Dihydropyridine- and voltage-sensitive Ca2+ entry in human parathyroid cells

Patch-clamp and fluorescence measurements of cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) were performed to directly detect extracellular Ca(2+) entry into cultured parathyroid cells from patients with secondary hyperparathyroidism. Cells loaded with fluo-3 AM or fluo-4 AM showed a transient increase in fluorescence (Ca(2+) transient) following 10 s exposure to 150 mm K(+) solution in the presence of millimolar concentrations of external Ca(2+). The Ca(2+) transient was completely inactivated after 30-40 s exposure to the high-K(+) solution, was reduced by dihydropyridine antagonists and was enhanced by FPL-64176, an L-type Ca(2+) channel agonist. The electrophysiological and pharmacological properties of the whole-cell Ca(2+) and Ba(2+) currents were similar to those of L-type Ca(2+) channels. The Ca(2+) transients induced by 10 s exposure to 3.0 mm extracellular Ca(2+) concentration ([Ca(2+)](o)) were inhibited by dihydropyridine antagonists and were partly inactivated following 30-40 s exposure to the high-K(+) solution. These results demonstrate, for the first time, that human parathyroid cells express L-type-like Ca(2+) channels that are possibly involved in the [Ca(2+)](o)-induced change in [Ca(2+)](i). This Ca(2+) entry system might provide a compensatory pathway for the negative feedback regulation of parathyroid hormone secretion, especially in hyperplastic conditions in which the Ca(2+)-sensing receptor is poorly expressed.