Evaluation of the alpha and beta adrenoceptor-mediated activities of the novel, orally active inotropic agent, ibopamine, in the cardiovascular system of the pithed rat: comparison with epinine and dopamine

The alpha and beta adrenoceptor-mediated effects of the novel, orally active inotropic prodrug, ibopamine, have been studied in the pithed rat and compared with those effects mediated by dopamine and the active form of ibopamine, epinine. All three agents produced alpha adrenoceptor-mediated pressor responses in pithed rats, and the vasopressor effects of ibopamine and epinine, but not dopamine, were potentiated by beta adrenoceptor blockade with propranolol (3 mg/kg i.v.). Catecholamine depletion with reserpine (5 mg/kg i.p.) did not affect the vasoconstrictor response elicited by any of these agents, indicating a direct effect in the vasculature. Epinine was 10 times more potent than ibopamine or dopamine. The pressor response to all three agents was antagonized by the alpha-1 adrenoceptor antagonist, prazosin (0.1 mg/kg i.v.) and the alpha-2 adrenoceptor antagonist, rauwolscine (0.5 mg/kg i.v.), suggesting the involvement of both alpha adrenoceptor subtypes in the vasopressor responses elicited by these compounds. After complete blockade of alpha adrenoceptors using a combination of phenoxybenzamine (3 mg/kg i.v.), prazosin (0.1 mg/kg i.v.) and rauwolscine (1 mg/kg i.v.), higher doses of ibopamine, epinine and dopamine produced a propranolol-sensitive, beta-1 adrenoceptor-mediated positive chronotropic response that was significantly reduced in reserpine-pretreated rats, indicating a significant indirect component in the activity of these compounds at the level of the myocardium. Epinine and dopamine were equipotent and were 10 times more potent than ibopamine as directly acting beta-1 adrenoceptor agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of aromatic fluorine substitution on the alpha and beta adrenoceptor-mediated effects of 3,4-dihydroxytolazoline in the pithed rat

The alpha and beta adrenoceptor-mediated effects of the catecholimidazoline, 3,4-dihydroxytolazoline, and its 2-, 5- and 6-aromatic fluorine-substituted derivatives have been studied in the cardiovascular system of the pithed rat. All four compounds produced vasopressor responses in beta adrenoceptor blocked (propranolol, 3 mg/kg i.v.) animals. The pressor responses produced by all four compounds were antagonized by the selective alpha-1 adrenoceptor antagonist, prazosin (0.1 mg/kg i.v.), and were relatively unaffected by the selective alpha-2 adrenoceptor antagonist, rauwolscine (0.5 mg/kg i.v.), indicating that vasoconstriction produced by the fluorinated imidazolines was mediated exclusively by postjunctional vascular alpha-1 adrenoceptors. The rank order of potency at the alpha-1 adrenoceptor was: 5-fluoro greater than 2-fluoro greater than desfluoro greater than 6-fluoro. At higher doses, 3,4-dihydroxytolazoline and its fluorinated derivatives produced an alpha-2 adrenoceptor-mediated inhibition of neurogenic tachycardia in animals pretreated with prazosin, with all four compounds being equipotent. In rats with complete alpha adrenoceptor blockade [phenoxybenzamine (3 mg/kg i.v.), prazosin (0.1 mg/kg i.v.) and rauwolscine (1 mg/kg i.v.)] whose blood pressure was elevated by constant infusion of angiotensin II (150 ng/kg/min i.v.), high doses of the 2-fluoro-, but not the 5-, 6- or desfluoro catecholimidazoline derivatives, produced a beta-2 adrenoceptor-mediated vasodepressor response. All four compounds produced a beta-1 adrenoceptor-mediated positive chronotropic response in pithed rats with the rank order of potency being: 2-fluoro = 5-fluoro greater than desfluoro greater than 6-fluoro.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pathophysiological and pharmacological mechanisms of acute cocaine toxicity in conscious rats.

