CA-2+ CHANNELS FROM THE SEA-URCHIN SPERM PLASMA-MEMBRANE

Ca2+ influx across the sea urchin sperm plasma membrane is a necessary step during the egg jelly-induced acrosome reaction. There is pharmacological evidence for the involvement of Ca2+ channels in this influx, but their presence has not been directly demonstrated because of the small size of this cell. Sea urchin sperm Ca2+ channels are being studied by fusing isolated plasma membranes into planar lipid brayers. With this strategy, a Ca2+ channel has been detected with the following characteristics: (a) the channel exhibits a high mainstate conductance (γMS) of 172 pS in 50 mM CaCl2 solutions with voltage-dependent decaying to smaller conductance states at negative Era; (b) the channel is blocked by millimolar concentrations of Cd2+, Co2+, and La3+, which also inhibit the egg jelly-induced acrosome reaction; (c) the γMS conductance sequence for the tested divalent cations is the following: Ba2+ > Sr2+ > Ca2+; and (d) the channel discriminates poorly for divalent over monovalent cations (Pca/PNa = 5.9). The sperm Ca2+ channel γMS rectifies in symmetrical 10 mM CaCl2, having a maximal slope conductance value of 94 pS at + 100 mV applied to the cis side of the bilayer. Under these conditions, a different single-channel activity of lesser conductance became apparent above the γMS current at positive membrane potentials. Also in 10 mM Ca2+ solutions, Mg2+ permeates through the main channel when added to the cis side with a Pca/PMg = 2.9, while it blocks when added to the trans side. In 50 mM Ca2+ solutions, the γMS open probability has values of 1.0 at voltages more positive than -40 mV and decreases at more negatives potentials, following a Boltzmann function with an E0.5 = -72 mV and an apparent gating charge value of 3.9. These results describe a novel Ca2+-selective channel, and suggest that the main channel works as a single multipore assembly. © 1990, Rockefeller University Press., All rights reserved.