Comparative analysis of beta-1 adrenoceptor agonist and antagonist potency and selectivity of cicloprolol, xamoterol and pindolol

The partial beta adrenoceptor agonist properties of cicloprolol, xamoterol and pindolol have been compared in vivo (anesthetized catecholamine-depleted or pithed rats) and in vitro (guinea pig or rat right atria and guinea pig tracheal muscle preparations) conditions. All three compounds increased heart rate in the former preparations, and their intrinsic activities relative to isoproterenol were 0.7, 0.65 and 0.45, respectively. The positive chronotropic effects of cicloprolol or xamoterol were competitively antagonized by betaxolol or propranolol; however, part of those induced by pindolol were resistant to these beta adrenoceptor antagonists. None of these compounds increased the spontaneous beating rate of isolated guinea pig atria; however, xamoterol only increased heart rate in isolated rat atria, and its intrinsic activity with respect to isoproterenol was 0.4. Pindolol, xamoterol and cicloprolol behaved as competitive beta-1 adrenoceptor antagonists against isoproterenol-induced tachycardia in a pithed rat model. In order to mimic the intrinsic effects of the partial agonist drugs, control dose-response curves for isoproterenol were determined in pithed rats in which the base-line heart rate was elevated by thoracic spinal cord stimulation. In this in vivo preparation, xamoterol and pindolol were more potent beta-1 adrenoceptor antagonists than cicloprolol; however, cicloprolol and xamoterol, in contrast to pindolol, were selective for beta-1 adrenoceptors. In isolated spontaneously beating guinea pig right atria, cicloprolol and xamoterol were equipotent beta-1 adrenoceptor antagonists but were about 50 times less potent than pindolol. In isolated rat atria, the beta-1 adrenoceptor antagonist potency of xamoterol was greater (pA2 = 8.7) than in guinea pig atria (pA2 = 7.8). The potencies of cicloprolol and pindolol did not vary between these species. In catecholamine-depleted rats, high i.v. doses of cicloprolol had vasodilator activity that was partly mediated by beta-2 adrenoceptors. In carbachol-contracted guinea pig trachea, cicloprolol and xamoterol, in contrast to pindolol, were relatively inactive against isoproterenol-induced relaxation. In conclusion, cicloprolol and xamoterol, similarly to pindolol, behave as agonists and antagonists of beta-1 adrenoceptors. However, only cicloprolol and xamoterol show an elevated degree of selectivity toward the beta-1 adrenoceptor subtype.     

Characterization of beta-1 and beta-2 adrenoceptors in guinea pig atrium: functional and receptor binding studies

The selective beta-2 adrenoceptor agonist procaterol produced positive inotropic and chronotropic responses over a concentration range of 1 nM to 0.1 mM in spontaneously beating right atria and in three of seven electrically driven left atria. The pD2 values (right atria, 7.30; left atria, 7.18) were midway between its known affinities at beta-1 and beta-2 adrenoceptors and are evidence that positive inotropic and chronotropic responses involve a minor beta-2 adrenoceptor component. The pKB values for procaterol against (-)-isoproterenol in the right atria (5.59) and left atria (5.29) were consistent with its affinity for beta-1 adrenoceptors and suggest that these are responsible primarily for positive inotropic and chronotropic responses. Receptor binding studies in right atrial homogenates showed that [125I]cyanopindolol binding was saturable (KD = 36.2 pM, maximal density of binding sites = 49.2 fmol mg-1 protein) and stereoselective with respect to the isomers of propranolol. Competition binding curves for the beta-1 adrenoceptor antagonist CGP 20712A and beta-2 selective antagonist ICI 118,551 against [125I]cyanopindolol binding were resolved into two components using iterative curve fitting techniques. Binding sites with the characteristics of beta-1 and beta-2 adrenoceptors were present in the proportions of approximately 75 to 25%. These studies indicate either that the beta-1 adrenoceptor is coupled more efficiently to the positive inotropic and chronotropic response than the beta-2 adrenoceptor or that a proportion of the beta-2 adrenoceptors subserve other functions.     

