Membrane structure as revealed by permeability studies 



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Discussion 



Chairman: J* Bracket 



R. D. Keynes. Are you inclined to doubt the existence of mechanisms for the active 

 transfer of water in other tissues ? There seems to be good evidence for the active 

 absorption of water in the intestine, for example. 



H. Ussing. I am quite convinced that active transport of water does occur in certain 

 organs. But I want to emphasize that a discrepancy between the water activity ratio 

 and the water flux ratio may be an indication of the presence of pores rather than 

 an indication of active water transport. It is curious that, when isotonic sucrose is 

 placed outside, there is no transfer of water across frog skin. 



H. Heller. Would you conclude that the effect of neurohypophysial hormone is on 

 pore size, and if so how is it produced ? 



H. Ussing. Yes, but the experimental evidence does not indicate the means. 



J. F. Danielli. To what extent would the occurrence of pinocytosis modify your 

 analysis ? 



H. Ussing. The analysis I have used is a purely formal one, and in certain cases 

 pinocytosis might simulate a porous membrane. Pinocytosis is likely, however, to 

 influence all solutes in the same way. It is our hope that, by performing the analysis 

 with a series of substances, certain concepts may be ruled out and others become 

 more likely. 



E. ^euthen. Pinocytosis should be unidirectional, but swelling and shrinkage indicate 

 a capacity of water to pass both ways. 



J. F. Danielli. Some tissue culture cells are continually taking up water by pinocytosis. 

 It must leave the cell again, but how? 



J. A. Kitching. The cells of Hydra are permeable to water, and the internal osmotic 

 pressure exceeds the external, so that the same problem arises here. 



R. J. Goldacre. I understand that the area occupied by the pores would be only a 

 small fraction of the total area of the membrane. In the frog skin, could the spaces 

 between the cells be adequate to account for the effect observed, or do you require 

 pores also in the plasma membrane of each cell ? 



H. Ussing. Neurohypophysial hormones decrease the resistance to active transport 

 of Na as well as the resistance to water flow. It therefore seems that water and Na 

 should follow in part the same paths. If, as seems likely, the active transport of Na 

 goes through the cell membrane, the latter would seem to have pores. Moreover, 



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