CARDIAC TRAXSAri'.MIil^WI-. I'OTI-.XTrAI.S-TIOFFMAN 



319 



tissues the duration of the action potential is decreased hy high potassium and the 

 duration of the refractory period is shortened. The threshold of the menil)rane to 

 external stimuli is first decreased and sul)sequently increased as the potassium con- 

 centration is progressively elevated.^'* 



The nature of these changes in excitahility and memhrane activity can he ex- 

 plained in terms of the effect of K on the resting potential and the known relation- 

 ships between resting potential, action |)otential and excital)ility (see above). If the 

 resting potential of a fiber depolarized by high potassium is raised to the normal 

 value by applied current, excitability is restored and action potentials showing the 

 usual configuration and ami)litude can be elicited.^- The change in action potential 

 duration most likely also results from the low resting potential ; however, a specific 

 effect of high extracellular K on the rate of repolarization has been suggested by 

 recent observations.^^ 



A decrease in the concentration of extracellular potassium also results in progres- 

 sive depolarization of the fiber.^^ In this case the changes in action potential con- 

 figuration and amplitude are similar to those produced by K excess (fig. 12). Ex- 

 citability and spontaneous rhythmicity, on the other hand, are greatly enhanced by 

 low K until the resting potential falls below 5o-50 mv. 



The effect of potassium on the transmembrane resting potential is dependent 

 upon the concentration of Ca in the extracellular fluid (fig. 13 j. High Ca protects 

 the fiber from the depolarizing effect of elevated K and increases the depolarization 

 resulting from a decreased K concentration. Low Ca has the opposite effect. ^^ This 

 relationship is similar to that found in skeletal muscle.'^' Changes in the calcium 

 concentration have several other important actions on the transmembrane poten- 

 tials. i^' "' ^-' "^^ Most important, perhaps, is the so-called stabilizing effect of ele- 



100 msec' 



100 msec 



Fig. 13. — Tracings of transmembrane action potentials of single ventricular fibers showing- 

 effects of potassium and calcium. (A) Ca = Vio normal, K ^ normal (solid line) =i normal 

 (dotted line) =3 times normal (dashed line); (B) Ca = 3 times normal, K = normal (solid 

 line) =:j normal (dotted line) =: 3 times normal (dashed line.) Note that high Ca protects 

 against high K, low Ca against low K. Arrows indicate peak reversal of appropriate action 

 potentials. Time and voltage calibrations shown in figure. 



