Ca2+ channels from the sea urchin sperm plasma membrane.

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Abstract
Ca2+ influx across the sea urchin sperm plasma membrane is a nececessary 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 bilayers. With this strategy, a Ca2+ channel has been detected with the following characteristics: (a) the channel exhibits a high mainstate conductance (.gamma.MS) of 172 pS in 50 mM CaCl2 solutions with voltage-dependent decaying to smaller conductance states at negative Em; (b) the channel is blocked by millimolar concentrations of Cd2+, Co2+, and La3+, which also inhbit the egg jelly-induced acrosome reaction; (c) the .gamma.MS 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 .gamma.MS 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 .gamma.MS 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, which it blocks when added to the trans side. In 50 mM Ca2+ solutions, the .gamma.MS 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.