Endothelial potentiation of relaxation response to beta adrenoceptor blocking agents

A number of beta adrenergic blocking drugs were evaluated on ring preparations of endothelium intact and denuded segments of the rat aorta. The preparations were preconstricted under isometric conditions with an EC80 dose of phenylephrine. Labetalol (10(-7)-10(-5) M), MK-761 10(-7)-10(-5) M), timolol (10(-7)-10(-4) M) and propranolol (10(-6)-10(-4) M) relaxed both endothelium intact and denuded vessels in a dose-dependent manner. Spirendalol (2.8 X 10(-8)-8.1 X 10(-6) M), a specific beta-2 receptor antagonist and L643717 (1.8 X 10(-7)-3.6 X 10(-6) M), a specific beta-1 receptor antagonist did not elicit relaxation. Labetalol, MK-761, timolol and propranolol promoted relaxation only when vascular segments were preconstricted with phenylephrine or norepinephrine and failed to do so when prostaglandin F2 alpha or U46619 were used. This indicates a possible displacement of alpha adrenergic agonists with the beta antagonists. The degree of relaxation induced by labetalol, MK-761, timolol and propranolol was significantly less (P less than .05) when the endothelium was removed. Eicosatetraynoic acid (3.2 X 10(-5) M) significantly attenuated the relaxation response to labetalol, MK-761 and timolol in the intact but not in denuded vascular preparations. These studies suggest that some of the vascular effects of beta blockers may relate to the endothelium.     

Possible involvement of membrane-stabilizing action in beneficial effect of beta adrenoceptor blocking agents on hypoxic and posthypoxic myocardium

The present study was designed to elucidate a possible involvement of membrane-stabilizing action of beta blocking agents in posthypoxic recovery of cardiac contractile function and myocardial metabolism. Propranolol and acebutolol, which possess a membrane-stabilizing action, and atenolol and metoprolol, which lack this action, were used in the isolated, perfused rabbit heart. The membrane-stabilizing effects of these agents were assessed on the basis of the effects on the maximal driving frequency of the left atria. Reoxygenation of hearts for 45 min following 20-min hypoxia resulted in little recovery of cardiac contractile force, sustained rise in resting tension, insufficient recovery of myocardial high-energy phosphates, accumulation of the tissue calcium and sodium and marked release of creatine kinase and ATP metabolites from the hearts. Treatment of hypoxic hearts with either 100 microM propranolol, 200 microM acebutolol, 200 microM atenolol or 100 microM metoprolol was commenced when the contractile force declined to 30% of the initial level and terminated at 20-min hypoxia. Treatment with either propranolol or acebutolol produced a significant posthypoxic recovery of cardiac contractile force, resting tension and myocardial high-energy phosphates, and a profound suppression of the tissue calcium and sodium accumulation and the loss of ATP metabolites from perfused hearts. In contrast, neither atenolol nor metoprolol affected these changes induced by the hypoxic insult and the following reoxygenation. The results suggest that membrane-stabilizing action of beta blocking agents plays an important role in the protection against posthypoxic cardiac contractile dysfunction and metabolic disturbances.     

Isoproterenol antagonism of cardioselective beta adrenergic receptor blocking agents: a comparative study of human and guinea-pig cardiac and bronchial beta adrenergic receptors.

pA2 values against isoproterenol were determined for a number of cardioselective and noncardioselective beta adrenergic receptor blocking agents using human and guinea-pig isolated atrial and bronchial or tracheal preparations to study possible species differences. No significant differences in pA2 values for propranolol, pindolol, Ro 3-4787, acebutolol, atenolol, practolol, metoprolol, H 87/07 and tolamolol on bronchial or tracheal beta adrenergic receptors of both species were found. With respect to atrial beta adrenergic receptors, significantly lower pA2 values for human preparations, as compared to guinea-pig preparations, were found for tolamolol and CI 775. These are the only two agents in the series that derive their cardioselectivities from specific nitrogen substitutents. The different potencies of only these two compounds in antagonizing isoproterenol on atrial beta adrenergic receptors of both species suggest a difference in an accessory receptor area close to the site that interacts with the nitrogen atom of beta adrenergic agents.     

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.     

Pamatolol: phase I evaluation of the pharmacodynamics of a cardioselective beta adrenoceptor blocking drug.

