Role of the alpha agonist activity of dobutamine in mediating cardiac output: effects of prolonged isoproterenol infusion

Hemodynamic activities of dobutamine enantiomers were studied in either control rats or those infused with isoproterenol (400 micrograms/kg/hr) for 4 days. In control animals prazosin attenuated the effects of (+/-)-dobutamine on cardiac output by approximately 50%; remaining activity was blocked by propranolol. After isoproterenol infusion (+/-)-dobutamine was less efficacious and the blocking effects of prazosin were greater than 90%. Isoproterenol infusion had no effect on (-)-dobutamine-mediated (alpha-1 adrenoceptor agonist) increases in cardiac output and these actions were blocked by prazosin. By contrast, compared to (+/-)- and (-)-dobutamine, effects of (+)-dobutamine (beta adrenoceptor agonist) on cardiac output were modest, not altered by prazosin and were blocked by propranolol; (+)-dobutamine was inactive after isoproterenol infusion. (-)-Dobutamine increased systemic vascular resistance in both control and isoproterenol infused rats, whereas (+)-dobutamine was inactive. (+/-)-Dobutamine increased systemic vascular resistance only in isoproterenol-infused rats. All increases in systemic vascular resistance were blocked by prazosin. Neither (+/-)- nor (+)-dobutamine significantly altered stroke volume. By contrast, (-)-dobutamine resulted in prazosin-sensitive increases in stroke volume in both control and isoproterenol-infused rats. In control animals, (+/-)-, (+)- and (-)-dobutamine increased heart rate in a dose-dependent manner; chronotropic effects of (-)-dobutamine were less than those of either (+/-)- or (+)-dobutamine. Chronotropic effects were not demonstrable in isoproterenol-infused animals. These data support the notion that in control rats cardiac output may be increased by either alpha or beta adrenoceptor stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo cardiovascular responses to isoproterenol, dopamine and tyramine after prolonged infusion of isoproterenol

Effects of prolonged in vivo infusion of isoproterenol on acute cardiovascular responses to isoproterenol, dopamine and tyramine were studied in pithed rats. Isoproterenol infusion resulted in a significant decrease in control values for maximum left ventricular dP/dt; heart rate and left ventricular systolic blood pressure were not altered. This treatment also depleted both atrial and ventricular stores of norepinephrine and caused cardiac hypertrophy. Isoproterenol infusion resulted in a desensitization of drug-induced cardiovascular responses. The acute in vivo effects of isoproterenol on maximum left ventricular dP/dt, heart rate and left ventricular systolic blood pressure responses to isoproterenol were severely attenuated. The ED50 for maximum left ventricular dP/dt was increased 36-fold and maximal responses were reduced by half; changes in heart rate occurred in a parallel fashion. By contrast, ED50 values for inotropic responses to tyramine and dopamine were increased 14- and 4-fold, respectively, whereas increases in heart rate were blunted. Tyramine and dopamine-mediated increases in heart rate were completely attenuated by desensitization; chronotropic effects were again evident after pretreatment with the selective alpha-1 blocker prazosin. In addition, prazosin blocked the inotropic responses to tyramine and dopamine after desensitization and this antagonism was only slightly enhanced by addition of propranolol (prazosin + propranolol); propranolol alone was ineffective. These results are consistent with the down-regulation of beta adrenoceptors after prolonged exposure to catecholamines and indicate that under such conditions the alpha-mediated cardiovascular responses may be unmasked. Compared to pure beta agonists, agents with a degree of alpha-1 activity might be superior inotropes in heart failure patients who characteristically present with depleted stores of myocardial norepinephrine and minimal beta adrenoceptor reserve.     