In conscious rats, continuous i.v. infusion of cocaine (2 mg/kg/min) produced a marked increase in blood pressure, an initial moderate increase followed by a decrease in heart rate, tonic-clonic convulsions and, finally, a lethal episode of status epilepticus. No change in rectal temperature was observed. Infusion of cocaine methiodide (2 mg/kg/min), a quaternary derivative of cocaine, also produced a lethal episode of status epilepticus, but it was 6 times less potent than cocaine on a molar basis. In pentobarbital-anesthetized, spontaneously breathing rats, cocaine produced death by respiratory failure. Artificial ventilation of pentobarbital-anesthetized rats elevated the lethal dose of cocaine by 15-fold and these animals died of marked hypotension. In conscious rats, pretreatment with dl-, d- or l-propranolol or the alpha 2-selective adrenoceptor antagonist yohimbine enhanced the convulsive and lethal effects of cocaine. In contrast, the alpha 2-selective adrenoceptor agonist clonidine or the alpha 1-selective adrenoceptor antagonist prazosin attenuated these effects. Yohimbine antagonized the protective effect of clonidine. The nonselective alpha adrenoceptor antagonist phentolamine, the autonomic ganglionic blocker chlorisondamine and various calcium channel blockers had no effect on the convulsive or lethal doses of cocaine. The pressor response to cocaine was attenuated by calcium channel blockers, clonidine, phentolamine and dl- or l-propranolol, but not by d-propranolol. The pressor response to cocaine was abolished by chlorisondamine, reversed to a depressor response by prazosin and enhanced by yohimbine. The initial tachycardiac response to cocaine was reversed to bradycardia by dl- and l-propranolol, prazosin, yohimbine or high doses of the calcium channel blockers, but was unaffected by phentolamine, d-propranolol, clonidine or chlorisondamine. These results indicate that in spontaneously breathing animals, acute i.v. infusions of lethal doses of cocaine produce death primarily by central effects, namely by status epilepticus in conscious rats and by respiratory arrest in pentobarbital-anesthetized rats. In artificially ventilated, pentobarbital-anesthetized rats, however, cocaine produces death by effects on the cardiovascular system. In conscious rats, endogenous alpha 1 adrenoceptors exert a deleterious influence on cocaine-induced convulsive and lethal effects, whereas alpha 2 adrenoceptors provide protective influence. Propranolol appears to enhance cocaine-induced acute lethality through a mechanism independent of beta adrenoceptors. Calcium channel blockers appear ineffective in antagonizing cocaine's lethality.     

Role of receptor reserve in the inhibition of alpha-1 adrenoceptor-mediated pressor responses by calcium antagonists in the pithed rat

The effect of the calcium channel antagonists nifedipine and FR 34235 on the vasopressor response to alpha-1 adrenoceptor stimulation in the pithed normotensive rat was investigated. The maximal pressor response elicited by the full alpha-1 adrenoceptor agonist SK&F l-89748 was slightly but significantly reduced by 1-mg/kg doses of nifedipine (21 +/- 2%) and FR 34235 (34 +/- 4%). In comparison, the maximal pressor response to alpha-1 adrenoceptor stimulation by the partial alpha-1 agonist SK&F 88444 was markedly inhibited by nifedipine (51 +/- 1%) and FR 34235 (65 +/- 3%). Partial inactivation of the postsynaptic alpha-1 adrenoceptors with phenoxybenzamine (0.1 mg/kg) resulted in a maximal increase in diastolic pressure to alpha-1 adrenoceptor activation by SK&F l-89748 less than that induced by SK&F 88444. After phenoxybenzamine treatment, nifedipine and FR 34235 produced even greater reductions in the maximal vasopressor response to alpha-1 adrenoceptor stimulation by SK&F l-89748 (77 +/- 8 and 85 +/- 1%, respectively). Moreover, an inverse linear correlation (r = 1.00) was observed between the sensitivity of the maximal vasopressor response to nifedipine and FR 34235 and the magnitude of the maximal pressor response. The data suggest that the sensitivity of the alpha-1 adrenoceptor-mediated pressor response to inhibition by calcium antagonists in the pithed rat is inversely related to the magnitude of the pressor response, and they are consistent with the notion that the presence of "spare" alpha-1 adrenoceptors may determine the sensitivity of the pressor response to calcium antagonists.     

Evaluation of the alpha-1 and alpha-2 adrenoceptor-mediated effects of a series of dimethoxy-substituted tolazoline derivatives in the cardiovascular system of the pithed rat

The alpha-1 and alpha-2 adrenoceptor-mediated effects of a series of dimethoxy-substituted tolazoline derivatives were investigated in the cardiovascular system of the pithed rat. The 2,5- and 3,5-dimethoxy-substituted tolazoline derivatives produced vasopressor responses that were inhibited by the alpha-1 adrenoceptor antagonist, prazosin (0.1 mg/kg i.v.), and were not affected by the alpha-2 adrenoceptor antagonist, yohimbine (1 mg/kg i.v.), suggesting that these derivatives selectively activate postsynaptic vascular alpha-1 adrenoceptors. The 2,5- and 3,5-dimethoxy-substituted derivatives of tolazoline did not produce an alpha-2 adrenoceptor-mediated inhibition of neurogenic tachycardia in cord-stimulated pithed rats and were therefore presumed to be devoid of alpha-2 adrenoceptor agonist activity. In contrast, 2,3-dimethoxytolazoline produced a vasopressor effect that was inhibited by yohimbine but not by prazosin, suggesting selective activation of postsynaptic vascular alpha-2 adrenoceptors. Consistent with this observation is the fact that 2,3-dimethoxytolazoline elicited a dose-dependent, alpha-2 adrenoceptor-mediated inhibition of neurogenic tachycardia in cord-stimulated pithed rat. 3,4-Dimethoxytolazoline was a weak alpha-1 adrenoceptor agonist in the vasculature of the pithed rat and was devoid of agonist activity at alpha-2 adrenoceptors. However, 3,4-dimethoxytolazoline was found to be an alpha-2 adrenoceptor antagonist of similar potency as yohimbine. The results of the present study indicate that dimethoxy-substituted derivatives of tolazoline possess different activities and selectivities at alpha-1 and alpha-2 adrenoceptors depending upon the positions of substitution.(ABSTRACT TRUNCATED AT 250 WORDS)     

