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.     

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.     

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.     

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)     

Altered muscarinic receptor properties and function in the heart in diabetes

The cardiac cholinergic system was studied in streptozotocin (STZ)-diabetic and age-matched control rats. STZ-diabetic rats (8-10 weeks) were supersensitive to the negative chronotropic effects of acetylcholine, carbamylcholine and bethanechol; inotropic responses to these muscarinic agonists were unaltered. This phenomenon was associated with a decrease in acetylcholinesterase activity but no change in the rate and extent of neuronal choline uptake. [3H]N-methylscopolamine bound to muscarinic receptors in atria from both groups of rats with the same high affinity. The density of [3H]N-methylscopolamine binding sites, however, was 34% lower in atria from STZ-diabetic rats. Agonist binding affinity was lower in diabetes; carbamylcholine had a lower affinity for both the high- and low-affinity receptors. These results indicate that cardiac cholinergic supersensitivity in right atria in diabetes occurs before the development of autonomic neuropathy insofar as neuronal [3H]choline uptake is unaltered at this stage of STZ diabetes. Changes in agonist binding conformation, without a concomitant change in antagonist binding affinity, suggest that supersensitivity of right atria to muscarinic agonist may be a consequence of altered coupling of muscarinic receptor to transduction mechanisms involved in chronotropism in diabetes.     

The sympathomimetic activity of N-isopropyloctopamine in vitro

Isolated right and left guinea-pig atria, guinea-pig tracheae and rabbit aortic strips were used to define the sympathomimetic properties of N-isopropyloctopamine. This compound was a highly beta selective, direct-acting adrenergic agonist, approximately 200- and 440-fold less potent than isoproterenol in cardiac and smooth muscle, respectively. It was nearly a full agonist in both cardiac and smooth muscle without appreciable selectivity for either beta-1 or beta-2 receptors and had no demonstrable beta blocking activity. No alpha adrenergic activity was detectable within the concentration range tested. The chronotropic effect of N-isopropyloctopamine was very persistent and resistant to repeated washing of the tissues, which may reflect unusually firm binding to beta receptors.     

Thyroid status and adrenergic receptor subtypes in the rat: comparison of receptor density and responsiveness

The density and functional responsiveness of adrenergic receptor subtypes were determined in tissues from control, hyperthyroid and hypothyroid rats. There was a decrease in sensitivity to isoproterenol in spontaneously beating right atria, electrically driven left atria and field-stimulated vas deferens associated with hypothyroidism, with no change in maximum response. Hyperthyroidism increased the potency of isoproterenol in right atria, but not in left atria or vas deferens. The maximal response to isoproterenol was greatly reduced in hyperthyroid left atria. The potency of procaterol, a partial agonist at beta adrenergic receptors in right atria, was unaltered in hyper- or hypothyroidism, although the maximum stimulation by procaterol was increased in hyperthyroidism. Scatchard analysis of specific [125I]pindolol binding showed that beta adrenergic receptor density was greater in hyperthyroidism than in hypothyroidism in left atria, right atria, ventricles, vas deferens and cerebral cortex, although the proportions of beta-1 and beta-2 adrenergic receptor subtypes did not change. There was no change in the responsiveness of alpha-1 adrenergic receptors mediating contraction of caudal artery and vas deferens or mediating [3H]inositol phosphate accumulation in cerebral cortex in hyperthyroid or hypothyroid rats, although the maximal contraction of caudal artery was significantly reduced in hyperthyroidism. Scatchard analysis of specific [125I]BE 2254 binding showed that alpha-1 adrenergic receptor density was significantly decreased in the ventricles from hyperthyroid rats and increased in the ventricles of hypothyroid rats, but was unchanged in vas deferens, caudal artery and cerebral cortex. Alpha-2 adrenergic receptor density in cerebral cortex, determined by Scatchard analysis of specific [3H] rauwolscine binding, was not altered in hyperthyroid or hypothyroid rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Altered vascular beta adrenoceptor-mediated relaxation in deoxycorticosterone-salt hypertensive rats

