Ca²⁺-dependent inhibition of smooth muscle adenylate cyclase activity

We have examined the inhibitory regulation by Ca²⁺ of the adenylate cyclase activity associated with microsomes isolated from bovine aorta smooth muscle. In the presence of 2 mm MgCl₂, Ca²⁺ (0.8-100 μm) inhibited in a noncompetitive manner activation of the enzyme by GTP, Gpp[NH]p, or forskolin. In all instances the value for half-maximal inhibition was between 2 and 3 μm. In contrast, Ca²⁺ inhibited the activation by MgCl₂ (2-50 mm), alone or in the presence of GTP, in a competitive manner. The inhibition of adenylate cyclase by 10 μm Ca²⁺ was reversed in the presence of either 5 or 25 μm calmodulin or troponin C. These data show that (i) Ca²⁺, at concentrations similar to those which activate smooth muscle contraction, inhibits the stimulation of adenylate cyclase by several activators; (ii) Ca²⁺ and Mg²⁺ compete for a common site on the smooth muscle adenylate cyclase complex; and (iii) the reversal of Ca²⁺-dependent inhibition by Ca²⁺-binding proteins may be produced by chelation of the metal by these proteins.