Effect of phorbol myristate acetate on secretion of parathyroid hormone

The influence of phorbol myristate acetate (PMA), an activator of protein kinase c, on the secretion of parathyroid hormone from collagenase-dispersed bovine parathyroid cells was tested. The cells were incubated at low (0.5 mM) or high (2.0 mM) concentrations of calcium in the medium, and the hormone secreted into the medium was measured by a radioimmunoassay that recognizes both intact and C-terminal fragments of hormone. At low calcium, the secretory rate averaged 32 ± 3.8 ng · h^-1 · (10⁵ cells)^-1. The addition of 1.6 μM PMA did not affect secretion. At high calcium there was a significant suppression of secretion by 38% to 19.8 ± 3 ng · h^-1 · (10⁵ cells)^-1. The addition of 1.6 μM PMA significantly stimulated hormone secretion to 35.8 ± 8 ng · h^-1 · (10⁵ cells)^-1, a rate indistinguishable from low calcium. This stimulatory effect of PMA at high calcium was seen at PMA concentrations as low as 1.6 nM, did not occur with a biologically inactive 4α-isomer of phorbol ester, and was independent of changes in cellular adenosine 3',5'-cyclic monophosphate levels. Examination of ³²P-labeled phosphoproteins by two-dimensional gel electrophoresis revealed acidic proteins of ~ 20,000 and 100,000 Da that were phosphorylated at low and high calcium + 1.6 μM PMA but not at high calcium alone. The protein kinase c activity associated with the membrane fraction of parathyroid cells significantly decreased 40% when the cells were incubated at high vs. low calcium. The data suggest that calcium may regulate parathyroid hormone secretion through changes in protein kinase c activity of the membrane fraction of the cell and protein phosphorylation.