Subtype-selective regulation of beta adrenergic receptor-adenylyl cyclase coupling by phorbol esters in 3T3-L1 fibroblasts

To study the epigenetic regulation of beta adrenergic receptor subtypes, we examined the effects of phorbol esters on beta adrenergic receptor coupling to adenylyl cyclase in 3T3-L1 fibroblasts, which express both beta-1 and beta-2 adrenergic receptor subtypes. Pretreatment of intact 3T3-L1 cells with the protein kinase C activator phorbol dibutyrate caused a dose- and time-dependent decrease in subsequent cyclic AMP (cAMP) accumulation mediated by the beta adrenergic agonist isoproterenol. This effect was selective for beta-adrenergic receptor-mediated responses because there was a potentiation of cAMP accumulation caused by other activators such as prostaglandin E1, forskolin or cholera toxin. The inactive phorbol, alpha-phorbol dibutyrate was ineffective at 1 microM in attenuating isoproterenol stimulation, and 25 nM of the protein kinase C inhibitor staurosporine blocked the effects of phorbol ester on beta adrenergic agonist responses. Stimulation of cAMP accumulation by isoproterenol occurred through a greater proportion of beta-2 adrenergic receptors in phorbol dibutyrate-treated cells than in control cells. This was demonstrated using the beta-1 adrenergic selective antagonist ICI 89.406 and the beta-2 adrenergic selective antagonist ICI 118.551 to inhibit competitively isoproterenol-stimulated cAMP accumulation. Beta-2 adrenergic receptor number and subtype in these cells are regulated by glucocorticoids and butyrate. Decreasing the proportion of beta-1 adrenergic receptors and concomitantly increasing beta-2 adrenergic receptors with either glucocorticoids or butyrate decreased the ability of phorbol ester pretreatment to attenuate cAMP accumulation by isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selective regulation of beta-1 and beta-2 adrenergic receptors by atypical agonists

The interactions of the atypical agonists pindolol and celiprolol with beta adrenergic receptors were compared with those of the full agonist, isoproterenol. Studies were carried out using intact cells as well as membranes prepared from C6 glioma cells. Computer-assisted analysis of dose-response curves resulting from the inhibition of the binding of [125I]iodopindolol by the beta-1 and beta-2 selective compounds ICI 89,406 and ICI 118,551 revealed that approximately one-third of the beta adrenergic receptors on these cells were beta-1 receptors. Addition of GTP to the binding assay simplified the dose-response curve for inhibition of the binding of [125I]iodopindolol by isoproterenol and diminished the potency of the agonist. GTP had no effect on the binding of pindolol or celiprolol, suggesting that these drugs do not induce the formation of a ternary complex with the receptor and the guanine nucleotide-binding protein for stimulation of adenylate cyclase activity. When added to the growth medium of intact C6 cells, isoproterenol induced a 40-fold increase in cyclic AMP accumulation. Pindolol and celiprolol, however, caused no elevation of enzyme activity. Addition of isoproterenol to the growth medium of intact cells resulted in an 80% decrease in the density of both beta-1 and beta-2 adrenergic receptors within 8 hr. Growing cells in the presence of pindolol or celiprolol induced a 50% decrease in the density of beta-2 receptors, which was inhibited by beta adrenergic antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo interactions between beta-1 and beta-2 adrenoceptors regulate catecholamine tachyphylaxia in human adipose tissue.

Catecholamine tachyphylaxia was investigated in human s.c. adipose tissue in situ by using microdialysis. The tissue was dialyzed with adrenergic agents (10(-8) mol/l) and the glycerol concentration (lipolysis index) was determined. Perfusion with adrenaline caused a 3-fold rise in the glycerol concentration, which peaked at 30 min and then (within 1 hr) declined to a level 75% higher than base line; the latter elevation was constant for at least 2 hr. Noradrenaline or isoprenaline in the absence and presence of a selective beta-2 receptor antagonist, or the selective beta-1 adrenergic agonist dobutamine, caused a 2- to 2.5-fold transient lipolytic response which also peaked at 30 min but then (within 3 hr) declined to the base-line level. On the other hand, isoprenaline plus a selective beta-1 receptor antagonist or the beta-2 selective adrenergic agonist terbutaline caused a constant lipolytic effect for at least 3 hr. Noradrenaline or adrenaline plus a nonselective beta adrenergic antagonist as well as the alpha-2 selective adrenergic antagonist clonidine caused a sustained antilipolytic action for at least 3 hr. In conclusion, the adrenoceptor subtypes involved in lipolysis regulation in humans have different in vivo sensitivities to homologous desensitization. Beta-2 and alpha-2 adrenoceptors are resistant in this respect whereas activation of beta-1 adrenoceptors leads to rapid desensitization. However, simultaneous beta-1 and beta-2 receptor activation is accompanied by different degrees of tachyphylaxia, indicating regulatory in vivo interactions within this receptor family in human adipose tissue.     

