Changes in intracellular calcium concentration in bovine oocytes following penetration by spermatozoa.

The pattern of changes in intracellular calcium concentration ([Ca2+]i) in bovine oocytes after penetration by spermatozoa was determined. Dynamic video imaging, using Fura-2 as a probe for intracellular free calcium, showed that activation of oocytes by spermatozoa induced multiple transient increases in [Ca2+]i with a spike interval of 24.2 +/- 7.3 min, and that the early transient increases were propagated throughout the oocytes in the form of a wave. Calcium transients at fertilization are involved in the induction of cortical granule exocytosis and the resultant block to polyspermy. The hypothesis that the inhibition of Ca2+ release from inositol trisphosphate-sensitive stores would inhibit exocytosis and increase polyspermy was tested by injecting oocytes before fertilization with heparin, a potent inhibitor of inositol trisphosphate-activated Ca2+ release. There was no significant difference after fertilization in either [Ca2+]i spikes or in polyspermy rates between control and experimental groups injected with low molecular mass heparin up to a final cytoplasmic concentration of 400 mumol l-1. We conclude that inositol trisphosphate-independent Ca2+ stores may be mobilized during the fertilization of bovine oocytes.