Steroidogenesis in isolated adrenocortical cells. Correlation with receptor-bound adenosine 3′:5′-cyclic monophosphate

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Abstract
Because several groups have recently questioned a mediating role for cyclic[c]AMP in adrenocortical steroidogenesis, the problem was analysed in more detail by measuring 3 different cAMP pools in cells isolated from decapsulated rat adrenals. Extracellular, total intracellular and bound intracellular cAMP were determined by radioimmunoassay in comparison with corticosterone production induced by low ACTH concentrations. The increase in extracellular and total intracellular cAMP with low ACTH concentrations was dependent on the presence of a phosphodiesterase inhibitor and short incubation times. Bound intracellular cAMP was less dependent on these 2 parameters. In unstimulated cells cAMP bound to its receptor represents only a small fraction of the total intracellular cAMP. After stimulation by a concentration of ACTH around the threshold for corticosterone production, an increase in bound cAMP was observed which correlated very well with steroidogenesis both temporally and with respect to ACTH concentration. This finding was complemented by measuring a concomitant decrease in free receptor sites. Full occupancy of the receptors was not necessary for maximal steroidogenesis. Binding kinetics of c[3H]AMP in concentrations equivalent to the intracellular cAMP concentration suggest the presence of at least 3 different intracellular cAMP pools. These observations are in agreement with a possible role for cAMP as a mediator of acute steroidogenesis induced by low ACTH concentrations.