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Human Reproduction, Vol. 7, No. 8, pp. 1131-1135, 1992
© 1992 European Society of Human Reproduction and Embryology

other

Regulation of cyclic adenosine monophosphate synthesis in human ejaculated spermatozoa. II. The role of calcium and bicarbonate ions on the activation of adenylyl cyclase

Francisco J. Rojas1, Maria E. Bruzzone2 and Ines Moretti-Rojas

Division of Reproductive Endocrinology and Infertility, Department of Obstetrics and Gynecology, University of California Irvine, 101 The City Drive, B-41 Orange, CA 92668, USA

Correspondence: 1To whom correspondence should be addressed

In this study, we have applied our previous data describing the experimental conditions necessary for expression of cyclic adenosine monophosphate (cAMP)-synthesizing adenylyl cyclase in human ejaculated spermatozoa, to investigate the direct effects of calcium (Ca2+) and bicarbonate (HCO3–) upon activation of the enzyme in vitro. We report that the effects of Ca2+ and HCO3– were significantly dependent on the status of the enzyme activity. Thus, at a near saturating (10 mM) concentration of MnCl2 giving high enzyme activity, addition of <10 mM HCO3– did not affect adenylyl cyclase activity and higher concentrations inhibited the enzyme, with 50 mM HCO3– reducing the activity by 33%. Also, addition of <20 mM CaCI2 alone or in combination with 10 mM HCO3– did not significantly change the enzyme activity. In great contrast, enzyme activation was highly responsive to Ca2+ and HCO3– when MnCl2 was present at a concentration giving submaximal enzyme activity. Thus, at 2 mM MnCl2, adenylyl cyclase was markedly increased by CaCl2 concentrations between 10 and 100 mM. The activation was further enhanced by increasing concentrations of HCO3–, with 50 mM HCO3– giving the highest activity at 50–100 mM CaCl2. Activation by CaCl2 was also observed in the absence of added MnCl2, being significantly greater than basal activity at 10 mM CaCl2 and maximal at 100 mM CaCI2. We conclude that (i) Ca2+ and HCO3– directly activate cAMP-synthesizing adenylyl cyclase in human ejaculated spermatozoa and may behave as the first physiological messengers for the enzyme; (ii) activation of sperm adenylyl cyclase involves a well concerted equilibrium in its requirements for Ca2+ and HCO3–; and (iii) Ca2+ alone can satisfy the enzyme's need for divalent cations. Since Ca2+ and HCO3– are present in seminal plasma and the female reproductive tract, our results provide new insights on the role of these ions as physiological modulators of sperm motility and function in the human.

Key words: adenylyl cyclase/bicarbonate/calcium/cyclic AMP/human spermatozoa


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