REGULATION OF PARATHYROID-HORMONE RELEASE BY PROTEIN-KINASE-C IS DEPENDENT ON EXTRACELLULAR CALCIUM IN BOVINE PARATHYROID CELLS

The purpose of this study was to evaluate regulation of PTH secretion by protein kinase-C (PKC) in adult bovine parathyroid cells. Extracellular calcium (Ca2+e) is the main physiological regulator of PTH secretion. Putative second messengers include intracellular calcium (Ca2+i), CAMP, inositol trisphosphate, and diacylglycerol (DAG). Both DAG and Ca2+i activate PKC. Certain phorbol esters mimic the effect of DAG and cause prolonged stimulation of PKC. The stimulatory phorbol esters 12-O-tetradecanoylphorbol acetate (1 muM) and phorbol-12,13-dibutyrate (1 muM) did not affect PTH secretion at low Ca2+e, but increased both individual cell secretion and recruitment of cells to secrete at high Ca2+e. The PKC inhibitors H7 (1 muM), tamoxifen (10 muM), and sphinganine (5 muM) inhibited PTH release at low Ca2+e (0.1 and 0.2 mM) and decreased cell recruitment over the physiological range of Ca2+e. The nonstimulatory phorbol esters 4alpha-phorbol-12,13-didecanoate (1 muM) and phorbol-13-monoacetate (1 muM) had no effect on PTH secretion. To assess the mechanism by which certain phorbol esters stimulated PTH secretion, in situ hybridization for PTH mRNA was performed. Phorbol-12,13-dibutyrate (1 muM) qualitatively increased steady state PTH mRNA levels compared to control values. We conclude that 1) PKC stimulation increased PTH secretion at high Ca2+e, but not at low Ca2+e; 2) PKC inhibition decreased PTH secretion at low Ca2+e; and 3) PKC stimulation increased steady state PTH mRNA levels. These data suggest that PKC plays an important regulatory role in the synthesis and secretion of PTH.