Insurmountable beta receptor blockade by ICI 147,798 in rabbits

In rabbits, the characteristics of cardiac beta-1 receptor blockade produced by ICI 147,798, a novel beta receptor blocking agent with diuretic properties, were evaluated and compared with those of propranolol. In conscious rabbits, i.v. injections of 0.31, 1.0 and 3.1 mg/kg of ICI 147,798 and 1.0 mg/kg of propranolol caused significant bradycardia. ICI 147,798 produced a dose-dependent shift to the right of the dose-response (chronotropic) curve of isoproterenol with suppression of the maximal tachycardia, an effect characteristic of insurmountable beta receptor blockade. Propranolol also produced a shift to the right of the dose-response curve of isoproterenol without affecting the maximal tachycardia. ICI 147,798-induced antagonism was specific for beta adrenoceptors as it failed to modify the effects of acetylcholine, angiotensin II, phenylephrine, adenosine, histamine and prostaglandin E2 on mean arterial pressure and heart rate. In rabbits with prior autonomic blockade, ICI 147,798, like propranolol, failed to inhibit the positive chronotropic effects of theophylline which are mediated by postreceptor mechanisms. In reserpinized rabbits, ICI 147,798 was found to have no intrinsic sympathomimetic activity. Unlike the effects of propranolol, which were attenuated by first-pass through the hepatic vascular bed, the effects of ICI 147,798 were unaffected suggesting an absence of first-pass metabolism. The effects of propranolol (1.0 mg/kg i.v.) were not detectable at 24 hr after injection, whereas significant beta receptor blocking activity was still present at 24 hr after ICI 147,798 (1.0 mg/kg i.v.). The results suggest that ICI 147,798 is a specific, long-acting, insurmountable beta-1 receptor blocking agent without intrinsic sympathomimetic activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

ICI 147,798: a novel diuretic agent with beta adrenoceptor blocking activity

ICI 147,798 is a novel compound which has both diuretic and beta adrenoceptor blocking properties in a single molecule. The natriuretic activity at 30 mg/kg p.o. was about 65% of the hydrochlorothiazide value at 10 mg/kg p.o. in saline-loaded rats; the corresponding kaliuretic activity was 42%. The natriuretic and the kaliuretic activity of ICI 147,798 in dogs were similar to that of hydrochlorothiazide over the doses 1 to 20 mg/kg p.o., although significantly less kaliuresis was obtained with ICI 147,798 at 1 mg/kg p.o. In the toad bladder preparation (analogous to the distal mammalian nephron), ICI 147,798 inhibited Na+ transport with mucosal and serosal IC50 values of 56 and 120 microM, respectively. ICI 147,798 inhibited isoproterenol-induced tachycardia in rats, cats, guinea pigs and dogs; these effects were associated with antagonism of isoproterenol vasodepressor responses. The pKB values of ICI 147,798 were 9.1 and 8.8 in isolated right atria and trachea of guinea pigs, respectively. ICI 147,798 did not exhibit local anesthetic activity in rabbit cornea and intrinsic sympathomimetic activity in catecholamine-depleted dogs and rats. Duration of beta blockade after a p.o. dose of 1 mg/kg in dogs followed the sequence: nadolol greater than ICI 147,798 greater than atenolol greater than timolol greater than propranolol. It is concluded that ICI 147,798 is a novel diuretic agent with nonselective beta blockade, and it appears to have the potential of a direct tubular action.     

Pharmacological analysis of the cardiac actions of xamoterol, a beta adrenoceptor antagonist with partial agonistic activity, in guinea pig heart: evidence for involvement of adenylate cyclase system in its cardiac stimulant actions

