Studies on the mechanism of arterial vasodilation produced by the novel antihypertensive agent, carvedilol

The mechanism(s) responsible for arterial vasodilation observed following acute administration of racemic carvedilol, a novel vasodilator/beta adrenoceptor antagonist, has been investigated in rats. In conscious spontaneously hypertensive rats, carvedilol (0.03-3.0 mg/kg, iv) produced a dose-dependent reduction in blood pressure with no significant effect on heart rate. Because cardiac output was relatively unaffected, the antihypertensive response of carvedilol was associated with a dose-dependent reduction in total peripheral vascular resistance. Submaximal antihypertensive doses of carvedilol were chosen for mechanism of action studies in pithed rats. Carvedilol (0.3 mg/kg, iv) produced a significant inhibition of the beta 1 adrenoceptor mediated positive chronotropic response to isoproterenol. This same dose of carvedilol also inhibited, but to a lesser degree, the beta 2 adrenoceptor mediated vasodepressor response to salbutamol in pithed rats whose blood pressure was elevated by a constant intravenous infusion of angiotensin II. Thus, carvedilol blocks both beta 1 and beta 2 adrenoceptors at antihypertensive doses, with modest selectivity being observed for the beta 1 adrenoceptor subtype. Carvedilol produced significant inhibition of the alpha 1 adrenoceptor mediated pressor response to cirazoline in the pithed rat, but had no effect on the alpha 2 adrenoceptor mediated pressor response to B-HT 933, suggesting that carvedilol is also an alpha 1 adrenoceptor antagonist at antihypertensive doses. Carvedilol had no effect on the pressor response elicited by angiotensin II, indicating a lack of nonspecific vasodilator activity. The vasopressor response to the calcium channel activator, BAY-K-8644, which is mediated through the opening of voltage dependent calcium channels and the subsequent translocation of extracellular calcium, was significantly inhibited by carvedilol (1 mg/kg, iv), suggesting that carvedilol is also a calcium channel antagonist, consistent with our previous in vitro studies. In anesthetized spontaneously hypertensive rats, the antihypertensive activity of carvedilol was nearly abolished by combined pretreatment of the rats with high doses of the alpha 1 adrenoceptor antagonist, prazosin (1 mg/kg, iv), and the nonselective beta adrenoceptor antagonist, propranolol (3 mg/kg, iv), suggesting that the majority of the antihypertensive response produced by carvedilol may be accounted for by blockade of beta and alpha 1 adrenoceptors. We therefore conclude that carvedilol, at antihypertensive doses, is an antagonist of beta 1, beta 2, and alpha 1 adrenoceptors, and also of calcium channels in vascular smooth muscle.(ABSTRACT TRUNCATED AT 400 WORDS)     

Prazosin selectively antagonizes norepinephrine contractions at low-affinity, non-alpha adrenoceptor sites (extraceptors) in arterial muscle

The characteristics of a low-affinity site activated by norepinephrine (NE), which is demonstrable in arterial smooth muscle pretreated with phenoxybenzamine (PBZ) to alkylate most alpha adrenoceptors were studied. Treatment with PBZ resulted in biphasic dose-response curves to NE. The initial phase associated with concentrations of NE up to 10(-5) M was due to its action in tissues where alpha adrenoceptors have been reduced to a number that would not support a maximum contraction. The second phase occurred with higher doses of NE. We postulate that this latter PBZ-resistant phase or contraction, which can represent more than 60% of the control maximal response, occurs at low-affinity sites for NE "extraceptors" distinct from alpha adrenoceptors in arterial smooth muscle. These residual responses to NE after PBZ treatment were inhibited selectively by prazosin and protected by prazosin from further inhibition by additional doses of PBZ. Effects due to prazosin at low-affinity sites were observed only with doses in excess of those required to competitively inhibit responses through alpha adrenoceptors in smooth muscle. The response to NE occurring through PBZ-resistant adrenoceptors was not dependent on the presence of the endothelium. This low-affinity site for NE was also evident after alkylation of alpha adrenoceptors with other irreversibly acting antagonists (e.g., benextramine and dibenamine). These observations are discussed in relation to the suggestion for a new adrenoceptor, the gamma-adrenoceptor. Differences and similarities of the extraceptor with this proposed adrenoceptor are discussed. Both sites have a high threshold to NE, and both may be influenced by prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of the endothelium on tone and the response of isolated pig coronary artery to norepinephrine

