DIADENOSINE POLYPHOSPHATE-ACTIVATED INWARD AND OUTWARD CURRENTS IN FOLLICULAR OOCYTES OF XENOPUS-LAEVIS

Ionic currents evoked by alpha,omega-adenine dinucleotides (Ap(X)A; X= 2-6) in follicular oocytes of Xenopus laevis were studied under voltag e-clamp conditions. Dinucleotides evoked inward and outward currents i n Xenopus oocytes by activating native P1 and P2 purinoceptors known t o be present on the follicle cell monolayer enveloping oocytes. Inward currents were mediated by a suramin-sensitive P2 purinoceptor which s howed an agonist potency order (at 10 mu M): Ap(4)A>ATP>Ap(3)A>>Ap(5)A , while Ap(2)A and Ap(6)A were inactive. Outward currents were mediate d by a novel theophylline-sensitive P1 purinoceptor which showed an ag onist potency order (at 10 mu M): Ap(2)A>ATP>>Ap(4)A=Ap(5)A=Ap(6)A>Ap( 3)A. Chromatographic analysis confirmed ectonucleotidase activity at t he follicle cell layer of oocytes but at a very low rate of dinucleoti de cleavage, indicating that currents evoked by dinucleotides resulted fi om a direct activation of oocyte P1 and P2 purinoceptors and not t hrough their breakdown to ATP, ADP and AMP. There was no evidence for specific receptors (i.e., P4 purinoceptors) for diadenosine polyphosph ates in Xenopus oocytes.