The pharmacological effects of xamoterol, a beta adrenoceptor antagonist with partial agonistic activity, were examined in guinea pig cardiac preparations and compared with those of isoproterenol to assess possible mechanisms of its cardiac stimulant actions. Xamoterol produced a positive inotropic effect in the papillary muscles and a positive chronotropic effect in the spontaneously beating right atria in a concentration-dependent manner. The maximum inotropic and chronotropic effects of xamoterol were about 33 and 35% of those of isoproterenol, respectively. Although xamoterol failed to produce a consistent increase in contractile force in the left atria, the positive inotropic effect of the agent was observed clearly in preparations obtained from reserpine-pretreated animals. The positive inotropic and chronotropic effects of xamoterol were antagonized by atenolol, but not by ICI 118,551. On the other hand, xamoterol antagonized competitively the positive inotropic and chronotropic responses to isoproterenol. In papillary muscles the increases in contractile force induced by xamoterol and isoproterenol were depressed markedly in the presence of carbachol or adenosine. In all of left atria, right atria and papillary muscles obtained from reserpine-pretreated animals, xamoterol caused a significant elevation in cyclic AMP levels, while inhibiting the isoproterenol-induced increase in cyclic AMP levels. Computer-assisted analysis of concentration-response curves for the inhibition by xamoterol of the binding of [125I]iodocyanopindolol in the membranes from guinea pig ventricles showed the existence of the 5'-guanylylimidodiphosphate sensitive, highly affinity site of beta adrenoceptors for xamoterol, suggesting that xamoterol may induce the formation of a ternary complex with the beta adrenoceptor and a stimulatory guanine nucleotide regulatory protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Further evidence for a homogeneous population of beta-1-adrenoceptors in bovine coronary artery

Isolated rings from the proximal and distal ends of the bovine epicardial anterior descending coronary artery and guinea pig tracheal rings were mounted in tissue baths for the measurement of isometric contraction and relaxation, and spontaneously beating rabbit atria were prepared for measurement of chronotropic response. All tissues were exposed to phenoxybenzamine to block tissue uptake of catecholamines and alpha adrenoceptors. The arterial and tracheal rings were contracted with 25 mM K+ in order to observe agonist-induced relaxation. Beta adrenergic agonist dose-response curves were obtained in all tissues in the presence and absence of the beta-2 selective antagonist, ICI 118,551. The orders of agonist potencies in the proximal and distal coronary arteries and the rabbit atria were the same: namely, isoproterenol (ISO) greater than norepinephrine (NE) much greater than procaterol. In contrast, that in the guinea pig trachea was procaterol greater than ISO greater than NE. Procaterol was a weak, partial agonist in the bovine coronary artery and rabbit atria and a potent, full agonist in the guinea pig trachea. The ICI 118,551 pA2 values in the proximal segment of the bovine coronary artery were the same against both NE and fenoterol, 7.38. In the distal segment, the values were 7.33 and 7.14, respectively. ICI 118,551 -log KB values in the rabbit atria, 6.81 and 6.83 for NE and ISO, respectively, were slightly below those in the coronary artery segments, whereas -log KB values in the guinea pig trachea were substantially higher, 8.19 and 8.81 for fenoterol and procaterol, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heterogeneity of beta adrenoceptors in right atria isolated from cold-exposed rats

The chronotropic response of right atria isolated from 5-day-cold-exposed rats to isoproterenol and norepinephrine was studied. A large increase in the sensitivity of the pacemaker to isoproterenol and a decrease in the sensitivity to norepinephrine occurred. Determination of pA2 values of propranolol and metoprolol using isoproterenol and norepinephrine as agonists and analysis of the slopes of Schild plots suggested that in atria isolated from control rats the chronotropic effect of isoproterenol and norepinephrine resulted from the preferential interaction of the catecholamines with a homogeneous beta-1 adrenoceptor population. After cold exposure the affinity of atrial adrenoceptors for propranolol increased when the agonist was isoproterenol and decreased when norepinephrine was used. The slopes of the Schild plots of metoprolol when the agonists were isoproterenol or norepinephrine were not unitary unless the experiments were performed in the presence of butoxamine. However, butoxamine prevented the demonstration of cold-induced super-sensitivity to isoproterenol, leaving the subsensitivity to norepinephrine unaffected. It is concluded that cold-induced heterogeneity of the atrial beta adrenoceptors is responsible for the increased sensitivity to isoproterenol. Probably, subsensitivity to norepinephrine resulted from conformational alterations of the atrial beta-1 adrenoceptors.     

