Induction of transporting sites in a sodium transporting epithelium.

Abstract

1. Frogs (Rana temporaria) were bathed for 1 week in solutions containing 1-1 mM sodium chloride and either one or both of amiloride (10(-4)M) and spironolactone (10(-5r both of amiloride (10(-4) M) and spironolactone (10(-5) M). This procedure was designed to deplete the sodium transporting compartment of the skin epithelium of sodium, while at the same time antagonizing the effects of endogenous aldosterone. 2. After 1 week the skins were used in vitro to measure the level of sodium transport (short-circuit current) and the density of sodium entry sites in the mucosal surface of the epithelium ([14C]amiloride binding). 3. Sodium deprivation for 1 week caused approximately a doubling of both sodium transport and the density of sodium entry sites in the mucosal surface of the epithelium compared to control skins. 4. When the results for sodium deprived and control skins were pooled there was a highly significant correlation between the density of sodium entry sites and sodium transport. 5. Mechanisms by which sodium deprivation leads to an increase in the density of sodium entry sites are discussed.

Role of basolateral membrane conductance in the regulation of transepithelial sodium transport across frog skin

“…As the concentration of amiloride was increased further, blockade of Na + entry became the dominating factor, which led to the expected decrease of the I sc and G c , while the intracellular potential increased moderately only. Our findings resemble earlier, hitherto unexplained observations by Cuthbert and Shum [1] who noted a small stimulation of the I sc in a few skins at lower amiloride concentrations and reported this finding as "curiosity". With the advent of intracellular measurements these enigmatic observations might be explained by reactions of the basolateral membrane due to an inherently very high sensitivity of the inwardly rectifying K + channels.…”

“…This, and the ensuing hyperpolarisation, occasionally led to an overshooting response of the I sc . The reason for this extreme voltage sensitivity of the K + channels is unclear but the observation that frog skin treated with spironolactones shows a higher occurrence of current increase at low amiloride concentrations [1] suggests an influence of steroids on the properties of the inwardly rectifying K + channels. The degree of inwards rectification in K + channels in animals under spontaneous conditions is unpredictable and can be so large that existing responses of transepithelial transport are blurred without information on specific membrane conductances.…”

Chemical stimulation of Na transport through amiloride-blockable channels of frog skin epithelium

“…There are indications that the pyrazine-diuretic amiloride not only blocks apical Na channels, but in addition stimulates transport at low concentrations, a phenomenon to which incidental reference has been made in the literature (Cuthbert & Wong, 1972;Cuthbert & Shum, 1976;Bevevino & Lacaz-Vieira, 1982). Recently Thurman and Higgins (1982) showed that in the toad urinary bladder amiloride develops a strong stimulatory effect on Na uptake when applied in low concentrations in the presence of Ca ions and a divalent cation chelator.…”

Potassium-Sparing Diuretics

“…The half-life of the Na+ channel in the apical membrane in the toad bladder epithe lium has been estimated to be within 2-3 days [31]. Treatment of the apical membrane b\ proteases reduces the amiloride-sensitive Na1 permeability [63.…”