8U» VII. LIPID DISTRIBUTION IN SPECIFIC TISSUES 



the thesis that a direct synthesis of fatty acids from acetate takes place in 

 the udder. Using the arteriovenous differences in the acetate content of 

 blood as the criterion, McClymont found that the acetate content of arterial 

 blood decreased from 2 to 8 milligram per cent as it passed through the 

 udder. The actual decrease in acetate depended upon its original level in 

 the arterial blood. 



e'. Proof of the Synthesis of Milk Fat from Glucose and from Acetate, 

 Based upon the Stimulatory Effect of Insulin on the Reaction, as Demon- 

 strated by the Respiratory Quotient : It is now well known that one of the 

 main functions of insulin is the stimulation of lipogenesis. A number of 

 workers 581 ' 582 demonstrated this effect by the use of liver slices. Since 

 mammary tissue possesses such outstanding potentialities in synthesizing 

 fat from small molecules in vitro, it should offer a favorable medium for 

 studying the insulin effect. 



On the basis of measurements of respiratory metabolism, Balmain and 

 associates 583,584 demonstrated that crystalline insulin exerted a marked 

 effect on the fat synthesis in mammary tissue of such non-ruminants as 

 the mouse, rat, and rabbit, provided that glucose was present in the medium. 

 Insulin increased the R.Q. and oxygen consumption. There was also a 

 concomitant rise in the rate of disappearance of glucose and acetate. Only 

 0.1 fig. (0.0022 I.U.) of insulin was required per ml. to initiate this re- 

 action. 518 On the other hand, insulin failed to exert a stimulatory effect 

 on sheep udder slices, either with acetate alone or with acetate plus glucose. 

 This indicates that a profound difference obtains in the action of insulin on 

 fat synthesis in non-ruminants as contrasted with ruminants. 518 



f . Proof of the Synthesis of Milk Fat from Glucose and from Acetate, 

 Based upon the Stimulatory Effect of Insulin on the Reaction, as Demon- 

 strated by Isotope Studies: The results of the respiration experiments have 

 been confirmed by studies with isotopic acetate. Thus, Balmain et al. b72 

 found that, when slices of the lactating mammary gland of the rat were 

 incubated with C 14 -acetate and unlabeled glucose, a synthesis of fat oc- 

 curred; the rate of this fat, formation could be augmented with insulin 

 in the in vitro experiments. Insulin was likewise able to cause an in- 

 crease in the incorporation of tritium into the fatty acids of rat mammary 

 slices incubated in tritiated water. 585 On the other hand, insulin was 



681 K. Bloch, Cold Spring Harbor Symposia Quant. Biol, 13, 29-34 (1948). 



682 R. O. Brady and S. Gurin, /. Biol. Chem., 186, 461-469 (1950). 



683 J. H. Balmain, T. H. French, and S. J. Folley, Nature, 165, 807-808 (1950). 



684 J. H. Balmain and S. J. Folley, Biochem. J., 48, i-ii (1951). 



685 J. H. Balmain, S. J. Folley, and R. F. Glascock, Nature, 168, 1083-1084 (1951). 



