Major role of dihydropyridine-sensitive Ca2+ channels in Ca(2+)-induced calcitonin secretion.

Endocrine cells are known to possess multiple types of Ca2+ channels. In neurons, omega-conotoxin-sensitive N-type Ca2+ channels have been shown to play a dominant role in neurotransmitter release, but uncertainty remains about the types of Ca2+ channels involved in stimulus-secretion coupling in endocrine cells. We investigated the relative contribution of 1,4-dihydropyridine-sensitive and omega-conotoxin-sensitive Ca2+ channels to Ca(2+)-induced calcitonin release in parafollicular cells of the thyroid (C cells). In whole cell voltage-clamp experiments, both 1,4-dihydropyridine-sensitive and omega-conotoxin-sensitive Ca2+ channel currents were identified. The dihydropyridine isradipine (1 microM) but not omega-conotoxin (1 microM) inhibited the steady-state Ca2+ influx at physiological membrane potentials, the spontaneous electrical activity, and calcitonin secretion (at 2-h incubations). Moreover, suppression of the spontaneous electrical activity by the Na+ channel blocker tetrodotoxin did not affect calcitonin release. We conclude that 1,4-dihydropyridine-sensitive Ca2+ channels play a major role in Ca(2+)-dependent calcitonin release and that calcitonin secretion due to Ca2+ influx proceeds even in the absence of action potentials.