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Abstract
Epinephrine stimulated adenylate cyclase in turkey erythrocyte ghosts is inhibited by calcium. The inhibition of adenylate cyclase is not apparent when intact erythrocytes are incubated with calcium and epinephrine. However, in the presence of the specific cation ionophore A 23187 and 5 m m Ca 2+, a 90% inhibition of epinephrine stimulated 3′,5′-adenosine monophosphate formation is found. The effect of catecholamines on calcium transport in the intact turkey erythrocyte was studied. Epinephrine causes a small but significant increase in Ca 2+ efflux. This effect is inhibited by propranolol. No effect of epinephrine on Ca 2+ uptake was observed. However, a 22% increase in Ca 2+ uptake in the presence of propranolol could be detected. The propranolol effect was found to possess high statistical significance ( p < .001). The absence of an epinephrine effect on influx probably reflects the presence of endogenous catecholamines in the control samples. It is proposed that the activation of adenylate cyclase by catecholamines occurs in two phases. The first phase is the increase of net Ca 2+ efflux from a crucial Ca 2+ pool, thus removing Ca 2+ from its inhibitory sites on the adenylate cyclase complex. The second phase is the activation of the deinhibited adenylate cyclase by the hormone.
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