Sperm Factor Initiates Capacitance and Conductance Changes in Mouse Eggs That Are More Similar to Fertilization Than IP3- or Ca2+-induced Changes

Abstract

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 ∼2 pF step increase in egg membrane capacitance and a minor increase in conductance with no change in [Ca2+]i at sperm fusion. This was followed ∼1 min later by a rise in [Ca2+]i that led to larger changes in capacitance and conductance. 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-μM peak [Ca2+]i favored capacitance decrease, while higher [Ca2+]i favored capacitance increase. The magnitude of accompanying conductance increases was variable and did not correlate well with peak [Ca2+]i. The intracellular introduction of porcine sperm factor reproduced the postfusion capacitance and conductance changes with a similar [Ca2+]i dependence. Raising [Ca2+]i by the intracellular introduction of IP3 initiated fertilization-like capacitance changes, but the conductance changes were 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 turnover of the mouse egg surface membrane is sensitive to [Ca2+]i and that sperm and the active component of sperm factor may be doing more than initiating the IP3-mediated release of intracellular Ca2+.