The present position in the field of facilitated diffusion and selective active transport 



concentration of cholinesterase associated with the membrane of motor end plates 

 (Holt, 1954); the high concentration of periodate-oxidizable carbohydrate at the 

 surface of secretory cells (Ruyter, 1953; Bell, unpublished). 



(5) Direct activators and inhibitors. It is thought probable, on somewhat slender 

 grounds, that a number of substances act directly on transfer mechanisms, e.g. 

 phosphate esters and acetylcholine (Danielli, 1953), insulin and anterior pituitary 

 hormone (Cori, 1945), phloridzin (Rosenberg and Wilbrandt, 1952), dinitrofluoro- 

 benzene (Bowyer, 1954) and possibly some of the oestrogens (Bullough, 1953). It is 

 to be hoped that more substantial evidence bearing on these interactions will soon 

 become available. 



(6) Potential studies. Where movement of ions is involved, the selective movement 

 of any one species will result in formation of an electrical potential difference. Such 

 potential differences must be compatible with, and quantitatively explained by, the 

 movements of the individual ions. Consequently, potential measurements have a 

 valuable place in transfer studies although, as Gasser (1933) stated, 'you cannot 

 determine a process from a potential'. 



EXAMPLES ILLUSTRATING THE PRESENT PROBLEM 



There have been several recent reviews,, e.g. Rosenberg and Wilbrandt (1952), 

 Goldacre (1952), Danielli (1953), Stadie (1954) and the recent S.E.B. Symposium 

 (Volume VIII, 1954). In this symposium Dr. Koch and Dr. Keynes will be con- 

 cerned with movements of ions: I shall limit myself to non-electrolytes. 



(A) The penetration of sugars into muscle 



When a substance is injected into an animal it rapidly becomes distributed through 

 the blood and extracellular spaces, but the extent to which it penetrates into intra- 

 cellular water is determined by ability to pass through cell plasma membranes. 

 Table I summarizes some of the main results obtained by Levine and his colleagues 

 (1950, 1953a, b). These results were obtained on animals which had been eviscerated 

 and nephrectomized, so that side effects due to metabolism and excretion might be 

 minimized. The data contained in the table effectively outline the problem as it 

 presents itself in mammals. Urea, which readily enters most mammalian cells, is 

 distributed in a volume of water equivalent to 70 per cent, of the body weight 

 (i.e. practically all the body water), whereas the non-penetrating substance sucrose 

 is distributed in an equivalent of 45 per cent, weight. Insulin has no effect on the 

 distribution of either sucrose or urea, (/-glucose is initially distributed in 45 per cent., 

 and there is an increase on adding insulin. But results with glucose are complicated 

 by metabolism, and the insulin effect is better seen with (/-galactose, /-arabinose and 

 ^/-xylose, with which a distribution-weight of 45 per cent, is raised to 70 per cent, 

 by insulin, /-rhamnose and (/-arabinose are not metabolized, and show no insulin 

 effect, and (/-fructose, (/-mannose and /-sorbose, though metabolized, show very much 

 less effect of insulin than does glucose. Thus insulin enables some substances to 

 penetrate readily into a volume of body water into which they move with great 

 difficulty, in the absence of insulin. The insulin action is structurally and sterically 

 specific — e.g. is positive for /-arabinose and negligible for (/-arabinose. Also it is not 



