Kenneth O. Turner

Progesterone acts at the plasma membrane of human sperm

There has been increasing interest in the relationship between rapid effects of steroids and steroid-plasma membrane interaction. This laboratory has previously reported that progesterone increases human sperm cytosolic free calcium ([Ca2+]i) and thereby initiates the human sperm acrosome reaction (AR) in less than 1 min. Herein, to test whether progesterone acts at the sperm plasma membrane, progesterone 3-(O-carboxymethyl)oxime: bovine serum albumin (BSA) conjugate (free of unconjugated progesterone) was added to capacitated human sperm. Fura-2 assays were used to detect less than 1 min changes in [Ca2+]i, and indirect immunofluorescence was used to assay the AR occurring 1 min after stimulus addition. The conjugate increased [Ca2+]i and the AR (though less than did unconjugated progesterone). Enzyme immunoassays demonstrated that the concentrations of unconjugated progesterone in conjugate-treated sperm suspensions did not increase over those of control suspensions. Since the progesterone: BSA conjugate presumably does not cross the sperm plasma membrane, progesterone must act at that membrane to increase [Ca2+]i and the AR.

Stimulation of an exocytotic event, the hamster sperm acrosome reaction, by cis-unsaturated fatty acids

This cis‐unsaturated fatty acids oleic, arachidonic and cis‐vaccenic stimulated the hamster sperm acrosome reaction in vitro (an exocytotic event which occurs in the sperm head and which is essential for fertilization). The trans‐isomers of oleic and vaccenic acids did not stimulate the acrosome reaction, nor did the cis‐unsaturated fatty acids petroselenic and docosahexaenoic or the saturated fatty acids lauric, myristic or stearic. This is the first report of a stimulatory effect of cis‐unsaturated fatty acids on an exocytic event in an intact viable cell.

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(A)/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.