The influence of the endothelium on smooth muscle tone and the response of the pig right coronary artery to norepinephrine (NE) was studied. Isolated rings of artery with and without endothelium were stretched in the presence of nitroprusside to a tension previously determined to be optimal for contraction. During wash out of the nitroprusside, rings without endothelium spontaneously generated tone representing 24% of the contraction caused by potassium (120 mM); in rings with endothelium no significant spontaneous tone was observed. Relaxations were caused by NE in rings with endothelium contracted with prostaglandin F2 alpha (PGF2 alpha). In rings without endothelium, NE relaxed spontaneously generated tone as well as that produced by PGF2 alpha. Independent of the mode or degree of contraction, rings with endothelium were more sensitive to NE than rings without endothelium. The difference in sensitivity to NE between rings with and without endothelium was likely due to endothelial cell alpha-2 adrenoceptors, inasmuch as the difference was abolished by rauwolscine. In the presence of propranolol and prazosin, endothelium-dependent relaxations were observed which were also inhibited by rauwolscine. Nevertheless, beta adrenoceptors are the predominant mediator of the relaxation to NE of pig coronary smooth muscle, because propranolol caused a greater shift to the right of the relaxation induced by NE compared to that caused by endothelium removal. Accordingly, under resting conditions, NE caused contractions only in the presence of propranolol. These contractions were attenuated by prazosin or rauwolscine, but blocked only by a combination of both alpha adrenoceptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms of the bepridil-induced vasodilation of the rabbit mesenteric artery

Bepridil, at a concentration over 10 microM, increased the membrane resistance and, over 20 microM, depolarized the smooth muscle membrane of the rabbit mesenteric artery. These changes in membrane properties may be due to inhibition of K and possibly Cl conductances, as examined by the current-voltage relationship. The spike potential evoked by current pulses in the presence of 3 mM tetraethylammonium or from the excitatory junction potential (e.j.p.) was somewhat inhibited by bepridil (over 10 microM). Bepridil (over 5 microM) reduced the amplitude of e.j.p. without changing the facilitation process of e.j.ps and, at concentrations beyond 20 microM, it inhibited the facilitation of e.j.ps. Bepridil (over 0.1 microM) inhibited and over 1 microM blocked completely the tonic response of the K-induced contraction. Thus, the voltage-dependent Ca channels activated by K-induced depolarization and during the spike generation differ in nature. Bepridil inhibited the caffeine-induced contraction in intact muscles and the Ca-induced contraction in skinned muscles prepared by saponin to the same extent, but more effectively inhibited the norepinephrine-induced contraction without altering the norepinephrine-induced depolarization. We conclude that bepridil mainly inhibits the voltage-dependent Ca channel (the K depolarization sensitive) in smooth muscle membrane. High concentrations of bepridil (over 10 microM) inhibit the Ca spike, the K and Cl conductances, contractile proteins and Ca release from intracellular stored sites of smooth muscle cells and Ca mobilization at nerve terminals. Actions of bepridil differ from those of other Ca antagonists.     

Differential effects of bucindolol and carvedilol on noradenaline-induced hypertrophic response in ventricular cardiomyocytes of adult rats

