Regulation of relaxin release from monodispersed porcine luteal cells: effect of calcium ionophore A23187 and calcium channel blockers.

Abstract

The role of calcium ion mobilization in modulation of secretion of the ovarian protein hormone relaxin by porcine luteal cells was examined by use of a reverse hemolytic plaque assay. In this assay, luteal cells were cocultured in monolayers with protein-A-coupled ovine erythrocytes. In the presence of porcine relaxin antiserum and complement, a zone of hemolysis, a plaque, developed around relaxin-releasing luteal cells. The rate of development of plaques in time-course experiments was used in this study as an index of the rate of relaxin release. Exposure of luteal cell-containing monolayers to the calcium-mobilizing agent A23187 (40 nM to 5 microM) resulted in a dose-related increase in the rate of relaxin-plaque formation. This effect [and the influence of a stimulatory secretagogue, prostaglandin E2 (PGE2; 1 microM)] was suppressed by coculture with Co2+ (5 mM), a calcium channel blocker. These results are consistent with the view that calcium ion redistribution within porcine luteal cells forms a pathway that subserves, at least in part, the rates of basal and stimulated relaxin release in vitro. However, A23187 was equally effective in enhancing the rate of plaque formation when the monolayers were bathed in a low calcium medium (mean +/- SEM, 6.61 +/- 0.92 microM Ca2+), rather than a calcium-replete medium (1.56 +/- 0.09 mM Ca2+). Likewise, neither basal nor PGE2-stimulated (1 microM) relaxin secretion was abrogated by culture of monolayers in low calcium medium. These data suggest that the stimulatory effect of A23187 (and perhaps PGE2) arose predominantly through redistribution of calcium stored within intracellular sites in luteal cells, rather than entrance of calcium into the cell from the extracellular medium. Yet, incompatible with this interpretation, we observed that TMB-8 [8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride; a putative inhibitor of intracellular calcium redistribution] enhanced rather than blocked A23187- or PGE2-stimulated relaxin release. This result is consistent with the possibility that TMB-8 mobilized (rather than blocked) calcium in this cell system or that it acted via calcium-independent mechanisms. We conclude that calcium mobilization has the potential to act as a molecular pathway that transduces secretion of an ovarian peptide hormone, relaxin. However, the exact nature and physiological role(s) of the Ca2+ pathway in the control of relaxin secretion and the interrelationships of this mechanism with other intracellular messengers that may also modulate ovarian peptide secretion remain to be more clearly defined.