Calcium and adenosine affect human sperm adenylate cyclase activity

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Abstract
The adenylate cyclase activity of human ejaculated spermatozoa in broken‐cell preparations was investigated. In the presence of 5 mM metal cations and 0.1 mM ATP, the relative enzyme activity with Mn2+, Ca2+, Mg2+, Ba2+ was 1.00, 0.28, 0.22, and 0.03, respectively. Added Ca2+ appeared to activate the enzyme in the presence of Mn2+ or Mg2+. The human sperm adenylate cyclase was stimulated by ∼ 2‐fold by free Ca2+ (lmM) in the presence of Mg2+ (5 mM). If the GTP analogue, 5′‐guanylyl imidophosphate (Gpp(NH)p) was added to the sperm homogenate in the presence of 200 μM ethylene‐glycol‐bis (β‐aminoethylether) N,N′‐tetraacetic acid (EGTA), the adenylate cyclase activity was increased by approximately 25%, but with the addition of 280 μM Ca2+ there was a decrease in enzyme activity. A similar response to low concentrations of Ca2+ was obtained after complementation of the sperm enzyme with the guanine nucleotide regulatory component from human erythrocytes, where the addition of 40 μM Gpp(NH)p, 200 μM EGTA, and Ca2+ (≤ 160 μM) stimulated the sperm enzyme ∼ 3–4‐fold, but the further addition of Ca2+ (280 μM, final) neutralized the stimulatory effect. The addition of adenosine, and the nucleotides 5′‐AMP and 5′‐ADP inhibited the enzyme, whereas guanine and 5′‐GMP had no appreciable effect. Human follicular fluid and serum also had little direct effect on the sperm adenylate cyclase. These resuls suggest that Ca2+ might be an important physiological modulator of the human sperm adenylate cyclase.