PROGESTERONE INITIATION OF THE HUMAN SPERM ACROSOME REACTION - THE OBLIGATORY INCREASE IN INTRACELLULAR CALCIUM IS INDEPENDENT OF THE CHLORIDE REQUIREMENT

The progesterone-initiated human sperm acrosome reaction (AR) requires a rise in intracellular Ca2+ ([Ca2+](i)), extracellular Cl- and apparently increased Cl- flux through a unique steroid receptor/Cl- channel resembling but not identical to a GABA(A)/Cl- channel complex. The present study uses fura-2 loaded human sperm, GABA,/Cl- channel blockers (picrotoxin and pregnenolone sulfate) and Cl--containing and Cl--deficient media to determine whether the progesterone-mediated increase in [Ca2+](i) is dependent on the Cl- requirement. There was no significant difference between the progesterone-mediated increases of [Ca2+](i) obtained in Cl--containing and Cl--deficient media. Picrotoxin did not significantly inhibit the progesterone-mediated increase in [Ca2+](i), and pregnenolone sulfate increased [Ca2+](i) to the same extent as progesterone. These results strongly suggest that the increase in [Ca2+](i) essential to the AR is independent of the AR Cl- requirement and could be explained by the existence of two different sperm plasma membrane progesterone receptors.