Activation of alpha-2 adrenergic receptors augments neurotransmitter-stimulated cyclic AMP accumulation in rat brain cerebral cortical slices

Norepinephrine-stimulated cyclic AMP (cAMP) accumulation in brain is mediated by both alpha and beta adrenergic receptors. The functional activity of these receptors can be differentiated by examining the response to isoproterenol alone and isoproterenol + 6-fluoronorepinephrine, and alpha adrenergic agonist. The present study was undertaken to define the pharmacological characteristics of the alpha adrenergic component associated with cAMP accumulation in brain. Using a prelabeling technique it was found that norepinephrine- or isoproterenol plus 6-fluoronorepinephrine-stimulated cAMP accumulation was only inhibited partially by an alpha-1 adrenergic receptor antagonist. In contrast, alpha-2 adrenergic receptor antagonists inhibited completely that portion of the response exceeding that produced by isoproterenol alone (alpha adrenergic augmentation). Furthermore, selective alpha-1 adrenergic agonists were incapable of enhancing the cAMP response to isoproterenol, whereas alpha-2 adrenergic agonists were active in this regard. Partial agonists for the alpha-2 adrenergic receptor were ineffective as augmentors. The data suggest that the alpha adrenergic component of the norepinephrine response in rat brain slices has characteristics of both alpha-1 and alpha-2 receptors, with the alpha-2 adrenergic component being a major contributor in this regard.     

Isolated parietal cells: adrenergic response and pharmacology

Isoproterenol (Iso), epinephrine and norepinephrine each stimulated isolated gastric mucosal parietal cells as shown by an increased accumulation of [14C]aminopyrine (AP), an indirect measure of acid secretion. The beta receptor selective agonists metaproterenol, terbutaline and zinterol stimulated AP accumulation to the same extent as Iso, whereas the beta-1 receptor selective agonist dobutamine was only 20% as effective. The general beta receptor antagonists oxprenolol and dl-propranolol and the beta-2 receptor antagonist H35/25 inhibited Iso-stimulated AP accumulation. Receptor stereoselectivity was shown by the approximately 100-fold difference in potency of the I- and d-isomers of propranolol. The alpha receptor antagonists phentolamine and phenoxybenzamine, the beta-1 receptor antagonists metoprolol and practolol and the muscarinic receptor antagonist atropine were without effect. The histamine H2-receptor antagonist cimetidine inhibited Iso-stimulated AP accumulation an average of 40% at a concentration which inhibits completely histamine-stimulated AP accumulation. The data demonstrate that cells of the rat gastric mucosa have adrenergic beta-2 receptors which when stimulated result in an increase in acid secretion. The results also show that the response is in part mediated indirectly by catecholamine-stimulated release of histamine.     

Beta adrenergic receptor subtypes in the atria: evidence for close coupling of beta-1 and beta-2 adrenergic receptors to adenylate cyclase

