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 μM) and phorbol-12,13-dibutyrate (1 μM) 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 μM), tamoxifen (10 μM), and sphinganine (5 μM) inhibited PTH release at low Ca²⁺_e (0.1 and 0.2 μM) and decreased cell recruitment over the physiological range of Ca²⁺_e. The nonstimulatory phorbol esters 4α-phorbol-12,13-didecanoate (1 μM) and phorbol-13-monoacetate (1 μM) 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 μM) 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.