Pharmacological profiles of YM-16151-1 and its optical isomers: a novel calcium entry blocking and selective beta-1 adrenoceptor blocking agent

The pharmacological properties of YM-16151-1 [(+/-)-dimethyl 4-[2-[4-(2-hydroxy-3-phenoxypropylamino)butoxyl]-5-nitrop hen yl]-2,6- dimethyl-1,4-dihydropyridine-3,5-dicarboxylate hydrochloride] and its optical isomers were evaluated in in vitro studies and radioligand binding assay. In isolated tissues, YM-16151-1 produced a competitive antagonism of CaCl2-induced contraction in the isolated rabbit aorta with a pKca-1 value of 8.17, and also produced a competitive antagonism of isoproterenol-induced positive chronotropic responses in the isolated rat atria with a pA2 value of 8.47. In rat brain membrane preparations, YM-16151-1 produced dose-dependent inhibitions of [3H]nitrendipine and [3H]dihydroalprenolol bindings with pKi values of 7.21 and 8.07, respectively. Calcium entry blocking activity of YM-16151-1 was 7 times weaker and 3 times greater than nifedipine and diltiazem, respectively. Beta-1 adrenoceptor blocking activity of YM-16151-1 was 2 times weaker than that of propranolol. YM-16151-1 showed about 900-fold selectivity for beta-1 adrenoceptor. YM-16151-1 also showed a weak alpha-1 adrenoceptor blocking activity and its potency was about 13 times weaker than that of phentolamine. S-(-)- and R-(+)-isomers of YM-16151-1 showed the same potency of calcium entry blocking activity. However, in beta-1 adrenoceptor blocking activity, the S-(-)-enantiomer was about 13 to 22 times more potent than the R-(+)-enantiomer. Oral administration of YM-16151-1 produced a dose-dependent blood pressure lowering effect without increasing heart rate in conscious spontaneously hypertensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta-2 adrenoceptor-mediated relaxation of the isolated human saphenous vein

On isolated strips of human saphenous vein, pretreated with 5 microM phenoxybenzamine and contracted with 10 mM KCl, the beta adrenoceptor mediating the relaxant effects of isoproterenol, procaterol and norepinephrine was characterized using the selective beta-1 adrenoceptor antagonist, bisoprolol, and the selective beta-2 adrenoceptor antagonist, ICI 118,551. All three agonists produced concentration-dependent relaxations of the isolated saphenous vein with an order of potency: procaterol (pD2 value, 7.69) greater than isoproterenol (pD2 value, 7.41) much greater than norepinephrine (pD2 value, 5.30). ICI 118,551 (3 X 10(-10) to 3 X 10(-9) M) was nearly 100 times more potent than bisoprolol (10(-7) to 10(-6) M) in antagonizing the relaxant effects of isoproterenol and procaterol. The slopes of the Schild plots for the antagonistic effects of ICI 118,551 and bisoprolol against isoproterenol- and procaterol-induced relaxations were not significantly different from unity indicating interaction with a homogeneous population of beta adrenoceptors. The pA2 value for ICI 118,551 amounted to 9.11 to 9.20 and for bisoprolol to 6.50 to 6.63. In addition, the concentration-response curve for the relaxant effect of norepinephrine was significantly shifted to the right by 10(-9) M ICI 118,551, but not affected by 10(-7) M bisoprolol. These results indicate that on the isolated strips of the human saphenous vein the beta adrenoceptor mediating relaxation is of the beta-2 subtype.     

Surgical sympathectomy of the heart in rodents and its effect on sensitivity to agonists

A new procedure for sympathetic denervation of the hearts of rats and guinea pigs is described. Bilateral removal of the inferior and medial cervical ganglia results in almost complete loss of catecholamines from atria and ventricles, disappearance of catecholamine-associated histofluorescence from the region of the sinoatrial node and marked depression of the chronotropic concentration-response curve for tyramine in right atria of both species. Seven days after bilateral sympathectomy, the chronotropic concentration-response curve for isoproterenol is shifted to the left by a factor of 3.3 in the rat and 1.7 in guinea-pig right atria. The chronotropic concentration-response curve for histamine was not shifted by sympathectomy in the guinea-pig right atrium. Inasmuch as the rat atrium does not respond to histamine, similar experiments could not be done in the rat. The inotropic concentration-response curve for isoproterenol in electrically driven left atria was not affected by 7 days of sympathectomy in either species. These results indicate that chronic surgical sympathectomy of the heart can be successfully accomplished in the rat and guinea pig. Such sympathectomy induces a postjunctional supersensitivity in guinea-pig right atria which is qualitatively and quantitatively similar to that described previously for chronic treatment with reserpine. Bilateral surgical sympathectomy provides a valuable tool for future investigations of the cellular basis of supersensitivity in the myocardium.     

