(1) Interaction of beta-adrenergic catecholamines with specific receptors on the cell membrane causes activation of adenylate cyclase, generation of intracellular cyclic 3',5'-AMP, and enhanced sodium and potassium transport. The effects of catecholamines on each of these functions are characteristic of beta1-type receptor interactions and are specific for 1(--)isomers. The order of effectiveness of agonists is isoproterenol greater than norepinephrine approximately equal to epinephrine. (2) exogenous cyclic 3',5'-AMP enhances sodium or potassium transport in a fashion identical to that caused by catecholamines. (3) the receptor contains an ethanolamine site that recognizes the stereospecific function of the agonist. Interaction at this site can be determined by measuring specific binding of a high specific activity-radioiodinated beta-blocker analogue (125-iodohydroxybenzylpindolol). Catecholamines bind at this site with affinities that parallel their biologic effectiveness as beta-adrenergic agonist. Beta-Blockers interact at this site with affinities that parallel their potencies as specific inhibitors of biologic function. (4) the adenylate cyclase complex contains a receptor site for guanyl nucleotides that is distinct from the catalytic site for ATP. Interaction at this site can be determined by measuring binding of radioactive guanylylimidodiphosphate (GMPPNP). GMPPNP markedly enhances the adenylate cyclase response to beta-adrenergic catecholamines with a 10-fold increase in apparent affinity of isoproterenol for activation of adenylate cyclase. Concomitantly there is a shift in Km and Vmax for ATP at the catalytic site. (5) Activation of adenylate cyclase by agonist or GMPPNP alone is reversible (GMPPNP remains bound even though the system returns to basal activity). The addition of isoproterenol plus GMPPNP, however, results in a state of enzyme activity which is apparently irreversible even though, under these conditions, the beta-adrenergic catecholamine freely dissociates from its receptor site. Under these conditions also the activated enzyme is not inhibited by the specific beta-blocker propranolol. (6) There is no evidence under adenylate cyclase assay conditions for a direct interaction of guanyl nucleotides with the beta-adrenergic receptor site nor for direct interaction of the agonist with the guanyl nucleotide site.
|Original language||English (US)|
|Number of pages||16|
|Journal||Advances in cyclic nucleotide research|
|State||Published - Jan 1 1975|
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