Role of Ecto-ATP Diphosphohydrolase in Ovarian Follicle Cells of the Starfish Asterina pectinifera

Abstract

A high activity of ecto-ATP diphosphohydrolase (ATPDase, EC 3.6.1.5) is present on the surface of ovarian follicle cells of the starfish Asterina pectinifera. It is known that follicle cells play an important role in oocyte maturation by producing 1-methyladenine (1-MeAde) as the maturation-inducing substance. This study was undertaken to determine if ecto-ATPDase is associated with 1-MeAde production. 1-MeAde production is induced by a gonad-stimulating substance (GSS) secreted from nervous tissue. It has also been shown that G-proteins and adenylyl cyclase are involved in the action of GSS in 1-MeAde production. When isolated follicle cells were incubated with the ecto-ATPDase metabolites, adenosine and AMP, a slight decrease in intracellular cyclic AMP level was observed. Adenosine also inhibited adenylyl cyclase in the presence of GTP. Pertussis toxin reversed adenosine-induced inhibition, suggesting that adenosine acts via the Gi receptor. On the other hand, GSS could induce 1-MeAde production and an increase in cyclic AMP level regardless of the absence or presence of AMP or adenosine. GSS-dependent 1-MeAde production markedly decreased after washing of follicle cells with seawater containing low concentrations of Ca2 , but the activity of ecto-ATPDase remained constant. Neither NaF nor adenylimidodiphosphate, which are inhibitors of ecto-ATPDase, had any effect on GSS-induced 1-MeAde production. Thus, it is unlikely that ecto-ATPDase is directly involved in 1-MeAde production in starfish ovarian follicle cells. Since an increase in cyclic AMP level is indispensable for 1-MeAde production, it may be possible that adenosine and AMP produced by ecto-ATPDase play a role in prevention of precocity before oocyte maturation stage.