Ca2+ Response to cADPr during Maturation and Fertilization of Starfish Oocytes

Abstract

During the reinitiation of the meiotic cycle (maturation) induced by the hormone 1-methyladenine (1-MA), starfish oocytes undergo structural and biochemical changes in preparation for successful fertilization. Previous work has shown that the sensitivity of internal Ca2+ stores to InsP3 increases during maturation of the oocytes. Since Astropecten auranciacus oocytes also respond to cADPr, we have studied whether the response to cADPr also changes during maturation. We have found that the photoactivation of injected cADPr in immature oocytes immediately induces multiple patches of Ca2+ release in the cortical region. The Ca2+ signal then spreads from these initial points of increase to the entire cell. In mature oocytes, the uncaging of cADPr induces instead a single (or at most a dual) initial point of Ca2+ release, which is immediately followed by the formation of a cortical Ca2+ flash and then by the globalization of the wave and by the elevation of the fertilization envelope. External Ca2+ plays a role in the Ca2+ responses. Inhibition of L-type Ca2+ channels does not affect the initial Ca2+ release, but abolishes the cortical flash and impairs the elevation of the fertilization envelope. External Ca2+ has other effects, as shown by the irregular appearance of the surface of oocytes incubated in Ca2+-free sea water. The sequence of Ca2+ responses induced by cADPr in mature oocytes mimics those seen at fertilization, i.e., a first localized Ca2+ increase followed by a cortical flash and by the globalization of the Ca2+ signal. As in the case of maturation, L-type Ca2+ channel blockers abolish the sperm induced cortical flash.