Influence of extracellular Cl concentration on Cl transport across isolated skin of Rana pipiens

Transepithelial Na+ transport and the intracellular fluids: a computer study

Computer simulations of tight epithelia under three experimental conditions have been carried out, using the rheogenic nonlinear model of Lew, Ferreira and Moura (Proc. Roy. Soc. London. B 206:53-83, 1979) based largely on the formulation of Koefoed-Johnsen and Ussing (Acta Physiol. Scand. 42: 298-308. 1958). First, analysis of the transition between the short-circuited and open-circuited states has indicated that (i) apical Cl- permeability is a critical parameter requiring experimental definition in order to analyze cell volume regulation, and (ii) contrary to certain experimental reports, intracellular Na+ concentration (ccNa) is expected to be a strong function of transepithelial clamping voltage. Second, analysis of the effects of lowering serosal K+ concentration (csK) indicates that the basic model cannot simulate several well-documented observations; these defects can be overcome, at least qualitatively, by modifying the model to take account of the negative feedback interaction likely to exist between the apical Na+ permeability and ccNa. Third, analysis of the strongly supports the concept that osmotically induced permeability changes in the apical intercellular junctions play a physiological role in conserving the body's stores of NaCl. The analyses also demonstrate that the importance of Na+ entry across the basolateral membrane is strongly dependent upon transepithelial potential, cmNa and csK; under certain conditions, net Na+ entry could be appreciably greater across the basolateral than across the apical membrane.

Intracellular ionic activities in frog skin

Intracellular Na+, K+, and Cl- activities (aiNa, aiK, aiCl) and transapical membrane potentials (V0) were measured with liquid ion-exchanger and open-tip microelectrodes in isolated short-circuited frog skins (R. pipiens) incubated at 23 degrees C in normal amphibian Ringer's solution. Under control conditions aiNa = 14 +/- 3 mM, aiK = 132 +/- 10 mM and aiCl = 18 +/- 3 mM (SD). The value of aiCl is 4.4 times the value corresponding to electrochemical equilibrium for this ion. Thus, Cl- is actively accumulated by epithelial cells of the frog skin. Shortly after addition of amiloride (2--5 microM) to the apical bathing medium, aiK, aiNa, and aiCl were essentially unchanged although V0 had hyperpolarized by about 30--40 mV. During long-term exposure to amiloride aiK and aiCl did not change significantly, V0 depolarized by about 16 mV from the maximal value and aiNa decreased to 8 +/- 3 mM. Immediately after exposure to amiloride the transmembrane driving force for Na+ increased from 124 to 154 mV. During further exposure to amiloride, despite changes in both V0 and aiNa, this driving force remained virtually constant. Since Isc during this period was close to zero, it is suggested that the observed driving force for Na+ under these condition approximates the maximal driving force generated by the Na+--K+ ATP-ase pump in the basolateral cell membrane.