Evidence for the Role of Calcium and Diacylglycerol as Dual Second Messengers in Thyrotropin-Releasing Hormone Action: Involvement of Ca+2*

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Abstract
The mechanism by which TRH acutely stimulates PRL [prolactin] secretion in GH3 pituitary cells was investigated. Studies of TRH stimulation of PRL secretion were conducted on a time scale which enables comparisons with other reported rapid effects of TRH on GH3 cells. TRH stimulation of secretion was extremely rapid in onset (.ltoreq. 10 s) and biphasic (phase I, 0-2 min; phase II, 5-60 min). The earliest (phase I) secretory response was observed to be independent of medium Ca+2 concentration or Ca+2 influx, but to be dependent on an intracellular Ca+2 pool. The phase II response to TRH depended, in part, on medium Ca+2. The phase I response to TRH could be mimicked only by agents known to influence Ca+2 translocation in GH3 cells (60 mM K+, A23187 [calcimycin], ionomycin, carbonyl cyanide p-trifluoromethoxyphenylhydrazone, and carbonyl cyanide m-chlorophenylhydrazone). These agents failed to promote sustained PRL release characteristic of phase II. The ability of TRH to rapidly stimulate PRL secretion (phase I) is correlated with its ability to rapidly promote a transient cytoplasmic Ca+2 concentration rise from an intracellular Ca+2 pool.

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