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

The Journal of Membrane Biology, ISSN: 0022-2631, Vol: 114, Issue: 1, Page: 37-52
1990
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We have studied a 25-pS nonselective cation channel from the apical membranes of cell line ST, derived from neonatal mouse mandibular glands. Its Cl permeability was not significantly different from zero. The permeabilities (relative to Na) for inorganic cations were NH(1.87)>K(1.12)>Li (1.02)>Na(1)>Rb(0.81)>Mg(0.07)>Ca(0.002), and for organic cations, guanidinium (1.61)4-aminopyridine (0.66)>diethylamine (0.54)>piperazine (0.25)>Tris (0.18)>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 Ca on the cytosolic surface (>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-Aminopyridine, 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. © 1990 Springer-Verlag New York Inc.

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