Stimulatory effect of phorbol diester on relaxin release by porcine luteal cells in culture

The role of protein kinase C (PKC) activation in the modulation of secretion of the peptide 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 anti-serum and complement, a zone of hemolysis--a plaque--developed around relaxin-releasing luteal cells. The rate of development of plaques in timecourse studies was then used as an index of the rate of relaxin release. The tumor-promoting agent, phorbol 12-myristate 13-acetate (PMA) activates a phospholipid- and calcium-dependent kinase, PKC. This enzyme is present in high concentrations in porcine luteal tissue, although its physiological role(s) is unknown. We report here that PMA exerted a time- and dose-dependent stimulatory effect on relaxin release by enzyme-dispersed porcine luteal cells in culture. Maximum stimulation was achieved by 50nM PMA. In contrast, the non-PKC-activating phorbol ester, 4 alpha-phorbol-12,13-didecanoate, exerted no significant effect on the rate of relaxin in doses up to 1 microM. We further observed that a synthetic 1,2-diacyl-glycerol (1-oleoyl-2-acetyl-rac-glycerol; 125 microM) mimicked the action of PMA in stimulating relaxin secretion. These results are consistent with the view that activation of PKC provides at least one intracellular mechanism that regulates relaxin secretion by porcine luteal cells.