CARDIAC TRANSMl'MBRANE POTKNTIALS— HOFFMAN 



315 



gests that the cliangc in rate during cooling does not result solely from the effect of 

 temperature on the diffusion of ions but that more complex reactions are concerned 

 with the membrane properties peculiar to pacemaker regions of cardiac fil)ers. 



Spontaneous activity ceases in isolated Purkinje fibers at temperatures between 

 25-15° C.^" in most instances, although occasional beats do occur at temperatures as 

 low as 10° C. In addition, if the progressive change in membrane potential of a 

 single fiber is studied during cooling, it is found most frccjuently that activity ceases 

 during diastole, the resting potential remaining at a low level (50-60 mv). In 

 certain cases, however, activity ceases during an action potential and the transmem- 

 brane potential of the fiber stays at the level of the plateau (near zero). In this 

 case repolarization of the membrane does not begin imtil the fiber has l)een re- 

 warmed.^" 



(c) Refractoriness. Changes in the duration of refractoriness resulting from a 

 decrease in heart temperature have been discussed in the preceding paper. In 

 auricle, ventricle, and specialized conducting tissues low temperature results in an 

 increase in duration of both the absolute and relative refractory periods.^" Studies 

 of the transmembrane action potentials of these three tissues during cooling reveal 

 changes in the time-course of repolarization mainly responsible for these alterations 

 in refractoriness. In the case of the Purkinje fiber, the duration of both the plateau 

 and the final phase of repolarization are increased 1)y low temperature.^"- *^' The 

 relative change in the plateau, however, is considerably greater than in the final 

 phase of repolarization. During rapid cooling of the isolated papillary muscle the 

 alterations in these two subdivisions of the action potential are similar in nature — 

 the relative prolongation of the plateau predominates (fig. 10). From these results 



100 msec 

 < — ^ 



— -100 mv 



Fig. 10. — Tracings of transmembrane action potentials of a single isolated ventricular fiber 

 in the dog papillary muscle showing the effect of rapid cooling. Temperature in degrees centi- 

 grade shown next to appropriate curves. Time and voltage calibration shown in figure. 