Effects of beta adrenoceptor down-regulation on the cardiovascular responses to the stereoisomers of dobutamine

Effects of prolonged in vivo infusion of either saline (control) or isoproterenol (beta adrenoceptor desensitization) on acute cardiovascular responses to (+) (beta agonist)-, (-) (alpha agonist)- and (+/-)-dobutamine were studied in pithed rats. Each form of dobutamine resulted in comparable dose-dependent increases in maximum left ventricular dP/dt (LVdP/dtmax) in control animals. Effects of (+)-dobutamine were blocked by propranolol whereas those of l-dobutamine were sensitive to prazosin; both alpha and beta antagonists were required to block the inotropic effects of the racemic mixture. Contractile responses to (+)- and (+/-)-dobutamine were accompanied by tachycardia (characteristic of beta adrenoceptor stimulation) whereas (-)-dobutamine enhanced LVdP/dtmax without altering heart rate (characteristic of alpha adrenoceptor stimulation). Isoproterenol infusion resulted in a pronounced desensitization to the inotropic effects (LVdP/dtmax) of (+/-)- and (+)-dobutamine. Ed30 values for (+/-)- and (+)-dobutamine were increased by approximately 15- and 50-fold, respectively, and maximal responses to both drugs were severely attenuated. Prazosin further blunted remaining inotropic responses to (+/-)-dobutamine and propranolol resulted in a complete block. Responses to (+)-dobutamine were only sensitive to propranolol. Attenuation of heart rate responses paralleled those observed for LVdP/dtmax. By contrast, the inotropic effects of (-)-dobutamine in either control or desensitized rats were both qualitatively and quantitatively comparable; responses were blocked by the alpha-1 antagonist, prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Selective desensitization of cardiac beta adrenoceptors by prolonged in vivo infusion of catecholamines in rats

The effects of prolonged in vivo infusion of isoproterenol (400 micrograms/kg/hr) or norepinephrine (200 micrograms/kg/hr) from a minipump on the physiological reactivity and binding properties of cardiac beta and alpha-1 adrenoceptors were tested in rats. Infusion of either catecholamine significantly reduced the in vitro inotropic and chronotropic potency of isoproterenol in isolated left and right atria, respectively; desensitization was near maximal as early as after 2 hr of infusion. No significant change in the density of [3H]dihydroalprenolol-labeled beta receptors was evident at this time point in either atrial or ventricular tissue, although isoproterenol did decrease binding site density after 7 days of infusion. There was no change in the binding affinity or physiological blocking potency of dihydroalprenolol after isoproterenol infusion. The inotropic potency of phenylephrine in the presence of dihydroalprenolol was unaffected by infusion of either isoproterenol or norepinephrine and methoxamine failed to increase right atrial rate either in control or in isoproterenol-infused rats. There was also no change in the density and affinity of [3H]prazosin binding sites after isoproterenol infusion. These results indicate selective desensitization of cardiac beta receptors without changes in alpha-1 receptors by prolonged in vivo stimulation with catecholamines. This reaction pattern is different from the well documented effects of hypothyroidism, which include decreased sensitivity of cardiac beta and increased sensitivity of cardiac alpha-1 receptor-mediated responses in rats. Thus, the mechanisms responsible for altered receptor function in the two conditions appear to be different.     

Type II cell response to chronic beta adrenergic agonist and antagonist infusions

Beta adrenergic agonists stimulate the secretion of lamellar bodies by lung alveolar type II cells. Most studies of such stimuli have been for short time periods; we wished to determine the effects of longer infusions of beta adrenergic agonists and antagonists upon biochemical and morphometric measures of type II cell surfactant pools. We implanted osmotic minipumps which infused 10 micrograms/kg/min of isoproterenol, 10 micrograms/kg/min of terbutaline or 0.2 microgram/kg/min of propranolol into the s.c. tissue of rats for 7 days. Saline-infused rats and untreated rats were studied concurrently. Seven days after pump implantation, we measured body, lung and heart weight and lamellar body and lavage disaturated phosphatidylcholine content. A group of rats with saline-, isoproterenol- or propranolol-filled osmotic minipumps had lungs fixed and processed for morphometric volume determination of type II cell organelles. Lung and body weights were unaffected by beta agonists and by propranolol, whereas heart weights were increased 14 to 47%. Lamellar body disaturated phosphatidylcholine was elevated 20 to 30% after 7 days of beta agonist infusion and was 20% lower after 7 days of propranolol, compared to saline infusion. Morphometric estimates of total lamellar body volume per lung showed a 22% increase after 7 days of isoproterenol and a 40% decrease after 7 days of propranolol. These changes were found to be due to an increase in lamellar body volume per cell after isoproterenol and a decrease in both lamellar body volume per cell and total number of type II cells per lung after propranolol infusion.     

Duration of cardiac effects of timolol and propranolol.

