HANS H. USSING 



proportional to the activity rather than to the concentration of the water. Even in 

 this case the net transfers of water were much larger than predicted from the water 

 activities, whether the gut contents were hypotonic or hypertonic with respect to the 

 blood. Visscher took this as evidence that the water movements across the intestinal 

 wall are due largely to active processes rather than to simple diffusion. 



A few years ago in the Zoophysiological Laboratory of Copenhagen we resumed 

 the study of water movements across the amphibian skin. The impetus to this study 

 was a wish to clarify the mechanism underlying the so-called Brunn reaction or 

 water balance reaction of anuran amphibians, which has been extensively studied in 

 recent years (for references compare Heller, 1945, and Jorgensen, 1950). The re- 

 action consists in an increased uptake of water through the skin following the in- 

 jection into the animal of small doses of posterior lobe hormones. The response can 

 also be elicited in the isolated skin of toads (Novelli, 1936) and frogs (Fuhrman and 

 Ussing, 1 95 1, Sawyer, 1951). 



Since the Brunn reaction is more pronounced in toads than in frogs, skins of the 

 former animal were used. An apparatus was designed which allowed the determina- 

 tion of the net water-transfer rates with an accuracy of ± 10 jul. and, simultaneously, 

 the measurement of the water-diffusion rate, using 5 per cent, heavy water as a 

 tracer. As inside medium ordinary Ringer solution was used, whereas the outside 

 medium was 1/10 Ringer. 



Some typical results are shown in Table I (Koefoed-Johnsen and Ussing, 1952). 

 The heavy-water diffusion figures are calculated as total influx values {M in ) ex- 

 pressed as the amount that would pass through unit area in unit time if the heavy- 

 water concentration were maintained at 100 per cent, in the outside compartment 

 and at zero in the inside compartment. The net water flux, A w , as well as the influx, 

 is given in /xl./cm. 2 /hr. 



The results confirm in every respect those of Hevesy, Hofer and Krogh (I.e.) on 

 live frogs. For the sake of argument, let us assume that the water uptake is due to 

 simple osmosis and that the net uptake is the difference between two diffusion 

 streams. The permeability coefficient as calculated from heavy-water diffusion, 

 namely P diff , is defined by the equation 



Mfa = " diff c w(o) 

 For M m = 532 /zl./hr. P dm works out to be 1 48 x io~ 4 cm./sec. In the same experi- 

 ment A w was 30 /u,l./hr. 

 Now, for A w we have 



A w = M- m — M ont = P osm C w(o) P OS m C w(i) = ° osm( C w(o) — C w(i)) 



Remembering that A w and (c w(o) — c w{i) ) should be expressed in the same units, 

 we get 



^osm =2 32 x 10- 3 



or nearly 1 6 times the figure for P diff . 



It is seen that the influx changes only slightly on the addition to the inside solution 

 of posterior lobe extract. The flux may even go down. But the net flux always in- 

 creases violently, often by more than 100 per cent. In the beginning we took this 

 finding as an indication that the hormone evokes an active transport of water, a view 



34 