A Phase I evaluation of a new adrenoceptor blocker, pamatolol, was performed in 10 healthy male volunteers. Minor reductions in standing and exercise and isoproterenol-induced increases in heart rate were observed with the 10-mg oral dose and appeared maximal with the 400-600 mg dose. The rate of decline of effect averaged 1.5% of the exercise heart rate/hr. Neither resting systolic time intervals nor post-exercise pulmonary function was affected by this dose range of pamatolol. Based on the latter responses and the isoproterenol dose response curve, we tentatively conclude that pamatolol is relatively cardioselective in man.     

Localization of beta adrenoceptor subtypes in rat kidney by light microscopic autoradiography

The characteristics and localization of beta adrenoceptor subtypes in rat kidney sections have been examined using [125I]cyanopindolol and in vitro labeling combined with autoradiography. Binding was stereoselective since the (-)-isomers of propranolol and pindolol were some two orders of magnitude more effective as competitors than the (+)-isomers. Competition curves obtained using the subtype selective antagonists ICI 118,551 (beta-2) and betaxolol (beta-1) had low pseudo Hill coefficients and were resolved into two distinct components representing beta-1 (63%) and beta-2 adrenoceptors (37%). Combined autoradiographic and histochemical studies using nuclear emulsion-coated coverslips and alkaline phosphatase staining showed that the majority of receptors were in the renal cortex and in the outer band of the medulla with fewer receptors associated with the inner medulla, papilla and renal blood vessels. Delineation of beta-1 and beta-2 adrenoceptor subtypes with the selective antagonists betaxolol and ICI 118,551 indicated that the highly localized receptors were predominately of the beta-1 subtype, associated with glomeruli and with tubules that from their staining characteristics and topography represent distal and cortical collecting tubules with few if any receptors associated with proximal tubules. Beta-2 adrenoceptors were more diffusely distributed in the cortex and there were minor areas of localization in the inner medulla. Although some glomerular beta adrenoceptors probably play a role in control of renin release, their distribution throughout this structure indicates that they also control other functions. The distribution of beta adrenoceptors in tubules corresponds well with the known distribution of beta adrenoceptor-stimulated adenylate cyclase in rat kidney and indicates that these receptors subserve a physiological function.     

Clebopride enhances contractility of the guinea pig stomach by blocking peripheral D2 dopamine receptor and alpha-2 adrenoceptor.

The mechanism of action of clebopride on the motility of guinea pig stomach was examined by the receptor binding assay for bovine brain membrane and by measuring gastric contractility and the release of acetylcholine from the stomach. The receptor binding assay revealed that clebopride bound to the D2 dopamine receptor with a high affinity and to the alpha-2 adrenoceptor and 5-HT2 serotonin receptor with relatively lower affinity, and not to D1 dopamine, alpha-1 adrenergic, muscarinic acetylcholine, H1 histamine, or opioid receptor. In strips of the stomach, clebopride at 10(-8) M to 10(-5) M enhanced the electrical transmural stimulation-evoked contraction and the release of acetylcholine. This enhancement was attributed to the blockade of the D2 dopamine receptor and alpha-2 adrenoceptor because: 1) Maximum responses obtained with specific D2 dopamine receptor antagonist, domperidone, and with specific alpha-2 adrenoceptor antagonist, yohimbine, were smaller than that with clebopride, and the sum of the effects of these two specific receptor antagonists is approximately equal to the effect of clebopride. 2) The facilitatory effect of clebopride was partially eliminated by pretreatment of the sample with domperidone or yohimbine, and the facilitatory effect of clebopride was not observed in preparations treated with the combination of domperidone and yohimbine. Clebopride also antagonized the inhibitory effects of dopamine and clonidine on the electrical transmural stimulation-evoked responses. These results indicate that clebopride acts on post ganglionic cholinergic neurons at D2 and alpha-2 receptors in this preparation to enhance enteric nervous system stimulated motility.     

The role of neuronal and extraneuronal plasma membrane transporters in the inactivation of peripheral catecholamines.

