Membrane potential regulates sea urchin sperm adenylylcyclase

Abstract

Adenylylcyclase (AC) from sea urchin sperm does not appear to be regulated by G proteins [Hildebrandt, J. D., Tash, J. S., Kirchick, H. J., Lipschunits, L., Secra, R. D., & Birmbaumer, L. (1985) Endocrinology 116, 1357-1366]. During sperm activation and the acrosome reaction, membrane potential changes and cAMP increases. Here we explore if membrane potential can modulate the sperm AC. Hyperpolarization of Lytechinus pictus sea urchin sperm either with valinomycin in artificial sea water (ASW) without K+ or with dilution in ASW without Na+ increased the [c-AMP] (2.2- and 5.8-fold, respectively). This increase also occurred in the absence of extracellular Ca2+ (1.9- and 3.1-fold, respectively) and was enhanced by 100 microM IBMX, a phosphodiesterase inhibitor. It has been suggested that sea urchin sperm AC is stimulated by increases in intracellular Ca2+ and intracellular pH. In ASW without Na+ and Ca2+ (0Na0CaASW), sea urchin sperm intracellular pH decreases, and intracellular Ca2+ cannot increase. Therefore, these observations taken together indicate that AC in these cells in modulated by membrane potential. Dilution of Strogylocentrotus purpuratus sperm in 0Na0CaASW hyperpolarized them and increased their cAMP levels (1.3-fold). This stimulation was enhanced by IBMX (1.6-fold). Addition of the egg peptide speract under this condition further hyperpolarized S. purpuratus sperm and synergistically increased [cAMP] above 0Na0CaASW. This stimulation became larger in the presence of IBMX (from 1.6- to 5.2-fold). Since speract cannot elevate intracellular pH or [Ca2+] in 0Na0CaASW, the increase in [cAMP] it causes must be due to sperm hyperpolarization.