The uptake of adenosine cyclic 3', 5'-inonophosphate (cAMP) and stimulation of membrane-associated protein kinase in mature human erythrocytes were investigated. cAMP transport across the membrane was temperature dependent, and cAMP binding to the isolated membrane had less temperature dependence. More than 99% of the [3H]-cAMP taken up by erythrocytes was nonmembrane bound. Maximal stimulation of membrane protein kinase and maximal occupancy of membrane cAMP binding sites by extracellular cAMP occurred at 30 °C within 30 min after initiation of the incubation of erythrocytes with cAMP. The concentration of extracellular cAMP that gave half-maximal stimulation of membrane protein kinase was 5.4 X 1CU1 M, a value consistent with the concentrations of cAMP (5.2 X 10“4 M) found to occupy half-maximally the membrane cAMP binding sites in erythrocytes. Extracellular cAMP and to a lesser extent guanosine cyclic S'.S'-monophosphate and inosine cyclic S', '-monophosphate stimulated membrane protein kinase in erythrocytes. The cAMP uptake by human erythrocytes as well as cAMP binding to membranes in the erythrocyte was blocked by an inhibitor [4, 4'-bis(isothiocyano)stilbene-2, 2-disulfonate] of the anion channel. These studies indicate that cAMP can be transported across membranes into human erythrocytes and can bind to membranes to activate membrane protein kinase. It appears that there is a shared transport channel for cAMP and anion transport.
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