Effects of inorganic ions on rabbit ciliary process adenylate cyclase

The interaction of several inorganic ions (Mn2+, Ca2+, VO3-, and F-) with adenylate cyclase in the albino rabbit ciliary process particulate fraction was studied. Effects of these substances were determined on three activity states of the enzyme (basal, Gs-protein stimulated activity via isoproterenol activation of beta-adrenergic receptors, forskolin-activated catalytic unit) in the presence of 3 mM Mg2+, 1 mM EDTA and 0.2 mM EGTA. A different pattern of effects was found for each of the four ions. Addition of Mn2+ (2 mM) increased all three responses; basal by up to ten-fold, isoproterenol and forskolin by four to five-fold. Added Ca2+ (1 mM) increased basal by two to three-fold, but inhibited isoproterenol and forskolin responses by 25-50%. Added vanadate (1-10 mM) increased basal by two to four-fold, had no effect on the isoproterenol response, but doubled the forskolin response at 3 mM. Added F- (10 mM) increased basal by 30-40-fold, decreased the isoproterenol response and potentiated the forskolin response. The response to F- which directly activates G-proteins was much greater than that of non-hydrolysable GTP analogs, which also directly activate G-proteins. The results suggest that more than one type of adenylate cyclase and/or several modes of regulation of adenylate cyclase with different ionic requirements may be present in ciliary process membranes.