Hormonal inhibition of adenylate cyclase. A crucial role for Mg2+

In several adenylate cyclase systems (anterior pituitary gland, human platelets, adipocytes, rat liver membranes), inhibitory hormones were shown to reduce basal adenylate cyclase activity by decreasing the "apparent affinity" of those systems for Mg2+ activation, without modifying the Vmax of the reaction. In the absence of hormones, the Mg2+ dose-activation curves were monophasic, whereas in the presence of hormones a clear heterogeneity was revealed. Therefore, inhibitory hormones induced a right-hand shift in the Mg2+ dose-activation curve. This hormonal effect was concentration-dependent. In human platelets, the inhibition of prostaglandin E1-stimulated adenylate cyclase by norepinephrine was also due to a decrease in the apparent affinity for Mg2+. In anterior pituitary gland, when Mg2+ was substituted by Mn2+, similar results were obtained. Thus, dopamine produced its inhibition by decreasing the apparent affinity for Mn2+ both under basal and vasoactive intestinal peptide-stimulated conditions. At Mg2+ or Mn2+ concentrations high enough to obtain saturation of the low apparent affinity state, hormone-induced inhibition was not observed. In anterior pituitary gland and in human platelet membranes, Na+ was not required in order to observe adenylate cyclase inhibition by catecholamines. In adipocytes and rat liver membranes, however, Na+ was required. In both systems, GTP was able to transform adenylate cyclase to a low Mg2+ apparent affinity state. Na+ was able to reverse (in a dose-dependent manner) the system to a high Mg2+ apparent affinity state. Once in this state, hormones were shown to inhibit adenylate cyclase activity by reverting the enzyme to a low apparent affinity state for Mg2+.