Regulation of Guinea Pig Sperm Adenylate Cyclase by Calcium1

Abstract

The effect of various metal ions on guinea pig sperm adenylate cyclase activity was determined. In the presence of 4.5 mM free metal, relative enzyme activity with Mn²⁺, Ca²⁺, Mg²⁺, Co²⁺, Zn²⁺, and Ba²⁺ was 1.00, 0.16, 0.10, 0.10, 0.05 and 0.02, respectively. Added Ca²⁺, specifically, appeared to activate the enzyme in the presence of Mn²⁺ or Mg²⁺. The guinea pig sperm adenylate cyclase was stimulated ∼4-fold by low concentrations (µM) of free Ca²⁺ in the presence of Mg²⁺ (5 mM). This Ca²⁺-dependent increase in adenylate cyclase activity was inhibited by trifluoperazine (0.3-0.5 mM), a known inhibitor of calmodulin. Basal adenylate cyclase activity measured in the presence of Mg²⁺ (5 mM) and in the absence of Ca²⁺ was not affected by the addition of trifluoperazine (0.5 mM). Treatment of the sperm homogenate with ethylene-glycol-bis (β-aminoethyl ether) N,N'-tetra-acetic acid (EGTA) under a variety of conditions failed to completely remove the Ca²⁺-sensitivity of the particulate adenylate cyclase; such treatment also failed to remove the membrane associated calmodulin. After detergent solubilization, the sperm Mg²⁺-dependent adenylate cyclase activity was less than 0.5% of the Mn²⁺-dependent activity and was not stimulated by added Ca²⁺. These results suggest that a component of the guinea pig sperm adenylate cyclase complex is regulated by Ca²⁺. Whether the Ca²⁺-sensitive component is calmodulin remains unclear.