α1D-Adrenoceptors contribute to the neurogenic vasopressor response in pithed rats

Summary— The aim of the present study was to assess the role of vascular α_1D-adrenoceptors in the sympathetic vasopressor response in vivo. Specifically, we evaluated the effect of a selective α_1D-adrenoceptor antagonist, BMY 7378 (8-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-8-azaspiro(4,5)decane-7,9-dione 2HCI), on the vasopressor response induced by preganglionic (T₇-T₉) sympathetic stimulation in the pithed rat. The vasopressor response was dose-dependently sensitive to inhibition by intravenous BMY 7378 (0.1, 0.31, 1 and 3.1 mg/kg), doses of 1 and 3.1 mg/kg being equally effective. Like BMY 7378, 5-methylurapidil (0.1, 0.31, 1 and 3.1 mg/kg) antagonized the vasopressor response to spinal stimulation; doses of 1 and 3.1 mg/kg were also equally effective. In combination experiments, BMY 7378 (1 mg/kg, iv) and the α_1A-adrenoceptor antagonist, 5-methylurapidil (1 mg/kg, iv), showed an additive effect. The present results demonstrate that the α_1D-adrenoceptor subtype plays an important role in the pressor response to sympathetic nerve stimulation in the pithed rat, and confirm the participation of the α_1A-adrenoceptor subtype in the same response.     

Undifferentiated effects of calcium antagonists on pressor responses to selective alpha-1 and alpha-2 adrenoceptor agonists in anesthetized, spinal dogs

Whether pressor responses mediated by alpha-2 adrenoceptors are more susceptible to calcium antagonists than those mediated by alpha-1 adrenoceptors was investigated in anesthetized, spinal dogs. All drugs were administered i.v. Methoxamine (3-100 mu/kg) or xylazine (3-300 micrograms/kg) produced a sustained increase in mean arterial pressure but almost no effect on heart rate. Both the initial and the sustained phase of the pressor response to methoxamine were selectively antagonized by prazosin, whereas those to xylazine were selectively antagonized by yohimbine. These results indicate that the peripheral arterial bed of the dog comprises alpha-1 and alpha-2 adrenoceptors and that both the initial and sustained phases of the pressor response to methoxamine are predominantly mediated by alpha-1 adrenoceptors, whereas those to xylazine are mediated by alpha-2 adrenoceptors. The calcium antagonists, i.e., nifedipine (0.3-3 micrograms/kg), diltiazem (10-100 micrograms/kg) and KB-944 (10-100 micrograms/kg), administered during the sustained phase of the pressor responses to equieffective doses of methoxamine (100 micrograms/kg) and xylazine (1000 micrograms/kg), lowered mean arterial pressure. The three calcium antagonists in these doses also lowered the baseline mean arterial pressure but to a lesser extent than the elevated one. These results altogether indicate that the calcium antagonists were more effective in lowering mean arterial pressure elevated by either an alpha-1 or alpha-2 adrenoceptor agonist than the base-line mean arterial pressure and that the sustained phase of the pressor response mediated by alpha-1 adrenoceptors would involve Ca++ influx as much as or more than those mediated by alpha-2 adrenoceptors.     

Alpha and beta adrenoceptor blocking action of carvedilol in the canine mesenteric artery and vein

We observed the effects of carvedilol, a novel beta adrenoceptor blocker, on electrical responses of smooth muscle cells produced by endogenous and exogenous norepinephrine (NE) in isolated canine mesenteric artery and vein. Carvedilol inhibited the NE-induced depolarization in the artery but not in vein, with potencies equivalent to prazosin, i.e., carvedilol blocked alpha 1 adrenoceptors in arterial smooth muscles. Stimulation of perivascular nerves evoked an excitatory junction potential (e.j.p.) and a slow depolarization in these vascular smooth muscles. Carvedilol inhibited the slow depolarization evoked in the artery but not in the vein, with no marked inhibition of the e.j.p.s. High concentrations (10(-5) M) of carvedilol inhibited the e.j.p., slow depolarization, and also the compound action potentials of sympathetic nerve bundles running along the mesenteric vessels, suggesting that these inhibitions were due to local anesthetic actions. The e.j.p. amplitude was increased by isoprenaline and was decreased by NE. The NE and isoprenaline actions were antagonized by yohimbine and propranolol, respectively. Carvedilol inhibited the isoprenaline-actions but not the NE actions on the e.j.p., suggesting that this drug blocked prejunctional beta adrenoceptors but not the alpha 2 adrenoceptors. These results indicate that carvedilol blocks alpha 1 and beta adrenoceptors but not alpha 2 adrenoceptors in vascular tissues.     

