Sodium and calcium action potentials in cells derived from a rat medullary thyroid carcinoma

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Abstract
Primary monolayer cultures were made from a rat medullary thyroid carcinoma. All the cultures produced calcitonin throughout the 4 wk experimental period. Two different parenchymal cell types could be distinguished, based on a marked size difference. Only the group of smaller cells was electrically excitable. The membrane potential and resistance of these cells were 55.3 .+-. 15.4 mV and 623 .+-. 275 M.OMEGA. (mean .+-. SD, n = 23), and current injections produced regenerative responses in all the cells. All-or-none action potentials were recorded in both normal and Na+-free solution. In Na+-free solution the maximum rate of rise of the action potential was reduced by about 70%, and the action potential was reversably blocked by D-600 [methoxyverapamil]. The action potentials have both a Na+ and a Ca2+ component, and circumstantial evidence suggests that recordings were obtained from the calcitonin-producing cells.