A cytosolic sperm factor triggers calcium oscillations in rat hepatocytes

Previously it has been shown that injecting a cytosolic sperm protein factor into mammalian eggs induces sustained repetitive transients of cytosolic free Ca²⁺ ([Ca²⁺]_i), or [Ca²⁺]_i oscillations [Swann (1990) Development 110, 1295-1302]. These sperm-factor (SF)-induced [Ca²⁺]_i oscillations are similar to those seen at fertilization. Here we demonstrate that injecting the same cytosolic extracts of mammalian sperm into single rat hepatocytes induces a series of [Ca²⁺]_i oscillations, as measured by aequorin luminescence. SF injection into hepatocytes induced [Ca²⁺]_i oscillations that were of longer duration, lower frequency and greater amplitude than those seen with the Ins(1,4,5)P₃-generating agonist phenylephrine. The SF-induced [Ca²⁺]_i responses appeared to be due to internal release of Ca²⁺, since transients could occur in Ca²⁺-free media. Addition of the phorbol ester phorbol 12,13-dibutyrate (PDBu) at low concentrations did not inhibit the SF-induced [Ca²⁺]_i oscillations; high concentrations of PDBu led to a sustained increase in [Ca²⁺]_i concentrations. These data demonstrate that sperm contain a protein factor capable of inducing a characteristic series of [Ca²⁺]_i oscillations in a somatic cell, the hepatocyte. Along with previous observations in dorsal root ganglion neurons, the data suggest a widespread efficacy of the factor in triggering Ca²⁺ oscillations.