34 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



TABLE I 

 Apparent Potassium Exchange Rates in the Isolated, Perfused Hindleg of the Cat 



'Fast' compartment 'Slow' compartment 



Tem- 

 perature 

 ° C. 



35° 



5° 



cellular K*. The partition of cell K* between the "rapidly exchanging" and "slowly 

 exchanging" compartments is essentially unaffected by temperature. The calculated 

 "exchange rate" between plasma and the "fast" compartment is reduced by slightly 

 more than half over a 30 degree experimental decrease in temperature, a low tem- 

 perature coefficient indeed, and for the "slow" compartment, there is scarcely any 

 efifect of temperature at all. 



Before we can interpret the "isotope exchange" data, we must examine what it is 

 that an isotope equilibration experiment measures. Consider a K*- ion in the plasma. 

 First, it is carried by the blood stream to the capillary bed, second, it diffuses across 

 the capillary wall, third, it diffuses through the interstitial fluid, and finally, it arrives 

 at the muscle cell membrane and is transported across. Thus a chain of transport 

 processes exists, and the slowest member of the chain will determine the overall 

 transport rate. As far as the "slow" compartment is concerned, blood flow appears 

 to be the limiting factor in determining the rate at which K*- equilibrates. If we 

 recalculate the K'' "exchange rates" as plasma clearances (exchange rate divided by 

 plasma concentration), we find that the slow compartment clearance is the same as 

 the clearances of Na'-"* and sucrose, two substances which differ greatly in their 

 distribution in the tissues and in their mechanisms of transport. Table II compares 

 plasma clearances of both cations at normal and low temperatures. Both slow com- 

 partment clearances are independent of temperature. We conclude therefore, that 

 the slowly equilibrating component for both ions (and also for sucrose) represents 

 a portion of the perfused hindleg which is poorly circulated, mean plasma flow 

 about 0.2 ml./min. per 100 gm., and in which the low rate of l)lood flow limits the 

 blood-tissue transport of these substances.* 



TABLE II 



Plasma Clearances of Potassium and Sodium, 

 Ml./min. x 100 gm. 



(a) 'Fast' compartment clearances 



Tem- 

 perature 



° C. Na^i K'2 



35° 1.23 1.54 



3°-10° 1.44 0.71 



(b) '.Slow' compartment clearances 



Tem- 



perature 



» C. Na'-^' K'- 



35° 0.19 0.23 



3°-10° 0.17 0.21 



