Fluctuation analysis of sodium channels in epithelia.

When compiling results obtained with various techniques, it appears that the apical Na translocators are channels of high Na, Li selectivity that permit large transport rates at low driving forces. The concentration dependence of the transport rate is linear at least up to 60 mM Nao. The voltage dependence is of the Goldman-Hodgkin-Katz type or closely related to it. Outward facing surface charges have little influence. The flux ratio exponent is close to unity, compatible with single-site channels or multisite channels of low occupancy. Noise analysis has proven particularly suited for following changes in the density of conducting channels in the living preparation, and has shown that cellular and hormonal regulatory mechanisms control Na transport by changing the channel density in a variety of ways.