The duration of the cardiac effects of single intravenous doses of the beta-antagonists, timolol and propranolol, was compared in 6 healthy male subjects. Timolol and propranolol were given in doses of 1 mg and 10 mg, respectively, and at specified times after their administration, beta-blockade was assessed by the reduction of maximal exercise-induced tachycardia and by the inhibition of the chronotropic and inotropic effects of isoproterenol. Inotropic effects were measured by changes in the pre-ejection period of left ventricular systole obtained from systolic time intervals. There was no statistically significant difference in the timolol and propranolol time-courses of beta-blockade. The change in exercise-tachycardia was maximal 5 min after beta-antagonist infusion but dissipated rapidly so that no statistically significant change was observed 9 hr later. The chronotropic and inotropic effects of isoproterenol were almost completely antagonized for 11/2 hr after beta-antagonist infusion, and significant beta-blockade could be demonstrated 9 hr later. There was no difference in the time-course of the negative chronotropic and inotropic effects of either beta-antagonist.     

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.     

Pharmacodynamic Studies of Beta Adrenergic Antagonism Induced in Man by Propranolol and Practolol

The pharmacodynamic activities of two beta adrenergic antagonists, propranolol and practolol, were compared in eight hypertensive patients. The activity of each antagonist was established in relation to its blood concentration at maximal and submaximal adrenergic blockade defined by inhibition of exercise tachycardia. Maximal inhibition of exercise tachycardia was comparable with both drugs and averaged 74+/-7% of the control value during drug treatment. This inhibition was achieved with a blood concentration of 2.5+/-0.4 mug/ml practolol and 0.10+/-0.08 mug/ml propranolol. The antagonist activities of these drugs against adrenergic stimulation with isoproterenol infusion indicated a much greater relative potency of propranolol against this stimulus, and in vivo estimates of PA(2) values differed by more than 600-fold. Relative antagonist activity of practolol during isoproterenol stimulation was equivalent both at cardiac (inotropic and chronotropic) and at vascular adrenergic receptors, whereas greater antagonist activity of propranolol was observed at vascular receptors than at cardiac receptors. Thus, the activity of practolol was not limited to cardiac receptors as previously suggested. Practolol did not reduce cardiac output at any dose level and the effect on resting blood pressure was small. Both practolol and propranolol had much greater hypotensive activity during exercise. These studies have defined the differing pharmacodynamic activities on the cardiovascular system of two effective beta adrenergic receptor antagonists and have established the blood levels of these antagonists necessary to achieve effective adrenergic blockade.     

Intrinsic sympathomimetic activity of the partial agonist prenalterol in relation to beta adrenoceptor interaction in various tissues, in vitro

The intrinsic sympathomimetic activity (ISA) of prenalterol was studied in isolated tissues from different species, including tissues containing predominantly beta-1 adrenoceptors (cardiac preparations from cat, rabbit, rat and guinea pig) and tissues characterized by beta-2 adrenoceptor predominance (cat skeletal muscle and rat uterus). The ISA of prenalterol, varying between 0 and 94% in the various tissues, was found to be positively correlated to the stimulatory potency (-log EC50) of isoproterenol and prenalterol. In the cardiac preparations from the rabbit there was an interindividual variation in the ISA of prenalterol, which was also positively correlated to the stimulatory potency of the beta agonists. The density of beta adrenoceptors in the tissues studied correlated neither to the variable ISA of prenalterol nor to the -log EC50 values of isoproterenol or prenalterol. The affinities of isoproterenol and prenalterol for the beta adrenoceptors were subject to less variation than were the stimulatory potencies of the agonists. The degree of separation between the concentration-effect curves for beta adrenoceptor occupancy and mechanical performance, expressed as the ratios Kd/EC50 for both agonists, were positively correlated to the corresponding ISA of prenalterol in various tissues. However, a considerably steeper relationship between occupancy/potency ratio and ISA was seen with prenalterol than with isoproterenol. The present data suggest that the level of ISA of the partial agonist, prenalterol, depends upon the efficiency of signal transmission from the activated receptor to the final end-organ response. The separation between the concentrations of the full agonist, isoproterenol, required for receptor occupancy and response serves as an index of the efficiency of coupling between the stimulus, elicited by activation of the receptor, and the response.     

