24 Hugh Davson 



excreting system fails at low temperature or in the presence 

 of cyanide. Thus, soaking a muscle at 0° certainly leads to 

 swelling, but this is completely accounted for by the gain of 

 Na+ and Cl~; warming the muscle causes an excretion of 

 these ions and it returns to its original volume. The same 

 argument will apply to other tissues, and conclusive proof 

 that this is the principal explanation for the changes taking 

 place on cooling was provided by the elegant experiments of 

 Deyrup (1953) who showed that if the tissues were bathed in 

 iso-osmotic sucrose (0 • 3 m) they failed to swell. If the swelling 

 in Ringer solution had been due to a failure of a water-excret- 

 ing mechanism, substitution of salt for sucrose should have 

 had no effect, whereas if the swelling had been due primarily 

 to a penetration of NaCl, substitution of a non-penetrating 

 substance like sucrose would have prevented it. It seems 

 safe to conclude, then, that very large differences of osmolarity 

 between cell contents and their environment, such as those 

 postulated by Opie (1949) and Robinson (1952), do not occur. 

 The detection of smaller differences, that would demand a 

 water pump continuously excreting water from the cell to 

 maintain an osmotic steady state between cells and their 

 environment, must rely on very precise measurements of 

 osmolarity. 



The depression of freezing point has been employed by a 

 number of workers with a view mainly to testing the claim 

 that mammalian tissues were hypertonic to plasma (Conway 

 and McCormack, 1953; Opie, 1954; Brodsky et al, 1953, 1956; 

 Conway, Geoghegan and McCormack, 1955 ; Itoh and Schwartz, 

 1956); but, as Conway's studies indicate, the interpretation 

 of the results is not easy, since an excised tissue, when ground 

 up at 0°, undergoes autolytic changes — in particular the 

 breakdown of adenosine triphosphate to inosinic acid, 

 ammonia and phosphate — that lead to a considerable increase 

 in osmolarity. It would seem from Conway's studies that 

 within the limits of accuracy of the cryoscopic method — 

 probably a few per cent — the tissue cells examined — liver, 

 kidney and muscle — are iso-osmotic with their environment. 



