A calcium-activated, large conductance and non-selective cation channel in Paramecium cell

A non-selective cation channel was found in mutant Paramecium cells (K115). This cell had been selected as a resistant mutant in a high-K+ solution. In patch clamp studies of these cells in the inside-out configuration, this channel was activated by bath applications of elevated Ca2+ concentrations. The channels became very active when the Ca2+ concentration was above 3.2 microM. The channel was also activated by depolarization. The voltage dependency was steep upon depolarization, whereas upon hyperpolarization the channel activity barely changed. This channel had poor selectivity for monovalent alkali cations. Using the Goldman-Hodgkin-Katz equation for the reversal potential, the permeability ratios with respect to K+ for Na+, Rb+, Cs+ and Li+ were nearly 1. Although the permeability ratios were similar for each cation, the single channel conductances differed. The single channel conductances were 467 pS with K+ as the charge carrier, 406 pS with Na+, 397 pS with Rb+, 253 pS with Cs+ and 198 pS with Li+ upon depolarization in 100 mM cation solutions. A similar calcium-activated large conductance channel was observed in the wild-type (G3) Paramecium cells but was very rare.