Peter F. Curran

Na Transport across Frog Skin at Low External Na Concentrations

Isolated frog skin was bathed with a dilute solution containing 1 mm NaCl on the outside and with normal Ringer’s solution on the inner surface. Net Na flux was determined by simultaneous measurement of unidirectional fluxes with Na22 and Na24 and intracellular electrical potentials were examined with microelectrodes. There was a net inward transport of Na under both open-circuit and short-circuit conditions. The short-circuit current was approximately 15% greater than the net Na flux; the discrepancy could be accounted for by a small outward flux of Cl. The electrical potential profile did not differ greatly from that observed in skins bathed on the outside with normal Ringer’s solution. Under open-circuit conditions, there were usually several potential steps and under short-circuit conditions the cells were negative relative to the bathing solutions. Estimates of epithelial Na concentrations utilizing radioactive Na suggested that if all epithelial Na were in a single compartment, an active entry step would be necessary to allow a net inward Na transport. The results could also be explained by a series arrangement of Na compartments without necessarily postulating an active Na entry. The behavior of the potential profile suggested that this latter alternative was more likely.

Effect of Sugars on Transport of Alanine in Intestine

The eflect of glucose and galactose on transport of alanine by rabbit ileum has been investigated. Transmural transport and cellular ac cumulation of the amino acid were in hibited by the sugars, but alanine in flux across the mucosal border of the cells was unaltered.

Alanine transport across isolated rabbit ileum

Abstract 1. (1) When both surfaces of isolated segments of rabbit ileum are bathed with a Ringers solution containing 140 mM Na + and 5 mM l -alanine, there is an average net flux of l -alanine from the mucosal to the serosal solution of 1.2 μmoles/h · cm 2 . Under these conditions the unidirectional flux from musoca to serosa is approximately ten times the flux in the opposite direction. The net flux of alanine is nearly equal to the previously observed increment in active Na + flux brought about by the addition of 5 mM alanine to the mucosal solution. 2. (2) Net transport of l -alanine is markedly inhibited if the Na + in the bathing solution is replaced with choline, or if 10 −4 M ouabain is added to the serosal solution in the presence of Na + . In both instances, the inhibition of net alanine transport is the result of a decline in the unidirectional flux from mucosa to serosa. Addition of ouabain to the mucosal solution alone does not significantly inhibit transport of the amino acid. 3. (3) There is net transport of d -alanine across rabbit ileum in the absence of a concentration difference and the tissue accumulates d -alanine to relatively high concentrations. This accumulation is markedly inhibited by ouabain and by replacement of the Na + in the incubation medium with choline.

Interpretation of Hexose-dependent Electrical Potential Differences in Small Intestine

ACTIVELY transported sugars added to the solution bathing the mucosal surface of the small intestine produce an increase in the electrical potential difference (p.d.) and short-circuit current (Isc) across the tissue1–4. Schultz and Zalusky1 have shown that the increase in the Isc (ΔIsc) across rabbit ileum is a function of both sugar and sodium concentrations in the bathing solution. When sodium was replaced with potassium, the maximum ΔIsc caused by addition of sugars was a linear function of sodium concentration. Recently, Barry et al.5 reported that the increase in the p.d. (Δp.d.) following addition of galactose to the mucosal bathing solution of everted sacs of rat mid-intestine is independent of sodium concentration when mannitol is used to replace sodium chloride, but is approximately a linear function of sodium concentration when potassium is used to replace sodium. They have concluded that the “galactose dependent potential is unaffected by sodium concentration but is reduced by increasing potassium concentration”. Lyon and Crane6 have reported that, in rat jejunum, the maximum Δp.d. resulting from addition of actively transported sugars is independent of sodium concentration when tris [2-amino-2(hydroxymethyl)-1,3-propanediol] is used to replace sodium, but decreases when potassium is used as replacement ion.