Catecholamines are translocated across plasma membranes by transporters that belong to two large families with mainly neuronal or extraneuronal locations. In mammals, neuronal uptake of catecholamines involves the dopamine transporter (DAT) at dopaminergic neurons and the norepinephrine transporter (NET) at noradrenergic neurons. Extraneuronal uptake of catecholamines is mediated by organic cation transporters (OCTs), including the classic corticosterone-sensitive extraneuronal monoamine transporter. Catecholamine transporters function as part of uptake and metabolizing systems primarily responsible for inactivation of transmitter released by neurons. Additionally, the neuronal catecholamine transporters, recycle catecholamines for rerelease, thereby reducing requirements for transmitter synthesis. In a broader sense, catecholamine transporters function as part of integrated systems where catecholamine synthesis, release, uptake, and metabolism are regulated in a coordinated fashion in response to the demands placed on the system. Location is also important to function. Neuronal transporters are essential for rapid termination of the signal in neuronal-effector organ transmission, whereas non-neuronal transporters are more important for limiting the spread of the signal and for clearance of catecholamines from the bloodstream. Besides their presynaptic locations, NET and DAT are also present at several extraneuronal locations, including syncytiotrophoblasts of the placenta and endothelial cells of the lung (NET), stomach and pancreas (DAT). The extraneuronal monoamine transporter shows a broad tissue distribution, whereas the other two non-neuronal catecholamine transporters (OCT1 and OCT2) are mainly localized to the liver, kidney, and intestine. Altered function of peripheral catecholamine transporters may be involved in disturbances of the autonomic nervous system, such as occurs in congestive heart failure and hypernoradrenergic hypertension. Peripheral catecholamine transporters provide important targets for clinical imaging of sympathetic nerves and diagnostic localization and treatment of neuroendocrine tumors, such as neuroblastomas and pheochromocytomas.     

The beta adrenoceptor blocker tertatolol causes vasodilatation in the isolated perfused vasoconstricted rat kidney

The activity of the beta adrenoceptor antagonist tertatolol on renal vasoconstrictions was investigated. Infusion of increasing concentrations of tertatolol (10(-8) to 10(-5) M) progressively inhibited the constrictor responses to bolus injections of norepinephrine and to electrical stimulation in isolated perfused kidneys of both normotensive and spontaneously hypertensive rats. Also, in kidneys of normotensive rats the vasoconstrictions caused by serotonin and barium chloride were inhibited by tertatolol. During sustained vasoconstrictions induced by infusion of norepinephrine (6 X 10(-7) M) increasing doses of tertatolol (2.5 X 10(-7) g to 2 X 10(-5) g) caused rapid, reversible dilatations in the rat kidneys. The inhibitory responses caused by tertatolol were not antagonized by propranolol, atropine, hexamethonium, SCH23390, metoclopramide, mepyramine, cimetidine, naloxone, cocaine or indomethacin. During constrictions caused by norepinephrine, methylene blue significantly inhibited the renal vasodilatations caused by tertatolol, acetylcholine, papaverine and nitroglycerin but not those caused by atrial natriuretic factor. Unlike the other vasodilators, tertatolol did not inhibit the constrictions induced by prostaglandin F2 alpha (5 X 10(-6) M) in the rat kidneys. In canine renal arteries with endothelium, tertatolol (10(-9) to 10(-5) M) did not cause relaxations during contractions induced by norepinephrine, electrical stimulation or prostaglandin F2 alpha. Our data illustrate that tertatolol has potent vasodilator properties in the isolated perfused vasoconstricted rat kidney. The dilator response to the beta blocker cannot be inhibited by a variety of classical receptor blockers but ultimately seems to depend on the formation of cyclic GMP.     