Cardiovascular responses to the stimulation of alpha-1 and alpha-2 adrenoceptors in the conscious dog

Hemodynamic responses to the selective stimulation of alpha-1 and alpha-2 adrenoceptors were examined in chronically instrumented, conscious dogs. Norepinephrine (0.02-0.1 micrograms/kg/min), a mixed alpha-1/alpha-2 adrenoceptor agonist, phenylephrine (0.2-1.0 micrograms/kg/min), a selective alpha-adrenoceptor agonist and B-HT 920 (0.5-2.0 micrograms/kg/min), a selective alpha-2 adrenoceptor agonist, were infused i.v. after ganglionic (hexamethonium, 30 mg/kg i.v.), beta adrenoceptor (propranolol, 1, mg/kg i.v.) and muscarinic receptor (atropine methylbromide, 0.1 mg/kg i.v.) antagonism. Each of the alpha adrenoceptor agonists increased mean arterial pressure and total peripheral resistance but had no significant effect on cardiac output, stroke volume or heart rate. Equipressor doses of the alpha adrenoceptor agonists caused similar increases in left ventricular systolic and end-diastolic pressure, but there were no significant changes in left ventricular dP/dt or heart rate with any of the alpha adrenoceptor agonists. Selective antagonism of alpha-1 adrenoceptors with prazosin (1 mg/kg i.v.) abolished the pressor and vasoconstrictor responses to phenylephrine but had a lesser effect on the response to B-HT 920. Antagonism of alpha-2 adrenoceptors with rauwolscine (0.1 mg/kg i.v.) caused a significantly greater attenuation of the pressor and vasoconstrictor responses to B-HT 920 than to phenylephrine. The responses to norepinephrine were significantly attenuated by antagonism of either alpha-1 or alpha-2 adrenoceptors. Thus, in the conscious dog with reflex pathways blocked, selective stimulation of either postsynaptic alpha-1 or alpha-2 adrenoceptors increases arterial pressure and total peripheral resistance but does not significantly change heart rate, left ventricular dP/dt, stroke volume or cardiac output.     

An investigation of the effects of intravenous injection of the 5-hydroxytryptamine 2 receptor antagonists ketanserin and LY 53857 on blood pressure in anesthetized spontaneously hypertensive rats

The mode of action of i.v. injected ketanserin, LY 53857 and other 5-hydroxytryptamine (5-HT2) receptor antagonists in lowering blood pressure was examined in anesthetized and pithed spontaneously hypertensive rats (SHR). In pithed SHR, LY 53857 (1 mg kg-1) had no effect on alpha-1 or alpha-2 adrenoceptors, but ketanserin (1 mg kg-1) had some potency as an alpha-1 adrenoceptor antagonist, being approximately 100 times less potent than prazosin. Both ketanserin and LY 53857 (0.01 mg kg-1) markedly antagonized the pressor response to 5-HT. In pentobarbitone-anesthetized SHR, ketanserin and LY 53857 (1 mg kg-1) were equieffective at lowering diastolic blood pressure (DBP) subsequent to prazosin (1 mg kg-1), although ketanserin (1 mg kg-1) was more effective at lowering DBP in the absence of prazosin. The blood pressure lowering effects of LY 53857 were unaffected by the peripherally acting 5-HT2 receptor antagonist BW 501C. Neither LY 53857 nor ketanserin lowered DBP in pithed rats. It is concluded that ketanserin in high doses lowers DBP in anesthetized SHR partly by alpha-1 adrenoceptor blockade, but that ketanserin and LY 53857 in high doses have additional blood pressure lowering properties, unrelated to peripheral 5-HT2 receptor blockade.     

