TRH mobilizes membrane calcium in thyrotropic cells as monitored by chlortetracycline

Chlortetracycline (CTC), a probe of membrane-bound divalent cations, was used to study the action of thyrotropin-releasing hormone (TRH) in mouse pituitary thyrotropic tumor (TtT) cells in culture. Cellular fluorescence of CTC was caused by both Ca2+- and Mg2+-CTC complexes and was influenced by the concentration of these cations in the incubation medium. TRH, but not other peptides, caused a rapid, transient, and concentration-dependent decrease in the CTC fluorescence intensity; half-maximal effect occurred with 10--30 nM TRH. The decrement in fluorescence intensity caused by TRH was not due to enhanced loss of CTC from the cells. The decrease in fluorescence elicited by TRH was specific for Ca2+-CTC complexes because preincubation of the cells with 1 mM EGTA or 1 mM EDTA plus 2.05 mM Mg2+ abolished the response, whereas preincubation with 1 mM EDTA plus 2.05 mM Ca2+ permitted the usual TRH response. Antimycin A and carbonyl cyanide m-chlorophenylhydrazone decreased cellular ATP content to 37 +/- 1 and 32 +/- 1% of control, respectively, and abolished the TRH-induced decrease in CTC fluorescence. We conclude that TRH displaced Ca2+ from an energy-dependent, membrane-bound pool(s) within TtT cells and that this may be one mechanism by which the concentration of intracellular free Ca2+ is raised so that is couples stimulation by TRH to TSH secretion.