The effect of phorbol ester on in vitro release of parathyroid hormone from abnormal human parathyroid cells.

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Intracellular events that regulate parathyroid hormone (PTH) release are not well understood. Cyclic AMP (cAMP) and cAMP-dependent protein kinases play a role in the regulation of release due to several agonists, but these factors do not fully explain PTH release that is mediated by extracellular ionized calcium (Ca++). A calcium-phospholipid-dependent (non-cAMP-dependent) protein kinase can be activated by 12-O-tetradecanoylphorbol-13-acetate (TPA). To determine whether this protein kinase regulates PTH release, we examined the effect of TPA on PTH release from human parathyroid tissue. Cell suspensions of abnormal parathyroid tissue removed at surgery were prepared by enzymatic dispersion and incubated for several hours with and without 10(-7) mol/L TPA at low and high calcium levels. In ten preparations in the absence of TPA, increasing Ca++ from 0.25 to 2.5 mmol/L reduced PTH release to an average of 39% of maximal release (range, 11% to 67%). The effect on TPA on Ca++-regulated PTH release appeared biphasic. At low (0.25 mmol/L) Ca++ level, TPA suppressed PTH release to an average of 78% of maximal release without TPA (95% confidence interval, 67% to 88%) (p less than 0.01 compared to cells incubated without TPA). At high (2.25 mmol/L) Ca++ level, TPA augmented PTH release from an average of 39% of maximal release without TPA to 62% of maximal release without TPA (95% confidence level, 48% to 78%), an average augmentation of 22% (95% confidence level, 9% to 36%) (p less than 0.01 compared with cells incubated without TPA). TPA appeared to make PTH release independent of Ca++. Both inhibitory and stimulatory effects were dose dependent. Incubations with TPA demonstrated no toxicity as judged by trypan blue dye exclusion, linearity of PTH release, and cellular incorporation of tritiated leucine. TPA had no effect on the radioimmunoassay for PTH. We conclude that a calcium/phospholipid-dependent, non-cAMP-dependent protein kinase may play a role in mediating Ca++-regulated PTH release from abnormal human parathyroid cells. Its site of action and integration with other regulatory pathways remain to be determined.