CARDIAC TRANSMEMBRANE POTENTIALS— HOFFMAN 313 



plction of repolarization prior to recovery of excitability — will l)e nieiitioncfl subse- 

 (juently in conjtiiuiidii wilh the effects of low teinperalure. 



The effects of temperature. The effect of temperature changes on the trans- 

 membrane potentials of single cardiac muscles have been studied extensively in 

 recent years: frog ventricle;'^ mammalian auricle;-" mammalian Purkinje 

 fibers;'"''''^ mammalian papillary muscle/''-^' y\n attempt will be made to sum- 

 marize the results of these studies and to relate changes in membrane activity of 

 the single cell to the phenomena associated with cooling of the intact heart in situ. 

 In certain instances reference will be made to studies of single nerve and skeletal 

 muscle fibers ; in these tissues certain fields of investigation have progressed beyond 

 the scope of present knowledge of cardiac muscle. 



(a) The magnitude of the resting and action potential. Cooling of an isolated 

 Purkinje fiber preparation results in a small decrease in the magnitude of the rest- 

 ing transmembrane potential (-5 mv) and a small but consistent increase in the 

 reversal of polarity at the peak of the action potential (+8 mv).-*° In the case of 

 the isolated cat auricle-" the change in action potential amplitude during cooling is 

 quite similar. In this tissue the niaxium change ( -1- 10 mv) is recorded between 

 24—28° C. and is associated with a slight increase (-I-2-3 mv) in the resting po- 

 tential. In the isolated papillary muscle of both cat**' and dog^" a decrease in tem- 

 perature results in a maximum action potential amplitude at approximately 25° C. 



If the temperature is lowered below 25° C. the changes in the action potential 

 and resting potential are much greater in magnitude. Moreover, below this tempera- 

 ture the magnitude action potential and reversal decreases rapidly. In Purkinje 

 fibers^" the resting potential falls to low levels (50-60 mv) in the temperature 

 range of 20-10° C. and simultaneously the action potential is greatly diminished in 

 height. Similar results are obtained from the isolated papillary muscle.^"' *^ In both 

 tissues the action potential often fails to show any reversal at the lower tempera- 

 tures. The cat auricle fails to conduct at temperatures below 2y C.-^ Information 

 concerning the action potential at lower temperatures is not available. 



The changes in both resting potential and action potential occur almost instan- 

 taneously during cooling and are completely reversed with equal speed by rewarm- 

 ing. This observation suggests that the changes in magnitude of the resting potential 

 and action potential induced by cooling do not result from changes in the concen- 

 tration gradients of ions across the fiber membrane. Attempts to explain the ob- 

 served changes in terms of other mechanisms, however, are uncertain. The increase 

 in magnitude of the reversal, at the peak of the action potential, probably results 

 quite simply from the relative difference in the efifect of cooling on factors causing 

 depolarization and repolarization. The rising phase is relatively insensitive to cool- 

 ing, while repolarization is markedly slowed (see below) ; the action potential thus 

 tends to approach an upper limiting value dependent upon the Na concentration 

 gradient. Slight changes in resting potential can be explained on the basis of the 

 effect of temperature on an ion concentration potential.*^ The marked decrease in 

 resting potential at lower temperatures is more difficult to account for. It has been 

 demonstrated that cooling decreases the activity of the sodium-pump and as a result 

 the rate of active transport of Na outward and K inward across the membrane is 

 diminished.^ ^ The decrease in resting potential thus might result from either an 



