Ca 2+-activated nonselective cation, maxi K + and Cl − channels in apical membrane of marginal cells of stria vascularis

Abstract

Patch clamp recordings on the apical membrane of marginal cells of the stria vascularis of the gerbil were made in the cell-attached and excised configuration. Marginal cells are thought to secrete K + into and absorb Na + from endolymph. Four types of channel were identified; the most frequently observed channel was a small, nonselective cation channel which was highly similar to that found in the apical membrane of vestibular dark cells (Marcus et al., (1992) Am. J. Physiol. 262, C1423–C1429). The small nonselective cation channel was equally conductive (26.7±0.3pS; N = 49) for K +, Na +, Rb +, Li + and Cs +, 1.6 times more permeable to NH 4 +, but not permeable to Cl −, Ca 2+, Ba 2+ or N-methyl- d-glucamine. This channel yielded linear current-voltage relations which passed nearly through the origin (intercept: -2.2±0.4 mV, N = 49) when conductive monovalent cations were present on both sides of the membrane in equal concentrations. Channel activity required the presence of Ca 2+ at the cytosolic face but not the extracellular (endolymphatic) face; there was essentially no activity for cytosolic Ca 2+ ⩽ 10 −7M Ca 2+ and full activity for ⩾ 10 −5M. Cell-attached recordings had a conductance of 28.6±2.2 pS ( N = 6) and a reversal voltage of -2.2±5.2 mV ( N = 3) which was interpreted to reflect the intracellular potential of marginal cells under the present conditions. The three other types of channel were a Cl − channel (∼ 50 pS; N = 2), a maxi-K + channel (∼ 260 pS; N = 1), and another large channel, probably cation nonselective (∼ 70 pS; N = 1). The 27 pS nonselective cation channel may be involved in K + secretion and Na + absorption under stimulated conditions which produce an elevated intracellular Ca 2+; however, consideration of the apparent channel density in relation to the total transepithelial K + flux suggests that these channels are not sufficient to account for K + secretion.