Regulation of intracellular free Ca2+ by the LH/CG receptor in an established cell line 293 expressing transfected rat receptor.

Luteinizing hormone (LH)/chorionic gonadotropin (CG) receptor (LHR) regulation of intracellular free Ca2+, [Ca2+]i, was studied by spectrofluorometric analysis and digital imaging of intracellularly trapped fura-2 fluorescence in a stable cell line 293 expressing transfected rat LHR. Exposure of the suspensions of 293-LHR cells to human CG (hCG) resulted in single, dose-dependent burst of elevated [Ca2+]i, the maximum being obtained at 0.1-1 microgram hCG/ml. The subconfluent individual 293-LHR cells responded to 1 microgram hCG/ml with either a single or oscillating [Ca2+]i bursts, the appearance of the oscillation being dependent upon the presence of extracellular Ca2+ ([Ca2+]e). 72% of the cells produced oscillation in response to hCG in the presence of [Ca2+]e and only 33% in the absence. Moreover, removal of [Ca2+]e from the incubation medium lowered the elevated basal [Ca2+]i level to or below the prestimulatory level and concomitantly damped out the oscillation, while its readdition without hCG was capable of re-elevating [Ca2+]i level and of gradually restoring the oscillation. The 293-LHR cells exposed to increasing doses of hCG also developed a dose-dependent desensitization of [Ca2+]i increase to renewed hormonal stimulation. The [Ca2+]i bursts within individual 293-LHR cells appeared in particular regions at the cell peripheries rather than distributed uniformly throughout the cytoplasm, pointing to compartmentation of the Ca2+ stores and to a local differences in receptor number in most cells.(ABSTRACT TRUNCATED AT 250 WORDS)