Sperm factor initiates capacitance and conductance changes in mouse eggs that are more similar to fertilization than IP3- or Ca2+-induced changes

We used patch clamp electrophysiology and concurrent imaging with the Ca2+- sensitive dye, fura-2, to study the temporal relationship between membrane capacitance and conductance and intracellular free Ca2+ concentration ([Ca2 +](i)) during mouse egg fertilization. We found an similar to2 pF step incr ease in egg membrane capacitance and a minor increase in conductance with n o change in [Ca2+](i) at sperm fusion. This was followed similar to1 min la ter by a rise in [Ca2+](i) that led to larger changes in capacitance and co nductance. The most common pattern for these later capacitance changes was an initial capacitance decrease, followed by a larger increase and eventual return to the approximate starting value. There was some variation in this pattern, and sub-muM peak [Ca2+](i) favored capacitance decrease, while hi gher [Ca2+](i) favored capacitance increase. The magnitude of accompanying conductance increases was variable and did not correlate well with peak [Ca 2+](i). The intracellular introduction of porcine sperm factor reproduced t he postfusion capacitance and conductance changes with a similar [Ca2+](i) dependence. Raising [Ca2+](i) by the intracellular introduction of IP3 init iated fertilization-like capacitance changes, but the conductance changes w ere slower to activate. Capacitance decrease could be induced when [Ca2+](i ) was increased modestly by activation of an endogenous Ca2+ current, with little effect on resting conductance. These results suggest that net turnov er of the mouse egg surface membrane is sensitive to [Ca2+](i) and that spe rm and the active component of sperm factor may be doing more than initiati ng the IP3-mediated release of intracellular Ca2+. (C) 2001 Academic Press.