Ca2+ channels from the sea urchin sperm plasma membrane.

Abstract

Ca ~+ 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 Ca 2+ channels in this influx, but their presence has not been directly demonstrated because of the small size of this cell. Sea urchin sperm Ca 2+ channels are being studied by fusing isolated plasma membranes into planar lipid brayers. With this strategy, a Ca P+ channel has been detected with the following characteristics: (a) the channel exhibits a high mainstate conductance (3'MS) of 172 pS in 50 mM CaCIp solutions with voltage-dependent decaying to smaller conductance states at negative Era; (b) the channel is blocked by millimolar concentrations of Cd 2+, Co P+, and La 3+, which also inhibit the egg jelly-induced acrosome reaction; (c) the "yMS conductance sequence for the tested divalent cations is the following: Ba P+ > Sr P+ > Ca2+; and (d) the channel discriminates poorly for divalent over monovalent cations (Pc~/PN~ = 5.9). The sperm Ca P+ channel 3'MS rectifies in symmetrical 10 mM CaC12, having a maximal slope conductance value of 94 pS at + 100 mV applied to the c/s side of the bilayer. Under these conditions, a different single-channel activity of lesser conductance became apparent above the "}'MS current at positive membrane potentials. Also in 10 mM Ca P+ solutions, Mg ~+ permeates through the main channel when added to the c/s side with a Pc~/PMg = 2.9, while it blocks when added to the trans side. In 50 mM Ca P+ solutions, the ")'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.s = -72 mV and an apparent gating charge value of 3.9. These results describe a novel CaP+-selective channel, and suggest that the main channel works as a single multipore assembly.

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

“…The channel poorly discriminated for divalent over monovalent cations (P Ca /P Na ϭ 6). La 3ϩ , Co 2ϩ , and Cd 2ϩ which inhibit the EJ-induced AR at millimolar concentrations blocked the channel with a similar potency (325). These findings suggested that this channel could participate in the AR (325).…”

Sperm channel diversity and functional multiplicity

“…Notably, a high-conductance, Ca 2þ -permeable channel, whose properties resemble some of those displayed by the PC1-PC2 channels, has been recorded in BLMs containing sea urchin or mouse sperm plasma membranes (Lievano et al 1990, Beltran et al 1994. The sea urchin sperm channel discriminates poorly between monovalent and divalent cations (PCa/PNa , 5; Lievano et al 1990). This evidence suggests that REJ and PC proteins may be involved in the ion fluxes that follow binding of FSP and lead to the AR.…”

“…Indeed, sea urchin sperm possess a PC2 homolog (suPC2) associated with suREJ3 in the sperm plasma membrane in a thin band overlying the acrosomal vesicle (Mengerink et al 2002, Neill & Vacquier 2004. Notably, a high-conductance, Ca 2þ -permeable channel, whose properties resemble some of those displayed by the PC1-PC2 channels, has been recorded in BLMs containing sea urchin or mouse sperm plasma membranes (Lievano et al 1990, Beltran et al 1994. The sea urchin sperm channel discriminates poorly between monovalent and divalent cations (PCa/PNa , 5; Lievano et al 1990).…”

“…In L. pictus sperm, binding of FSP induces a fast K þ -dependent hyperpolarization (Gonzalez-Martinez & Darszon 1987) which precedes and probably leads to the activation of a Ca 2þ -dependent Na þ /H þ exchange that in turn increases pH i (Gonzalez-Martinez et al 1992). This hyperpolarization could remove inactivation from Ca v channels (Gonzalez-Martinez & Darszon 1987, Lievano et al 1990.…”

Perforated Patch Clamp in Non-Neuronal Cells, the Model of Mammalian Sperm Cells

“…The plasma membrane of spermatogonial cells is functionally and structurally similar to the membrane of mature sperm, but there are many differences in types and cell distribution of ion channels (Serrano et al, 1999). The third alternative is the reconstitution of ion channels in artificial lipid bilayers; however, this technique removes the ion-channels from their natural lipidic environment and therefore, their biophysical properties are not precisely the same as in the cell plasma membrane (Lievano et al, 1990). The probability of obtaining a high resistance seal of the sperm head by microscopy using perforated patch clamp is about 45%; this is 15 times greater than the probability reported in studies using conventional "patch-clamp".…”