Clinical pharmacology of carvedilol in normal volunteers

The mechanism of the vasodilatory action of carvedilol (BM 14190), a new antihypertensive agent, was investigated in normal volunteers. Intra-arterial blood pressure and ECG were monitored continuously. Carvedilol (1 mg/min for 15 minutes) produced a rapid reduction in blood pressure and a transient increase in heart rate. At the end of infusion, systolic and diastolic blood pressure were reduced by 23% (-32.3 mm Hg) and 18% (-13.6 mm Hg), respectively, whereas heart rate was not different from baseline. At the doses used, the hypotensive effect of carvedilol was greater than that of labetalol (36 and 72 mg in 15 minutes). Carvedilol and labetalol antagonized isoproterenol-induced hypotension and tachycardia, at serum levels greater than or equal to 8 and 20 mg/ml, respectively. Both drugs antagonized phenylephrine pressor effects. A similar degree of inhibition (25% of control) of pressor effects was observed for carvedilol and labetalol when their respective serum concentrations were 23 ng/ml and 80 ng/ml. Neither carvedilol nor labetalol had any effect on AII pressor responses. Carvedilol serum levels as high as 150 ng/ml failed to inhibit AII-induced pressor responses. Our results suggest that at the doses used in this study, carvedilol has both alpha 1-and nonselective beta-receptor blocking properties. Moreover, carvedilol is approximately three to five times more potent than labetalol in blocking alpha 1-and beta-receptors and in reducing blood pressure.     

The antihypertensive effect of the angiotensin II receptor antagonist DuP 753 may not be due solely to angiotensin II receptor antagonism.

The angiotensin II (AII) receptor antagonist, DuP 753 (10 mg/kg intraduodenal), produced a sustained and long-lasting antihypertensive effect in conscious renin-dependent hypertensive rats. Blood pressures were still reduced markedly 24 to 72 hr after administration of a single dose of DuP 753. However, pressor responses elicited by either angiotensin I or AII were not blocked at these times despite the continued antihypertensive effect of DuP 753. In a model of orthostatic hypotension, DuP 753 and the selective alpha-1 adrenoceptor antagonist prazosin produced a marked orthostatic hypotension response in renin-dependent hypertensive rats as demonstrated by potentiation of the decrease in blood pressure induced by a 90 degrees tilt. The nonpeptide AII receptor antagonist SK&F 108566 (10 mg/kg intraduodenal) did not produce orthostatic hypotension and the angiotensin converting enzyme inhibitor enalapril produced only a slight orthostatic response to tilting. In conscious spontaneously hypertensive rats (SHR), allowed 3 to 4 days to recover from surgery, administration of either enalapril (1 mg/kg i.v.) or SK&F 108566 (10 mg/kg i.v.) did not significantly effect blood pressure. In SHR tested within 24 hr of surgery, enalapril was effective in lowering blood pressure. In contrast, in surgically recovered SHR, DuP 753 (10 mg/kg i.v.) produced an antihypertensive effect that was slow in onset, sustained and extremely long in duration. Blood pressures did not return to predrug levels until 48 hr after administration of DuP 753. Stimulation of the thoracolumbar sympathetic outflow in pithed rats produced frequency-dependent pressor responses that were significantly potentiated by continuous infusion of a subpressor dose of AII.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of the novel inotropic agent, ASL-7022, with alpha and beta adrenoceptors in the cardiovascular system of the pithed rat: comparison with dobutamine and dopamine

Three selective inotropic agents, ASL-7022, dobutamine and dopamine, were evaluated for their effects at alpha and beta adrenoceptors in the cardiovascular system of the pithed rat. ASL-7022, dobutamine and dopamine were equipotent as pressor agents in propranolol- and reserpine-pretreated pithed rats; however, the mechanisms involved in their alpha adrenoceptor-mediated pressor effects were markedly different. The pressor response of ASL-7022 was mediated entirely by postsynaptic vascular alpha-2 adrenoceptors, whereas the pressor response of dobutamine was mediated exclusively by postsynaptic vascular alpha-1 adrenoceptors. The pressor response of dopamine was mediated by both postsynaptic vascular alpha-1 and alpha-2 adrenoceptors. All three compounds elicited beta-2 adrenoceptor-mediated vasodepressor responses in pithed rats when vascular tone was elevated by a constant infusion of angiotensin II. In contrast to the equal vasopressor potencies of these compounds, the vasodepressor activities varied by more than two orders of magnitude with ASL-7022 being the most potent and dopamine the least potent. Based on ratios of relative potencies for alpha adrenoceptor-mediated vasopressor effects and beta-2 adrenoceptor-mediated vasodepressor effects, it appears that dobutamine possesses an equal balance between its vasopressor and vasodepressor potencies, such that the net effect in the vasculature is a physiological antagonism with little or no change in blood pressure, consistent with clinical observations and experiments in animals. In contrast, the vasopressor potency of dopamine exceeds its potency as a depressor agent, such that the net effect is vasoconstriction, consistent with clinical and animal studies.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of pertussis toxin on α1-adrenoceptor-mediated vasoconstriction by the full agonist, cirazoline, and the partial agonist, (-)-dobutamine, in pithed rats

