Effect of HCO3 and pH on ion transport in the posterior intestine of the freshwater-adapted teleost Anguilla anguilla

ABSTRACT

The intestinal mucosa may be exposed to acidic or alcaline solutions due to liberation of digestive secretions. In several situations blood pH may also change. Consequently, the effects of HCO-3, CO2, and pH variation of medium on the ion transport acrross the posterior intestine of the eel (Anguilla anguilla) adapted to freshwater were studied in terms of fractional values of short-circuit current (SCC), transepithelial potential difference (TPD) and conductance (G). Immature eels weighing 100-200 g were used. The control physiological solution contained 118.5 mM NaCl, 25.0 mM NaHCO3, 3.0 mM CaCl2.2H2O, 4.7 mM KCl, 1.0 mM MgCl2.6H2O, 5.0 mMD-glucose, 10.0 mM D-mannitol, pH 7.80, and was gassed with 98 percent O2-2 percent CO2. Control values(N = 21) were SCC = 51.90 + or - 51.90 + or - 2.8 µA.cm-2, TPD = 2.33 + or - 0.1 mV, G = 22.43 + or - 0.6 mS.cm-2. At constant pH, the reduction of HCO-3 concentration to 50 percent and 10 percent did not alter the values of SCC and TPD, but G increased with HCO-3 reduction to 10 percent. In the absence of HCO-3, SCC, TPD and G (slightly) decreased, but 1.5 mM HCO-3 still maintained the ion transport within control values, at constant pH. Comparing PH values from 6.65 to 8.61, higher values of SCC and TPD were observed at PH 7.45, but were little affected below and above this pH. There was a significant correlation between pH and SCC and TPD values; from the regression equations (1) SCC was zero at pHs below 6.62 and above 8.78 and (2) TPD was zero below 6.50 and above 8.71