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This study was designed to determine whether HCO3- and Cl- are required for the activation of the GABAA receptor/Cl- channel (GBRC) by GABA and the subsequent capacitation of rat sperm. Spermatozoa from adult Sprague Dawley rats were incubated in four different media: modified complete rat fertilization medium (mRFM), Cl--deficient (Cl--DF) mRFM, HCO3--DF mRFM, and Cl--DF HCO3--DF mRFM, with or without GBRC agonists (GABA and progesterone) or GBRC antagonists (bicuculline and picrotoxin) for 0-6 h under capacitating conditions. Sperm capacitation and hyperactivation were assessed by chlortetracycline staining and computer-assisted sperm analysis, respectively. The results showed that GABA added to the mRFM accelerated capacitation and hyperactivation, followed by increase in the acrosome reaction, reaching maximum value after 5 h. Progesterone also accelerated sperm capacitation and hyperactivation. Bicuculline and picrotoxin, antagonists of GABA, blocked the effects of both GABA and progesterone acceleration of sperm capacitation and hyperactivation. Sperm capacitation required both Cl- and HCO3-. These results indicate that activation of GBRC may contribute to sperm capacitation and hyperactivation, and that both HCO3- and Cl- are essential. This is the first report of a close relationship between HCO3-/Cl- transport and the activation of GBRC in rat sperm capacitation and hyperactivation. [ABSTRACT FROM AUTHOR]