Saturation kinetics of sodium efflux across isolated frog skin

Abstract

Measurement of Na efflux across the frog skin epithelium from the serosal side to the outside (JNa 3 leads to 1) in a new chamber specifically designed to avoid edge damage shows that JNa 3 leads to 1 exhibits saturation kinetics with a maximal efflux (Jmax) of 31.8 nmol/cm2 per h and an apparent KNa of 4.0 mM. In contrast, JNa 3 leads to 1 measured in conventional chambers and efflux determinations in the new chamber of substances that pass the epithelium via extracellular pathways (polyethylene glycol 900, sucrose, mannitol) exhibit a linear relationship between the efflux of the substance in question and its concentration in the bath. In addition, changes in external Na concentration do not cause substantial changes in JNa 3 leads to 1. The saturation remains but both Jmax and KNa increase after application of ouabain. Amiloride, as well as dinitrophenol, eliminates the saturation and JNa 3 leads to 1 becomes a linear function of Na concentration. The separate effects of ouabain and amiloride suggest that these two inhibitors which are known to affect two distinctly different steps in the active transport pathway act also on two separate steps of JNa 3 leads to 1: the passage across the inward- (serosal) and outward-facing (apical) cell membranes of the epithelial cells, respectively. The action of dinitrophenol indicates the involvement of metabolism in JNa 3 leads to 1 probably at the latter of the two steps. The results suggest strongly that JNa 3 leads to 1 proceeds not via a paracellular but via a transcellular pathway that interacts with the active transport pathway.

Effect of external cation and anion substitutions on sodium transport in isolated frog skin

“…We have found a mean flux ratio for choline of 1.05 +_ 0.12 (n = 24) with the high ionic strength Ringer's. Although we found no evidence for the active transport of choline, the permeability was significantly higher than that demonstrated for Na in a previous paper (Biber & Mullen, 1976). The high permeability of choline cannot be explained by molecular size but has been observed previously in frog skin (Kirschner, 1960) and the kidney tubule (Boulpaep, 1976).…”

“…The high permeability of choline cannot be explained by molecular size but has been observed previously in frog skin (Kirschner, 1960) and the kidney tubule (Boulpaep, 1976). It has been suggested that the high permeability of choline in relation to Na might be due to radiolysis of choline (Kirschner, 1960), and there is some evidence that the low permeability values for Na in frog skin might be due to an interaction of the active transport system with J~3~ (Biber & Mullen, 1976;Huf & Howell, 1974;Kirschner, 1959).…”

Transport of sodium and chloride across rat gastric mucosa in vitro.

“…In studies on frog skin it has been shown that a significant component of the sodium blackflux utilized the transcellular channel, and that this component of the backflux was inhibited by amiloride (Biber & Mullen, 1976). In the present experiments J~a was not affected by a concentration of amiloride that abolished net transport of sodium, suggesting that in the rat gastric mucosa the backflux of sodium utilized a GASTRIC ION TRANSPORT shunt pathway independent of the sodium transport system, and it may be that the paracellular channel constituted a major component of this pathway.…”

Mechanism of epithelial lithium transport. Evidence for basolateral Na:Na and Na:Li exchange.

“…In the absence of vasopressin, sodium backflux was unaffected by amiloride . Biber and Mullen (2,3) showed that in frog skin the sodium backflux exhibited saturation kinetics, but was not affected by 6 mM serosal lithium. An amiloride-sensitive component of the sodium backflux was only observed at reduced (6 mM) mucosal sodium concentrations, however .…”