Cardiac inotropic beta adrenoceptors are fully active at low temperature.

The purpose of this study was to test the adrenoceptor interconversion hypothesis of Kunos and Nickerson (Brit. J. Pharmacol. 59: 603--614, 1977) which claims that lowering temperature from 31 to 17 degrees C converts inotropic beta adrenoceptors in rat atria to alpha adrenoceptors. If lowering temperature transforms the adrenoceptor from a beta type to an alpha type, one should expect that the sympathomimetic drug phenylephrine would be more potent at the low than at the high temperature, that the reverse would be true for the sympathomimetic drug isoproterenol, and that the blocking ability of the beta adrenoceptor blocking drug propranolol might be reduced and that of the alpha adrenoceptor blocking drug phentolamine increased by lowering temperature. This was not observed. It was impossible to obtain satisfactory inotropic effects at 17 degrees C and the calcium concentration in the incubation medium had to be reduced, which lowered contractility and permitted strong inotropic effects. At 17 degrees C the sympathomimetic drug effects were inhibited only by propranolol and not by phentolamine, and there was no evidence of "adrenoceptor interconversion."     

Beta adrenergic antagonists inhibit serotonin uptake by pulmonary vascular cells in culture.

Beta adrenergic antagonists have been demonstrated to show stereoselective affinity for serotonin (5-HT) receptors in the central nervous system, and competitively inhibit 5-HT transport of platelets, but have not previously been evaluated for their influence on either 5-HT receptors or 5-HT transport systems of vascular cells. We have reported stimulation of 5-HT uptake by subjection of endothelial (EC) and smooth muscle cells (SMC) to anoxia. We now report that (+/-)-propranolol inhibits 5-HT uptake by both room air- and anoxia-exposed EC and SMC in culture. The effect on SMC was more marked than that on EC and showed a similar inhibition as ketanserin. The relative inhibitory potencies of beta adrenergic antagonists and ketanserin on uptake of 5-HT by SMC were as follows: (+/-)-propranolol = ketanserin greater than alprenolol = pindolol greater than oxprenolol = atenolol. The beta adrenergic receptor agonist, isoproterenol, in the presence of isobutylmethylxanthine, an inhibitor of phosphodiesterase, produced a high elevation of cyclic AMP in SMC along with a cellular configurational change and partially inhibited uptake of 5-HT. Propranolol inhibited 5-HT uptake both in the presence and absence of isoproterenol plus isobutylmethylxanthine, suggesting that its inhibitory effect was not mediated through its interaction at the beta adrenergic receptor. Kinetic analyses of the effect of propranolol on 5-HT uptake indicated that propranolol reduced 5-HT uptake by noncompetitive inhibition. We conclude that beta adrenergic antagonists block vascular cell uptake of 5-HT through an action other than interaction with the beta adrenergic receptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

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

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

Beta adrenoceptor blocking properties of MK-761.

MK-761 is a new class of compound which has beta adrenoceptor antagonist and vasodilating properties in a single molecule. The compound has in vitro beta adrenoceptor blocking activity in the isolated cat heart papillary muscle and isolated rat atria. Unlike propranolol, it did not depress contactile force. Intrinsic sympathomimetic activity was not observed in vitro, (isolated cat papillary muscle, isolated atria) but the compound has some intrinsic sympathomimetric activity because it slightly increased heart rate in reserpinized rats. MK-761 was found to be approximately as potent as timolol and pindolol in blocking cardiac and vascular beta adrenergic receptors in anesthetized dogs. The drug was not, however, cardioselective. Oral beta adrenoceptor blocking activity was observed in rats at doses similar to those which decreased blood pressure.     

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.     

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.     