Effect of beta adrenergic blocking agents on erythropoiesis in rats

The purpose of the present study was to analyze the effect of two different beta blocking agents, metipranolol (Trimepranol) and dl-propranolol (Inderal), upon erythropoietin production, erythropoietin responsive cell compartment and erythrocyte production. The effect on erythropoietin production: in rats metipranolol or dl-propranolol was injected daily in a single dose of 10 mg/kg b.wt. On the 5th day, the rats were exposed to an atmospheric pressure of 353.7 torr (47.1 kPa, 6000 m) to stimulate erythropoietin production. The onset of hypoxic exposure was started 1, 4, 12 or 24 hr after the last injection of beta blocking drugs. After the termination of hypoxia, erythropoietin plasma levels were determined in polycythemic mice. The rats treated with metipranolol 1 and 4 hr before hypoxia produced significantly less erythropoietin in response to hypoxia than saline-treated control animals. The effect on erythropoietin responsive cell compartment: polycythemic mice were treated with metipranolol or dl-propranolol in doses of 10 to 40 mg/kg b.wt. and with an erythropoietin standard. Radioiron incorporation into red blood cells in control mice and in mice pretreated with beta blocking agents was not significantly different. The effect on erythrocyte production: in rats treated daily with metipranolol in a single dose of 10 mg/kg b.wt., for a period of 10 days, the rate of erythropoiesis measured by the incorporation of 59Fe into red blood cells was decreased, dl-propranolol (10 mg/kg b.wt.) did not reduce the rate of erythropoiesis. Our results demonstrate that the changes in erythropoiesis after administration of metipranolol may be caused by reduction of erythropoietin production with a consequent reduction of the erythrocyte production rate.     

ICI 147,798: a slowly dissociable beta adrenoceptor antagonist that causes insurmountable beta-1 and surmountable beta-2 adrenoceptor antagonism in isolated tissues

ICI 147,798 has been shown to exhibit both diuretic and beta-antagonist properties in vivo. The present study investigated the nature and selectivity of the beta-antagonism in a variety of isolated tissues. ICI 147,798 produced a concentration-dependent suppression of the maximum chronotropic response of norepinephrine in guinea pig right atria (beta-1 adrenoceptor). ICI 147,798 caused a concentration-dependent shift to the right of the salbutamol concentration-response curve in the guinea pig trachea (beta-2 adrenoceptor), and Schild analysis suggested competitive inhibition. Propranolol produced parallel shifts to the right of the norepinephrine concentration-response curve in guinea pig right atria, except at relatively high concentrations. The inhibitory effects of propranolol in guinea pig right atria were reversed by greater than 95%, whereas the effects of ICI 147,798 were only slightly reversed after a 6-hr washout period. Preincubation of propranolol with ICI 147,798 in guinea pig right atria prevented completely the suppression of the norepinephrine maximum chronotropic response. Postincubation of propranolol with ICI 147,798 partially reversed the suppression of the maximum chronotropic response. ICI 147,798 had no effect on the maximum chronotropic responses of either histamine (H2-receptor) or forskolin (adenylate cyclase activation) in guinea pig right atria and had no effect on agonist responses in a variety of other receptor systems. The insurmountable beta-1 adrenoceptor antagonism was evaluated based on the assumptions of irreversible competitive antagonism, mixed competitive and noncompetitive antagonism and slowly dissociating competitive antagonism ("hemi-equilibrium" conditions). Concentration-dependent changes in norepinephrine KA values suggested the first three possibilities were unlikely.(ABSTRACT TRUNCATED AT 250 WORDS)     

Central effect of beta adrenergic blocking agents on arterial blood pressure.

dl-Sotalol, dl-pindolol, d-propranolol and dl-propranolol were evaluated for central hypotensive activity in chloralose-anesthetized vagotomized cats. Blood pressure and average evoked potentials recorded from one postganglionic renal nerve were measured during intraventricular (lateral ventricle) or intravenous (radial vein) infusion of each agent. Potentials were evoked in the sympathetic nerve by single shocks to the sciatic nerve. Three concentrations (1, 3, 5 mM) of each compound were infused consecutively for 20 minutes per concentration. The decrease in blood pressure after intraventricular infusion of 1 mM pindolol, 5 mM d-propranolol and 3 mM dl-propranolol was significantly greater than the decrease after intravenous infusion of the same concentrations of each agent. The decrease in the average evoked potential after intraventricular infusion of the 5 mM concentration was greater than the response after intravenous infusion. Consecutive (1, 3, 5 mM) intraventricular or intravenous infusions of sotalol did not significantly change either parameter. During intraventricular infusion of 3 mM dl-propranolol blood pressures was markedly decreased at a time when the average evoked potential was unchanged; however, parallel changes were observed during infusion of the drug in vagotomized cats in which the carotid sinus baroreceptors were also denervated. The results indicate a) that intraventricular administration of d-propranolol, dl-propranolol and dl-pindolol, but not of dl-sotalol, decreased blood pressure and discharge evoked in the postganglionic renal nerve by an action on central sympathetic structures, and b) that baroreceptor activity during hypotension can overcome the depressant effect of dl-propranolol on potentials evoked reflexly in the renal nerve.     

