Beta-adrenergic receptors and adenylate cyclase activity in rat reticulocytes and mature erythrocytes

Abstract

The adenylate cyclase activity in membranes from rat reticulocytes exhibits a markedly greater stimulation (relative to basal activity) by catecholamines than does the activity in membranes from mature erythrocytes; this increased responsiveness of reticulocytes to catecholamines is not observed for other hormonal adenylate cyclase activators such as prostaglandins E 1and E 2 and human growth hormone. From the order of potency ofcatecholamine-stimulated adenylate cyclase activity (isoproterenol > epinephrine > norepinephrine), and from the selective action of butoxamine compared with practolol, both in inhibiting isoproterenol-stimulated adenylate cyclase activity and in blocking the binding to membranes of 125I-labeled hydroxybenzylpindolol, it is concluded that both reticulocytes and mature rat erythrocytes possess adrenergic receptors of the beta-2 type. Practolol acts as a partial agonist in both cell types, stimulating adenylate cyclase activity. The guanine nucleotides, GTP and 5′-guanylylimidodiphosphate (GMP-PNP), affect the adenylate cyclase activity in membranes from both cell types. GTP stimulates basal activity 2-fold, but does not affect stimulation by fluoride. In reticulocyte membranes, GTP enhances isoproterenol stimulation of adenylate cyclase to a level approximately 2-fold greater than that achieved by fluoride; in contrast, in mature erythrocyte membranes, isoproterenol stimulation of adenylate cyclase in the presence of GTP is equivalent to that of fluoride. In reticulocyte membranes, the stimulation of adenylate cyclase by GMP-PNP alone is equivalent to the stimulation in the presence of either isoproterenol or fluoride. In contrast, in membranes from mature erythrocytes the stimulation by GMP-PNP alone is equivalent to stimulation by fluoride, but is markedly lower than that obtained in the simultaneous presence of isoproterenol. It is concluded that whereas the beta-2 nature of the catecholamine receptor on rat erythrocytes remains unaltered as the reticulocyte matures, there is a pronounced change during maturation in the coupling of adrenergic-receptor occupation to adenylate cyclase activation, as indicated by the effects of guanine nucleotides.