Hormone-induced loss of surface membrane during maturation of starfish oocytes: differential effects on potassium and calcium channels.

Prior to fertilization, starfish oocytes undergo meiotic maturation, triggered by the hormone 1-methyladenine (1-MA). Maturation involves a variety of complex biochemical, morphological, and electrical changes, many of which are similar to those caused by progesterone in vertebrates. Using voltage-clamp and ultrastructural techniques to study maturation in starfish, we have discovered a novel process by which 1-MA alters the electrical properties of the oocyte. The surface area of the oocyte decreases by more than 50% during the first hour of maturation, due to the elimination of microvilli, but the calcium and potassium currents present are affected differently by the loss of membrane. The amplitudes of both the transient K current ("A-current") and the inwardly rectifying K current decrease, following the time course of the decrease in surface area, while the Ca current amplitude remains virtually unaffected, and may even increase in some oocytes. The kinetics of the currents do not change. This selective removal of K channels results in a larger and more rapidly rising action potential in the mature egg, which may aid in the fast block to polyspermy. The differential accessibility of various ion channels to mechanisms of membrane removal and insertion may play an important role in the development of excitable cells.