The influx of Cl- across isolated frog skin bathed on the outside by 0.8 mM NaCl is about 100 nmol cm-2 h-1, which is approximately twice the Cl- influx in intact animals. The influx consists of diffusion (1%), exchange diffusion (38%), and active transport (60%). About 80% of the influx is independent of Na+ in the outer bath and is also independent of concomitant inward movement of cations. Chloride is exchanged for anions, probably HCO-3. The Cl- transport system is saturable; Vmax is about 200 nmol cm-2 h-1, and Ks is about 0.7 mM Cl-. High external concentrations of NaCl increase unidirectional fluxes of Cl- and urea, indicating a change in paracellular pathways. Active transport of Cl- is temperature sensitive (Q10 equals 2.68) and is inhibited by cyanide, dinitrophenol, iodoacetic acid, iodide, thiocyanate, and acetazolamide. The Na-independent component of JClin was unaffected by amiloride, ouabain, or eserine, all of which inhibit Na+ transport.

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.

Frogs kept in dilute solutions of Cl- maintain a steady state with respect to this ion. Chloride is exchanged at a rate of about 15 mumol 100 g-1 h-1 (47 nmol cm-2 h-1). Over 90% of the efflux is integumentary of which about 50% is diffusion of the total influx. The rest is carrier mediated, half of which is exchange diffusion and half active transport. The chloride transport system displays saturation kinetics and is inhibited by acetazolamide. Uptake of Cl- is not dependent on concomitant uptake of cations. Salt-depleted frogs accumulate Cl- from dilute KCl or choline chloride in exchange for an endogenous base, probably HCO-3. High bath concentrations of NaCl (greater than 5 mM) abolish the active uptake of Cl- and increase the passive permeability of the skin to Cl-.

The urinary bladder of Amphiuma exhibits stable transport properties and an electrical potential difference in vitro. The lumen is significantly negative to the serosa and under short-circuited conditions flux rations for Na and Cl of 5.92 +/- 0.42 and 1.81 +/- 0.20, respectively, were observed. The close agreement between the short-circuit current and net Na flux suggests that most, if not all, of the current is carried by Na. Both ouabain and amiloride decreased the short-circuit current and the mucosal-to-serosal (M leads to S) flux of Na. Furosemide caused a transient increase in the M leads to S flux of Na and Cl but ADH was without effect. In bladders that had high transmural resistance, a net movement of K in the M leads to S direction under short-circuited conditions with flux ratios of up to 2 could be observed. The epithelium of the Amphiuma bladder consists of three cell types: granular cells, basal cells, and mitochondria-rich cells. No goblet cells are present. The mitochondria-rich cells comprise less than 5% of the population of the surface epithelium in Amphiuma in contrast to other amphibian bladders, where it accounts for up to 25% of the population.