In adult rat ventricular cardiomyocytes, noradrenaline exerts dual effects on protein synthesis: increases via alpha(1)-adrenoceptors and decreases via beta(1)-adrenoceptors. Carvedilol and bucindolol are beta-blockers with additional alpha(1)-adrenoceptor blocking activities. We studied the effects of carvedilol and bucindolol on noradrenaline-induced protein synthesis (assessed by [(3)H]phenylalanine incorporation) in adult rat ventricular cardiomyocytes. Radioligand binding studies with [(125)I]iodocyanopindolol and [(3)H]prazosin revealed that carvedilol had a much higher affinity to alpha(1)-adrenoceptors than bucindolol (beta(1)-/alpha(1)-adrenoceptor ratio for carvedilol, 1:2.7; for bucindolol, 1:43). Noradrenaline-evoked increases in protein synthesis were enhanced by propranolol (1 microM) and beta(1)-adrenoceptor-selective antagonists bisoprolol (1 microM) and CGP 20712A [1-[2-((3-carbamoyl-4-hydroxy)phenoxy)-ethyl-amino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl)phenoxy]-2-propranol methanesulfonate] (300 nM). Carvedilol (100 pM-10 microM) inhibited 1 microM noradrenaline-induced increase in protein synthesis with monophasic concentration-inhibition curves independent of whether CGP 20712A was present or not; K(i) values for carvedilol were 5 to 6 nM. In contrast, bucindolol (100 pM-10 microM) inhibited l microM noradrenaline-induced increase in protein synthesis with a bell-shaped concentration-inhibition curve; it increased noradrenaline-induced protein synthesis at 10 nM, although at concentrations >100 nM it was inhibited. In the presence of 300 nM CGP 20712A or 1 microM propranolol, however, bucindolol inhibited 1 microM noradrenaline-induced increase in protein synthesis with monophasic concentration-inhibition curves; K(i) values were 40 to 75 nM. On the other hand, both carvedilol and bucindolol inhibited 1 microM phenylephrine-induced protein synthesis with monophasic concentration-inhibition curves; K(i) values were 4 (carvedilol) and 45 nM (bucindolol). These results indicate that, at low (beta-adrenoceptor blocking) concentrations, bucindolol can enhance noradrenaline-induced protein synthesis whereas it is inhibited by carvedilol.     

Beta-1 and beta-2 adrenoceptor-mediated responses in preparations of pulmonary artery and aorta from young and aged rats

Rat pulmonary artery contains both alpha adrenoceptors mediating contraction and beta-1 and beta-2 adrenoceptors mediating relaxation. Neither alpha nor beta adrenoceptor-mediated responses of this vessel to norepinephrine or epinephrine were potentiated by cocaine, despite evidence for the presence of some adrenergic nerves. Beta adrenoceptor-mediated relaxation of pulmonary artery, but not aorta, modulated alpha adrenoceptor-mediated contractions to epinephrine but not norepinephrine. Rat aorta also contains both beta-1 and beta-2 adrenoceptors mediating relaxation, in that Schild plots for atenolol (beta-1 selective antagonist) using a beta-1 selective agonist (norepinephrine) and a beta-2 selective agonist (fenoterol), respectively, were not superimposed. The Schild plot data for lCl 118,551 (beta-2 selective) indicated that the minor population (beta-1) was less important in aorta than in pulmonary artery. On preparations from 18-month-old rats, the maximum relaxation to isoproterenol (20.4%, aorta and 67.9%, pulmonary artery) was less than in preparations from young rats (79.8%, aorta and 96.9%, pulmonary artery), i.e., aging had reduced the beta adrenoceptor-mediated relaxation in both vessels, but particularly in aorta. Also, the negative log EC50 of fenoterol, but not of isoproterenol or norepinephrine, was less than in preparations from young rats. This could indicate that aging had affected beta-2 more than beta-1 adrenoceptor-mediated responses and may explain why aging depressed the maximum relaxation of aorta more than that of pulmonary artery.     

Carvedilol in treating primary pulmonary hypertension patients: effect on severity of cardiac failure,degree of pulmonary hypertension,concentration of catecholamines in blood plasma and dependence of cyclic AMP synthesis in lymphocytes on beta-adrenergic receptors

AIM: To evaluate carvedilol effect on clinical state, norepinephrine (NE) and epinephrine (E) plasma concentration and lymphocyte beta 2-adrenoreceptor dependent cAMP synthesis in patients with primary pulmonary hypertension (PPH). MATERIAL AND METHODS: 14 patients with PPH were included in the study; 9 patients were treated by carvedilol; 5 patients were in the control group. The primary efficiency parameters were submaximal exercise measured by 60 min walk test, systolic pulmonary artery pressure (SPAP) measured by Doppler echocardiography, heart rate (HR) measured by 24-hour ECG-monitoring. RESULTS: One month carvedilol treatment of PPH patients resulted in a significant HR reduction. There was a tendency to SPAP decrease, the 6-minute walk test results improved in IIINYHA patients. CONCLUSION: Carvedilol therapy improves clinical state and evoked lymphocyte beta 2-adrenoceptor resensitization in PPH patients.     

