Electrophysiological properties of calcitonin‐secreting cells derived from human medullary thyroid carcinoma

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Abstract
Primary cultures of human medullary thyroid carcinoma tissue were prepared from lymph node metastases in two patients. The parenchymal, cultured cells displayed positive immunocytochemical staining for CT, and the cells also released the hormone into the culture medium. The membrane potential and resistance of the CT‐producing cells were 50.1 ± 8.9 mV and 634 4±154 Mf2 (mean ± SD, n = 46). TTX sensitive action potentials with maximum rate of rise up to 51 V s‐1 were evoked by current injection in Na+‐containing solution, whereas TTX insensitive action potentials with maximum rate of rise up to 9 V s‐I were generated in Na+‐free solution. These action potentials were reversibly blocked by D‐600. We conclude that the action potentials of the human MTC cells have both a Na+ and a Ca2+ component. Ejection of Ca2+‐free solution close to the cells caused membrane hyperpolarization associated with decreased membrane resistance. The reversal potential of this response was —66.2 ± 10.9 mV (n = 10), indicating that a permeability increase to Cl‐ and/or K+ may be involved. We suggest that elevated plasma Ca2+ concentration in vivo may cause increased excitability due to membrane depolarization and resistance increase, thus leading to enhanced Ca2+ influx and hormone secretion.