The relationship between occupancy of beta adrenergic receptors and stimulation of adenylate cyclase in dog atrial tissue was examined by studying the binding of [125I]iodopindolol and the activation of adenylate cyclase. Computer-assisted nonlinear regression analysis was used to analyze the inhibition of isoproterenol-stimulated adenylate cyclase activity by beta-1- or beta-2-selective antagonists. The Ki values for each subtype of receptor for the selective antagonists resulting from studies of the inhibition of adenylate cyclase activity were similar to those determined in studies of the inhibition of the binding of [125I]iodopindolol. To compare further the occupancy of beta-1 or beta-2 adrenergic receptors with the activation of adenylate cyclase mediated by each class of receptor, computer modeling of the stimulation of adenylate cyclase by the beta-1-selective agonist norepinephrine was carried out. The EC50 values of norepinephrine for each receptor subtype, as measured in studies of norepinephrine-stimulated adenylate cyclase activity, were similar to the Ki values for the inhibition by norepinephrine of the binding of [125I]iodopindolol to each receptor subtype. The data led to the conclusion that beta-1 adrenergic receptors make up about 70% of the total number of beta adrenergic receptors and mediate 70% of the increase in adenylate cyclase activity produced by isoproterenol. These results suggest that the relationship between occupancy of each class of receptor and activation of adenylate cyclase is linear and that, when agonist-stimulated adenylate cyclase activity is used as a functional response, neither spare beta-1 nor spare beta-2 adrenergic receptors exist in the atrium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Desensitization of adenylate cyclase and down regulation of beta adrenergic receptors after in vivo administration of beta agonist

In vitro incubation of cells with catecholamines leads to both down regulation of beta adrenergic receptor number and desensitization of agonist-stimulated adenylate cyclase activity. These same parameters, down regulation of beta adrenergic receptor number and desensitization of adenylate cyclase activity were assessed in rat lung membranes after in vivo administration of metaproterenol, a beta-2 selective agonist. In vivo treatment with metaproterenol leads to: 1) reduced beta adrenergic receptor number; 2) reduced isoproterenol-stimulated adenylate cyclase activity; 3) unaffected NaF or 5'-guanylylimidodiphosphate-stimulated adenylate cyclase activity; and 4) reduced affinity of the receptor for isoproterenol similar to the affinity observed in the presence of 5'-guanylylimidodiphosphate. The date suggest that in vivo metaproterenol administration results in an uncoupled receptor-adenylate cyclase complex. The effects of in vivo administration of the glucocorticoid, methylprednisolone, to metaproterenol-pretreated animals were also assessed. Glucocorticoid treatment was associated with 1) increased beta adrenergic receptor number in rats in which the receptors have been down regulated, 2) increased isoproterenol responsiveness in agonist-desensitized rats and 3) no effect on agonist affinity in desensitized animals. These data suggest that the restoration of agonist responsiveness by glucocorticoids in the catecholamine refractive state is not simply a reversal of receptor down regulation or adenylate cyclase desensitization.     

Pharmacological characterization of chick and frog beta adrenergic receptors in primary cultures of myocardial cells.

In membrane preparations derived from primary cultures of chick myocardial cells, beta adrenergic receptors modeled for a single low-affinity site for both betaxolol (beta-1-selective) and ICI 118551 (beta-2-selective) displacement of [125I]iodocyanopindolol (ICYP), indicating that the chick beta receptor is pharmacologically distinct from both mammalian beta-1 and beta-2 adrenergic receptors with respect to these antagonists. However, the highly beta-1-selective compound CGP 20712A was able to distinguish two binding sites on ICYP competition curves, a high-affinity "beta-1 site" (75%) and a low-affinity "beta-2 site" (25%). Also, in chick heart cell membranes the relative ability of agonists to displace ICYP produced a profile typical of beta-1 adrenergic receptors with a rank order of potency or efficacy of: isoproterenol greater than epinephrine = norephinephrine. When agonist-mediated adenylyl cyclase stimulation was assessed the order of potency was slightly different, isoproterenol greater than epinephrine greater than or equal to norepinephrine. Additionally, antagonism of isoproterenol stimulation of adenylyl cyclase by CGP 20712A yielded a Kb value (1.16 +/- 0.35 x 10(-7) M) intermediate between the high and low-affinity binding sites of CGP 20712A, suggesting that the low-affinity site is coupled to adenylyl cyclase. In membrane preparations of frog myocardial cells, ICYP/antagonist competition curves modeled for a mixed population of receptors, with subtype percentages varying from 50:50 beta-1:beta-2 to 100% beta-2 depending on the specific antagonist used and the individual cell preparation. For ICYP/agonist competition binding experiments the relative ability to displace ICYP was isoproterenol greater than epinephrine = norepinephrine, a profile typical of beta-1 adrenergic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Denervation-induced changes in alpha and beta adrenergic receptors of the rat submandibular gland.

