Divergent effects of forskolin on 3',5' cyclic adenosine monophosphate production and parathyroid hormone secretion.

Forskolin, a diterpene which directly stimulates adenylate cyclase, markedly stimulated cAMP production in intact rat parathyroid glands and dispersed cells from hyperplastic and adenomatous human parathyroid tissues. Stimulation of cAMP production in human parathyroid adenomas occurred as early as 2 min and continued for at least 2 h; furthermore, a dose-response relationship was observed, with a maximal 80-fold cAMP response occurring at 100 microM forskolin. When PTH secretion by rat or human parathyroid tissues was studied at low (0.5 mM) and high (2.5 mM) extracellular Ca2+ in either the presence or absence of forskolin, no significant stimulation by forskolin was observed at 15 min, 1 h, and 2 h. When 10 human parathyroid specimens were studied with varying concentrations of forskolin at 1 mM Ca2+, 6 failed to show stimulation of PTH secretion and 4 showed modest but detectable increases in PTH that did not appear dose-related. We conclude that (1) at low and high Ca2+ levels, marked stimulation of cAMP production by forskolin can occur without a corresponding increase in PTH secretion; (2) inhibition of PTH secretion by high extracellular Ca2+ levels continues unchanged despite stimulation of cAMP production by forskolin; and (3) at intermediate Ca2+ levels (1.0 mM), PTH secretion is affected either minimally or not at all by forskolin in human hyperparathyroid tissue preparations. The marked stimulation of parathyroid adenylate cyclase by forskolin without concomitant increases in PTH secretion in the majority of tissues suggests that the level of cAMP production is not a primary or sufficient determinant of hormone secretion.