Determination of the steady-state intracellular chloride concentration in capacitated human spermatozoa

Chloride channels participate in the mammalian sperm acrosome reaction (AR). However, the mechanism by which sperm regulate intracellular Cl- concentration ([Cl-]i) is not known. The steady-state [Cl-]i has also never been reported for mammalian spermatozoa. Therefore, using chloride-sensitive fluorescent dyes, we sought to determine the steady-state [Cl-]i of capacitated human spermatozoa by a null-point measurement technique. We found that the [Cl-]i was sufficiently elevated (a conservative estimate of > or = 41 mM) such that the opening of chloride channels should result in a Cl- efflux and, hence, in depolarization. Moreover, the [Cl-]i does not remain constant under varying extracellular Cl- concentrations ([Cl-]o).