Strophanthidin sensitive electrogenic mechanisms in frog sartorius muscles exposed to barium

Potassium transport across the frog retinal pigment epithelium

Previous experiments indicate that the apical membrane of the frog retinal pigment epithelium contains electrogenic Na:K pumps. In the present experiments net potassium and rubidium transport across the epithelium was measured as a function of extracellular potassium (rubidium) concentration, [K]0 ( [Rb]0). The net rate of retina-to-choroid 42K(86Rb) transport increased monotonically as [K]0 ( [Rb]0) increased from approximately 0.2 to 5 mM on both sides of the tissue or on the apical (neural retinal) side of the tissue. No further increase was observed when [K]0 ( [Rb]0) was elevated to 10 mM. Net sodium transport was also stimulated by elevating [K]0. The net K transport was completely inhibited by 10-4 M ouabain in the solution bathing the apical membrane. Ouabain inhibited the unidirectional K flux in the direction of net flux but had no effect on the back-flux in the choroid-to-retina direction. The magnitude of the ouabain-inhibitable 42K(86Rb) flux increased with [K]0 ( [Rb]0). These results show that the apical membrane Na:K pumps play an important role in the net active transport of potassium (rubidium) across the epithelium. The [K]0 changes that modulate potassium transport coincide with the light-induced [K]0 changes that occur in the extracellular space separating the photoreceptors and the apical membrane of the pigment epithelium.

Periodic paralysis and the sodium-potassium pump

Analysis of the pathophysiology of hypokalemic paralysis, as it occurs in barium poisoning, chronic potassium deficiency, and thyrotoxicosis, suggests that these disorders may have a similar mechanism. An increased ratio of muscle sodium permeability to potassium permeability reduces the ionic diffusion potential, while the resting membrane potential is sustained by an increase of Na-K pump electrogenesis. The result is that potassium entry (the sum of active and passive influx) exceeds potassium efflux; this causes a large shift of extracellular potassium into muscle until the Na-K pump turns off, leading to depolarization and paralysis. The primary defect in familial hypokalemic periodic paralysis, as in the example of barium poisoning, may be a marked reduction of muscle permeability to potassium.

Coupled transepithelial sodium and potassium transport across isolated frog skin: effect of ouabain, amiloride and the polyene antibiotic filipin

Addition of the polyene antibiotic filipin (50 microM) to the outside bathing solution (OBS) of the isolated frog skin resulted in a highly significant active outward transport of K+ because filipin per se increases the nonspecific Na+ and K+ permeability of the outward facing membrane. The K+ transport was calculated from the chemically determined changes in K+ concentrations in the solution bathing the two sides of the skin. The active transepithelial K+ transport required the presence of Na+ in the OBS, but not in the inside bathing solution (IBS), and it was inhibited by the Na+, K+-ATPase inhibitor ouabain. The addition of Ba++ to the IBS in the presence of filipin in the OBS resulted in an activation of the transepithelial K+ transport and in an inhibition of the active Na+ transport. This is in agreement with the notion that Ba++ decreases the passive K+ permeability of the inward facing membrane. In the presence of amiloride (which blocks the specific Na permeability of the outward facing membrane) and Ba++ there was a good correlation between the active Na+ and K+ transport. It is concluded that the active transepithelial K+ transport is carried out by a coupled electrogenic Na-K pump, and it is suggested that the pump ratio (Na/K) is 1.5.

Barium inhibition of sodium ion transport in toad bladder

The effect of Ba2+ on Na+ transport and electrical characteristics of toad bladder was determined from change produced in short circuit current (Isc), epithelial, apical and basal-lateral potentials (psit, psia, psib), epithelial and membrane resistances (Rt, Ra, Rb) and shunt resistance (Rs). Mucosal Ba2+ had no effect. Serosal Ba2+ reduced Isc, psit, psia, and psib, but had no effect on Rt, Ra, Rb and Rs. Minimal effective Ba2+ concentration was 5-10(-5) M. The phenomenon was reversed by Ba2+ removal, but not by 86 mM serosal K+. Ba2+ inhibition of Isc did not impair the response to vasopressin which was quantitatively the same as controls. Psia with Ba2+ equalled psib. After Ba2+ inhibition, ouabain produced no further decrease in psit and Isc. Ba2+ exposure after ouabain did not decrease psit and Isc. The results suggest that Ba2+ inhibits the basal-lateral electrogenic Na+ pump.