Sj. Publicover et Clr. Barratt, Voltage-operated Ca2+ channels and the acrosome reaction: which channels are present and what do they do?, HUM REPR, 14(4), 1999, pp. 873-879
Evidence from pharmacological studies suggests that induction of the acroso
me reaction of mammalian spermatozoa by solubilized zona pellucida, and pos
sibly by progesterone, is dependent upon Ca2+ influx through voltage-operat
ed Ca2+ channels. Studies on Ca2+ accumulation and membrane potential in li
gand-stimulated or artificially depolarized spermatozoa support such conclu
sion. Electrophysiological studies on rodent spermatogenic cells have revea
led the presence of a 'T' type voltage-operated Ca2+ current. This current
has pharmacological attributes consistent with those of the putative channe
l responsible for Ca2+ influx mediating the acrosome reaction. However, use
of molecular techniques to study human and rodent testis and spermatogenic
cells has detected the presence of three different voltage-operated Ca2+ c
hannel subunits. One of these (alpha l(E)) may generate T-currents, though
this is currently disputed. Voltage-operated Ca2+ channel structure and the
relationship between channel subunit expression and the characteristics of
consequent Ca2+ currents is briefly reviewed, The nature and function of T
-channel-mediated Ca2+ influx is examined in the context of the time-course
of ligand- and depolarization-induced elevation of [Ca2+](i) in mammalian
spermatozoa. It is likely that a secondary Ca2+ response (mobilization of s
tored Ca2+ or activation of a second Ca2+-influx pathway) is required for t
he acrosome reaction. Evidence for the existence and participation of vario
us candidates is discussed (including voltage-operated Ca2+ channels, which
may be functionally expressed only in mature spermatozoa), the available e
vidence favouring a secondary Ca2+-influx pathway. Immediate priorities for
future research in this area are proposed.