SEARCH · PubMed Health
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Injection of frogs with beta-adrenergic catecholamines produced a selective desensitization (loss of responsiveness) of the erythrocyte membrane adenylate cylase to subsequent stimulation in vitro by isoproterenol. Basal, prostaglandin E1- and fluoride-sensitive enzyme activities were unaffected. A 77% (p less than 0.001) decline in isoproterenol-responsive enzyme activity in the cells from the treated animals was observed with no change in the Km for isoproterenol stimulation of the enzyme (concentration causing 1/2 maximal enzyme activation). The decrease in catecholamine-sensitive adenylate cyclase was accompanied by a parallel 68% (p less than 0.001) fall in the apparent number of beta-adrenergic receptors in the erythrocyte membranes, assessed by (-) (3H)alprenolol binding studies. There was no change in the affinity of the receptor binding sites. The catecholamine-induced desensitization and fall in the beta-adrenergic receptor number were both concentration and time-dependent and displayed beta-adrenergic specificity. Isoproterenol was more potent in desensitizing cells and in lowering the receptor number than was norepinephrine. The beta-adrenergic antagonist propranolol, but not the alpha-adrenergic antagonist phentolamine, blocked the desensitizing effects of isoproterenol. Propranolol itself, however, did not cause desensitization. Cells became resensitized to the stimulatory effects of catecholamines, in association with a return in beta-receptor number, when propranolol was injected into previously desensitized animals. The changes in receptor number in membranes from desensitized and resensitized animals were also reflected in soluble receptor preparations. The protein synthesis inhibitor cycloheximide did not affect either desensitization, resensitization, or the changes in receptor number which accompanied the changes in adenylate cyclase sensitivity to catecholamines. These findings suggest that the chronic occupancy of beta-adrenergic receptors by beta-adrenergic agonists (but not antagonists) decreases the number of functional beta-adrenergic receptor binding sites and, hence, lowers the responsiveness of adenylate cylase to catecholamine stimulation. The lack of effort of cycloheximide on these regulatory effects suggests that "inactivation" and subsequent "reactivation" of the receptors, rather than changes in receptor turnover, are involved.
pA2 values against isoproterenol were determined for a number of cardioselective and noncardioselective beta adrenergic receptor blocking agents using human and guinea-pig isolated atrial and bronchial or tracheal preparations to study possible species differences. No significant differences in pA2 values for propranolol, pindolol, Ro 3-4787, acebutolol, atenolol, practolol, metoprolol, H 87/07 and tolamolol on bronchial or tracheal beta adrenergic receptors of both species were found. With respect to atrial beta adrenergic receptors, significantly lower pA2 values for human preparations, as compared to guinea-pig preparations, were found for tolamolol and CI 775. These are the only two agents in the series that derive their cardioselectivities from specific nitrogen substitutents. The different potencies of only these two compounds in antagonizing isoproterenol on atrial beta adrenergic receptors of both species suggest a difference in an accessory receptor area close to the site that interacts with the nitrogen atom of beta adrenergic agents.
The effects of the muscarinic cholinergic agonist methacholine on affinity of beta-adrenergic receptors for isoproterenol and on isoproterenol-induced stimulation of adenylate cyclase activity were assessed in canine myocardium. GTP and guanyl-5'-yl imidoiphosphate both decreased the affinity of beta-adrenergic receptors for isoproterenol without altering the affinity of these receptors for propranolol. Methacholine (10 nM to 10 micronM) antagonized the guanine nucleotide-induced reduction in beta-adrenergic receptor affinity for isoproterenol. This effect of methacholine was reversed by atropine. The choline ester had no effect on the affinity of beta-adrenergic receptors for isoproterenol in the absence of guanine nucleotides. Likewise, methacholine had no effect on the affinity of beta-adrenergic receptors for propranolol, either in the presence or absence of guanine nucleotides. Methacholine also attenuated GTP-induced activation of adenylate cyclase or isoproterenol-induced activation of the enzyme in the presence of GTP. The effects of methacholine on myocardial adenylate cyclase activity were apparent only in the presence of GTP. These effects were also reversed by atropine. The choline ester had no effect on adenylate cyclase activity in the presence of guanyl-5'-yl imidodiphosphate or NaF. The results of the present study suggest that muscarinic cholinergic agonists can regulate both beta-adrenergic receptors and adenylate cyclase by modulating the effects of GTP.
