PROTEIN-KINASE-C MODULATES HORMONE-SECRETION REGULATED BY EXTRACELLULAR POLYCATIONS IN BOVINE PARATHYROID CELLS

1. The role of protein kinase C (PKC) in the regulation of parathyroid hormone (PTH) secretion was examined in dissociated bovine parathyroi d cells. 2. Increasing the concentration of extracellular Ca2+ from 0. 5 to 2 mm inhibited PTH secretion by 60-80 %. Similar depressive effec ts on secretion were obtained by increasing the concentration of extra cellular Mg2+ from 1 to 7 mm or by adding La3+ (to 40 mum). The PKC ac tivator phorbol myristate acetate (PMA) depressed PTH secretion at the lower and potentiated secretion at the higher concentrations of extra cellular Ca2+, Mg2+ or La+. The inhibitory effect of PKC on secretion correlated positively with the magnitude of the inhibitory effect elic ited by elevated extracellular Ca2+. 3. The stimulatory effects of PKC activators on PTH secretion were reversed completely and the inhibito ry effects were reversed partially by the PKC inhibitor staurosporine. Staurosporine alone did not affect secretion at low (0.5 mm) or high (2 mm) concentrations of extracellular Ca2+ but it did depress secreti on at intermediate concentrations (around 1 mm) of extracellular Ca2+. 4. The stimulatory effects of PKC activators on secretion were overco me by increases in the concentration of extracellular Ca2+ (to 5 or 10 mm) or La3+ (to 100 muM). In contrast, increasing the concentration o f extracellular Mg2+ to 11 or 19 mm did not alleviate the potentiating effects of PKC activators. The different results obtained with Ca2+ a nd Mg2+ could not be explained by their different effects on cytosolic Ca2+ and suggests that different cations can have varying degrees of efficacy to activate functional responses linked to the Ca2+ receptor on bovine parathyroid cells. 5. PTH secretion stimulated by isoprenali ne was not affected by PKC activators or staurosporine. Similarly, the inhibitory effects of extracellular ATPgammaS on secretion were unaff ected by PKC activators. These results show that PKC activators affect specifically PTH secretion regulated by extracellular polycations. 6. The stimulatory effect of PKC activators on secretion parallels its i nhibitory effects on [Ca2+]i and inositol trisphosphate formation, sho wing that PKC blunts the mechanisms associated with extracellular Ca2-induced inhibition of secretion. The specificity of these actions sug gests that PKC acts at a very early step of stimulus-secretion couplin g in parathyroid cells, specific to that used by extracellular polycat ions and perhaps involving the Ca2+ receptor.