Electrophysiologic Effects of Carvedilol: Is Carvedilol an Antiarrhythmic Agent?

The cardiovascular drug carvedilol is characterized by multiple pharmacological actions, which translate into a wide-spectrum therapeutic potential. Its major molecular targets are membrane adrenoceptors, ion channels, and reactive oxygen species. Carvedilol's favorable hemodynamic effects are due to the fact that the drug competitively blocks beta(1)-, beta(2)-, and alpha(1)- adrenoceptors. Several additional properties have been documented and may be clinically important, including antioxidant, antiproliferative/antiatherogenic, anti-ischemic, and antihypertrophic effects. The antiarrhythmic action of carvedilol may be related to a combination of its beta-blocking effects with its modulating effects on a variety of ion channels and currents. Several studies suggest that the drug may be useful in reducing cardiac death in high-risk patients with prior myocardial infarction and/or heart failure, as well as for primary and secondary prevention of atrial fibrillation. This article will review experimental data available on the electrophysiologic properties of carvedilol, with a focus on their clinical relevance.     

Effects of YM-12617, an alpha adrenoceptor blocking agent, on electrical and mechanical properties of the guinea-pig mesenteric and pulmonary arteries

The effects of YM-12617 on the electrophysiological properties of smooth muscle membranes and prejunctional nerve terminals and contractions evoked by different procedures were studied using guinea-pig mesenteric and pulmonary arteries. In concentrations over 1 nM, YM-12617 inhibited the depolarization induced by norepinephrine in both muscle tissues. Yohimbine had no effect whereas prazosin inhibited the norepinephrine-induced depolarization to a lesser extent than YM-12617. When YM-12617, in concentrations over 1 microM, was applied to the mesenteric artery the amplitude of the first excitatory junction potential evoked by a train stimulation of perivascular nerves was inhibited, but the facilitation of excitatory junction potentials evoked by frequencies over 0.1 Hz was enhanced. As a consequence, the amplitude of the excitatory junction potentials after completion of the facilitation exceeded the control, as was expected to occur with a typical alpha-2 adrenoceptor blocker. YM-12617 inhibited the contraction evoked by exogenously applied norepinephrine or perivascular nerve stimulation, with a higher potency than seen with prazosin, but this agent had no effect on the contraction evoked by excess concentrations of K+ or by direct muscle stimulation. These results indicate that YM-12617 possesses a more potent alpha-1 adrenoceptor blocking action than does prazosin, and is more selective for alpha-1 than for alpha-2 adrenoceptors.     

Failure of propranolol to inhibit the epinephrine-induced enhancement of responses to sympathetic nerve stimulation in the rat mesenteric vascular bed

Experiments were designed to characterize the nature of the epinephrine-induced potentiation of responses to sympathetic nerve stimulation in the Hooded Wistar rat. The responses to sympathetic nerve stimulation were determined in the isolated perfused mesenteric vascular bed preparation before and after infusion of epinephrine (at 0.27 or 2.7 microM); at the conclusion of the experiment the content of epinephrine in the mesenteric artery was determined. The intraluminal infusion of epinephrine at both high and low concentrations potentiated the responses of the preparation to sympathetic nerve stimulation. Mesenteric artery concentrations of this catecholamine were unchanged at the lower concentration (0.27 microM), but were increased after perfusion of epinephrine at the higher concentration (2.7 microM). The beta adrenoceptor antagonist propranolol (0.5 microM) did not prevent the epinephrine-associated potentiation of responses to sympathetic nerve stimulation, nor did it influence the pressor effects of exogenous norepinephrine. The results suggest that beta adrenoceptors do not play a role in the epinephrine-induced potentiation of responses to sympathetic nerve stimulation in the rat mesenteric vascular bed preparation. This potentiation may, however, be related to a desensitization of presynaptic inhibitory alpha adrenoceptors.     

