but failed to affect [Ca2+]i in 45 and 90 mM KCl-PSS. 3. Cromakalim only partly inhibited (-45%) the 30 mM KCl-induced contractures, whereas pinacidil and nicorandil nearly abolished contractions produced by 45 mM, 90 mM and 30 mM KCl-PSS. 4. Tetrabutylammonium (TBA), a nonselective K+ channel blocker, or glibenclamide, a supposed adenosine 5'-triphosphate (ATP)-sensitive K+ channel blocker, abolished the reduction of [Ca2+]i caused by the three K+ channel openers and the relaxant effect of cromakalim, whereas they only slightly attenuated the relaxant effects of pinacidil and nicorandil. 5. The increase in [Ca2+]i produced by 45 or 90 mM KCl-PSS in the presence of pinacidil or nicorandil was abolished by 10(-5) M verapamil, indicating that the increase in [Ca2+]i was caused by the influx of extracellular Ca2+ and that pinacidil and nicorandil did not affect the voltage-dependent Ca2+ channel directly. 6. The [Ca2+]i-force relationship in the presence of cromakalim was not distinguishable from that of control. 7. The [Ca2+]i-force curve was shifted to the right by pinacidil and nicorandil. 8. These results show that cromakalim is a more specific K+ channel opener than pinacidil and nicorandil, and that vasodilatation produced by cromakalim in this study is predominantly a result of a reduction of [Ca2+]i due to the closure of voltage-dependent Ca21 channels by hyperpolarization. In contrast, additional mechanisms are involved in the vasodilator actions of pinacidil and nicorandil. One of these is related to a reduction in the sensitivity of contractile proteins to Ca2 . The latter mechanism of nicorandil is akin to that of nitroglycerin. K+ channels opened by these K+ channel openers may be ATP-sensitive ones which are blocked by glibenclamide.

Hyperpolarization and relaxation of arterial smooth muscle caused by nitric oxide derived from the endothelium

Stimulation of the endothelial lining of arteries with acetylcholine results in the release of a diffusible substance that relaxes and hyperpolarizes the underlying smooth muscle. Nitric oxide (NO) has been a candidate for this substance, termed endothelium-derived relaxing factor. But there are several observations that argue against the involvement of NO in acetylcholine-induced hyperpolarization. First, exogenous NO has no effect on the membrane potential of canine mesenteric arteries. Second, although haemoglobin (believed to bind and inactivate NO (refs 11-15)) and methylene blue (which prevents the stimulation of guanylate cyclase) inhibit relaxation, neither has an effect on hyperpolarization. Finally, nitroprusside, thought to generate NO in vascular smooth muscle, relaxes rat aorta without increasing rubidium efflux. Nevertheless, nitrovasodilators, nitroprusside and nitroglycerin cause hyperpolarization in some arteries. NO might therefore be responsible for at least part of the hyperpolarization induced by acetylcholine. We now report that hyperpolarization and relaxation evoked by acetylcholine are reduced by NG-monomethyl-L-arginine, an inhibitor of NO biosynthesis from L-arginine. Thus NO derived from the endothelium can cause hyperpolarization of vascular smooth muscle, which might also contribute to relaxation by closing voltage-dependent calcium channels. Our findings raise the possibility that hyperpolarization might be a component of NO signal transduction in neurons or inflammatory cells.

Tetrodotoxin-Insensitive Relaxation of Coronary Arterial Smooth Muscle to Electrical Stimulation: Possible Involvement of a Dopaminergic Mechanism

Experiments were designed to determine the mechanism by which electrical stimulation causes tetrodotoxin-insensitive relaxation in isolated arteries. Rings of left anterior descending coronary arteries of dogs, pigs and calves were suspended in organ chambers between platinum electrodes. Experiments were performed after treatment with phenoxybenzamine and in the presence of propranolol. Calcium-free solution and calcium antagonists reduced the relaxation. Chemical denervation with 6-hydroxydopamine reduced the relaxation induced by electrical stimulation; in the presence of pargyline, the inhibitor of monoamine oxidase, it was virtually abolished. The nonselective dopaminergic antagonist droperidol and the selective DA1-dopaminergic antagonist SKF R83566 caused a concentration-dependent inhibition of the relaxation; the DA2-dopaminergic antagonist domperidone was ineffective. High concentrations of dopamine induced relaxation of the coronary smooth muscle; the relaxation was inhibited by SKF R83566 but not by droperidol. These results suggest that electrical stimulation causes relaxation by liberating an endogenous vasodilator substance, which acts on DA1-dopaminergic receptors of the coronary smooth muscle.