General Discussion 77 



it was possible in many instances to obtain growth of fibroblasts in 

 tissue cultures from segments of aortas stored for periods of thirty to 

 fifty days at 4>° C in a salt solution-serum medium. 



Franklin: You haven't used a piece of the old vessel for repair? 



Kirk: No, I have done no transplantation studies. 



McCance: I think there is more in it than whether the tissue is alive 

 sufficiently to use for "repair" purposes. If, for instance, you take a 

 piece of kidney and cut thin slices of it and immerse it in the solution 

 containing no oxygen, the cells swell, but they can be restored to nor- 

 mality by replacing them in oxygen. The cells can also be made to swell 

 and become very abnormal by putting them for a time in cyanide 

 solution or some other poison: yet they may recover after they have 

 been removed from the cyanide and placed in a good oxygenated saline. 

 It is not so much a question of what the aorta will do later, as its exact 

 condition at the moment at which Dr. Kirk is studying it. And I would 

 feel that the value of this work would be enormously enhanced if he 

 could establish that his tissue was, when he was studying it, absolutely 

 normal, or as normal as it can be when it is old. 



Kirk: Of course, it is very difficult to establish that a tissue is 

 absolutely normal. We have measured the thickness of the aortic mem- 

 branes before and after the experiments, and the variations we have 

 found have only been of the magnitude of + 5 per cent. We believe that 

 one reason for our ability to keep the membrane of the same thickness 

 for some period of time has been the use of the buffer medium indicated 

 by Aebi {Helv. physiol. acta, 10, 184, 1952). This buffer has a rather high 

 potassium and calcium concentration. Aebi's studies have shown that 

 with a buffer of such composition no appreciable swelling of the tissue 

 occurs. Furthermore, the loss of nitrogenous material from the tissue is 

 very small. Our observations on aortic tissue (J. Gerontol., 9, 10, 1954) 

 have confirmed Aebi's findings. 



Aub: Your CO2 diffusion is lower than the oxygen and nitrogen; isn't 

 CO2 much more diffusible? 



Kirk: This has been discussed in the literature for a long time. As far 

 as I can see, the conflicting statements with regard to the diffusion rate 

 of carbon dioxide gas in solution compared with that of oxygen are due 

 to the fact that two different definitions of the diffusion have been 

 employed. One of these definitions is based on the difference in partial 

 pressure of the gas on the two sides of the membrane (Krogh, 1918-19, 

 J. Physiol., 52, 391), whereas the other is based on the difference in the 

 quantity of gas per volume of fluid (i.e. the concentration) (Hifl, Proc. 

 roy. Soc, B, 1928-29, 104, 39). 



If one employs the definition of the diffusion coefficient introduced by 

 Krogh the value for carbon dioxide wall be much greater than that for 

 oxygen. This is due to the fact that the absorption coelficient of carbon 

 dioxide in water at 38° C is 0-550, whereas that of oxygen is only 

 • 023. For the same partial pressure of the gases the carbon dioxide 

 content of the w ater will therefore be 23 • 8 times greater than the content 

 of oxygen. Since the diffusion of gases in water is inversely proportional 

 to the square root of their molecular weights, the diffusion rate of carbon 



