Direct Measurement of Uptake of Sodium at the Outer Surface of the Frog Skin

Abstract

A combination of the methods described by Schultz et al. (6) and by Ussing and Zerahn (9) was used to measure directly the unidirectional uptake of sodium from the outside solution into the frog skin, under shortcircuit conditions. The sodium uptake was determined at six sodium concentrations ranging from 3.4 to 114 m. NaCI was replaced by choline chloride in the solutions bathing both sides of the skin. Sodium uptake is not a linear function of sodium concentration but appears to be composed of two components, a saturating one and one that varies linearly with concentration. The sodium uptake is inhibited by the addition of lithium to the outside solution. The effect appears to be primarily on the saturating component and has the characteristics of competitive inhibition. In addition, lithium uptake by the skin is inhibited by sodium. The effects of lithium cannot be ascribed to changes in electrical potential difference. Measurements with microelectrodes indicate that under short-circuit condition there is no change in the intracellular potential when lithium chloride is added to the outside solution.The functional model for Na transport across frog skin proposed by KoefoedJohnsen and Ussing (1) has frequently served as a basis for consideration of transport across other epithelial tissues. One of the important features of this model was that entry of Na into the skin from the outside solution is due to simple diffusion. This concept was based primarily on the observation that in open-circuit conditions the outer surface of the skin behaved like a Na electrode. However, some recent observations are rather difficult to explain in terms of simple diffusion of Na across the outer surface. For example, Cereijido et al. (2) concluded from indirect estimates of rate coefficients for Na movement that the apparent Na permeability of the outer barrier decreased with increasing Na concentration. On the basis of studies of Na transport by skins bathed on the outside with dilute solution containing I nm NaCl, Biber et al. (3) suggested that if all epithelial Na were in a single 83

Enhancement of electrogenic Na+ transport across rat inner medullary collecting duct by glucocorticoid and by mineralocorticoid hormones.

“…In this process, Na+ enters the cell via an apical membrane channel and is extruded by the basolateral membrane Na+-K+ ATPase. The frog skin (26,27), toad urinary bladder (28), rabbit urinary bladder (29), and the turtle colon (30) kidney distal nephron, and particularly the collecting duct, also possess an electrogenic Na+ transport system but the processes are more complex. For example, the rat distal nephron has an electroneutral NaCl cotransport system in addition to an electrogenic system (31).…”

The effect of antidiuretic hormone on Na movement across frog skin

“…While the values of most parameters discussed in this paper were measured by several authors in both species, comparative values of JOT are not available because the technical possibility of a direct measurement of JOT is relatively new Biber & Curran, 1970), and (2) the technique for the evaluation of JOT used in this paper required a large exposed area (8-9 cm2) and this makes it impossible to obtain two pieces of M. CEREIJIDO AND C. A. ROTUNNO white abdominal skin for paired experiments out of the same L. ocellatus. R. pipiens are available in sizes as large as 4-5 in.…”

Characteristics of the entry process for sodium in transporting epithelia as revealed with amiloride.

“…There is a hyperbolic relationship between sodium influx (or at least that component of influx associated with sodium transport) and sodium concentration, indicative of a saturable phenomenon. At a given sodium concentration antidiuretic hormone increases influx, whereas amiloride inhibits influx (Rotunno, Villalonga, Fernandez & Cereijido, 1970;Biber & Curran, 1970;Cereijido & Rotunno, 1971;Biber, 1971;Ferguson & Smith, 1972; Moreno, Reisin, Rodriguez-Boulan, Rotunno & Cereijido, 1973;Biber & Cruz, 1973;Erlij & Smith, 1973;Biber & Sanders, 1973). One of the sources cited above (Biber & Sanders, 1973) is of particular interest since it was shown that the extent of sodium influx at a given sodium concentration can be influenced by the transepithelial potential.…”