Analysis of time dependent states of solute coupled water transport

Abstract

At transepithelial equilibrium water uptake by toad skin epithelium is linearly correlated with the active Na+ flux. The present study aimed at analyzing the coupling between these fluxes during transition from one steady state to another. The short‐circuited isolated toad skin epithelium was studied in an Ussing setup with a horizontal capillary tube on the inside chamber. The volume was clamped by keeping the position of the meniscus in the capillary tube constant via a motor‐driven syringe the position of which was continuously recorded. When the isoproterenol stimulated short‐circuit current (ISC) and volume flow (JV) were stationary, 100 μM amiloride was added on the outside while the time courses of ISC and JV were followed until a new steady state was obtained. Initially, ISC = 18±4.1 μA/cm2 and JV = 1.3±0.4 nl/cm2/s (mean ± SEM, n = 9). Amiloride produced a relatively fast decrease of ISC with T1/2 = 1.6± 0.1 min, and a slow decrease of JV with T1/2 = 14.1± 2.3 min towards the new steady states of, ISC = ‐0.1±0.4 μA/cm2 and JV = 0.39±0.11 nl/cm2/s. Theoretical quantitative analysis was performed by solving the set of equations of our mathematical‐physical model for time dependent states, indicating that also during the imposed transient state JV is driven by the lateral Na/K‐pumps governed by the rate at which the cellular Na+ pool is emptied. Supported by The Danish Natural Science Research Council 272‐05‐0417.