DIFFERENT G-PROTEINS ARE INVOLVED IN THE BIPHASIC RESPONSE OF CLONAL RAT PITUITARY-CELLS TO THYROTROPIN-RELEASING-HORMONE

In rat anterior pituitary tumour cells (GH(3)/B-6) thyrotropin-releasi ng hormone (TRH) elicits a biphasic response. First, a release of intr acellularly stored Ca2+ induces a hyperpolarization of the cell. Secon d, a depolarization thought to be induced by a reduction of the inward -rectifying K+ current (K-IR) causes an increase in action potential f requency and a plateau-like increase in [Ca2+](i). It has been propose d that the two phases are induced by the actions of inositol 1,4,5-tri sphosphate (InsP(3)) and protein kinase C (PKC), respectively, but we demonstrate here that PKC is not responsible for the second phase incr ease in [Ca2+](i) and suggest that the pathways diverge at the level o f receptor and G protein coupling. Both phases of the TRH response wer e insensitive to pertussis toxin, but cholera toxin (CTX) selectively affected the second phase. After CTX pretreatment cells had a high spo ntaneous spiking frequency and smaller K-IR amplitude. In these cells TRH failed to increase the action potential frequency after the first phase hyperpolarization, elicited only a transient peak increase in [C a2+](i) with no plateau phase and could only slightly reduce K-IR. The se effects of CTX are not mediated by its ability to increase cAMP via activation of G(s), as increased cAMP levels neither inhibit K-IR nor prevent its reduction by TRH. In addition, inhibition of protein kina se A activation did not block the second phase increase in [Ca2+](i) i nduced by TRH, suggesting that the CTX-sensitive G protein mediating t he second phase of the TRH response is not G(s).