Concentration dependence of currents through single sodium-selective pores in frog skin

Abstract

Many transport processes in biological membranes show a saturation of transport rate when the concentration of the transported ions or molecules is sufficiently increased. Saturation will result when at high concentrations a translocation step which does not directly depend on bulk concentration becomes rate limiting, and it is expected for all multi-step translocation mechanisms. However, a possible alternative explanation for saturation is a regulatory process which decreases the number of translocating channels in response to an increase in bulk concentration of the transported species. In the frog epidermis, Na⁺ uptake from the extracellular space into the epithelial cytosol occurs through Na⁺-selective, passively conducting channels located in the apical membrane of the cells directly below the cornified outer cell layer. Saturation of transport occurs with increasing outer Na⁺ activity ([Na]_o): the net Na⁺ current^1,2, membrane conductance^2,3 and unidirectional Na⁺ influx⁴ increase only subproportionally with [Na]₀. The apical Na⁺ current can transiently be larger than the saturation value, suggesting that a fixed upper single channel transport rate is not responsible for the saturation, and it has, therefore been suggested that in this case saturation occurs through a [Na]_o-dependent decrease in channel density². The Na⁺-translocation process can be investigated by a statistical evaluation of spontaneous transport fluctuations. The reversible transport blocker amiloride can be used to induce random fluctuations of the Na⁺ current at the apical membrane. The power density spectrum of these fluctuations is of the lorentzian type and permits an estimation of the Na⁺ current which passes through individual translocators in the time intervals between blocks⁵. We have recorded the current fluctuations at different [Na]_o values and different amiloride concentrations in the [Na]_o range in which saturation of total Na⁺ current is observed. We report here that the density of the conducting pores decreases with increasing [Na]_o.