Beta adrenoceptor-mediated relaxation was studied on femoral and mesenteric arteries and thoracic aorta from deoxycorticosterone-salt hypertensive and age-matched normotensive rats. Maximum relaxations to isoproterenol (ISO), fenoterol and norepinephrine were less in hypertensive than in normotensive tissues. Mean negative log EC50 values of ISO and norepinephrine, but not fenoterol, were smaller in femoral and mesenteric strips from hypertensive rats than those from oil-water-treated rats. In thoracic aortas from hypertensive rats, the mean negative log EC50 values of ISO and fenoterol decreased, whereas those of norepinephrine increased as compared with those in oil-water-treated rats. In oil-salt- and deoxycorticosterone-water-treated rats, hypertension did not develop nor were the responses to ISO reduced. Relative potencies of the beta adrenoceptor agonists and Schild plot data for atenolol and butoxamine indicate that femoral arteries of normotensive and hypertensive rats possess beta-1 adrenoceptors mediating relaxation. It also is suggested that the mesenteric artery and thoracic aorta of both normotensive and hypertensive rats predominantly possess beta-2 adrenoceptors mediating relaxation. The present results suggest that an elevation in blood pressure is associated with the reduction of beta-1 adrenoceptor-mediated relaxation in femoral arteries and of beta-2 adrenoceptor-mediated relaxation in mesenteric arteries.     

Developmental changes in the accumulation of norepinephrine and the chronotropic effects of catecholamines in isolated rat atria as influenced by dietary lipid

Pregnant rats were fed semisynthetic diets enriched in either saturated fat (coconut oil) or polyunsaturated fat (sunflower oil) and sympathetic function was observed in the resulting neonatal pups. The neuronal accumulation of [3H]norepinephrine was significantly decreased in atria from neonates (11, 24 and 37 days of age) of dams receiving dietary sunflower oil compared with coconut oil diet. The nonspecific accumulation of [3H]norepinephrine occurring in the presence of desipramine (5 X 10(-5)M) was not changed by dietary lipid treatment. No differences in the accumulation of [3H]norepinephrine were observed in atria from day 7 neonates or in atria from adults which had received the diets since birth. When the beating rate of isolated atria was studied upon exposure to norepinephrine and isoproterenol, it was observed that the EC50 for these agonists was not changed upon exposure to the diets but that the maximal effect of both agonists was increased in atria from adult animals fed coconut oil relative to the sunflower oil diet. No significant differences were observed in the chronotropic effects of either norepinephrine or isoproterenol in atria from 11-day-old rats. A developmental analysis of the chronotropic response of isolated atria to a maximally effective dose of norepinephrine (10 microM) revealed a gradual onset of the altered maximum effects found in adult rat atria. The results of this study indicate that sympathetic nerve regulation of myocardial function can be modified during development by changing the dietary fat composition. The results are discussed in reference to the perturbation of specific membrane-associated processes that may be brought about by dietary lipid treatment and the possible importance of each of these effects in the changes observed in the accumulation of norepinephrine and the chronotropic effects of catecholamines in isolated rat atria.     

Cellular potentials, electrogenic sodium pumping and sensitivity in guinea-pig atria

