Correlation Between Transepithelial Na+ Transport and Transepithelial Water Movement Across Isolated Frog Skin (Rana esculenta)
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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 (I sc ) and the transepithelial water movement (TEWM) were measured simultaneously. It has been shown repeatedly that the I sc 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 I sc and TEWM. Under control conditions I sc 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 I sc and TEWM, and a linear correlation between I sc 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 I sc 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.
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