Different roles of IP 4 and IP 3 in the signal pathway coupled to the TRH receptor in microinjected Xenopus oocytes

Abstract

Different effects of injected IP 4 and IP 3, on membrane Cl − currents were observed in Xenopus oocytes previously microinjected with GH 3, cell poly(A) + RNA. IP 3 induced a biphasic current response similar to that induced by TRH, whereas IP 4 regularly evoked a monophasic current response consisting of a prolonged inward current with superimposed oscillations. Membrane currents elicited by TRH were enhanced by prior injection of IP 4, but not of IP 3. In contrast to IP 3-induced membrane currents, those evoked by IP 4 were independent of injection depth. Phorbol ester led to enhancement of the initial current after injection of IP 3, whereas the current oscillations were greater following injection of IP 4. The results indicate that, in Xenopus oocytes, IP 4 plays a role different from that of IP 3 within the signal transduction pathway initiated by ligand stimulation of a neuropeptide receptor.