Bicarbonate- and Calcium-Dependent Induction of Rapid Guinea Pig Sperm Acrosome Reactions by Monovalent lonophores

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Abstract
The monovalent cationic ionophores, monensin and nigericin, stimulated rapid guinea pig sperm acrosome reactions in the presence of extracellular Na+, Ca2+ and bicarbonate (HCO3-/CO2). Extracellular K+ (mM concentrations), in contrast, was not required for the stimulatory effect of the ionophores. The effect of HCO3-/CO2 is concentration, pH and temperature dependent, with maximal responses obtained with 50 .mu.M monensin or 25 .mu.M nigericin at a concentration of 30 mM HCO3-, 2.5% CO2 and pH 7.8 at 25.degree. C. At a constant HCO3- concentration (30 mM), monensin stimulated acrosome reactions within the pH range 7.5-7.8, whereas a higher or lower pH did not support acrosome reactions at 25.degree. C. At constant extracellular pH (7.8), monensin stimulated acrosome reactions in the presence of 30 mM HCO3-, whereas higher and lower concentrations did not support acrosome reactions at 25.degree. C. The permeant anions pyruvate and lactate were essential to maintain sperm motility when treated with monensin under these conditions. NH4Cl, sodium acetate and 4.41-diisothiocyano-2, 21-disulfonic acid stilbene (DIDS; 25 .mu.M), an anion transport inhibitor, blocked the ability of monensin to stimulate acrosome reactions. Verapamil (100 .mu.M), a putative Ca2+ transport antagonist, in contrast, did not prevent the monensin-induced acrosome reactions. Physiological concentrations of Na+ were needed for monensin to stimulate acrosome reactions, but high concentrations of Mg2+ prevented the monensin stimulation. The Ca2+ ionophore A23187 [calcimycin] (75 nM) also required physiological concentrations of Na+ for the rapid induction of maximal acrosome reactions at an elevated pH (8.3) but did not require the presence of extracellular HCO3-. A monovalent ionophore-induced rise in sperm intracellular Na+ concentrations is a pre-Ca2+ entry event; it stimulates an endogenous Ca2+/Na+ exchange that allows a Ca2+ influx which in turn induces the acrosome reaction. The possible regulatory role of the sperm intracellular pH and Na+, K+-ATPase during the capacitation process under physiological conditions is discussed.