Activation of coronary arterial guanylate cyclase by nitric oxide,nitroprusside, and nitrosoguanidine—Inhibition by calcium,lanthanum, and other cations,enhancement by thiols

Although reports that certain vasodilate s activate soluble guanylate cyclase, especially in the presence of thiols, and elevate cyclic GMP levels in smooth muscle suggest that cyclic GMP is involved in vascular smooth muscle relaxation, earlier reports that Ca²⁺ activates guanylate cyclase and that Ca²⁺ -dependent contractile agents elevate cyclic GMP levels are seemingly at odds with this hypothesis. The objective of this study was to examine the effects of Ca²⁺ related cations, and thiols on bovine coronary arterial soluble guanylate cyclase. Guanylate cyclase activity was detected in the presence of Mg²⁺ or Mn²⁺ but not of other cations. Basal activity was greater in the presence of Mn²⁺ than of Mg²⁺. Activity of guanylate cyclase stimulated by nitroprusside, nitric oxide, or nitrosoguanidine, however, was greater with Mg²⁺, although the requirement of activated enzyme for Mn²⁺ was reduced about 10-fold. Ca²⁺ markedly inhibited guanylate cyclase activation in the presence of Mg²⁺ but not of Mn²⁺. La²⁺ inhibited enzyme activation in the presence of Mg²⁺ or Mn²⁺. Neither Ca²⁺ nor La³⁺ altered basal enzymatic activity. Results that were qualitatively similar to those indicated above were observed with partially purified, heme-free, coronary arterial soluble guanylate cyclase. Nitric oxide and nitroso compounds activated partially purified enzyme, and thiols enhanced enzyme activation by nitroprusside and nitrosoguanidine without appreciably altering basal activity. Irreversible sulfhydryl binding agents such as ethacrynic acid and gold inhibited both basal and activated guanylate cyclase. These results suggest that changes in intracellular concentrations of free Ca²⁺ and sulfhydryl groups could influence the rate of formation of cyclic GMP by vasodilators and that this, in turn, could alter smooth muscle tone.