Receptor density determines secretory response patterns mediate by inositol lipid-derived second messengers. Comparison of thyrotropin-releasing hormone and carbamylcholine actions in thyroid-stimulating hormone-secreting mouse pituitary tumor cells.

Signal transduction by thyrotropin-releasing hormone (TRH) and carbamylcholine (CCH) in some cells is mediated by inositol lipid hydrolysis forming the second messengers, inositol 1,4,5-trisphosphate (I-1,4,5-P3) and 1,2-diacylglycerol, and causing elevation of cytoplasmic free Ca2+ concentration [( Ca2+]i). In mouse thyrotropic tumor (TtT) cells, maximally effective doses of TRH caused biphasic stimulation of thyroid-stimulating hormone (TSH) secretion, whereas CCH stimulated monophasic sustained TSH secretion without a burst phase. TRH, at maximally effective doses, stimulated a rapid marked increase in I-1,4,5-P3 which was associated with a rapid elevation of [Ca2+]i to approximately 1000 nM, whereas maximally effective doses of CCH caused little increase in I-1,4,5-P3 and no burst elevation of [Ca2+]i. Both TRH and CCH caused sustained modest (to 210-280 nM) elevations of [Ca2+]i which were inhibited by voltage-sensitive channel-blocking agents and stimulated sustained hydrolysis of inositol lipids. CCH-like responses were observed when TtT cells were stimulated by low doses of TRH. In TtT cells prepared from five tumors, the ratio of the number of TRH receptors to muscarinic receptors ranged from 10 to 40:1. Lastly, CCH-like responses were observed with maximally effective doses of TRH when the TRH receptor number was down-regulated to a level similar to that of muscarinic receptors. These data suggest that the kinetic pattern of stimulated TSH secretion caused by secretagogues that use the inositol lipid signal transduction pathway is determined by the density of receptors. In particular, there appears to be a minimal number of receptor-ligand complexes which is required to generate rapidly sufficient I-1,4,5-P3 to release intracellular Ca2+ and cause a secretory burst.