Pharmacokinetics of propranolol isomers and their relationships with beta adrenoceptor blocking activity in rabbits administered with dl-propranolol

Plasma concentrations of dl- and l-propranolol were determined in rabbits after the i.v. injection of 200, 400 and 800 micrograms/kg of dl-propranolol. The data conformed to a biexponential equation. The beta adrenoceptor blocking activity was measured by the percent reduction in isoproterenolol-induced tachycardia after the administration of dl-propranolol. The l-isomer had a longer plasma half-life than dl-propranolol at each dose during the beta-phase. At a dose of 800 micrograms/kg, both dl- and l-propranolol had longer plasma half-life than those of 200 and 400 micrograms/kg doses and a smaller total plasma clearance was observed to 800 micrograms/kg, although statistically not significant. At steady state, plasma concentrations of both dl- and l-propranolol correlated well with beta adrenoceptor blocking activity (r = 0.913 for dl-propranolol and r = 0.939 for the l-isomer). These results demonstrate that after the administration of the racemic drug l-propranolol had a longer plasma half-life. The plasma concentration of dl- and l-propranolol is a good parameter for beta adrenoceptor blocking activity. The pharmacologic activity of propranolol at the highest dose persists longer than expected, probably due to hemodynamic alterations, which causes a decrease in liver blood flow with the resultant reduction in the elimination rate of the drug.     

The role of the fourth cerebroventricle in nicotine-stimulated prolactin release in the rat: involvement of catecholamines.

The central mechanism(s) whereby peripherally administered nicotine (N) stimulates prolactin secretion has not been clarified. The current studies showed that an i.c.v. injection of N into the fourth ventricle (IV) produced a significant dose-dependent elevation of plasma prolactin [buffer less than N 0.125 micrograms = N 0.25 micrograms (P less than .05) less than N 0.5 micrograms = N 2.5 micrograms (P less than .01)]. Injecting the nicotinic cholinergic (NAch) antagonist, mecamylamine, into the IV (20 or 40 micrograms) or i.v. (0.5 mg/kg b.wt.) before the administration of N by the alternate route indicated that nicotine was activating NAch receptors accessible from the IV. Because brain stem catecholaminergic cell groups, adjacent to the IV, project to hypothalamic regions involved in modulating prolactin release, the involvement of IV catecholaminergic neurons in N-stimulated prolactin release was investigated. Ablation of central catecholaminergic neurons by 6-hydroxydopamine abolished the prolactin response to N injected i.c.v. (1 or 2.5 micrograms; P less than .05) or i.v. (0.03 or 0.05 mg/kg b.wt.; P less than .05). To assess the role of norepinephrine and epinephrine, LY 10853, an inhibitor of dopamine-beta-hydroxylase, was given i.p. 4 h before i.v. nicotine; the prolactin response was attenuated (P less than .01). Selective inhibitors of epinephrine synthesis, SKF 64139 or 2,3-dichloro-alpha-methylbenzylamine, administered on the same schedule, reduced (P less than .01) the prolactin response to N without altering responsiveness to the dopamine receptor antagonist, domperidone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Histamine mediation in muscular vasodilatation induced by beta adrenoceptor stimulation in dogs.

This study was designed to investigate the possibility of a histamine mediation in muscular vasodilation induced by beta adrenoceptor stimulation. Accordingly, in seven dogs the effects of isoproterenol administration on the release of 14C-histamine from the perfused gracilis muscle were studied. Beta adrenoceptors stimulation induced a vasodilatation, as shown by a decrease in perfusion pressure(-43 +/- 12 mm Hg); simultaneously, a significant increase of the radioactivity measured in the venous blood effluent from the gracilis muscle was observed. Both these events were blocked by propranolol. In the other five dogs, chlorpheniramine was able to reduce the vasodilatation induced by the injection in the gracilis muscle of isoproterenol. Under control conditions, isoproterenol induced a fall in perfusion pressure of 44 +/- 5 mm Hg while, after chlorpheniramine, perfusion pressure decreased by only 24 +/- 4 mm Hg. The results of this study seem to confirm the possibility of a histamine mediation in isoproterenol-induced vasodilatation. However, further investigation is needed in order to identify the exact role of histamine in the geneis of this phenomenon.