title: Inositol trisphosphate mediates thyrotropin-releasing hormone mobilization of nonmitochondrial calcium in rat mammotropic pituitary cells.
abstract: Thyrotropin-releasing hormone (TRH) stimulation of prolactin secretion from GH3 cells, cloned rat pituitary tumor cells, is associated with 1) hydrolysis of phosphatidylinositol 4,5-bisphosphate to yield inositol trisphosphate (InsP3) and 2) elevation of cytoplasmic free Ca2+ concentration [( Ca2+]i), caused in part by mobilization of cellular calcium. We demonstrate, in intact cells, that TRH mobilizes calcium and, in permeabilized cells, that InsP3 releases calcium from a nonmitochondrial pool(s). In intact cells, TRH caused a loss of 16 +/- 2.7% of cell-associated 45Ca which was not inhibited by depleting the mitochondrial calcium pool with uncoupling agents. Similarly, TRH caused an elevation of [Ca2+]i from 127 +/- 6.3 nM to 375 +/- 54 nM, as monitored with Quin 2, which was not inhibited by depleting mitochondrial calcium. Saponin-permeabilized cells accumulated Ca2+ in an ATP-dependent manner into a nonmitochondrial pool, which exhibited a high affinity for Ca2+ and a small capacity, and into a mitochondrial pool which had a lower affinity for Ca2+ but was not saturated under the conditions tested. Permeabilized cells buffered free Ca2+ to 129 +/- 9.2 nM when incubated in a cytosol-like solution initially containing 200 to 1000 nM free Ca2+. InsP3, but not other inositol sugars, released calcium from the nonmitochondrial pool(s); half-maximal effect occurred at approximately 1 microM InsP3. Ca2+ release was followed by reuptake into a nonmitochondrial pool(s). These data suggest that InsP3 serves as an intracellular mediator (or second messenger) of TRH action to mobilize calcium from a nonmitochondrial pool(s) leading to an elevation of [Ca2+]i and then to prolactin secretion.
mesh_terms: D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002460:Cell Line; D002999:Clone Cells; D003593:Cytoplasm / Q000378:metabolism; D004347:Drug Interactions; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology*; D008928:Mitochondria / Q000378:metabolism; D010911:Pituitary Neoplasms / Q000378:metabolism*; D011388:Prolactin / Q000378:metabolism*; D051381:Rats; D013403:Sugar Phosphates / Q000494:pharmacology*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D014475:Uncoupling Agents / Q000494:pharmacology
chemical_list: D007295:Inositol Phosphates; D013403:Sugar Phosphates; D014475:Uncoupling Agents; D000001:Calcimycin; D013973:Thyrotropin-Releasing Hormone; D015544:Inositol 1,4,5-Trisphosphate; D011388:Prolactin; D000252:Calcium-Transporting ATPases; D002118:Calcium