HANDBOOK OF PHYSIOLOGY ^ CIRCULATION I 



3 and the return of the membrane potential to normal. 

 Thus, any factor which tends to raise [K+] at the 

 membrane should increase Pk and diminish the time 

 needed for repolarization; whereas, a low external 

 [K+] should lengthen this time. Consistent with the 

 theory are the findings that a) in a K-free medium 

 repolarization is greatly prolonged (184); /;) close 

 intra-arterial injection of K"*" shortens the action po- 

 tential (315); and finally c), interventions which cause 

 accumulation of K+ at the outer surface of the mem- 

 brane, either by inhibiting the rate of re-entry (digi- 



talis) or diminishing the time available for re-entry 

 between contractions (high frequency), also are asso- 

 ciated with a shortened action potential. 



Dudel & Trautwein (66) have reported that 

 cardiac glycosides cause an increase in the membrane 

 resistance of the Purkinje fibers of the cat during both 

 rest and activity. This finding suggests that in addi- 

 tion to an effect on active transport the glycosides 

 alter the permeability of the cell membrane. The 

 data of Glynn (96) on red cells are consistent with 

 such a conclusion. 



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