Fluctuation analysis of short-circuit current in a warm-blooded sodium-retaining epithelium: Site current,density, and interaction with triamterene

Summary Density and conductance of the Na-site in hen coprodeum were studied by employing fluctuation analysis of shortcircuit current at sodium concentrations from 26 to 130mm. Fluctuations of current in the frequency range 2–800 Hz were induced by triamterene, a reversible blocker of conducting epithelial Na-sites. At 130mm Na the site density was 5.8±1.0 m^–2 and the site conductance was 4 pS. This conductance is equal to that of the frog skin (W. Van Driessche and B. Lindemann, 1979,Nature (London) 282:519–520). Extrapolation of site density to zero sodium renders a total of 38±28 sites m^–2, which is compared with other estimates for the coprodeum. The site-triamterene association and dissociation constants were 9.5±0.4 rad sec^–1 m ^–1 and 255±20 rad sec^–1 and they were independent of external sodium concentration. An analysis of the affinity constant for triamterene based on the DC-short-circuit current was found to be unrelated to the external sodium concentration and identical to that obtained from fluctuation analysis indicating a noncompetitive interaction between sodium and triamterene. Due to the oxygen demand of the epithelium we have developed an experimental method using short data processing times. A new analytical approach using integration of the power density spectrum proved necessary because of low signal-to-noise ratios.