CARDIAC TRANSMEMBRANE POTENTIALS— HOFFMAN 305 



The action potential recorded from the Pnrkinje iil)er shows a more i)rominent 

 spiked reversal of polarity which lasts only a few msec, a prolonged plateau, and a 

 phase of repolarization which is slower than that of ventricular muscle. ^"^ While the 

 duration of depolarization is (juite sinn'lar in auricle and ventricle at similar heart 

 rates, the Pnrkinje fiber action potential lasts approximately twice as long as that 

 recorded from plain ventricular muscle. ^^' ^'' This difference in action potential 

 duration is present at both slow and rapid heart rates. One additional factor, not 

 apparent in the figure, is the considerable difference in rising velocity of the action 

 potentials recorded from specialized and plain cardiac muscle. The maximum rising 

 velocity of the action potential recorded from a single Pnrkinje fiber amounts to 

 500-1000 v/sec ;^^ this is considerably greater than values obtained for undiffer- 

 entiated ventricular fibers. ^^ 



Records obtained from isolated preparations of dog auricle and papillary muscle 

 are similar to those obtained from the intact heart in sitit'-^ and are comparable to 

 the transmembrane potentials of the intact human heart. -^ 



Ionic basis of transmembrane potentials. The distribution of ions between 

 intracellular and extracellular water is such that, in the case of cardiac muscle, the 

 concentration ratios of the major cations between the inside (I) and outside (O) 

 are: Ki:Ko = 30:1; Nai: Nao=l : lO;^^- ^'^^ 2* and Cai : Cao= 1 : 1.^^' ^-^ Much less 

 is known about the actual concentrations of the intracellular anions ; however, the 

 concentration of CI within the fiber is probably considerably less than that in the 

 extracellular fluid."' Under resting conditions the fiber membrane is somewhat 

 permeable to all of these ions, and during activity the net fluxes of K and Na in- 

 crease considerably (see below). An explanation of the concentration gradients thus 

 cannot depend solely on membrane impermeability. 



In the case of the isolated squid giant axon^^ experimental evidence indicates that 

 Na is actively extruded from within the axoplasm across the membrane by a trans- 

 port mechanism referred to as the "sodium pump." This transport takes place 

 against both the concentration and potential gradients. Such transport of positive 

 charge from inside to outside a membrane which behaves as a double-layer ca- 

 pacity-® would tend to create a potential difference across the membrane and thus 

 might be responsible for the intracellular concentration of K. The magnitude of 

 the resting transmembrane potential (—90 mv) is in accord with an activity ratio 

 for K of approximately 30: 1 and thus the distribution of K ions might be passive 

 in nature. On the other hand anoxia and certain enzymatic inhibitors in proper 

 concentration cause a decrease not only in the rate of Na extrusion but also a simi- 

 lar change in the rate at which K enters the fiber." However, under these conditions 

 the rate of K loss from the fiber and the magnitude of the resting transmembrane 

 potential are not significantly altered. These results suggest that while the accumu- 

 lation of K depends on metabolic activity and may be related to active Na transport 

 the outfiux of K is passive in nature. 



A direct dependence of the resting potential on the K concentration gradient and 

 outward diffusion of this ion across the membrane is suggested by the observation 

 just mentioned. Additional evidence in support of this proposition is afforded by 

 the demonstration that the magnitude of the resting potential is inversely proper- 