Norepinephrine-sensitive, phenoxybenzamine-resistant receptor sites associated with contraction in rabbit arterial but not venous smooth muscle: possible role in adrenergic neurotransmission

The possibility that not all contractions of rabbit blood vessels to norepinephrine (NE) are mediated through alpha adrenoceptors sensitive to phenoxybenzamine (PBZ) was investigated. Dose-response curves (DRCs) to NE were made in the absence and presence of PBZ pretreatment which minimized the contribution of alpha adrenoceptors. In all arteries studied (saphenous, renal, femoral and central ear arteries), after PBZ-treatment, NE produced biphasic DRCs. The initial component of these DRCs corresponded to doses of NE which in the absence of PBZ were supramaximal. Under conditions of our experimentation the plateau-phase usually occurred at between 5 and 40% of the pre-PBZ maximal response to NE. The second phase occurred with further additions of NE, and achieved a mean of 72 (+/- 4)% of the pre-PBZ maximal contraction to NE. The latter component presumably represented contractions mediated through low-affinity sites for NE which were insensitive to doses of PBZ sufficient to alkylate alpha adrenoceptors. In veins (saphenous and inferior vena cava), we found no evidence for such sites. Our results are discussed in light of current ideas of adrenergic neurotransmission in vascular smooth muscle as proposed by Hirst and Neild (1980a) and others who suggest that response to high concentrations of neuronally released NE occur through PBZ-resistant receptors termed gamma adrenoceptors located exclusively at the postsynaptic membrane. We were able to demonstrate PBZ-resistant, low-affinity sites for NE contractions in the femoral artery, a vessel with very sparse adrenergic innervation, and conclude that such sites for NE are present in a number of arteries (and not veins) irrespective of their innervation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Norepinephrine release and vascular response elicited by nerve stimulation in rats with chronic neurogenic hypertension

In rats with chronic neurogenic hypertension induced by sinoaortic baroreceptors denervation (SAD), dl-[3H]norepinephrine (dl-[3H]NE) release, metabolism and vascular response to nerve stimulation were studied in isolated perfused mesenteric arteries. Thirty weeks after SAD, systolic arterial pressure was increased and, in isolated mesenteric arteries, vascular resistance, vascular response to nerve stimulation and responses to exogenous NE were higher in hypertensive than in sham-operated normotensive rats. However, in the isolated mesenteric arteries labeled with dl-[3H]NE, the total increase in radioactivity elicited by nerve stimulation was similar in both SAD and sham-operated groups. Unmetabolized [3H]NE released in mesenteric arteries was higher than that observed in other peripheral noradrenergic tissues (such as cat nictitating membrane, cat spleen, guinea-pig atria) and deaminated glycol 3,4-dihydroxyphenylglycol was the main metabolite of [3H]NE released by nerve stimulation from the mesenteric arteries of both groups. Blockade of prejunctional alpha adrenoceptors with phentolamine increased [3H]NE overflow to the same extent in controls and hypertensive rats. These findings indicate that the negative feedback regulatory mechanism for [3H]NE release mediated by prejunctional alpha adrenoceptors is not affected in chronic neurogenic hypertension. Blockade of postsynaptic alpha receptors by phentolamine reduced the vascular response equally in both groups. In hypertensive rats, a higher fraction of [3H]NE released by nerve stimulation was collected as the extraneuronally formed metabolite 3H-normetanephrine. The results suggest that, in rats with chronic neurogenic hypertension, sympathetic neurotransmission is not altered in the mesenteric arteries. Increased vasoconstrictor response to a given level of sympathetic nerve stimulation and higher resting vascular resistance could, in part, maintain the high blood pressure after 30 weeks of SAD.     

