Starfish oocyte maturation: 1-Methyladenine triggers a drop of cAMP concentration related to the hormone-dependent period

Abstract

Oocyte maturation (meiosis reinitiation) in starfish is induced by the natural hormone 1-methyladenine (1-MeAde). Oocytes of Evasterias troschelii contain 0.43 pmole cyclic AMP/mg protein and 0.47 pmole cyclic GMP/mg protein. Upon stimulation by 1-MeAde the oocytes undergo a moderate (10–30%) decrease in their cAMP concentration. The concentration of cGMP remains unaltered. Oocytes treated with forskolin, an activator of adenylate cyclase, increase their cAMP concentration over 35-fold, up to 16 pmole cAMP/mg protein. When stimulated by 1-MeAde these forskolin-pretreated oocytes undergo a major (50–70%) decrease in their cAMP concentration. A similar decrease is triggered by mimetics of 1-MeAde, such as dithiothreitol, arachidonic acid (AA), and 8-hydroxyeicosatetraenoic acid (8-HETE), but not by adenine which is inactive. 1-MeAde-stimulated oocytes of Pisaster ochraceus also undergo a decrease in cAMP content, the size of which is increased by forskolin. Although a decrease in cAMP begins at sub-threshold 1-MeAde concentrations, the maximal decrease occurs at the same concentration of 1-MeAde needed for maturation induction and a further 1000-fold increase of the 1-MeAde concentration has no further effect. Upon removal of 1-MeAde, the cAMP concentration immediately increases to its original level. Sequential addition and removal of 1-MeAde triggers a sequential decrease and increase of the cAMP concentration, illustrating the continuous requirement for 1-MeAde for eliciting the decrease. Successive additions of 1-MeAde, however, do not trigger further decreases of the cAMP concentration. The temperature dependences of the cAMP concentration decrease and of the hormone-dependent period (HDP; the time of contact with 1-MeAde required for induction of maturation) are closely related. Forskolin, which increases the cAMP concentration, also increases the duration of the HDP (2.5-fold), delays the time course of protein phosphorylation burst and germinal vesicle breakdown, and inhibits AA- and 8-HETE-induced maturation. We conclude that 1-MeAde triggers a drop in cAMP concentration, which is tightly associated with the hormone-dependent period of oocyte maturation.