Incubation of purified frog erythrocyte membranes with beta-adrenergic agonists at 25 degrees produces relatively rapid (half-time about 10 min) desensitization (inactivation) of about 60% of the beta-adrenergic receptor binding sites. The desensitized receptors no longer bind the specific beta-adrenergic ligand (-)[3H]dihydroalprenolol. The decrease in the number of functional beta-adrenergic receptors is also manifest as a decreased ability of isoproterenol to stimulate the membrane-bound adenylate cyclase.
Antibodies raised against purified beta-adrenergic receptors themselves specifically bind beta-adrenergic ligands. Digitonin-solubilized frog (Rana pipiens) erythrocyte beta-adrenergic receptors, purified 100- to 200-fold by adsorption to an alprenolol-agarose affinity support and specifically eluted from the affinity resin by 1-100 mM (+/-)-isoproterenol, were used to immunize six rabbits. All immune sera, in contrast to preimmune sera, bound the beta-adrenergic antagonist [(3)H]Dihydroalprenolol binding activity was due to immunoglobulins. By competition studies, antibody [(3)H]dihydroalprenolol binding was found to display a specificity and stereoselectivity resembling that of the beta-adrenergic receptor, [i.e., (-)-isoproterenol > (-)-epinephrine > (-)-norepinephrine; alprenolol approximately propranolol >> phentolamine = aloperidol; and (-) isomers of both agonists and antagonists 10-100 times more potent than (+) isomers]. A portion of the [(3)H]dihydroalprenolol binding antibodies could be specifically adsorbed onto purified frog erythrocyte membranes, whereas Xenopus and human erythrocyte membranes, both of which are almost devoid of beta-adrenergic receptors, were ineffective in adsorbing [(3)H]dihydroalprenolol binding antibodies. We suggest that the likely immunogen was a beta-adrenergic receptor-isoproterenol complex and that immunization with drugs noncovalently bound to their receptors might be a means of raising antibodies to biologically active otherwise nonimmunogenic small molecules. Such antibodies, whose specificity mimics that of a receptor, should also provide useful models for the study of the structure of the receptor binding sites.
Explore the source record for details and available documents.
Protein kinases play a pivotal role in the propagation and modulation of transmembrane signaling pathways. Two major classes of receptors, G-protein-linked and tyrosine kinase receptors not only propagate signals but also are substrates for phosphorylation in response to stimulation by agonist ligands. Insulin (operating via tyrosine kinase receptors) and catecholamines (operating by G-protein-linked receptors) are counterregulatory with respect to lipid and carbohydrate metabolism. How, on a cellular level, these two distinct classes of receptors may cross-regulate each other remains controversial. In the present work we identify a novel cross-talk between members of two distinct classes of receptors, tyrosine kinase (insulin) and G-protein-linked (beta-adrenergic) receptors. Treatment of DDT1 MF-2 hamster vas deferens smooth muscle cells with insulin promoted a marked attenuation (desensitization) of beta-adrenergic receptor-mediated activation of adenylylcyclase. Measured by immune precipitation of beta 2-adrenergic receptors from cells metabolically labeled with [32P]orthophosphate, the basal state of receptor phosphorylation was increased 2-fold by insulin. Phosphoamino acid analysis revealed that for insulin-stimulated cells, the beta 2-adrenergic receptors showed increased phosphorylation on tyrosyl and decreased phosphorylation on threonyl residues. Phosphorylation of the beta-adrenergic receptor was rapid and peaked at 30 min following stimulation of cells by insulin. beta-Adrenergic receptor phosphorylation and attenuation of catecholamine-sensitive adenylylcyclase provide a biochemical basis for the counterregulatory effects of insulin upon catecholamine action.
Beta-adrenergic agonists, adenosine and prostaglandin E1 increased the level of adenosine 3':5'-monophosphate (cAMP) in glial cultures prepared from rat cerebral cortical tissue. In addition to these physiological effectors, cholera toxin also increased cAMP levels in these cultures. The accumulation of cAMP in response to each of these agen-s, including cholera toxin, was partially blocked (50--80%) by simultaneous alpha-adrenergic receptor stimulation. Basal levels of cAMP were not affected by alpha-adrenergic agonists. These results indicate that in glia, alpha-adrenergic receptors may serve to modulate the level of cAMP which normally accumulates in response to a number of neurohumoral substances. The modulatory effect of alpha-adrenergic agents does not appear to reduce cAMP accumulation by activating phosphodiesterase since the effect was not blocked by a potent inhibitor of this enzymemthe results suggest that the modulatory effect of alpha-adrenergic receptor activation results from an interaction which takes place at some point in between adenylate cyclase-associated-membrane receptors and the enzymatic degradation of cAMP.