Pharmacological characterization of the postsynaptic alpha adrenoceptors in vascular smooth muscle from canine and rat mesenteric vascular beds

Postsynaptic alpha adrenoceptors have been characterized in rat superior mesenteric artery and branches of canine mesenteric artery. l-Phenylephrine, naphazoline and clonidine were used as agonists and prazosin and yohimbine were used as antagonists. The pA2 values for prazosin and yohimbine were determined against these agonists in canine and rat mesenteric arteries. The pA2 values for either prazosin or yohimbine were similar when l-phenylephrine and naphazoline were used as the agonists. The slope of the Schild plot was less than unity for prazosin and yohimbine acting as antagonists to l-phenylephrine in canine mesenteric artery whereas in the rat mesenteric artery, the slopes obtained for either phenylephrine or naphazoline against the antagonists were not significantly different from unity. In rat mesenteric artery, the pA2 value for yohimbine against clonidine was not statistically different from values obtained with the other agonists; however, in the presence of prazosin the dose-response curve to clonidine was biphasic, suggesting that the action of clonidine is mediated through two distinct sites to which prazosin has different affinities. However, these results do not provide definitive support for the concept of functionally independent subtypes of postsynaptic alpha adrenoceptors in rat and canine mesenteric artery.     

Reduced function of the stimulatory GTP-binding protein in beta adrenoceptor-adenylate cyclase system of femoral arteries isolated from spontaneously hypertensive rats

Arterial relaxant responses to beta adrenoceptor agonists are decreased in spontaneously hypertensive rats (SHR) when compared with normotensive Wistar-Kyoto rats (WKY). To determine which component of the beta adrenoceptor-adenylate cyclase (AC) system is involved in the decreased beta adrenoceptor responses, effects of two activators of AC-forskolin and cholera toxin and of dibutyryl cyclic AMP (DBcAMP) were compared between the strips of femoral arteries isolated from 13-week-old SHR and age-matched WKY. Arterial relaxant responses to either forskolin, an activator of AC or DBcAMP were not significantly different between the SHR and WKY, whereas the relaxant responses to norepinephrine (NE) via beta adrenoceptors were significantly weaker in the SHR than in the WKY. In the absence of timolol, a beta adrenoceptor antagonist, contractile responses to NE were significantly greater in the SHR than in the WKY. Timolol augmented the contractile responses to NE to a greater extent in the WKY than in the SHR. After the blockade by timolol of beta adrenoceptors, contractile responses to alpha adrenoceptor stimulation with NE were not significantly different between the two strains. The pretreatment of the strips with cholera toxin, an activator of stimulatory GTP-binding protein (Gs), antagonized the alpha adrenoceptor-mediated contractions much greater in the WKY than in the SHR. The alpha adrenoceptor-mediated contractions after the pretreatment with cholera toxin were comparable to the contractile responses to NE determined in the absence of timolol in either the SHR or the WKY. Forskolin and DBcAMP also antagonized the alpha adrenoceptor-mediated contractions. However, these antagonisms were not significantly different between the two strains. The cellular cAMP content in arterial strips after the stimulation with NE was significantly less in the SHR than in the WKY, whereas the cAMP contents were similar in arterial strips from both strains which were stimulated with forskolin. These results suggest that the reduced function of Gs is involved in the abnormality of beta adrenoceptor-AC system in the SHR femoral artery.     

A beta adrenoceptor with atypical characteristics is involved in the relaxation of the rat small intestine

In several studies in guinea pig ileum or rat colon a beta adrenoceptor with characteristics distinct from beta-1 or beta-2 receptors has been observed and has been denoted as "atypical" beta adrenoceptor. In this study the relaxation of the rat small intestine was investigated, using isolated segments of the rat jejunum. Several beta-1 or beta-2 agonists and antagonists were tested on the rat jejunum preparation, and it was found that nonselective and selective antagonists for beta-1 or beta-2 receptors showed a relatively low affinity, compared to their affinity for beta-1 or beta-2 receptors. BRL 37344, an agonist which has been reported to be selective for the atypical beta adrenoceptor, was more potent although a partial agonist compared to isoprenaline, whereas it is clearly less active than isoprenaline on beta-1 or beta-2 receptors. These findings indicate that beta adrenergic relaxation of the rat small intestine is mediated via atypical beta adrenoceptors. Efforts were made to confirm these findings with binding studies on small intestinal 45,000-g membranes. Competition radioligand binding experiments were performed with the radiolabeled ligand [125I]iodocyanopindolol and the various antagonists which were also tested in the intact rat jejunum preparations. Receptor binding experiments only revealed beta adrenoceptors of the beta-2-subtype, which does not correspond with the results obtained in the jejunum relaxation. Probably the beta-2 receptors found in the binding studies are located on circular smooth muscle cells or on epithelial cells, whereas longitudinal smooth relaxation is mediated by atypical beta adrenoceptors. Atypical beta adrenoceptors were not measured in binding studies probably because [125I]iodocyanopindolol is an unsuitable ligand to label atypical intestinal beta adrenoceptors.     

