§ 5.212 CARBOHYDRATE MHTABOLISM 197 



allows of a more rapid movement of glucose both in and out of the 

 cells. Since the cell membranes are not permeable to the phos- 

 phorylated hexose, the formation of this within the cell would 

 automatically have the role of trapping the glucose and rendering 

 the transfer virtually a one-way system. Even though insulin had 

 no effect upon the rate of phosphorylation, such a system "would 

 be consistent with the fact that glucose utilization [in contrast to 

 galactose distribution, Fig. 5-9] is a summated phenomenon of 

 both insulin-stimulated cellular entry and of metabolic disposal". 



Fish. In some teleosts and elasmobranchs insulin maintains a 

 relatively low level of sugar in the blood; but the results of pan- 

 createctomy are inconsistent, presumably because the source of 

 glucagon (§ 5.211) is removed as well as that of the insulin. 



Amphibia. In Rana and Bufo the loss of insulin by pancre- 

 atectomy results in hyperglycaemia, or increased blood-sugar, 

 because the source of the diabetogenic hormone in the adrenal 

 cortex is not removed (Houssay, 1959). 



AvES. Injected insulin is effective in lowering the level of blood- 

 sugar in pigeons, which can survive higher doses of this hormone 

 than can mammals (Riddle et al., 1947). 



Mammalia. Experiments have been carried out on anaesthetized 

 and eviscerated dogs and rats to test the effect of insulin on the 

 level of blood-sugars, in the absence of utilization and storage in 

 the liver or loss by excretion (Levine et ah, 1950). Glucose is 

 steadily consumed in the tissues; but another monosaccharide, 

 galactose, is not utilized. When a known quantity of the latter is 

 injected, it is found to distribute itself in a volume equivalent to 

 about 45 per cent of the animal's weight. This is equal to the 

 volume of circulating blood. After injection of insulin the con- 

 centration of galactose in the blood drops, as though it were 

 distributed in a volume equal to 70 per cent of the body weight 

 (Fig. 5-9). This is equivalent to the volume of all the body water, 

 and it is assumed, therefore, that the sugar has been allowed to pass 

 into the intracellular fluid of the cells. By comparing a number 

 of monosaccharides, it can be further shown that insulin only 

 facilitates their passage into the tissues from the blood if the 

 configuration of their molecules is such that the side-chains 

 attached to the first three carbon atoms are the same as those of 



