Characterization of a 25-pS nonselective cation channel in a cultured secretory epithelial cell line

We have studied a 25-pS nonselective cation channel from the apical membranes of cell line ST885, derived from neonatal mouse mandibular glands. Its Cl- permeability was not significantly different from zero. The permeabilities (relative to Na+) for inorganic cations were NH4+ (1.87) greater than K+ (1.12) greater than Li+ (1.02) greater than Na+ (1) greater than Rb+ (0.81) greater than Mg2+ (0.07) greater than Ca2+ (0.002), and for organic cations, guanidinium (1.61) greater than ethanolamine (0.70) greater than 4-aminopyridine (0.66) greater than diethylamine (0.54) greater than piperazine (0.25) greater than Tris (0.18) greater than N-methylglucamine (0.12). The Tris and N-methylglucamine permeabilities differed significantly from zero. Fitting the Renkin equation indicated that the channel had an equivalent pore radius of 0.49 nm. The channel was activated by Ca2+ on the cytosolic surface (greater than 0.1 mmol/liter) with a Hill coefficient of 1.2; it was also activated by depolarization. Open- and closed-time histograms indicated that it had at least two open and two closed states. The channel was blocked by cytosolic AMP or ATP (0.1 mmol/liter). It was also blocked by the Cl- channel blocker, diphenylamine-2-carboxylate (DPC; 0.1 mmol/liter), applied to the extracellular but not the cytosolic surface. 4-Amino-pyridine, which permeated the channel when applied to the extracellular surface, blocked it when applied in low concentrations (5 mmol/liter) to the cytosolic surface. Quinine (0.1 mmol/liter) blocked from both the extracellular and cytosolic surfaces, blockade from either side being enhanced by depolarization. The channel was held open by application of SITS (0.1 mmol/liter) to the cytosolic surface. The channel shows striking similarities to the nicotinic acetylcholine receptor channel, viz., both channel types are abnormally permeable to 4-aminopyridine applied externally, and their selectivity sequences for inorganic ions are similar and for organic cations are identical.