Protein-bound cyclic AMP and steroidogenesis in the adrenal gland of the rat.
Adrenocortical cyclic AMP (both total and protein-bound), and adrenal and plasma concentrations of corticosterone were measured in male rats killed at selected intervals throughout the day. The animals had previously been synchronized for 3 weeks in natural lighting. Adrenal and plasma levels of corticosterone showed similar circadian fluctuations and the onset of their ascending phases started at 13.00 h, maximum concentrations being reached at 21.00 h. On the other hand, a time-lag between the circadian variations of total cyclic AMP and protein-bound cyclic AMP could be seen in adrenocortical tissue. The onset of an increase in adrenocortical protein-bound cyclic AMP was apparent at 15.00 h and the peak occurred at 21.00 h, while total adrenocortical cyclic AMP did not begin to rise before 19.00 h and was maximal at 04.00 h. No direct link between total cyclic AMP and protein-bound cyclic AMP could be seen during the dark phase, suggesting a functional compartmentalization of cyclic AMP in the adrenal gland of the rat. While the ascending phase of the rhythm in steroidogenesis preceded the rise in total adrenocortical cyclic AMP by about 4 h an excellent synchrony between the respective patterns of corticosterone concentration and protein-bound cyclic AMP was noticed.