L. Bonaccorsi et al., Low-voltage-activated calcium channels are not involved in capacitation and biological response to progesterone in human sperm, INT J ANDR, 24(6), 2001, pp. 341-351
The involvement of voltage-dependent calcium channels in the biological eff
ects exerted by progesterone (P) on human spermatozoa is still a controvers
ial issue. We have investigated the involvement of T-type calcium channels
[voltage-operated calcium channels (VOCCT)] in two biological functions of
human sperm, responsiveness to P and capacitation, by employing three diffe
rent pharmacological antagonists of VOCCT, namely mibefradil (Ro 5967), pim
ozide and amiloride. Intracellular calcium [Ca2+](i) increase in response t
o P was essentially unaffected by pre-treatment with mibefradil and pimozid
e at concentrations previously shown to prevent [Ca2+](i) increase in respo
nse to zona proteins. Amiloride could not be tested in these experiments be
cause it was found to interfere with fura-2 fluorescence. The increase in t
yrosine phosphorylation stimulated by P in a protein of about 97 kDa was un
affected by the three antagonists. Acrosome reaction (AR) induced by P was
also unaffected by mibefradil or pimozide but was significantly inhibited b
y amiloride at high concentrations (100 and 500 but not 10 mum). At 100 and
500 mum amiloride also inhibited Na/H exchanger as assessed by a fluorimet
ric method. We conclude that VOCCT are not involved in calcium increase and
AR stimulated by P in human sperm.
We next investigated the effect of the three VOCCT inhibitors on sperm capa
citation by evaluating tyrosine phosphorylation and AR in basal conditions
and in response to P. We found that the presence of pimozide and amiloride
during capacitation stimulated a higher increase of tyrosine phosphorylatio
n, whereas mibefradil was less effective. The ability of P to induce the AR
, considered an index of occurrence of capacitation, was not affected by pi
mozide and mibefradil, whereas was inhibited by amiloride at concentrations
that inhibit Na/H exchanger. In conclusion, our results do not support a m
ajor role of low-voltage-activated calcium channels in capacitation and res
ponse to P of human spermatozoa.