Beta-1 and beta-2 adrenoceptors mediate smooth muscle relaxation in bovine isolated mesenteric lymphatics.

The relaxant effects of beta adrenoceptor agonists were investigated in isolated bovine mesenteric lymphatics which had been contracted by 5-hydroxytryptamine. Addition of isoproterenol (a nonselective beta agonist), denopamine (a selective beta-1 agonist) and procaterol (a selective beta-2 agonist) caused concentration-dependent relaxations in the lymphatic preparations. There was no significant difference in the relaxant responses to the beta adrenoceptor agonists between the preparations with and without endothelium. Treatment with 10(-7) to 3 x 10(-6) M metoprolol (a selective beta-1 antagonist) shifted the concentration-response curve for denopamine to the right, whereas 10(-9) to 3 x 10(-8) M ICI 118,551 (a selective beta-2 antagonist) did not affect the relaxant response to denopamine. The relaxations of bovine mesenteric lymphatics induced by isoproterenol were suppressed by both metoprolol and ICI 118,551. The procaterol-induced relaxations were inhibited by 10(-9) to 3 x 10(-8) M ICI 118,551 but not by 10(-7) to 3 x 10(-6) M metoprolol. Schild plot analyses showed that the slope and pA2 values for metoprolol against denopamine were 1.10 and 7.59, respectively, and that those for ICI 118,551 against procaterol were 0.91 and 9.96. These results suggest that both beta-1 and beta-2 adrenoceptors are located on the smooth muscle cells in bovine mesenteric lymphatics and that stimulation of either receptor produces a marked relaxation.     

Presynaptic alpha adrenoceptor modulation of neurally mediated cholinergic excitatory and nonadrenergic noncholinergic inhibitory responses in guinea pig trachea

Cholinergic and nonadrenergic noncholinergic (NANC) excitatory nerves in guinea pig trachea are subject to presynaptic alpha-2 adrenoceptor inhibitory control. Although the trachea is also innervated by NANC inhibitory nerves, little is known about their presynaptic regulation. The present study assessed the capacity of alpha-1 and alpha-2 adrenoceptor agonists to modulate NANC inhibitory nerves and for comparison, cholinergic excitatory nerves in guinea pig trachea. To eliminate effects of intrinsic sympathetic nerve stimulation and prostanoid production, tissues were pretreated with guanethidine, propranolol and indomethacin. The alpha-2 adrenoceptor agonist, clonidine (1 microM), induced a 12-fold rightward shift of the frequency-response curve for neurally mediated cholinergic contractions but had no effect on the concentration-response curve for exogenously administered acetylcholine. This action of clonidine was inhibited in a concentration-dependent manner by the alpha-2 adrenoceptor antagonist, yohimbine, and by the alpha-1 adrenoceptor antagonist, prazosin, NANC inhibitory responses were unaffected by clonidine (1 microM). The alpha-1 adrenoceptor agonist, phenylephrine (1 microM), failed to influence responses induced by cholinergic or NANC inhibitory nerve stimulation, acetylcholine or vasoactive intestinal peptide. Furthermore, in tissues treated with propranolol but not subjected to adrenergic neuronal blockade with guanethidine, neurally mediated cholinergic responses were not altered by yohimbine (0.3 microM) treatment. These results indicate that in guinea pig trachea: 1) cholinergic nerves are modulated by presynaptic, prazosin-sensitive inhibitory presynaptic alpha-2 adrenoceptors and 2) NANC inhibitory nerves do not possess presynaptic, modulatory alpha adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

A study designed to explore the hypothesis that beta-1 adrenoceptors are "innervated" receptors and beta-2 adrenoceptors are "hormonal" receptors

