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

Primary monolayer cultures were made from a rat medullary thyroid carcinoma. All the cultures produced calcitonin throughout the 4 week 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 +2- S.D., 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 reversibly blocked by D-600. It is concluded that the action potentials have both a Na+ and a Ca2+ component, and circumstantial evidence suggests that our recordings were obtained from the calcitonin-producing cells.