HANS H. USSING 



the larger molecules may not be able to penetrate the pores at all. In order to 

 measure the effect one ought therefore to use molecules as large as possible among 

 those which do penetrate. This imposes the experimental difficulty that the concen- 

 tration changes in the solutions in contact with the membrane are likely to be im- 

 measurably small. 



But here the double-labelling tracer technique may prove useful. In order to study 

 the water drag effect we have prepared (Andersen and Ussing, in preparation) 

 thiourea labelled with 14 C which, together with the commercially available 35 S- 

 labelled thiourea, gives a suitable pair. Thiourea is very water-soluble and hardly 

 soluble in lipoids. If pores are present it is therefore likely to follow these. Another 

 interesting feature is that since it is much larger than the water molecule it is likely 

 to penetrate mostly through the larger pores where the linear rate of water flow and 

 thus the drag effect is larger. 



The experimental approach is the following: A toad skin is placed as a membrane 

 with Ringer solution on the inside and i/io Ringer on the outside. 14 C-labelled 

 thiourea (20 mg. per cent.) is added to the outside solution and 35 S-thiourea of equal 

 concentration is added to the inside. It is then possible to measure both influx and 

 outflux of the substance although the transfers are far too small to be measured 

 chemically. The first experiments of this type were completed only a few days ago. 

 It is therefore only possible to give a few examples. In one experiment the influx and 

 the outflux were both 7-9 x io~ 10 mol/cm 2 /hr. Then posterior lobe hormone (1 

 unit per 20 ml.) was given to the inside solution. In the following three hours the 

 influx rose to 56 8 x io -10 whereas the outflux rose relatively less, to 43 «i x io -10 . 

 Thus the flux ratio (M in /Af out ) was i-oo in the first period but 1-31 during the 

 period of the hormonally stimulated water flow. 



Experiments of this type have, of course, to be performed under varying conditions 

 and also with various test substances. We hope, however, that it will prove possible 

 by this approach to obtain quantitative measures of some properties of living mem- 

 branes which have been hitherto very difficult to obtain. 



REFERENCES 



Capraro, V. and Bernini, G. (1952). Nature, Lond. 169, 454. 

 Collander, R. (1937). Trans. Faraday Soc. 33, 985. 



Davson, H. and Danielli, J. F. (1943). The permeability of natural membranes. Cam- 

 bridge University Press. 

 Fuhrman, F. and Ussing, H. H. (1951). J. cell. comp. Physiol. 38, 109. 

 Heller, H. (1945). Biol. Rev. 20, 147. 



Hevesy, G., Hofer, E. and Krogh, A. (1935). Skand. Arch. Physiol. 72, 199. 

 Huf, E. G. (1936). Pfliig. Arch. ges. Physiol. 238, 97. 

 Jorgensen, C. Barker (1950). Acta physiol. scand. 22, Suppl. 78. 

 Koefoed-Johnsen, V. and Ussing, H. H. (1952). Acta physiol. scand. 28, 60. 

 Novelli, A. (1936). Rev. Soc. argent. Biol. 12, 163. 

 Orr, W. and, Butler, J. (1935). Journ. Chem. Soc. p. 1273. 

 Pappenheimer, J. (1953). Physiol. Rev. 33, 387. 

 Prescott, D. M. and Zeuthen, E. (1952). Acta physiol. scand. 28, 77. 



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