Ca²⁺ mobilization by the lh receptor expressed in Xenopus oocytes independent of 3′,5′-cyclic adenosine monophosphate formation: Evidence for parallel activation of two signaling pathways

The cDNAs encoding the murine LH receptor (LHR) and the human β₂-adrenoceptor (hβ₂AR) were cloned and RNAs complementary to their sense strands (cRNAs) were injected into defolliculated Xenopus oocytes. This led to expression, respectively, of LH-and isoproterenol-stimulable adenylyl cyclase activities, indicating that functionally active receptor cDNAs had been cloned. In oocytes injected with LHR cRNA, but not in control or hβ₂AR cRNA-injected oocytes, human CG and LH increased a Ca²⁺-activated Cl^- current, as measured by the two-micorelectrode voltage-clamp method. This effect was not seen with isoproterenol in control or hβ₂AR cRNA-injected oocytes, it was also not observed in response to forskolin or (Bu)₂cAMP. The response to human CG could be obtained in the absence of extracellular Ca²⁺ but was abolished by injection of EGTA, indicating that it was caused by mobilization of Ca²⁺ from intracellular stores. The response was unaffected by overnight treatment with 1 μg/ml pertussis toxin. The experiments show that a glycoprotein hormone receptor can be expressed as a functionally active molecule in Xenopus oocytes, and that the LHR has the ability of activating two separate intracellular signaling pathways: one forming the second messenger cAMP, and the other mobilizing Ca²⁺ from intracellular stores. It is proposed that the latter is secondary to a primary activation of phospholipase C by the LHR, which elevates intracellular Ca²⁺ via intermediary elevation of inositol phosphates, presumably (1,4,5)inositol trisphosphate.