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-releasing hormone (TRH) elicits a biphasic response. First, a release of intracellularly stored Ca2+ induces a hyperpolarization of the cell. Second, a depolarization thought to be induced by a reduction of the inward-rectifying K+ current (K-IR) causes an increase in action potential frequency and a plateau-like increase in [Ca2+](i). It has been proposed that the two phases are induced by the actions of inositol 1,4,5-trisphosphate (InsP(3)) and protein kinase C (PKC), respectively, but we demonstrate here that PKC is not responsible for the second phase increase in [Ca2+](i) and suggest that the pathways diverge at the level of receptor and G protein coupling. Both phases of the TRH response were insensitive to pertussis toxin, but cholera toxin (CTX) selectively affected the second phase. After CTX pretreatment cells had a high spontaneous 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 [Ca2+](i) with no plateau phase and could only slightly reduce K-IR. These 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 kinase A activation did not block the second phase increase in [Ca2+](i) induced by TRH, suggesting that the CTX-sensitive G protein mediating the second phase of the TRH response is not G(s).