Gonadotropin-Releasing Hormone-Stimulated Sperm Binding to the Human Zona Is Mediated by a Calcium Influx1

Abstract

The mechanism by which GnRH increases sperm-zona pellucida binding in humans was investigated in this study. We tested whether GnRH increases sperm-zona binding in Ca(2+)-free medium and in the presence of Ca(2+) channel antagonists. We also examined the GnRH effect on the intracellular free Ca(2+) concentration ([Ca(2+)](i)). Sperm treatment with GnRH increased sperm-zona binding 300% but only when Ca(2+) was present in the medium. In Ca(2+)-free medium or in the presence of 400 nM nifedipine, 80 microM diltiazem, or 50 microM verapamil, GnRH did not influence sperm-zona binding. GnRH increased the [Ca(2+)](i) in the sperm in a dose-dependent manner. The maximum effect was reached with 75 nM GnRH. The GnRH-induced increase in [Ca(2+)](i) was fast and transient, from a basal [Ca(2+)](i) of 413 +/- 22 nM to a peak value of 797 +/- 24 nM. The GnRH-induced increase in [Ca(2+)](i) was entirely due to a Ca(2+) influx from the extracellular medium because the increase in [Ca(2+)](i) was blocked by the Ca(2+) chelator EGTA and by the Ca(2+) channel antagonists nifedipine and diltiazem. These antagonists, however, were not able to inhibit the progesterone-activated Ca(2+) influx. On the contrary, T-type calcium channel antagonists pimozide and mibefradil did not affect GnRH-activated Ca(2+) influx but inhibited the progesterone-activated Ca(2+) influx. Finally, the GnRH-induced Ca(2+) influx was blocked by two specific GnRH antagonists, Ac-D-Nal(1)-Cl-D-Phe(2)-3-Pyr-D-Ala(3)-Arg(5)-D-Glu(AA)(6)-GnRH and Ac-(3,4)-dehydro-Pro(1),-p-fluoro-D-Phe(2), D-Trp(3,6)-GnRH. These results suggest that GnRH increases sperm-zona binding via an elevation of [Ca(2+)](i) through T-type, voltage-operated calcium channels.

The Laminin-Induced Acrosome Reaction in Human Sperm Is Mediated by Src Kinases and the Proteasome1

“…Our finding that Ln induces the AR and Ca 2þ influx supports the hypothesis that a6b1 integrin has an outside-in signaling function in the sperm. Furthermore, the existence of L-type calcium channels has already been reported in human sperm [80][81][82]. We also found that epoxomicin, an inhibitor of proteasome activity, decreased the plateau phase of the Lninduced calcium entry into the sperm cells, while the transient initial peak was unaffected.…”

Cumulus Oophorus-associated Glycodelin-C Displaces Sperm-bound Glycodelin-A and -F and Stimulates Spermatozoa-Zona Pellucida Binding

“…A recent report demonstrated that when glycodelin binds to its receptor on T cells, CD45, it can act as a calcium-dependent lectin to bind other T cell surface glycoproteins to mediate its immunoregulatory activities (70). Gonadotropin-releasing hormone increases spermatozoa-zona binding via an elevation of intracellular calcium through T-type voltage-operated calcium channels (71). It is also possible that glycodelin-C induces signal transduction leading to the formation of more functional zona pellucida receptors.…”

Intra-Testicular Signals Regulate Germ Cell Progression and Production of Qualitatively Mature Spermatozoa in Vertebrates

“…Accordingly, GnRH antagonists inhibit, in vivo and in vitro , fertilization in rodents (14) whereas sperm binding to the human zona pellucida and calcium influx in response to GnRH and progesterone have been reported (13), providing evidence of functional role of GnRH-Rs in human SPZ.…”