The role of pertussis toxin-sensitive guanine nucleotide regulatory proteins (G-proteins) in the signal transduction process(es) involved in postjunctional vascular alpha 1-adrenoceptor-mediated vasoconstriction produced by the full agonist, cirazoline, and the partial agonist, (-)-dobutamine, have been investigated in the cardiovascular system of the pithed rat. Pertussis toxin pretreatment (50 micrograms/kg, iv, 3 days prior to experimentation) only slightly inhibited the pressor response of cirazoline, and the degree of inhibition produced by pertussis toxin was roughly equivalent to the inhibition produced by the calcium channel antagonist, nifedipine (1 mg/kg, ia). In contrast, pertussis toxin pretreatment produced marked inhibition of the alpha 1-adrenoceptor-mediated pressor response to the partial agonist, (-)-dobutamine, and this large degree of inhibition was qualitatively and quantitatively similar to the degree of inhibition produced by nifedipine. The differential pattern of inhibition of full and partial alpha 1-adrenoceptor agonists by pertussis toxin suggests that the vasoconstrictor response of an alpha 1-adrenoceptor partial agonist, which is more dependent upon the translocation of extracellular calcium than a full agonist, as evidenced by its sensitivity to inhibition by nifedipine, involves a pertussis toxin-sensitive G-protein that couples the alpha 1-adrenoceptor to the calcium channel. Furthermore, for alpha 1-adrenoceptor-mediated vasoconstriction by full agonists with high intrinsic efficacy, which involves both intracellular and extracellular pools of calcium, and particularly the former, pertussis toxin only inhibits that component of the alpha 1-adrenoceptor response which is dependent upon the translocation of extracellular calcium, accounting for the limited degree of inhibition of the response to cirazoline by pertussis toxin and by nifedipine. By inference, the other component of the alpha 1-adrenoceptor-mediated pressor response to a full agonist, which is dependent upon the mobilization of intracellular stores of calcium through a process believed to involve the activation of phospholipase C, likely utilizes a pertussis toxin insensitive G-protein that is distinct from that which we propose couples the alpha 1-adrenoceptor to the calcium channel. We conclude, therefore, that the alpha 1-adrenoceptor in the vasculature of the pithed rat may be coupled to 2 distinct G-proteins, only one of which is sensitive to inhibition by pertussis toxin and links the alpha 1-adrenoceptor to the membrane calcium channel, and which may be utilized by both full agonists and partial agonists.     

Alpha-1 adrenoceptor selectivity of phenoxybenzamine in the rat kidney

The dose-dependent selectivity of an irreversible binding antagonist, phenoxybenzamine (POB), for renal alpha-1 adrenoceptors, was biochemically and physiologically characterized. Receptors were quantified with the radioligands [3H]prazosin and [3H]rauwolscine for alpha-1 and alpha-2 adrenoceptors, respectively. Alpha-1 adrenoceptor function was quantified by the shift of the norepinephrine and phenylephrine pressor response in vivo and the vasoconstrictor response in the isolated perfused kidney preparation. A renal alpha-2 adrenoceptor response was demonstrated by showing that epinephrine could reverse the effect of vasopressin on water and sodium in the presence of beta blockade and alpha-1 destruction by POB. Doses of POB from 0.1 to 10.0 mg/kg progressively reduced [3H]prazosin binding to renal alpha-1 adrenoceptors until there was no specific binding at the 10.0-mg/kg dose. At this dose more than 60% of [3H]rauwolscine binding to renal alpha-2 adrenoceptors was still present. POB 1.0 mg/kg/hr decreased specific binding to renal alpha-1 adrenoceptors by approximately 40% (P less than .05) but reduced the alpha-1 adrenoceptor-induced vasoconstriction in the nonrecirculating isolated perfused kidney to 10 to 20% of the control. This was the maximal dose of POB studied which did not affect the alpha-2 adrenoceptor-induced antagonism of vasopressin. Higher doses of POB (3.0 mg/kg/hr) demonstrated some alpha-2 adrenoceptor binding as indicated by an attenuation of the antagonism by alpha-2 adrenoceptors. Thus, POB displays selectivity for renal alpha-1 over alpha-2 adrenoceptors. Our data indicate that a dose of 1.0 mg/kg/hr of POB will leave alpha-2 adrenoceptors intact while functionally incapacitating alpha-1 adrenoceptors to 10 to 20% of the control value.     