It has been demonstrated that the chronotropic responses to sympathomimetic amines in rat atria were mediated only by beta-1 adrenoceptors. This was like guinea pig (beta-1) but unlike cat (beta-1 and beta-2). Extraneuronal uptake of isoproterenol into rat atrial myocardial cells was detected by fluorescence histochemistry but uptake was less than was seen in cat atria. Furthermore, it did not modulate responses of rat atrial preparations to isoproterenol. The lack of specific extraneuronal uptake in guinea-pig atrial myocardial cells was confirmed. Collation of data on the atria of these three species with data already available on guinea-pig trachea allowed the following conclusions. All four tissues contained a population of beta-1 adrenoceptors and were adrenergically innervated. Beta-2 adrenoceptors were present in cat atria and guinea-pig trachea and these tissues possessed a functionally effective extraneuronal metabolizing system for catecholamines. Beta-2 adrenoceptors were not detected in guinea-pig or rat atria and in these tissues extraneuronal uptake was either absent (guinea pig) or not functionally effective (rat). It is suggested that these data could support the hypotheses that 1) beta-1 adrenoceptors are "innervated" receptors mediating responses to neuronally released norepinephrine, whereas beta-2 adrenoceptors are "hormonal" receptors mediating responses to circulating epinephrine and 2) extraneuronal metabolizing systems are of particular importance in the dissipation of circulating catecholamines acting on beta-2 adrenoceptors.     

Cardiac responsiveness to alpha and beta adrenergic amines: effects of carbachol and hypothyroidism

Previous reports of cardiac beta to alpha adrenoceptor interconversion secondary to hypothyroidism left open the alternative possibility of a functional influence by hypothyroidism on the inotropic and chronotropic effects of adrenergic amines through a different mechanism. To test this possibility, the effects of hypothyroidism (thyroidectomy) were compared with those of acute carbachol pretreatment on the responses of isolated rat atria to the selective beta and alpha adrenoceptor agonists isoproterenol and methoxamine. Both hypothyroidism and acute carbachol pretreatment (3 X 10(-7) -10(-6) M): 1) reduced basal right atrial rates and left atrial tensions; 2) caused an apparent decrease in the inotropic and chronotropic potencies of isoproterenol; 3) reduced the degree of antagonism by propranolol of the responses to isoproterenol; 4) increased the maximum inotropic response of left atria to methoxamine; and 5) converted a lack of response to a positive chronotropic response of right atria to methoxamine. Equivalent reductions of basal rates by hypothermia, or of basal tensions by lowered calcium ion concentrations, did not affect the responses to isoproterenol or methoxamine. The results suggest that both carbachol pretreatment and hypothyroidism functionally antagonize the responses to isoproterenol and enhance the responses to methoxamine by means other than adrenoceptor interconversion.     

In vitro characterization of the adrenoceptor activity of AY-28,925

The adrenoceptor activity of AY-28,925 (5-[1-hydroxy-2-[(1-methyl-3-phenylpropyl)amino]ethyl]-1 H-indole-7-carboxamide) was evaluated on various isolated vascular and nonvascular tissues. Based upon pA2 values derived from either the antagonism of the effect of isoproterenol on paced guinea pig left atria (pA2 = 7.3), spontaneously beating guinea pig right atria (pA2 = 7.4) or canine saphenous vein (pA2 = 7.3), AY-28,925 demonstrated nonselective beta adrenoceptor antagonist activity. Studies using the spontaneously beating guinea pig right atria and the prostaglandin F2 alpha-contracted canine saphenous vein indicated that AY-28,925 also possessed nonselective beta-adrenoceptor intrinsic efficacy. AY-28,925 was shown to be a selective alpha-1 adrenoceptor antagonist with a pA2 value which was greater against phenylephrine in the rabbit aorta (pA2 = 6.6) than against clonidine in the rat vas deferens (pA2 = 5.4) or B-HT 920 in the canine saphenous vein (pA2 = 5.3). Only in concentrations greater than 2000 times those required for adrenergic activity was AY-28,925 able to inhibit potassium chloride- or prostaglandin F2 alpha-induced contractions of the rabbit aorta. The compound had no significant negative intropic activity. These experiments indicate that AY-28,925 possesses, in decreasing orders of activity: nonselective beta-adrenoceptor properties; a selective alpha-1 adrenoceptor inhibitory effect; and a nonspecific direct relaxing action on vascular smooth muscle.     

