Effect of calcium on adenylate cyclase of rat anterior pituitary gland

The effects of free calcium (Ca2+) on adenylate cyclase (AC) activity of rat anterior pituitary gland have been investigated in order to shed some light on the interrelationships between the two second messengers (cAMP and calcium) which operate in pituitary cells. Anterior pituitary homogenates or crude membranes preparations (obtained using buffers free of divalent cation chelators) were assayed and the concentrations of Ca2+ in the assay mixture containing EGTA were calculated by a computer program for each addition of CaCl2. A wide range of Ca2+ concentrations (from 2 x 10-9 to 6 x 10-4 M) was spanned. Ca2+ was found to markedly inhibit pituitary AC and the mathematical analysis of data indicated the presence of two inhibition sites. The two K(i)s were: 1.78 ± 0.48 x 10-7 M and 2.47 ± 0.52 x 10-4 M for the homogenates and 1.71 ± 0.45 x 10-7 M and 3.15 ± 0.85 x 10-4 M for the membrane preparations. No stimulation of the enzyme could be detected at any Ca2+ concentration tested. Furthermore, because of our experimental conditions it is unlikely that there was substantial loss of endogenous calmodulin, or other calcium binding protein(s) required to mediate AC stimulation by calcium. The lack of calcium-calmodulin stimulation of pituitary AC was confirmed by experiments with anticalmodulin drugs (trifluoperazine and calmidazolium, R24571) and experiments with EGTA-washed membranes in the presence of exogenous calmodulin. At any Ca2+ concentration, the same AC activity was observed in the presence and in the absence of anticalmodulin drugs or added calmodulin. The mechanism of pituitary AC inhibition by Ca2+ was investigated focusing on a range of Ca2+ concentrations near the K(i) for the high affinity calcium site and thus similar to the intracellular Ca2+ concentrations. Ca2+ was found to act as a competitive inhibitor of the Mg2+ activation of AC and as a noncompetitive inhibitor with respect to the MgATP2-, the substrate of the enzyme. The effects of Ca2+ on AC were also studied in cell populations and tissues extremely rich in PRL-secreting cells (cell fractions purified from rat anterior pituitaries and human prolactinomas). The pattern of Ca2+ action was found to be nearly superimposable on that observed in total pituitary. The fact that Ca2+ can inhibit AC at concentrations as low as those known to be present within the cells as free ionized calcium suggests that the ion might be involved in the physiological regulation of AC in pituitary cells, by inhibiting, at least in some conditions, the activity of the enzyme. By this mechanism cells might control the intracellular level of one messenger (cAMP) by the other (calcium).