264 HANDBOOK OF PHYSIOLOGY ^ CIRCULATION I 



TIME (Sec) 



1.0 2.0 iO TIME (Sec) 



FIG. 20. Superimposability of cardiac action potentials. All potentials were recorded with an 

 intracellular electrode from a perfused frog ventricle. A: eflFect of stimulus interval on duration of 

 AP. Five superposed tracings, each consisting of a control, labeled o, and other AP evoked at a 

 particular interval after o, labeled i, 2, 3, 4, or 5. Vertical marks just above base line indicate the 

 time of the second stimulus. Line o consists of five superimposed control tracings ; the variation among 

 the controls was about the width of the line. B: AP's i to 5 of part .-1 shifted in time so that their 

 most rapidly falling phases coincide. All the records superimpose throughout their period of overlap 

 except that the shortest one (i) has a faster rate of repolarization for S's near Sr. The early difference 

 between o and 5 is little more than the variation between successive o's. C: critical test of super- 

 imposability. Lower tracing shows transmembrane potential of another cell in the same ventricle 

 as a function of time. The upper tracings show data of the same two action potentials but in this 

 case the negative of the rate of repolarization ( — E) is plotted as a function of 8. The two records 

 are of the same shape but the G of the b W is about 20% greater than that of n for all £'s. (Wood- 

 bury & Kirk, unpublished data.) 



Figure 20C illustrates a much more sensitive test 

 of superimposability. The lower record shows two 

 successively evoked action potentials, the second 

 being much shorter than the first. The upper record 

 shows the negative of the slope of the action po- 

 tential, — S plotted against S, i.e., an — S,S diagram 

 for the two potentials. The two curves are almost 

 identical in shape, but the shorter one {b) is about 

 15 per cent steeper than the longer {a) at all S's. 

 There is not this much difference in slopes between 

 long and short action potentials in all records; 

 rather, the figure represents the maximum deviation 

 ordinarily seen. 



The records in figure 20 indicate that, regardless 

 of t,vp, repolarization is relatively stereotyped in 

 that the sequence of events during the third phase 

 is fixed. This fixed behavior could result if ionic 

 conductances vary with voltage in an appropriate 



manner. Thus the ts,t^p relationship possibly results 

 from largely time-dependent conductance changes 

 during the second phase and superimposability 

 from largely voltage-dependent conductance changes 

 in the third phase. This latter possibilit)' is much less 

 attractive when examined in the light of the effects 

 of applied currents on membrane potentials. 



SLOPE CONDUCTANCE DURING REPOLARIZ.\TION. In 



■95'> Weidmann (125) estimated the slope con- 

 ductance of the membrane during the action po- 

 tential of ungulate Purkinje cells. He polarized the 

 membrane by flowing a current pulse several time- 

 constants long and of strength Is through one intra- 

 cellular electrode and measured the resultant changes 

 in membrane voltage (AS) at the end of the pulse 

 with an adjacent intracellular electrode. Is does not 

 flow through the membrane with constant densitv 