Localization and characterization of alpha-2 adrenoceptors in the isolated canine pulmonary vein

Alpha-1 and alpha-2 adrenoceptor-mediated contractile responses were studied in isolated canine pulmonary veins. Norepinephrine elicited concentration-dependent contractile responses which were antagonized in a competitive manner by the selective alpha-2 adrenergic antagonist, rauwolscine, with a dissociation constant of 15.7 nM, whereas the selective alpha-1 adrenoceptor antagonist, prazosin, produced nonparallel rightward shifts in the norepinephrine concentration-response curve with a marked depression in the maximal response, indicating noncompetitive antagonism. B-HT 933, a selective alpha-2 adrenoceptor agonist, also produced a concentration-dependent contraction in canine pulmonary vein, with the maximal contraction elicited by B-HT 933 being approximately 45% of that produced by norepinephrine. The response mediated by B-HT 933 was antagonized in a competitive manner by rauwolscine with a dissociation constant of 4.4 nM, whereas prazosin again behaved in a noncompetitive manner producing nonparallel rightward shifts in the B-HT 933 concentration-response curve with a marked depression in the maximal response. However, another alpha-1 adrenoceptor antagonist, corynanthine, weakly blocked the response produced by B-HT 933 with a dissociation constant of 1400 nM, and this low affinity for corynanthine is consistent with interaction at alpha-2 adrenoceptors. Cirazoline, a selective alpha-1 adrenoceptor agonist, also produced a concentration-dependent vasoconstrictor response in canine pulmonary veins which was antagonized competitively by both alpha-1 adrenoceptor antagonists, corynanthine and prazosin, with dissociation constants of 180 and 1.4 nM, respectively, indicative of an interaction with alpha-1 adrenoceptors. Rauwolscine (10 nM) did not significantly affect the response produced by cirazoline.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta adrenoceptor subtype binding activity in plasma and beta blockade by propranolol and beta-1 selective bisoprolol in humans. Evaluation with Schild-plots

In the present study we investigated whether the beta adrenoceptor subtype binding activity in plasma samples can predict selective and nonselective beta blockade in humans. From the right shifts of isoprenaline dose-response curves 0 to 84 hr after administration of propranolol and the beta-1 selective bisoprolol, in vivo beta blockade was assessed. In an in vitro radioreceptor assay with membrane preparations of beta-1 or beta-2 adrenoceptors, plasma samples were assayed for subtype selective blocking activity. After propranolol administration, in vitro beta-1 and beta-2 adrenoceptor occupancy declined from initially 97% to less than 10% within 48 hr. An isoprenaline dose ratio (DR)-1 of 1 coincided with a 50% occupancy of the beta-1 or the beta-2 subtype in vitro. In Schild-plots using plasma concentrations (radioreceptor assay) and the isoprenaline DR-1 for heart rate, diastolic blood pressure and inotropy (QS2C), slopes of unity were observed. After bisoprolol administration, in vitro beta-1 occupancy shifted from initially 95% to less than 10% within 72 hr. For the beta-2 subtype, an occupancy of greater than 10% was detectable only within the first 12 hr. An isoprenaline DR-1 of 1 coincided with a 50% occupancy of beta-1 adrenoceptors. The bisoprolol Schild-plots yielded a slope of unity for inotropy, but less than unity for the heart rate and diastolic blood pressure. From an extended analysis of subtype selective antagonism in Schild-plots, the fractions of the beta-2 adrenoceptor subtype participating in the isoprenaline response were calculated: heart rate 0.45 +/- 0.12 and diastolic blood pressure 0.23 +/- 0.13. It is concluded that in vitro receptor occupancy can predict beta blockade in humans for propranolol. Beta adrenoceptor subtype-mediated effects in humans can be evaluated with a selective antagonist and a refined analysis of Schild-plot data.     