The synthesis of two high-affinity fluorescent beta-adrenergic blockers is described: dl-N(1)-[2-hydroxy-3-(1-naphthyloxy)propyl]-N(2)-(9-acridyl)-1,2-propanediamine (9-aminoacridylpropanolol, 9-AAP) and dl-N-[2-hydroxy-3-(1-naphthyloxy)propyl]-N'-dansylethylenediamine (dansyl analogue of propranolol, DAPN). Both 9-AAP and DAPN inhibit competitively the l-epinephrine-dependent adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in turkey erythrocyte membranes without affecting the fluoride-stimulated adenylate cyclase activity. Similarly, 9-AAP and DAPN inhibit in a competitive manner the binding of [(125)I]-iodohydroxybenzylpindolol to these beta-adrenergic receptors. The two fluorescent beta-adrenergic blockers 9-AAP and DAPN probe specifically beta-adrenergic receptors in the central nervous system as well as in other organs when injected into rats. The fluorescence pattern can be monitored by fluorescence microscopy performed on cryostat slices of these organs. The appearance of the characteristic fluorescence pattern can be blocked in a stereospecific fashion by a prior injection of l-propranolol and not by a prior injection of d-propranolol. These compounds therefore offer a powerful means to map beta-adrenergic receptors in vivo. The stereospecific displacement of 9-AAP from the beta-adrenergic receptors of turkey erythrocyte membranes by l-propranolol and by l-epinephrine can be detected in vitro using front-face fluorescence. The potential use of these compounds to probe beta-receptors in vitro and in vivo is discussed.
Alpha- and beta-adrenergic receptors were studied by measuring the binding of 3H-dihydroergocryptine and 3H-dihydroalprenolol, respectively, to membranes prepared from homogenized rabbit iris--ciliary bodies. The binding of 3H-dihydroergocryptine appears to be specific for alpha-adrenergic receptors, since adrenergic agents displace this radioligand with the following order of potency: phentolamine greater than epinephrine greater than or equal to norepinephrine greater than or equal to isoproterenol = propranolol. The binding of 3H-dihydroalprenolol appears to be specific for beta-adrenergic receptors, since adrenergic agents displace this radioligand with the following order of potency: propranolol greater than or equal to isoproterenol greater than or equal to epinephrine greater than norepinephrine greater than or equal to phentolamine. Clonidine and dopamine bind to the alpha-adrenergic receptor but have little activity at the beta-adrenergic receptor. Timolol, d-isoproterenol, and dipivalyl epinephrine bind to the beta-adrenergic receptor but have little activity at the alpha-adrenergic receptor. The results demonstrate that in vitro binding assays for alpha- and beta-adrenergic receptors are useful for studying the mechanism of drug action.
Explore the source record for details and available documents.
Propranolol, alprenolol and sotalol given intraventricularly (ivc) to rats reduce depressing effects of isoprenaline (IPS) and antagonize stimulating effects of noradrenaline (NA). Phentolamine given by the same route of administration does not affect IPS action.
Isoprenaline, propranolol, alprenol and sotalol administered intraventricularly (ivc) to anaesthetized rats induce hypotension. Propanolol and alprenolol given ivc to rats pretreated with isoprenaline increase blood pressure up to the values equal to those, previous to isoprenaline administration; another isoprenaline dose acts slightly hypotensively.
Explore the source record for details and available documents.
The beta-adrenergic agonist L-isoproterenol stimulated the enzymic synthesis of phosphatidyl-N-monomethylethanolamine and phosphatidylcholine in rat reticulocyte ghosts containing the methyl donor S-adenosyl-L-methionine. The stimulation was stereospecific, dose-dependent, and inhibited by the beta-adrenergic agonist propranolol. The addition of GTP inside the resealed ghosts shifted the dose-response of phospholipid methylation by L-isoproterenol to the left by 2 orders of magnitude. Direct stimulation of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] with sodium fluoride or cholera toxin did not increase the methylation of phospholipids. At a concentration of S-adenosyl-L-methionine that stimulates synthesis of phosphatidyl-N-monomethylethanolamine, the activity of isoproterenol-sensitive adenylate cyclase was increased 2-fold without changes in the basal activity of adenylate cyclase and the number of beta-adrenergic receptors. The increase of phospholipid methylation by L-isoproterenol decreased membrane viscosity and increased translocation of methylated lipids. These findings indicate that enhancement of phospholipid methylation by L-isoproterenol decreases membrane microviscosity and thus increases lateral movement of the beta-adrenergic receptors and coupling with adenylate cyclase.
Explore the source record for details and available documents.