Receptor number determines latency and amplitude of the thyrotropin-releasing hormone response inxenopus oocytes injected with pituitary RNA

TRH evokes depolarizing membrane electrical responses in Xenopus laevisoocytes injected with RNA from pituitary cells. We have shown previously that the amplitude of this response is directly proportional to the dose of TRH and the amount of RNA injected. Herein we show that the number of TRH receptors expressed on oocytes after injection of rat pituitary (GH₃) cell RNA or mouse thyrotropic (TtT) tumor RNA determines the latency as well as the amplitude of the response. In oocytes injected with a maximally effective amount of GH₃ cell RNA, the latency of the response decreased from a maximal duration of 103 ± 16 to 10 ± 1 sec when the TRH concentration was increased from 5 to 3000 nM. When oocytes injected with different amounts of GH₃ cell RNA were stimulated with 3000 nM TRH, the latency decreased from 31 ± 4 to 11 ± 0.5 sec when the amount of RNA injected was increased from 30 to 400 ng. Specific binding of [³H]methylhistidine-TRH increased when increasing amounts of TtT poly(A)⁺ RNA was injected, and binding correlated with increased response amplitude. To show that these effects were caused by mRNA for the TRH receptor and did not depend on other mRNAs, TtT poly(A)⁺ RNA was fractionated on a sucrose gradient. Using RNA from each fraction, there was an inverse correlation between response amplitude and latency. For size-fractionated RNA, as for unfractionated RNA, there was a direct correlation between specific [³H]methylhistidine-TRH binding and response amplitude. Lastly, in oocytes, as in pituitary cells, TRH stimulates an increase in inositol tris-phosphate (IP₃), leading to elevation of cytoplasmic Ca²⁺ and stimulation of Ca²⁺-dependent processes, and injection of IP₃ and Ca²⁺ into oocytes has been shown to evoke depolarizing responses. In contrast to the response to TRH, however, there was no latency when responses in oocytes were evoked by injections of IP₃ or Ca²⁺. Hence, these data show that in oocytes TRH receptor density determines the magnitude of the response amplitude, which varies directly, and latency, which varies inversely. We suggest that the rate-limiting step in the TRH response occurs after formation of the TRH-receptor complex and before generation of IP₃.