Effect of pertussis toxin treatment on postjunctional alpha-1 and alpha-2 adrenoceptor function in the cardiovascular system of the pithed rat

The role of pertussis toxin sensitive guanine nucleotide regulatory proteins (G-proteins) in the signal transduction processes involved in postjunctional vascular alpha-1 and alpha-2 adrenoceptor-mediated vasoconstriction has been investigated in the cardiovascular system of the pithed rat. Pertussis toxin pretreatment (50 micrograms/kg i.v., 3 days before experimentation) produced a marked inhibition of the alpha-2 adrenoceptor-mediated pressor response to B-HT 933. In contrast, pertussis toxin treatment had only a small effect on the alpha-1 adrenoceptor-mediated pressor response to cirazoline. However, after elimination of the alpha-1 adrenoceptor reserve for cirazoline with phenoxybenzamine (0.1 mg/kg i.v.), the pressor response to this agonist became highly sensitive to inhibition by pertussis toxin treatment. This pattern of inhibition of alpha-1 and alpha-2 adrenoceptor-mediated pressor responses by pertussis toxin is identical to that produced by inhibition of extracellular calcium influx by a high dose of the calcium channel antagonist, nifedipine (1.5 mg/kg i.a.), suggesting that those components of the alpha-1 and alpha-2 adrenoceptor-mediated vasoconstrictor processes that are dependent upon the translocation of extracellular calcium may involve a pertussis toxin sensitive G-protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activation of capsaicin-sensitive sensory neurons by carvedilol, a nonselective beta-blocker, in spontaneous hypertensive rats

We performed a study in spontaneous hypertensive rats (SHR) to determine whether carvedilol, a nonselective beta-adrenoceptor antagonist, activates capsaicin-sensitive sensory neurons (CSSNs), thereby promoting the release of calcitonin gene-related peptide (CGRP), a neuropeptide with an important role in maintenance of cardiovascular homeostasis. Carvedilol given intravenously at a dose of 0.3 mg/kg transiently decreased the mean arterial blood pressure (MABP) and increased renal tissue blood flow with increases in CGRP levels in plasma and kidney. These effects induced by carvedilol were not seen in animals pretreated with capsazepine, an antagonist of capsaicin. Although 1.0 mg/kg cavedilol markedly decreased MABP, it neither increased renal tissue blood flow nor CGRP levels in plasma and kidney. Prazosin, a selective alpha(1)-adrenoceptor antagonist, and bisoprolol, a selective beta(1)-adrenoceptor antagonist, decreased MABP with capsazepine, showing no antagonistic action in either cases, and these agents increased neither renal tissue blood flow nor levels of CGRP in plasma and kidney. Both ICI 118,551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol], a selective beta(2)-adrenoceptor antagonist, at a dose of 0.25 mg/kg and capsaicin mimicked effects induced by 0.3 mg/kg carvedilol. Administration of 1.0 mg/kg ICI 118,551 produced effects similar to those induced by 1.0 mg/kg carvedilol. These observations strongly suggested that the low dose of carvedilol might activate CSSNs in SHR to increase the release of CGRP, thereby decreasing blood pressure with an increase in renal tissue blood flow. The effects induced by carvedilol seemed to be mediated by its beta(2)-adrenoceptor blockade activity.     

Naloxone-induced bradycardia in pithed rats: evidence for an interaction with the peripheral sympathetic nervous system and alpha-2 adrenoceptors.

Earlier experiments performed in this laboratory have demonstrated that naloxone infusion (1 mg/kg/min i.v.) into conscious rats results in a bradycardia that has a peripheral component, is dependent on a certain level of sympathetic activity and is sensitive to alpha adrenoceptor blockade (5 mg/kg of phentolamine i.v.). The main objective of this investigation was to examine the underlying mechanism(s) responsible for the peripherally mediated naloxone-induced bradycardia, and to test the hypothesis that naloxone interacts with peripheral inhibitory alpha adrenoceptors associated with depression of peripheral sympathetic activity. Naloxone infusion (1 mg/kg/min i.v.) in pithed rats, in the absence of sympathetic nerve activation, resulted in a bradycardia that could not be blocked by 1 mg/kg (i.v.) of atropine, 5 mg/kg (i.v.) of phentolamine, 0.1 mg/kg (i.v.) of prazosin or 0.5 mg/kg (i.v.) of rauwolscine. Isoproterenol or norepinephrine-induced tachycardia was not blocked by naloxone infusion, suggesting that naloxone does not antagonize the postjunctional activation of cardiac adrenoceptors to cause bradycardia. In the presence of sympathetic nerve activity, naloxone depresses neurogenic tachycardia. This effect was blocked completely by 5 mg/kg (i.v.) of phentolamine or 0.5 mg/kg (i.v.) of rauwolscine, but not 0.1 mg/kg (i.v.) of prazosin or 1 mg/kg (i.v.) of atropine. The results of this investigation suggest that the naloxone-induced bradycardia in pithed rats is mediated postjunctionally and prejunctionally, and that this prejunctional effect is dependent on sympathetic nerve activity and inhibitory alpha-2 adrenoceptors. Furthermore, these results confirm results obtained from conscious rats in an earlier investigation.     

