Activation of ionic currents in Xenopus oocytes by corticotropin-releasing peptides

Oocytes of the African frog Xenopus laevis are shown by electrophysiological methods to possess receptors for corticotropin-releasing factor (CRF), arginine-vasopressin (AVP) and cholecystokinin (CCK). Oocytes surrounded by their follicular cell envelope responded to CRF or AVP with an outward hyperpolarizing current. This current was mediated by an increased conductance of K⁺ ions. Pretreatment with the adenylate cyclase activator forskolin or with the cAMP phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) potentiated the responses to these peptides indicating that the cAMP second messenger system may mediate the responses. Oocytes stripped of the follicular envelope, which cannot generate cAMP-dependent K⁺ currents, did not respond to either CRF or AVP. Oocytes exposed to CCK responded with an inward depolarizing current. This current was carried by an increased conductance to Cl^- ions. Removal of the follicular cell layer did not affect the response to CCK. The shape, time course, and reversal potential of the Cl^- current suggest that CCK acts through the phosphatidylinositol pathway.