Correlation between transepithelial Na+ transport and transepithelial water movement across isolated frog skin (Rana esculenta).

In the present work the coupling under short-circuited conditions between the net Na(+)-influx across isolated frog skin and the transepithelial transport of water was examined i.e., the short-circuit current (Isc) and the transepithelial water movement (TEWM) were measured simultaneously. It has been shown repeatedly that the Isc across isolated frog skin is equal to the net transepithelial Na+ transport. Furthermore the coupling between transepithelial uptake of NaCl under open-circuit conditions and TEWM was also measured. The addition of antidiuretic hormone (AVT) to skins incubated under short-circuited conditions resulted in an increase in the Isc and TEWM. Under control conditions Isc was 9.14 +/- 2.43 and in the presence of AVT 45.9 +/- 7.3 neq cm-2 min-1 (n = 9) and TEWM changed from 12.45 +/- 4.46 to 132.8 +/- 15.8 nL cm-2 min-1. The addition of the Na+ channel blocking agent amiloride resulted in a reduction both in Isc and TEWM, and a linear correlation between Isc and TEWM was found. The correlation corresponds to that 160 +/- 15 (n = 7) molecules of water follow each Na+ across the skin. In another series of experiments it was found that there was a linear correlation between Isc and the increase in apical osmolarity needed to stop the TEWM. The data presented indicate that the observed coupling between the net transepithelial Na+ transport and TEWM is caused by local osmosis.