Modulation of pyloric motor activity via adrenergic receptors

The influences of adrenergic nerves on pyloric motor function and the locus and types of receptors involved were examined. Using glyoxylic acid fluorescence a dense adrenergic innervation of the inner pyloric muscle ring was demonstrated. Pyloric motor activity was monitored while close i.a. injections of a selective alpha-1 adrenoceptor agonist, phenylephrine, or a selective alpha-2 adrenoceptor agonist, B-HT-920, were given. Neither agonist affected basal pyloric motor activity, but both inhibited pyloric activity when it was stimulated by duodenal field stimulation or by intraduodenal acid infusion. The actions of each of the inhibitory agonists, phenylephrine or B-HT-920, were blocked selectively by prazosin or rauwolscine, respectively. Injection of isoproterenol usually had no effect or excited basal pyloric activity. This excitation could be antagonized selectively by propranolol or by atropine. Injection of isoproterenol after neural blockade by tetrodotoxin inhibited pyloric motor activity. Receptor binding studies carried out with subcellular nerve or muscle enriched membrane fractions of canine pyloric muscle with [3H]prazosin, [3H]rauwolscine and [125I]cyanopindolol revealed a dual location of alpha-1, alpha-2 and beta receptors on both nerve membranes and smooth muscle membranes. These results suggest that adrenergic effects on the pyloric muscle can be exerted by pre- and postsynaptic beta receptors which, respectively, excite by releasing acetylcholine and inhibit by acting on receptors on the pyloric muscle. Also inhibitory alpha-1 and alpha-2 receptors are present on cholinergic nerves. The functions of postsynaptic alpha-1 and alpha-2 adrenoceptor binding sites on smooth muscle are so far unknown.     

Nonspecific supersensitivity induced by reserpine in guinea pig cardiac ventricle tissue

The depletion of norepinephrine stores by chronic reserpine pretreatment is thought to be responsible for the development of supersensitivity in cardiac tissue. The present study was designed to determine if reserpine-induced supersensitivity could be demonstrated in isolated guinea pig right ventricular strips, if it was associated with beta adrenoceptor proliferation and was specific for beta adrenoceptor agonists. In addition, reserpine dose-dependency of the phenomenon was tested for by using two pretreatment regimens to determine if supersensitivity was the result of direct or possibly toxic effects of reserpine. Pretreatment of guinea pigs with reserpine (0.1 mg/kg/day) for 7 days resulted in over 90% depletion of cardiac norepinephrine stores. Supersensitivity to the inotropic effects of isoproterenol, impromidine and forskolin was demonstrated. Associated with the supersensitivity was a significant increase in beta adrenoceptor density. Muscarinic receptor density was actually decreased. This pretreatment regimen also was associated with a significant (30%) drop in body weight suggesting that the phenomenon might be the result of a direct toxic effect of reserpine. Pretreatment of guinea pigs with a lower dose of reserpine (0.03 mg/kg/day) for 7 days produced the same degree of norepinephrine depletion, nonspecific inotropic supersensitivity and beta adrenoceptor proliferation in the absence of a significant reduction in body weight. The degree of supersensitivity induced by the two pretreatment regimens was between 2- to 3-fold for all three agonists tested.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition by 5-hydroxytryptamine of the beta adrenoceptor-mediated positive inotropic responses to catecholamines in rabbit papillary muscles: direct interaction with beta adrenoceptors.

The effects of 5-hydroxytryptamine (5-HT) on the positive inotropic responses to catecholamines were investigated in isolated rabbit papillary muscles. 5-HT produced a concentration-dependent positive inotropic effect, an effect which was antagonized by prazosin, but not by propranolol. The positive inotropic effect of 5-HT diminished greatly in muscles from rabbits pretreated with 6-hydroxydopamine. Thus, it is likely that 5-HT causes a release of norepinephrine and increases force of contraction indirectly through alpha-1 adrenoceptors. In the presence of prazosin, 5-HT exerted a concentration-dependent inhibition of the positive inotropic response to isoproterenol. The positive inotropic responses to tyramine and a beta-1 adrenoceptor agonist T-1583 were also inhibited by the addition of 5-HT. The inhibitory effect of 5-HT on the beta adrenoceptor-mediated responses was unaffected by methysergide, ketanserin, ICS 205-930 or atropine. Pretreatment with pertussis toxin did not block the inhibitory effect of 5-HT on the inotropic response to isoproterenol, while abolishing the cholinergic interaction against the isoproterenol response. In contrast to its antagonizing effect on the inotropic response to isoproterenol, 5-HT produced an additive effect on the positive inotropic response to norepinephrine. However, when neuronal amine uptake was blocked by cocaine, the positive intropic response to norepinephrine was suppressed by the addition of 5-HT. 5-HT inhibited (-)-[125I]iodocyanopindolol binding to the membranes from rabbit ventricles with a monophasic displacement curve.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)