Denervation resulted in a marked increase in the beta adrenergic response and isoproterenol-stimulated cyclic adenosine 3':5'-monophosphate accumulation of dispersed cells prepared from a rat submandibular gland. This increase in beta adrenergic response was paralleled by an increase in the density of beta adrenergic receptors in membranes prepared from these glands. Denervation also produced a moderate increase in alpha adrenergic receptor density in membranes prepared from whole glands. However, the alpha adrenergic response in cells, epinephrine-induced release of potassium, appeared unaltered by denervation. Furthermore, membranes prepared from denervated dispersed cells did not show the increase in alpha adrenergic receptor density seen in membranes from an intact denervated gland. These data demonstrate that removal of noradrenergic nerve terminals affects alpha and beta adrenergic receptors differently. While it is clear that beta adrenergic membrane receptors participate in denervation-induced supersensitivity, the changes in alpha adrenergic membrane receptors are more complex and may not contribute to the supersensitivity seen after denervation. On the basis of competitive binding studies, the adrenergic receptors in membranes from intact submandibular glands were subclassified as beta-1 and alpha-2. Denervation did not alter the binding characteristics of the alpha-2 receptors in this gland, demonstrating that alpha-2 adrenergic membrane receptors can be postsynaptic in this adrenergically innervated tissue.     

Beta adrenergically mediated release of renin in the dog is not confined to either beta-1 or beta-2 adrenoceptors

The role of the renal beta-1 and beta-2 adrenoceptor subtypes in renin release was evaluated in anesthetized dogs. The renal baroreceptor and macula densa mechanisms of renin release were inhibited by pretreatment with indomethacin (8 mg/kg i.v.). Thus, the beta adrenergic mechanism of renin release was functionally isolated. Intrarenal infusion of isoproterenol at two infusion rates elicited local renal effects and systemic effects at the low and high infusion rates, respectively. Renin secretion was stimulated at both infusion rates. Isoproterenol elicited a significant renin release in the presence of beta-1 adrenoceptor blockade with atenolol and in the presence of selective beta-2 adrenoceptor blockade with IPS-339. In addition, intrarenal infusion of albuterol, a selective beta-2 adrenoceptor agonist, stimulated renin release in the absence and presence of atenolol. These results lead to the conclusion that beta adrenergically mediated release of renin in the dog is not confined to either the beta-1 or beta-2 adrenoceptor subtypes.     

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.     

Hypercalcemic effect of catecholamines and its prevention by thyrocalcitonin.

Earlier work by others has shown that the catecholamines, epinephrine and isoproterenol, can raise blood calcium levels in parathyroidectomized but not intact rats, and can restrict the hypocalcemic effect of injected thyrocalcitonin (TCT). The present findings support this earlier work, further showing that such catecholamines can produce hypercalcemia in rats after removal of the thyroid gland by acute thyroparathyroidectomy (TPTX) and indicating that these drugs may raise blood calcium by mobilizing calcium from bone. Rats were fasted overnight, subjected to TPTX and concurrently injected with adrenergic agonist or antagonist drugs alone or in combination. Epinephrine, isoproterenol, and the beta-2 adrenergic agonist, salbutamol, in doses greater than or equal to 1 mg/kg raised blood calcium from low normal levels (approximately 9-10 mg/100 ml) by 1.5 to 2 mg/100 ml (p less than 0.01). Hypercalcemia was apparent by 1 hour after injection and lasted for 1-4 hours. The extent of Ca elevation was dose-related. Pretreatment of rats with the alpha-adrenergic antagonist, phenoxybenzamine, enhanced the effect of epinephrine while pretreatment with the beta-antagonist, propranolol, reduced the effect of isoproterenol. The more selective beta-2 antagonist, butoxamine, but not the beta-1 antagonist, practolol, also reduced the hypercalcemic effect of isoproterenol in TPTX rats. These results suggest that catecholamine-induced hypercalcemia in TPTX rats is mediated by beta-2 adrenergic receptors. Related studies using rats prelabeled with 45Ca further suggest that the catecholamines, like parathyroid hormone, may act to raise blood calcium by mobilizing calcium from bone. The fact that these catecholamines could induce marked hypercalcemia in acutely TPTX rats but not in intact rats indicated that endogenous TCT protects the thyroid intact rat against hypercalcemia. The present findings support this idea in showing that isoproterenol and salbutamol raised levels of immunoreactive rat TCT in both thyroid venous and peripheral blood. Catecholamines apparently can promote TCT secretion, either directly or by a small transient increase in blood calcium. This, in turem, acts to combat hypercalcemia in thhroid-intact rats.     

