Voltage-gated sodium and potassium currents and their variation in calcitonin-secreting cells of the chick

Abstract

1. The electrical properties of dissociated ultimobranchial cells from chick embryos (18-20 days after fertilization) were studied using whole-cell patch electrodes. Antibodies for immunohistological identification of calcitonin-secreting cells in the preparation were obtained by immunizing rabbits with a conjugated analogue of eel calcitonin. 2. In a proportion of cells, spike-like action potentials were generated in response to depolarization when cells were immersed in standard saline containing 140 mM-Na+ but no Ca2+. When the membrane potential was shifted from a holding potential (-83 - -103 mV) to a test depolarization (-50 mV or more positive) under voltage-clamp conditions, a transient inward current was produced which was followed by a slowly developing outward current. 3. The inward current was identified as a Na+-carried current, since (1) the kinetics of the current seemed fast and the amplitude consistently depended on the holding potential, (2) replacement of external Na+ with choline ions reversibly abolished the current, and (3) external application of tetrodotoxin (1 microM) abolished the current completely. The cells in which inward currents were detected showed intense to intermediate degrees of staining with anti-calcitonin antibodies. 4. In some other cells, no regenerative potentials were evoked even with intense depolarization, but a delayed decrease in membrane depolarization during the current pulse was observed. Voltage-clamp experiments in these cells revealed the existence of slowly developing outward currents, and the cells showed an intermediate degree of antibody staining. 5. The outward currents in both types of cells were selectively diminished in the presence of K+ channel blockers such as tetraethylammonium (1-10 mM) or 4-aminopyridine (1 mM). When the pipette contained 120 mM-CsCl, none of the dissociated cells exhibited any appreciable outward currents. Thus, the outward currents were most likely to be membrane potential-dependent K+ currents. The potential dependency of activation and inactivation of the currents were consistent with those of delayed K+ rectifier. 6. In the remaining cells, only passive responses of membrane potentials were observed with current injection. No discernible voltage-dependent inward or outward currents were detected under voltage-clamp conditions. Although these cells had a similar appearance to the two types of cells previously mentioned under phase-contrast microscopy, none of them showed significant antibody staining. These cells were presumed to represent non-secretory or supporting cells within the gland.(ABSTRACT TRUNCATED AT 400 WORDS)