Kinetic studies on the effects of ouabain on Na+ fluxes in frog skin

Salt and acid studies on canine lingual epithelium

A section of dog tongue just anterior to the circumvallate papillae was placed in an Ussing chamber and characterized with respect to monovalent cation transport pathways, response to HCl, and the interaction between salt and acid for the purposes of determining whether ion transport through the tongue is related to the transduction process. The primary findings of this work follow. The stimulation of the short-circuit current (Isc) with decreasing pH on the dorsal surface results from a net Cl- efflux through an amiloride- and ouabain-insensitive pathway that changed its selectivity from cationic at neutral pH to anionic at lower pH values. The stimulation is abolished at isotonic NaCl concentrations. This behavior would not be predicted from the proton-binding model to a "sour receptor" as suggested by Beidler (Handbook of Sensory Physiology; Berlin: Springer-Verlag, 1971). In symmetrical solutions of Krebs-Henseleit buffer, Isc is inhibited 30% by 10(-4) M amiloride on the mucosal side and 100% by 10(-3) M ouabain on the serosal side. These results coupled with net Cl- and Na+ flux measurements show that Isc is due to a net Cl- efflux. With 1.0 M NaCl on the dorsal side, 10(-4) M amiloride inhibits Isc 84%. In contrast, when the mucosal surface is bathed in KCl (1.0 M), CsCl (1.0 M), or HCl (10(-3) M), Isc is insensitive to both 10(-4) M amiloride and/or 10(-3) M ouabain. These data suggest that K+, Cs+, and H+ traverse the tongue through different pathways from Na+ and are not energized by a Na+ gradient.

Compartmental analysis of the Na+ flux ratio with application to data on frog skin epidermis

In this computer simulation study, the role of the topological factor on the Na+ influx/backflux (efflux) ratio in multicompartmental model membranes with active Na+ transport has been investigated. As in the classical "three compartment model", so also in multicompartment models with series order of compartments (series topology), the flux ratios are time-independent. By contrast, in models with series-parallel order of compartments (series-parallel topology), inclusive shunt pathways, the flux ratios are time-dependent. The values of the ratios can increase, or decrease with time, reaching steady state values, depending on the nature of the chosen topology. In a similar manner, the apparent value of the driving force, ENa, of the Na+-pumps, calculated from the Ussing-Teorell flux ratio equation and using global flux ratios, can vary in models with series-parallel topology. This is not the case in models with series topology. On the other hand, the true value of the driving forces of the Na+ pumps, calculated from local flux ratios, are higher, and time-independent. In the absence of Na+ pumps (simulated ouabain effect) the flux ratios have in all cases the values of 1.0. These theoretical results are in good agreement with the theoretical results recently published by Sten-Knudsen & Ussing (1981) whose analysis utilized principles differing from those used here. In the design of the multicompartment model and the choice of kinetic parameters, frog skin epidermis served as a guide, such that simulated outputs closely agreed with experimental data in the literature. This includes the realization of a "fast" paracellular, and a "slow" cellular pathway for transepidermal flow of Na+.

Effects of standard diuretics and ortho-vanadate on sodium transport across isolated frog skin

Frog (Rana temporaria) skins were studied in an Ussing type lucite chamber adapted to diminish tissue edge damage. The transepithelial electrical potential difference, short circuit current and direct current (DC) resistance of skins mounted in this chamber were 56, 20 and 24% higher, respectively, than those of skins mounted in a conventional chamber. Amiloride, triamterene, ouabain and ortho-vanadate inhibited short circuit current and net mucosal to serosal flux of 22Na. Amiloride and triamterene had rapid onsets of action and were effective only when administered to the mucosal (pond) side of the skin. Ouabain and ortho-vanadate had slower onsets of action and were effective only when administered to the serosal side of the skin. Steady state of effects of these drugs was not reached within the three-hour period of the experiments. The inhibitory effect of ortho-vanadate was blocked by adding a disulfonic stilbene derivative (DIDS) to the serosal side of the skin. Serosal prostaglandin E2 stimulated the short-circuit current and decreased the DC resistance. Thiazides, acetazolamide and loop diuretics had no effects on Na+ transport by frog skin. Thus, frog skin seems to be a useful model only in studies of the mode of action and the structure-activity relationship of diuretic which act by inhibiting sodium entry or Na+-K+-ATPase activity.