Effects of selective and nonselective beta adrenergic agents on renin secretion in isolated perfused rat kidney

The effects of selective and nonselective beta adrenergic drugs on the renin secretion rate (RSR) in isolated perfused rat kidneys were studied. Both isoproterenol (Ipr) (nonselective agonist) and salbutamol (Salb) (beta-2 selective agonist) stimulated RSR in a dose-dependent manner. The lowest doses able to induce a significant RSR increase were 5 nM for Ipr and 50 nM for Salb. A 5-fold increase in RSR was induced by 500 nM Ipr and a 3.2-fold increase by 5 microM Salb. Renin stimulation by both agonists was suppressed by propranolol (nonselective) and by acebutolol and its derivative M&B 16,942 (beta-1 selective antagonists). Thus, renin release was stimulated or inhibited effectively by all the drugs tested, regardless of the beta selectivity of agonists and antagonists. There were no consistent relationships observed between changes in renal hemodynamics and in RSR, suggesting that the used drugs affected renin release from in vitro-perfused rat kidney through their direct effects on juxtaglomerular cells. These results indicate that renal beta adrenoreceptors involved in renin release do not fall into two distinct beta subtypes.     

The positive chronotropic effect induced by BRL 37344 and CGP 12177, two beta-3 adrenergic agonists, does not involve cardiac beta adrenoceptors but baroreflex mechanisms.

The presence of a beta-3 adrenoceptor (in addition to the classical beta-1 and beta-2 adrenoceptors) and its involvement in the control of heart rate was investigated in the dog. Experiments were carried out in conscious normal and sinoaortic denervated dogs (i.e. animals deprived of baroreceptor pathways). In normal dogs, infusion of isoproterenol, BRL 37344 (4-[-[(2-hydroxy-(3-chlorophenyl) ethyl)-amino]propyl]phenoxyacetate) (a beta-3 adrenergic agonist) or CGP 12177 (4-[3-t-butylamino-2-hydroxypropoxy]benzimidazol-2- one) (a beta-1 beta-2 adrenergic antagonist reported to act as an agonist for the beta-3 adrenergic receptor) increased heart rate with an order of potency: BRL 37344 > isoproterenol > CGP 12177. [125I]Cyanopindolol binding (2-2000 pM) was saturable and Scatchard analysis indicated the presence of an homogenous population of binding sites. KD was 12.8 +/- 18.5 pM and maximum binding was 94.2 +/- 12.5 fmol/mg of protein. Competition binding studies on dog heart membranes using 150 pM [125I] cyanopindolol indicated an order of potency (CGP 12177 > isoproterenol > BRL 37344) different from that observed in cardiovascular studies. Isoproterenol stimulated adenylate cyclase activity in heart membranes from normal dogs, whereas CGP 12177 and BRL 37344 were without any stimulating action. The positive chronotropic effects of isoproterenol, BRL 37344 and CGP 12177 were accompanied with a reduction in arterial blood pressure. In sinoaortic denervated animals, isoproterenol infusion provoked tachycardia and hypotension. BRL 37344 and CGP 12177 were without any significant effect on heart rate but induced a rapid and dramatic hypotension.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta-2 adrenergic control of ornithine decarboxylase activity in brain regions of the developing rat