Pre- and postsynaptic alpha adrenoceptor selectivity studies with yohimbine and its two diastereoisomers rauwolscine and corynanthine in the anesthetized dog

The selectivity of yohimbine and its two diastereoisomers rauwolscine and corynanthine for pre- and postsynaptic alpha adrenoceptors has been investigated in the anesthetized dog. Antagonism of the inhibitory effect of clonidine on the tachycardia produced by electrical stimulation of the ansa subclavia was used as a measure of presynaptic alpha-2 adrenoceptor blockade. Inhibition of the diastolic pressor response to phenylephrine in ganglion and beta blocked dogs was used as a measure of postsynaptic alpha-1 adrenoceptor blockade. All three of the isomers reduced, and at higher doses reversed, the inhibitory effect of clonidine Yohimbine and rauwolscine were equipotent in this respect and were approximately 100-fold more potent than corynanthine. However, all the isomers were equipotent as antagonists of the diastolic pressure response to phenylephrine. Yohimbine and rauwolscine were approximately 30 times more potent as alpha-2 adrenoceptor than alpha-1 adrenoceptor antagonists, whereas corynanthine was 10-fold more potent at alpha-1 adrenoceptors than at alpha-2 adrenoceptors. These results are in broad agreement with those previously reported from in vitro experiments showing yohimbine and rauwolscine to be preferential alpha-2 adrenoceptor antagonists and corynanthine to be a preferential alpha-1 adrenoceptor antagonist. It is concluded that the high affinity of the antagonists yohimbine and rauwolscine for alpha-2 adrenoceptors is responsible for their selectivity because at the level of blockade of postsynaptic alpha-1 adrenoceptors both isomers were equipotent with corynanthine.     

Urethane inhibits cardiovascular responses mediated by the stimulation of alpha-2 adrenoceptors in the rat

Clonidine and oxymetazoline (4.0 microgram/kg i.v. or i.a.) evoked a marked bradycardia in either methylatropine-pretreated conscious or pentobarbital-anesthetized (55 mg/kg i.p.), vagotomized rats. Urethane (1.2 g/kg i.p.) inhibited by more than 50% this effect which is mediated through the stimulation of peripheral and/or central neuronal alpha-2 adrenoceptors. However, in adrenalectomized rats only the inhibition of oxymetazoline by urethane was significantly less pronounced. In pithed rats in which the adrenal glands were either left untouched or surgically removed, urethane significantly attenuated the clonidine or oxymetazoline-induced decreases in experimental neural sympathetic tachycardia although it neither changed the base-line nor the experimentally elevated heart rate. Urethane, in contrast to pentobarbital, increased plasma epinephrine concentrations in intact but not in adrenalectomized or in pithed rats. Elevation of plasma epinephrine did not result from the low arterial pressure level associated with urethane anesthesia since the increase of this parameter with vasopressin did not abolish the effect of urethane. Furthermore, guanethidine-pretreated rats, when anesthetized with urethane, exhibited a higher heart rate and plasma adrenaline value than those anesthetized with pentobarbital. The elevated heart rate was decreased by either propranolol or adrenalectomy. The bradycardia produced by injecting clonidine into the lateral cerebral ventricles of either intact or adrenalectomized rats was markedly less in urethane- than in pentobarbital-anesthetized animals. Whereas in pentobarbital-anesthetized rats the peak heart rate effects of i.v. or i.c.v. clonidine were similar, in urethane-anesthetized animals the effects of clonidine were more inhibited when it was given centrally than when it was given peripherally. In pithed rats, the cumulative dose-pressor response curves elicited by the relatively selective alpha-2 adrenoceptor agonists, B-HT 930 and M-7, were depressed by urethane significantly more than those produced by the relatively selective alpha-1 adrenoceptor agonists, phenylephrine and cirazoline, or by angiotensin II. Urethane also decreased the pressor responses evoked by clonidine, oxymetazoline and norepinephrine which stimulate both alpha-1 and alpha-2 adrenoceptors. However, the extent of this inhibition was less than that of B-HT 920 and M-7 but greater than that of cirazoline and phenylephrine. These results show that urethane inhibits cardiovascular responses that are mediated by peripheral and central alpha-2 adrenoceptors. Furthermore, urethane increases the central drive to the adrenal medulla and this leads to the secretion of epinephrine. This may be partly responsible for the inhibitory activity of urethane on oxymetazoline-induced bradycardia. Although the basic mechanism by which urethane impairs responses mediated by alpha-2 adrenoceptors remains to be determined, it is advised that urethane anesthesia should be avoided, particularly for cardiovascular studies.