A SOLUBLE SPERM FACTOR GATES CA2-ACTIVATED K+ CHANNELS IN HUMAN OOCYTES()

Purpose: Our goal was to study the activation current in physiological ly competent metaphase II human oocytes, ie., nor previously exposed t o spermatozoa or aged in vitro, and, in particular to determine whethe r a soluble sperm factor triggers a fertilization current comparable t o that observed with intact spermatozoa and to characterize the curren t involved. Methods: The whole-cell voltage-clamp technique was used o n spare metaphase II oocytes, obtained with patient consent from NF pr ograms. In this configuration a soluble fraction from human spermatozo a was microinjected, and the current recorded. Results: Metaphase II h uman oocytes generate bell-shaped outward currents of 400-1000 pA (X = 706 +/- 322; n = 10), following injection of a cytosolic extract from human spermatozoa. The amount of sperm extract injected was less than 10% of the total oocyte volume and was equivalent to 1-10 spermatozoa . A similar current was generated following exposure to 20 mu M of the calcium ionophore A23187 (n = 10). The steady-state conductance of th e oocyte increased from 10 to 19.8 nS (n = 10) following injection of the sperm factor and from 5.3 to 27.7 nS following ionophore exposure. Both sperm factor- and ionophore-induced currents were reduced in amp litude when the unfertilized oocyte was preexposed to 25-75 mu M iberi otoxin (n = 8) and eliminated at a concentration of 100 mu M iberiotox in. Conclusions: The data support the hypothesis of a soluble sperm fa ctor involved in the activation of human oocytes and shows that the in itial activation response in the human oocyte is the gating of Ca2+-ac tivated K+ channels.