P. Morales et al., Gonadotropin-releasing hormone-stimulated sperm binding to the human zona is mediated by a calcium influx, BIOL REPROD, 63(2), 2000, pp. 635-642
The mechanism by which GnRH increases sperm-zona pellucida binding in human
s was investigated in this study. We tested whether GnRH increases sperm-zo
na binding in Ca2+-free medium and in the presence of Ca2+ channel antagoni
sts. We also examined the GnRH effect on the intracellular free Ca2+ concen
tration ([Ca2+](i)). Sperm treatment with GnRH increased sperm-zona binding
300% but only when Ca2+ was present in the medium. In Ca2+-free medium or
in the presence of 400 nM nifedipine, 80 mu M diltiazem, or 50 mu M verapam
il, GnRH did not influence sperm-zona binding. GnRH increased the [Ca2+](i)
in the sperm in a dose-dependent manner. The maximum effect was reached wi
th 75 nM GnRH. The GnRH-induced increase in [Ca2+](i) was fast and transien
t, from a basal [Ca2+](i) of 413 +/- 22 nM to a peak value of 797 +/- 24 nM
. The GnRH-induced increase in [Ca2+](i) was entirely due to a Ca2+ influx
from the extracellular medium because the increase in [Ca2+](i) was blocked
by the Ca2+ chelator EGTA and by the Ca2+ channel antagonists nifedipine a
nd diltiazem. These antagonists, however, were not able to inhibit the prog
esterone-activated Ca2+ influx, On the contrary, T-type calcium channel ant
agonists pimozide and mibefradil did not affect GnRH-activated Ca2+ influx
but inhibited the progesterone-activated Ca2+ influx. Finally the GnRH-indu
ced Ca2+ influx was blocked by two specific GnRH antagonists, Ac-D-Nal(1)-C
l-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 in
creases sperm-zona binding via an elevation of [Ca2+](i) through T-type, vo
ltage-operated calcium channels.