Voltage-clamp study of the conductance activated at fertilization in the starfish egg.

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Abstract
Ionic currents underlying the fertilization potential of the egg of the starfish M. aequalis were studied using a 2 micro-electrode voltage clamp. Mature eggs were fertilized in vitro under voltage-clamp conditions. The fertilization current, IF, was induced by adding sperm to sea water bathing the egg. At a holding potential of -70 mV, IF was inward. It reached a peak within 2-4 min and then decayed over the next .apprx. 20 min with a rate which depended on the holding membrane potential. Instantaneous current-voltage relations measured at different times during IF were approximately linear and reversed at a potential of +6.0 .+-. 5.8 mV (mean .+-. SD, n = 11). Membrane chord conductance was highest at the peak of inward current and the declining phase of IF was due to a decrease in conductance towards the pre-fertilization level. When the membrane potential was rapidly stepped to levels more positive than .apprx. -45 mV, the conductance underlying IF decreased in a manner which depended on both membrane potential and time. The fertilization-specific conductance showed a sigmoidal activation curve between -50 and +10 mV with a half-activation level of -25 mV. Analysis of the steady-state voltage dependence indicated that at the peak of the fertilization potential (+10 to +15 mV) only 4-5% of the total available channels would be open. Current relaxations followed 1st-order kinetics and the relaxation time constant depended upon the membrane potential during the voltage pulse. The relation between the time constant and voltage was bell-shaped, decreasing at potentials more negative than -40 and more positive than 0 mV. Both the steady-state conductance-voltage relation and the kinetics of the current relaxations were consistent with a simple 2-state gating model in which the probability of a channel being open is determined by a single gating particle with an effective valency of -1.7 moving through the entire membrane field. The shifts in reversal potential with changes in external Na (at 10 mM-external K) were analyzed using the constant field expression, which gave a relative permeability of Na to K of .apprx. 0.6. Comparison of reversal potentials for IF measured in sea water in which Na was replaced with Li, Cs or Rb, yielded the permeability sequence Rb > K > Cs, Na > Li. In the starfish egg the process of fertilization induces a relatively non-selective increase in cation permeability which is controlled by both fast and slow voltage-dependent processes.