Membrane potential depolarization and increased intracellular pH accompany the acrosome reaction of sea urchin sperm.

The intracellular pH and membrane potential in sperm of the sea urchin Strongylocentrotus purpuratus were investigated by using fluorescent and radiolabeled probes. The weak bases [14C]methylamine, [14C]diethylamine, and 9-aminoacridine were concentrated within sperm 5-fold or greater. The weak acid [14C]dimethyloxazolidine-2,4-dione (DMO) was excluded from sperm. These data suggested that the apparent intracellular pH is acidic with respect to seawater (pH 8.0). Induction of the acrosome reaction caused efflux of the amines and uptake of DMO, consistent with an increase in apparent intracellular pH of 0.1-0.2 pH unit. The presence of an internally negative membrane potential was indicated by estimating the distribution of [3H]tetraphenylphosphonium (Ph4P+) and [14C]SCN-. From SCN- exclusion we estimated a value of about -30 mV for the nonmitochondrial membrane potential, whereas from Ph4P+ accumulation an apparent potential of -90 to -150 mV was demonstrated. The membrane potentials obtained with Ph4P+ and SCN- were dependent upon the external K+ concentration, with increasing K+ leading to depolarization. Induction of the acrosome reaction led to efflux of Ph4P+ and uptake of SCN- for an approximate depolarization of about 30 mV, primarily due to the collapse of the plasma membrane potential.