Intracellular recording techniques in guinea-pig atrial pacemaker and nonpacemaker cells were used to investigate 1) the role of membrane potential changes in postjunctional supersensitivity, 2) the electrogenicity of the Na+,K+ pump and 3) the role of electrogenic pumping in sensitivity of the atria to agonists. In nonpacemaker cells, ouabain (10(-6) M) had no effect on resting membrane potential (left atria) or maximum diastolic potential (right atria). However, ouabain effectively suppressed the transient hyperpolarization that followed cessation of electrical stimulation. In pacemaker cells, ouabain and chronic treatment with reserpine (0.1 mg/kg/day) produced quite different patterns of changes in intracellular potentials. Chronic treatment with reserpine induced chronotropic supersensitivity to isoproterenol but not to histamine. Ouabain did not alter the chronotropic sensitivity to either agonist. The effects of isoproterenol and histamine on intracellular potentials in pacemaker cells were investigated in the presence and absence of ouabain and in control atria vs. atria from guinea pigs chronically pretreated with reserpine. Analysis of the data indicated that 1) electrophysiological measurements do not provide a discernible explanation for chronotropic supersensitivity, 2) the Na+ pump has the capacity for electrogenic pumping under conditions of Na+ loading, but demonstrates little indication of electrogenicity under basal conditions and 3) chronic treatment with reserpine does suppress the Na+,K+ pump in some areas of the right atrium, but this activity probably does not contribute to chronotropic supersensitivity. Other possible mechanisms of postjunctional supersensitivity in atria are discussed.     

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.     

Characterization of beta adrenoceptor subtypes in canine airway smooth muscle by radioligand binding and physiological responses

Beta adrenoceptor subtypes in canine tracheal smooth muscle have been investigated by radioligand binding and by physiological responses to beta agonists and sympathetic nerve stimulation in vitro. Specific binding of [3H]dihydroalprenolol to tracheal smooth muscle membranes was of high affinity (Kd = 1.0 +/- 0.08 nM), as in peripheral lung membranes from the same animals, but the concentration of binding sites (95.6 +/- 4.7 fmol/mg of protein) was much lower than in lung (532 +/- 48 fmol/mg of protein). Binding was stereoselective and agonists competed with the rank order of potency isoproterenol greater than epinephrine greater than norepinephrine, signifying a preponderance of beta-2 receptors. Using selective beta antagonists, we determined the ratio of beta-1/beta-2 receptors in tracheal smooth muscle membranes to be 1:4. The relaxation response of tracheal smooth muscle strips to exogenous beta agonists was mediated by beta-2 receptors, with a very small contribution from beta-1 receptors. However, the relaxation response to electrical field stimulation of sympathetic nerves was mediated predominantly by beta-1 receptors. Our results suggest that most beta receptors in dog tracheal smooth muscle are of the beta-2 subtype and mediate responses to circulating catecholamines, but there is a small concentration of beta-1 receptors which mediate the response to neurally released norepinephrine.     

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.     

Contractile and arrhythmic effects of endothelin receptor agonists in human heart in vitro: blockade with SB 209670

It is known that binding sites with endothelin(A) (ET)(A) and ET(B) receptor characteristics coexist in human heart but little is known about the receptors that mediate cardiostimulant effects of ET receptor agonists or their consequences. Functional studies were performed on isolated human cardiac tissues. The maximal positive inotropic effects of ET-1 were right atrium > left atrium = right ventricle. The rank order of potencies of agonists in right atrium was sarafotoxin S6c > ET-1 = ET-2 > or = ET-3. The ET(A) receptor-selective compounds BQ123 (10 microM) and A-127722 (1 microM) only slightly blocked (<0.5 log-unit shift) the effects of lower concentrations of ET-1, and the ET(B) receptor antagonist Ro46-8443 (10 microM) did not cause blockade. SB 209670 caused concentration-dependent rightward shifts of ET-1 and sarafotoxin S6c concentration-effect curves with Schild slopes not different from one and affinities (-logM K(B)) of 7.0 and 7.9, respectively. ET-1 caused arrhythmic contractions in right atrial trabeculae that were prevented by 10 microM SB 209670 but not 10 microM BQ123 or 1 microM A-127722, precluding ET(A) receptors. ET-1 caused a higher incidence of arrhythmic contractions in tissues taken from patients treated with beta-blockers before surgery than in tissues from non-beta blocker-treated patients. Sarafotoxin S6c produced arrhythmias that were prevented by SB 209670. The positive inotropic effects of ET-1 in human right atrial myocardium are mediated mostly by a non-ET(A), non-ET(B) receptor. Ventricular inotropic ET receptors differ from atrial inotropic ET receptors. ET-1 induced arrhythmic contractions in human atria do not appear to be mediated by an ET(A) receptor.     