Potentiative effects of alpha agonistic sympathomimetic amines on vasoconstriction by adrenergic nerve stimulation

The effects of catecholamines and other sympathomimetic amines with alpha adrenergic activity on vasoconstriction were studied. Short ring segments were prepared from five rabbit blood vessels including the mesoduodenal, brachial, central ear and pulmonary lobar arteries and the saphenous vein. Constriction was elicited by electrical field stimulation of adrenergic neurons. This response was markedly potentiated in the mesenteric preparation by low concentrations of norepinephrine, epinephrine, phenylephrine, methoxamine, naphazoline and oxymetazoline. The potentiation occurred with amine concentrations not sufficient to elevate the spontaneous transmitter release or basal smooth muscle tone. Propranolol, cocaine and metanephrine did not prevent the potentiation. Isoproterenol and dopamine potentiated the response only in high concentrations and tyrosine was without an effect. An unidentified extraneuronal action was probably responsible for the potentiation. The mesenteric artery was unique for its marked potentiation as the brachial artery was potentiated only slightly and the other three vessels were inhibited by the alpha adrenergic agents in the constrictor response to stimulation.     

Responses to dopamine of isolated human and monkey veins compared with those of the arteries.

Isolated human gastroepiploic vein tributaries responded to dopamine only with contractions, whereas the gastroepiploic artery branches in the same region of the omentum responded with relaxations. Treatment with phentolamine converted the vein contraction to a relaxation, which was not influenced by propranolol but was abolished by droperidol. The relaxation was converted to a contraction by SCH23390 but unaffected by domperidone. Endothelium denudation abolished the relaxation caused by substance P but did not significantly alter the dopamine-induced relaxation. Dopamine increased the cyclic AMP content in the human veins. Monkey mesenteric, renal and portal veins and vena cava contracted in response to dopamine. Treatment with phentolamine converted the contraction to a slight relaxation in the mesenteric and renal veins; however, even in the presence of high concentrations of the alpha adrenoceptor antagonist, no relaxation was induced in the portal vein and vena cava partially contracted with prostaglandin F2 alpha. It is concluded that gastroepiploic veins and arteries in the human omentum respond quite differently to dopamine; the alpha adrenoceptor-mediated contraction predominates over the relaxation mediated via dopamine D1 receptor subtype in the veins, and vice versa in the arteries. Dopamine relaxes the human vein, possibly as a result of increased production of intracellular cyclic AMP by stimulation of D1 receptors. The predominant action of dopamine on alpha adrenoceptors may contribute to increasing venous return and cardiac output.     

Carvedilol, a new vasodilator and beta adrenoceptor antagonist, is an antioxidant and free radical scavenger.

The antioxidant effect of carvedilol, a new vasodilating, beta adrenoceptor blocker was studied and compared with five other beta blockers. Carvedilol rapidly inhibited Fe(++)-initiated lipid peroxidation, measured as thiobarbituric acid reactive substance (TBARS), in rat brain homogenate with an IC50 of 8.1 microM. Under the same conditions, the IC50 values of atenolol, pindolol propranolol, celiprolol and labetalol were over 1.0 mM. Carvedilol protected against Fe(++)-induced alpha-tocopherol depletion in rat brain homogenate with an IC50 of 17.6 microM; propranolol, celiprolol and labetalol, up to 200 microM, did not show any effect. Using dihydroxyfumarate/Fe(++)-ADP as a OH.radical generating system and 5,5-dimethyl pyrroline-N-oxide (DMPO) as a trapping agent, the characteristic DMPO-OH signals were monitored by electron paramagnetic resonance. Carvedilol dose-dependently decreased the intensity of the DMPO-OH signal, with an IC50 of 25 microM, whereas propranolol, at 500 microM, and U74500A, a 21-aminosteroid, at 100 microM, had no effect. The antioxidant effect of carvedilol mainly resides in the carbazole moiety, and the substitution of a hydroxyl group at certain positions on the phenyl ring of either carbazole or the ortho-substituted phenoxylethylamine part of carvedilol resulted in an increase in antioxidant activity. Furthermore, the protective effect of carvedilol analogs against OH.-mediated neuronal death positively correlated to their antioxidant effect. We conclude that carvedilol is a far more potent antioxidant than other commonly used beta blockers. The apparent mechanism of carvedilol's inhibition of lipid peroxidation is mainly via scavenging free radicals. This novel property of carvedilol may contribute to the known cardioprotective activity of this compound.     

