Ligand-Activated Signal Transduction in the 2-Cell Embryo

Platelet-activating factor (PAF) is an autocrine trophic/survival factor for the preimplantation embryo. PAF induced an increase in intracellular calcium concentration ([Ca2+]i) in the 2-cell embryo that had an absolute requirement for external calcium. L-type calcium channel blockers (diltiazem, verapamil, and nimodipine) significantly inhibited PAF-induced Ca2+ transients, but inhibitors of P/Q type ((omega)-agatoxin; (omega)conotoxin MVIIC), N-type ((omega)-conotoxin GVIA), T-type (pimozide), and store-operated channels (SKF 96365 and econazole) did not block the transient. mRNA and protein for the (alpha)1-C subunit of L-type channels was expressed in the 2-cell embryo. The L-type calcium channel agonist (plus-minus) BAY K 8644 induced [Ca2+]i transients and, PAF and BAY K 8644 each caused mutual heterologous desensitization of each otherʹs responses. Depolarization of the embryo (75 mM KCl) induced a [Ca2+]i transient that was inhibited by diltiazem and verapamil. Whole-cell patchclamp measurements detected a voltage-gated channel (blocked by diltiazem, verapamil, and nifedipine) that was desensitized by prior responses of embryos to exogenous or embryo-derived PAF. Replacement of media Ca2+ with Mn2+ allowed Mn2+ influx to be observed directly; activation of a diltiazem-sensitive influx channel was an early response to PAF. The activation of a voltage-gated L-type calcium channel in the 2-cell embryo is required for normal signal transduction to an embryonic trophic factor.