Development of brain tissue is thought to be regulated, in part, by biogenic amines. We examined the role of noradrenergic stimulation in regulation of ornithine decarboxylase (ODC), an enzyme whose activity is obligatory for neuronal development and which has been used as a biochemical marker for cellular maturation. Intracisternal administration of adrenergic agonists produced a prompt increase in ODC in neonatal rat cerebellum, an effect mediated through beta-2 receptors: the rank order of activity was isoproterenol greater than epinephrine greater than norepinephrine greater than methoxamine; the effect could be blocked by propranolol but not phenoxybenzamine; and zinterol (a beta-2 selective agonist) was equipotent to isoproterenol whereas prenalterol (a beta-1 agonist) was ineffective. The elevation of ODC caused by adrenergic stimulation was cyclic AMP-dependent, as evidenced by: direct measurement of cyclic AMP levels after isoproterenol administration; comparisons of the dose-response curve for stimulation of cyclic AMP with that of ODC; examination of the time course of effect on the two variables; stimulation of ODC by administration of cyclic AMP analogs; demonstration of identical kinetic mechanisms for ODC stimulation by dibutyryl-cyclic AMP and isoproterenol; and potentiation of the actions of isoproterenol on both cyclic AMP and ODC by RO-201724, a specific inhibitor of phosphodiesterase. Examination of the ontogenetic pattern of phosphodiesterase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of the prejunctional beta adrenoceptors in canine bronchial smooth muscle

Prejunctional beta adrenoceptors in canine bronchi (3rd to 6th order) were characterized by observing the effects of beta receptor agonists and antagonists on field stimulation-induced contractions and excitatory junction potentials (EJPs). Contractions were antagonized by norepinephrine (IC50 = 9.4 X 10(-7) M), isoproterenol (IC50 = 1.9 X 10(-8) M) or salbutamol (IC50 = 4.0 X 10(-8) M). EJPs were also decreased by all three agonists, with little or no effect on resting membrane potential or on carbachol-induced depolarization when used at concentrations sufficient to eliminate EJPs. These inhibitory effects were blocked by propranolol or timolol, as well as by the selective antagonists ICI 89,406 (beta-1-selective) and ICI 118,551 (beta-2-selective); pA2 values for the selective antagonists were 8.4 and 7.2 (norepinephrine as agonist) or 6.5 and 9.0 (salbutamol as agonist), respectively. Control responses were also sometimes potentiated by the nonselective antagonists. Schild plot analysis of the data indicated clearly that both beta-1 and beta-2 receptors are involved in the inhibitory effect. Electron microscopic studies showed this tissue to be densely innervated by adrenergic and cholinergic nerves with close apposition of adrenergic and cholinergic nerve varicosities, providing a structural basis for prejunctional interactions between them. From the data presented, we conclude that catecholamines act on prejunctional beta-1 and beta-2 receptors leading to inhibition of cholinergic neurotransmission in canine bronchi.     

Beta-2 adrenoceptor-mediated effects on sinus rate and atrial and ventricular contractility on isolated, blood-perfused dog heart preparations

Changes in the sinus rate, right atrial contractile force and left ventricular contractile force in response to isoproterenol, epinephrine, dobutamine, salbutamol and procaterol were studied in isolated, blood-perfused right atrial or left ventricular cardiac preparations of the dog. Each substance elicited dose-dependent increases in the three parameters and the ranking of the potency (ED50) for each effect was isoproterenol greater than epinephrine greater than dobutamine greater than or equal to salbutamol greater than or equal to procaterol. The ED50 of procaterol for changing sinus rate was lower than for altering atrial and ventricular contractile force, whereas the ED50 of dobutamine for changing sinus rate was higher. Ranking on the basis of the ratio of increase in sinus rate to increase in atrial tension induced by the agonists gave the following order: procaterol greater than or equal to salbutamol greater than epinephrine greater than or equal to isoproterenol greater than dobutamine. Procaterol-induced increases in sinus rate and atrial contractile force were dose-dependently inhibited by the beta-2 adrenoceptor antagonist, ICI 118,551, but only attenuated slightly by the beta-1 antagonist, atenolol. On the other hand, the positive chrono- and inotropic effects on the right atrium induced by dobutamine and isoproterenol were blocked completely by atenolol. The epinephrine- or salbutamol-induced positive chrono- and inotropic responses in the right atrium were inhibited moderately by both antagonists, but ICI 118,551 inhibited sinus rate increases more effectively than the atrial tension increases.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enhancement of prostaglandin output during activation of beta-1 adrenoceptors in the isolated rabbit heart

