CARL F. CORI 



heart muscle. Following glucose injection, heart muscle 

 proved to be much more permeable than skeletal muscle. This 

 was attributed to the difference in the number of open capil- 

 laries in the active cardiac muscle as compared to resting 

 skeletal muscle. In heart, after glucose injection, the ratios, 

 tissue sugar/plasma sugar, ranged from 0.4 to 0.6. Had the 

 sugar been present only in the extracellular fluid space, the 

 ratio would have been about 0.2. Part of the sugar was there- 

 fore intracellular, and the increase in the intracellular concen- 

 tration with rising plasma concentration indicated that the rate 

 of penetration was greater than the rate of utilization. When 

 skeletal muscle was stimulated, thereby imitating heart muscle, 

 an intracellular distribution of sugar could be demonstrated (6). 



Following the intravenous injection of a nonutilizable sugar 

 into a nephrectomized animal, to take relatively simple ex- 

 perimental conditions, the following factors come into play in the 

 distribution of the sugar in the body: (7) rate of penetration 

 through capillary wall; (2) rate of distribution in the extra- 

 cellular fluid space ; (3) rate of penetration into different tissue 

 cells ; and (4) selectivity of various cell membranes. In general, 

 (7) is a very rapid process, to judge from the initial rate of dis- 

 appearance of the sugar from the blood. The rate of (2) and 

 hence of (3) will depend on the cross section of open capillaries, 

 on blood pressure, and on rate of blood flow; for this reason 

 different tissues will attain a state of equilibrium at different 

 rates. 



In regard to (4), definite information is available with re- 

 spect to erythrocytes, small intestine, kidney tubules, and 

 peritoneal cavity. The latter shows no selectivity with respect 

 to the rate of absorption of various hexoses and pentoses, whereas 

 intestine and kidney are highly selective. The capillaries of the 

 choroid plexus (representing the so-called blood-brain barrier) 

 are also selective, whereas the glomerular capillaries allow the 

 passage even of substances of relatively large molecular weight, 

 such as inulin. 



Whether the mechanism of uptake of sugars by tissue cells 



202 