Influence of papaverine on spontaneous activity of isolated right atria from small mammals

The effects of papaverine and verapamil were studied in spontaneously beating right atria of guinea pigs, rabbits and rats and on segments of sinoatrial node tissue from rabbits. Papaverine (10(-4)M) produced a significant negative chronotropic effect in guinea pigs and rats, with a lesser effect in rabbits. Papaverine antagonized the positive chronotropic response to transmural nerve stimulation (TNS) of the sinoatrial node of rabbit atria in a concentration-dependent manner. The chronotropic response to 5 X 10(-7)M norepinephrine was enhanced in the presence of 10(-6)M papaverine, but was noncompetitively antagonized at concentrations above 10(-5)M. The negative chronotropic response to TNS was unaltered by papaverine except at concentrations at or above 5 X 10(-5)M; however, the negative chronotropic response to 5 X 10(-6)M methacholine was not altered by 10(-4)M papaverine. A local anesthetic (lidocaine) inhibited both the positive and negative chronotropic TNS response at a concentration that had no effect on the response to exogenous norepinephrine. Verapamil (10(-7) to 10(-6)M) had no significant effect on either the positive or negative chronotropic response to TNS. The effects of papaverine on TNS are attributed to the influence of at least three actions: 1) phosphodiesterase inhibition, 2) local anesthetic activity and 3) physiological antagonism of norepinephrine. The effects of 2 X 10(-4)M papaverine on action potentials of rabbit sinoatrial node cells were studied. Papaverine decreased the maximum diastolic potential, increased the action potential overshoot and transformed the normal ramp configuration of slow diastolic depolarization into a concave shape.     

Beta adrenoceptor mediation of the enhancing effect of norepinephrine on the murine primary antibody response in vitro

The beta adrenoceptor has been identified in this study to be the receptor responsible for the enhanced immunoglobulin M antibody response produced by norepinephrine in mouse spleen cells immunized with sheep erythrocytes in vitro. The magnitude and kinetics of the enhanced antibody response to norepinephrine alone, or to norepinephrine in the presence of phentolamine, were more closely mimicked with a beta-2 adrenoceptor agonist (terbutaline) than with a beta-1 adrenoceptor agonist (dobutamine). Norepinephrine alone, norepinephrine in the presence of phentolamine, or terbutaline exposure produced a number of spleen cells secreting immunoglobulin M antibody that is equal to control on day 4 after immunization and which is enhanced above control on days 5, 6 and 7. Dobutamine causes no change when compared to control on days 4 and 5, but causes a delayed decline in the response on days 6 and 7. All drug responses were concentration-dependent and propranolol antagonized the enhanced response observed in the presence of terbutaline or dobutamine alone. When norepinephrine was added to immunized spleen cell cultures in the presence of propranolol, an alpha adrenoceptor-mediated component was unmasked which produced an enhanced response on day 4 after immunization and returned to control levels on days 5, 6 and 7. These results suggest that antibody responses can be modulated positively by a sympathetic neurotransmitter. This up-modulation by norepinephrine is beta adrenoceptor-mediated at the time of, and after, peak control response and alpha adrenoceptor-mediated 1 day before peak control response.     

Supersensitivity and changes in the active population of beta adrenoceptors in rat right atria in early sepsis.

We have investigated the effect of sepsis induced by cecal ligation and puncture on the chronotropic actions of beta adrenoceptor agonists on isolated right atria. The present findings show that right atria obtained from rats in an early stage of sepsis were supersensitive to the chronotropic actions of the beta-agonists, isoproterenol (ISO), fenoterol (FEN) and prenalterol (PREN). The supersensitivity to the chronotropic actions of ISO and FEN was much greater than that which developed to PREN. The positive chronotropic actions of isobutylmethylxanthine and forskolin were not affected by sepsis. The receptor subtypes mediating the responses to ISO, FEN and PREN by control and septic right atria were characterized by functional assays using selective beta-1 and beta-2 antagonists. The results showed that the chronotropic response produced by all three agonists on right atria obtained from control rats were mediated by beta-1 receptors. In contrast, the chronotropic actions of ISO and FEN on atria from septic rats were mediated by what appears to be beta-2 receptors and those of PREN by beta-1 receptors.