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

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

Effects of K+-channel blockers on A1-adenosine receptor-mediated negative inotropy and chronotropy of guinea-pig isolated left and right atria

Adenosine has previously been shown to stimulate K(+)-efflux and to block L-type calcium channels in atrial myocytes. The aim of the present study was to evaluate the contribution of K(+)-channels in the development of the negative inotropic and chronotropic responses to adenosine agonists in guinea-pig left and right atria, respectively. Tetraethylammonium (TEA) potentiated the negative inotropic and chronotropic responses to R-(-)-N6-(2-phenyl-isopropyl)-adenosine (R-PIA), seen as leftward shifts of the concentration-response curves. Glibenclamide had no effect on the negative inotropic response to R-PIA but increased the rate of onset of the negative chronotropic response in right atria. 4-Aminopyridine (4-AP, 10 mM), potentiated the left atrial inotropic responses to R-PIA, seen as a leftward shift of the concentration-response curve, but slowed the speed of onset of the response to a single concentration (10(-6) M) of R-PIA. This reduction in speed of onset of the response can explain the differences in effects of 4-AP on concentration-response curves reported here and previously. In the right atria, 4-AP (10 mM) inhibited the negative chronotropic responses to R-PIA, seen as a rightward shift of the concentration-response curve and reduction of the maximum response. 4-AP also slowed the onset of the right atrial rate response to R-PIA. These results support the theory that K(+)-efflux plays only a minor role in the negative inotropic responses of guinea-pig left atria to R-PIA. This apparently controls the speed of onset of the response. The negative chronotropic response of guinea-pig right atria to R-PIA appears to be much more dependent upon K(+)-efflux than for the negative inotropic response of the left atria.     

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)     

Modulation of peripheral beta-1 and alpha-2 receptor sensitivities by the administration of the tricyclic antidepressant, imipramine, alone and in combination with alpha-2 antagonists to rats

The purpose of the present investigation was to determine whether peripheral beta-1 and alpha-2 receptors are regulated by the tricyclic antidepressant, imipramine. The administration of imipramine alone decreased the sensitivity of peripheral beta-1 receptor activity in anesthetized rats (isoproterenol-induced positive chronotropy) after 11 days, but not after 4 days. The coadministration of the selective alpha-2 antagonists, yohimbine or idazoxan (RX 781094), with imipramine resulted in a decrease in beta-1 receptor sensitivity in anesthetized rats within only 4 days. In isolated spontaneously beating right atria taken from drug-treated rats, beta-1 receptor sensitivity (isoproterenol positive chronotrophy) was not affected by the administration of imipramine for 4 days; however, administration of imipramine for 11 days resulted in significant decrease in beta-1 receptor sensitivity. The coadministration of yohimbine or idazoxan with imipramine resulted in a decrease in beta-1 receptor function in only 4 days. Beta-2 receptor sensitivity (isoproterenol-induced hypotension in anesthetized rats) was not significantly altered under conditions in which beta-1 receptor sensitivity was decreased. The destruction of central catecholamine-containing neurons with 6-hydroxydopamine prevented the subsensitivity of peripheral beta-1 receptor function in anesthetized rats induced by the coadministration of imipramine plus yohimbine (4 days). The sensitivity of peripheral alpha-2 receptors (clonidine-induced inhibition of electrically stimulated rat vas deferens) was depressed after the administration of imipramine alone (4 days) and imipramine plus yohimbine or idazoxan (4 days).(ABSTRACT TRUNCATED AT 250 WORDS)     

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.