Fertilization and early embryology: A cytosolic sperm factor triggers calcium oscillations and membrane hyperpolarizations in human oocytes

Abstract

Intracellular free Ca2+ concentrations ([Ca2+]₁) and membrane potentials were measured in mature human oocytes. Injection of cytosolic extracts made from human or hamster spermatozoa triggered oscillations in [Ca2+]₁ in human oocytes similar to those described previously in mouse and hamster oocytes. In contrast, injection of carrier buffer caused no [Ca2+]₁ increase and injection of Ca2+-containing solutions caused only a single [Ca2+]₁ transient. Injection of human sperm extracts also triggered [Ca2+]₁ oscillations in mature mouse oocytes. The [Ca2+]₁ oscillations in human oocytes were accompained by hyperpolarizations in membrane potential. Perfusing oocytes with the sulphydry1 reagent thimerosal also caused oscillations in the free [Ca2+]₁ concentration simultaneously with membrane potential hyperpolarizations. These data suggest that human oocytes possess a similar mechanism for generating [Ca2+]₁ oscillations to those described in other mammalian oocytes and a membrane potential response similar to that seen previously specifically in hamster oocytes. The data also support the view that human oocytes are activated at fertilization by diffusion of a protein from the spermatozoa into the ooplasm after gamete membrane fusion.