Electrophysiological analysis of adrenergic neurotransmission and its modulation by chronic denervation in canine saphenous veins

The present experiments were undertaken to investigate the electrophysiological responses of the canine saphenous vein evoked by perivascular nerve stimulation, norepinephrine or selective alpha adrenergic agonists before and after chronic sympathetic denervation. Unilateral sympathectomy was performed from T12 to L9 in adult female dogs. After 3 to 5 weeks, the denervated saphenous veins were removed. Innervated saphenous veins were obtained from unoperated dogs. In innervated but not in denervated veins, electrical stimulation generated excitatory junction potentials and a slow depolarization. The slow depolarization was inhibited by rauwolscine or phentolamine, but not by prazosin, whereas excitatory junction potentials were not inhibited by alpha adrenergic blockers. Exogenously applied norepinephrine caused a depolarization of the membrane that was inhibited by rauwolscine but not by prazosin. The selective alpha-1 adrenergic agonist, phenylephrine, and the selective alpha-2 adrenergic agonist, UK 14,304, caused depolarization. In denervated veins, the threshold concentrations of norepinephrine or UK 14,304 required to depolarize the smooth muscle cell membrane were reduced. Responses to phenylephrine were not affected by denervation. These results indicate that in the canine saphenous vein norepinephrine, whether added exogenously or released from sympathetic nerves, causes predominant depolarization by activating alpha-2 adrenergic receptors. Denervation augments selectively the electrical response to alpha-2 adrenergic stimulation.     

BRL 34915 (Cromakalim) stimulation of 42K efflux from rabbit arteries is modulated by calcium

BRL 34915 (Cromakalim) is a novel benzopyran that has anti-hypertensive actions. Contractile and 42K efflux responses to BRL 34915 were measured in blood vessels from the rabbit to determine whether Ca-dependent K-channels [K(Ca)] were involved. BRL 34915 increased 42K efflux from all blood vessels tested with an EC50 of 0.2 to 0.7 microM in the superior mesenteric artery (SMA). BRL 34915 also inhibited norepinephrine (NE)-stimulated 42K efflux (96%) and contraction (50%) in SMA with IC50 values (0.2-0.3 microM) which were similar to the EC50. Comparison of two vascular sites showed that the portal mesenteric vein required 10-fold higher BRL 34915 to achieve a 42K response equivalent to the maximal effect on SMA, yet the maximum response to NE was 4-fold greater in portal vein than SMA. The 42K efflux stimulated by BRL 34915 exhibited a significant decrement with time. Ca-removal increased the magnitude and stability of the BRL 34915 response in SMA and femoral arteries, whereas reapplication of Ca in the presence of BRL 34915 caused the 42K efflux to decrease rapidly. These effects of Ca were not altered by nisoldipine (NIS). The 42K efflux stimulated by BRL 34915 and by K-depolarization was additive when applied in combination in SMA. BRL 34915 had no effect on the NIS-inhibited 42K efflux (a Ca-dependent component) during K-depolarization. It is concluded that BRL 34915 acts on K-channels which differ significantly from the known properties of K(Ca)-channels and that BRL 34915 indirectly inhibits NE stimulation of contraction and K(Ca)-channels by hyperpolarizing the smooth muscle membrane.