Thyrotropin-releasing hormone mobilizes Ca2+ from endoplasmic reticulum and mitochondria of GH3 pituitary cells: characterization of cellular Ca2+ pools by a method based on digitonin permeabilization

Abstract

Treatment of 45Ca2+-loaded GH3 pituitary cells with various concentrations of digitonin revealed discrete pools (I and II) of cellular 45Ca2+ defined by differing detergent sensitivities. Markers for cytosol and intracellular organelles indicated that the two 45Ca2+ pools were correlated with the two major cellular Ca2+-sequestering organelles, endoplasmic reticulum (I) and mitochondria (II). Studies with various inhibitors were consistent with these assignments. Mitochondrial uncouplers preferentially depleted 45Ca2+ pool II while trifluoperazine selectively depleted 45Ca2+ pool I. Control experiments indicated that translocation of in situ organellar 45Ca2+ during and after permeabilization was negligible. We used the digitonin-permeabilization method to examine the effect of thyrotropin-releasing hormone (TRH) treatment on intracellular Ca2+ pools of GH3 pituitary cells. TRH was found to rapidly deplete both endoplasmic reticulum and mitochondrial exchangeable Ca2+ by 25-30%. The 45Ca2+ loss from both pools was maximal by 1 min after TRH addition and was followed by a recovery phase; mitochondrial 45Ca2+ content returned to control levels by 30 min. Previous treatment of cells with the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxy-phenylhydrazone blocked TRH-induced 45Ca2+ efflux from mitochondria, while previous treatment with valinomycin, an agent that depleted both 45Ca2+ pools, blocked any additional effect of TRH on these pools. We conclude that TRH rapidly promotes a net loss of exchangeable Ca2+ from GH3 cells as a result of hormone-induced mobilization of Ca2+ from endoplasmic reticulum and mitochondria.