The purpose of this study was to elucidate the type of adrenoceptor that mediates the effect of adrenergic stimuli on prostaglandin (PG) synthesis in the isolated rabbit heart and to determine the relationship of the released PGs to the mechanical changes elicited by catecholamines and stimulation of the cardiac sympathetic nerves. The output of 6-keto PGF1 alpha, PGE2 and PGF2 alpha was increased by electrical stimulation of the sympathetic nerves, norepinephrine, isoproterenol, dobutamine and angiotensin II, but not by phenylephrine or isoetharine. Propranolol or atenolol, but not phentolamine or butoxamine, blocked the output of PGs elicited by adrenergic stimuli. Indomethacin prevented the increase in PG formation caused by all stimuli. Moreover, the adrenergically induced release of PGs was not related to changes in heart rate, systolic tension or vascular tone elicited by the adrenergic stimuli. These data indicate that the adrenergically induced release of PGs in the isolated rabbit heart is due to the activation of beta-1 adrenoceptors and is independent of the mechanical effects produced by the adrenergic stimuli.     

Prostaglandin synthesis and renal vasoconstriction elicited by adrenergic stimuli are linked to activation of alpha-1 adrenergic receptors in the isolated rat kidney

The authors have investigated the effect of norepinephrine (NE) and selective alpha-1, alpha-2 and beta adrenergic receptor agonists and antagonists on prostaglandin (PG) output and vascular tone to determine the type of adrenergic receptor involved in these biological actions of NE in the isolated rat kidney perfused at a constant flow rate with Tyrode's solution. Renal arterial administration of NE (0.32-3.2 nmol) and the selective alpha-1 adrenergic receptor agonists phenylephrine (3-29.5 nmol), cirazoline (0.5-4.6 nmol) and amidephrine (4.1-41 nmol) produced dose-related increases in PG output and perfusion pressure. Administration of the selective alpha-2 adrenergic receptor agonists B-HT 933 (2-20 nmol) and guanabenz (1.7-17 nmol) into the kidney produced only small increases in PG output and perfusion pressure, whereas another selective alpha-2 adrenergic receptor agonist xylazine (1-20 nmol) failed to increase perfusion pressure or PG output. Infusion of the beta adrenergic receptor agonist isoproterenol reduced perfusion pressure, but failed to increase the output of PGs. The selective alpha-1 adrenergic receptor antagonist prazosin (2.7 X 10(-6) M) inhibited PG output and renal vasoconstriction elicited by phenylephrine, cirazoline and amidephrine, but not that caused by B-HT 933 and guanabenz. In contrast, the selective alpha-2 adrenergic receptor antagonist rauwolscine (1.3 X 10(-6) M) inhibited the small rise in PG output and perfusion pressure elicited by B-HT 933 and guanabenz, but not that caused by NE, phenylephrine, cirazoline or amidephrine. The beta adrenergic receptor antagonist propranolol (3.86 X 10(-6) M) did not alter PG output or renal vasoconstriction produced by NE or alpha-1 and alpha-2 adrenergic receptor agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vitro effects of beta adrenoceptor agonists and antagonists on the rat ovarian suspensory ligament

The mesovarian suspensory ligament of the rat was used to compare the activities of beta adrenoceptor agonists and antagonists. The following beta adrenoceptor agonists, in descending order of potency, inhibited spontaneous activity in a dose-related manner: zinterol greater than isoproterenol much greater than dobutamine. Several noncardioselective, beta-2 adrenoceptor antagonists with intrinsic sympathomimetic activity (ISA) also inhibited the activity of the ligament: pindolol greater than alprenolol = bucindolol = oxprenolol greater than labetalol. Maximal relaxation induced by the antagonists was equivalent to that caused by the beta receptor agonists. Two cardioselective, beta adrenoceptor antagonists with ISA, acebutolol and practolol, did not inhibit the activity of the suspensory ligament but did increase the rate of the isolated right atrium of the rat. The maximal increases in atrial rate evoked by the antagonists were significantly less than those induced by the beta adrenoceptor agonists. Studies with ICI 118,551 or atenolol as beta-2 or beta-1 selective adrenoceptor blockers, respectively, suggest that the beta adrenoceptors of the suspensory ligament are predominantly of the beta-2 subtype. The possible relevance of these results to the induction of mesovarian leiomyomas in rats by noncardioselective beta adrenoceptor agonists and antagonists with ISA is discussed.