No abstract

Summary. The operation of a seven-compartment model is described with respect to flows of Na + within and across this system, simulating published results obtained on frog skin. The seven compartments represent: one outside and one inside solution compartment; the subcorneal space; the first reacting cell layer (1.RCL); the remaining cell compartment; the non-, or slowly exchangeable Na + compartment; the extracellular space. Assuming reasonable volumes for the epidermal compartments and further chosing, by trial and error, appropriate rate constants, a set of seven simultaneous linear differential equations was solved by the application of the Continuous System Modeling Program (CSMP), using an IBM 1130 computer. Initial conditions for influx, backflux and net flux were taken which correspond to [Na+]0; [Na+]i= 115 mM. Print-out data were obtained at 0.5-min intervals for 30 min, when steady states were obtained in 13 models studied, varying certain k's thus simulating actions of chemical agents (hormones; drugs). Simulation was achieved with regard to rate of influx, backflux and net flux, steady-state time (30 min), and electrical potentials. In addition, this approach gave detailed information on Na + pool sizes and their variations with changes in k's. These results are compared to published data on frog skin and good agreement between operation of skin epidermis and model was found.During the last decade the three-compartment model proposed by Curran, Herrera and Flanigan [14] has frequently been used in kinetic studies of movement of ions across epithelial membranes. Some applications have recently been reviewed [38]. In this model, compartments 1 and 3 represent, respectively, the outside and the inside fluid compartments, and compartment 2 is an epithelial compartment located between two major barriers to Na § movement. From light-and electron-microscopic studies on frog skin epidermis [18,19,[32][33][34] it would appear that in inward and outward movement of Na + several epithelial compartments should be considered as significant parts of a flow model. For instance, Zerahn [43] has found it useful to consider three epithelial compartments in the interpreta-

Active ion (NaCl) transport across isolated frog skin is discussed in relation to sodium and potassium composition and to O3 consumption of skin. A distinction is made between processes in skin related to "unidirectional active ion transport" and processes related to "maintenance electrolyte equilibrium;" i.e., ionic composition of skin. Several metabolic inhibitors were found that could be used in separating maintenance electrolyte equilibrium from unidirectional active ion transport. Fluoroacetate (up to 1 X 10-M/liter) did not affect maintenance electrolyte equilibrium, but severely diminished the rate of active ion transport. This could also be accomplished with azide and diethyl malonate when 1 X 10 -3 molar concentrations were used. When applied in higher concentrations, these two inhibitors, and several others, diminished active ion transport, but this was associated with changes in maintenance electrolyte equilibrium (gain of Na + by and loss of K + from skin). Similar observations were made when skins were subjected to K+-deficient media. Mersalyl and theophylline, in low concentrations, stimulated active ion transport without leading to changes in maintenance electrolyte equilibrium. Inhibition of'active ion transport was found accompanied by decrease, increase, and unaltered over-all 03 consumption, depending on the kind of chemical agent used. A provisional scheme of the mechanism of unidirectional active ion transport is proposed. It is conceived as a process of metabolically supported ion exchange adsorption, involving a carrier, forming complexes with K + and Na +, a trigger, K + ions, and two spatially separated metabolic pathways.