A cellular activator of catecholamine-sensitive adenylate cyclase in rat reticulocytes and erythrocytes: changes during reticulocyte development and effects on the beta receptor

Rat reticulocytes contain an isoproterenol-sensitive adenylate cyclase activity which is lost with maturation to erythrocytes despite no change in the density of β-adrenergic receptors. To explore this observation, a cytosol factor, previously shown to be important in the expression of catecholamine-sensitive adenylate cyclase in the reticulocyte, was compared to a cytosol factor obtained in a similar manner from mature erythrocytes. The cytosol factor from reticulocytes augmented isoproterenol-responsive adenylate cyclase activity in reticulocyte and erythrocyte membranes half-maximally at 0.7 ± 0.1 (SEM) and 1.1 ± 0.3 μg/ml, respectively. These concentrations of reticulocyte-derived cytosol factor were significantly lower (P < 0.01) than those concentrations of the factor from erythrocytes necessary to augment isoproterenol-responsive adenylate cyclase activity in reticulocyte (9.7 ± 2.3) and erythrocyte (7.5 ± 1.0) membranes. Cytosol factor from reticulocytes also caused greater total isoproterenol responsiveness than that from erythrocytes both in reticulocyte (784 ± 107 vs 525 ± 65 pmol/mg protein) and in erythrocyte membranes (54 ± 6 vs 36 ± 3); P < 0.05. Neither reticulocyte nor erythrocyte cytosol factor affected the concentration at which isoproterenol half-maximally stimulated adenylate cyclase in either set of membranes. However, the cytosol factor from reticulocytes markedly decreased the binding affinity of isoproterenol for β receptors in reticulocytes from 0.8 ± 0.2 to 6.9 ± 1.4 μm; P < 0.001. This reticulocyte factor had no significant effect on the binding affinity of isoproterenol for erythrocyte membranes. Erythrocyte factor did not change the binding affinity for isoproterenol in either reticulocyte or erythrocyte membranes.