CELLULAR ELECTROPHVSIOLOGV OF THE HEART 



because of the cable properties of the filler. The 

 AS that would be produced by a constant current 

 density (Im) can be calculated; I,n/AS is proportional 

 to (Is/as)- (125). In, AS is an experimental approxi- 

 mation of dlm'd& at I,„ = o, a quantity closely re- 

 lated to the total slope conductance (G) at I,,, = o. 

 G is defined as clI; 5S. The partial difTerentiation 

 indicates tliat time is held constant. Since Weid- 

 mann's results have been widely quoted and nearl)' 

 as widely misinterpreted, it must be emphasized 

 that slope conductance, G = (5Ii/6'E, is not the same 

 as chord conductance, gg = Ig'(S — Sg). Slope 

 and chord conductances have been repeatedly equated 

 in the literature (e.g., 3, 68, 106) even though Hodg- 

 kin and Huxley carefully distinguished between the 

 two when defining the chord conductances gx,, and 

 gK [(57> P- 461); see also (14; 15; 19, appendix A)]. 

 The effects of current flow on S are illustrated in 

 figure 21 and Weidmann's results in figure 22. To 

 obtain the records in figure 21, the current was 



0,2 0.4 



TIME (sec) 



FIG. 21. Effects of polarizing current pulses on the trans- 

 membrane potentials (S) of dog or cat papillary muscle cells. 

 The currents were applied through several external electrodes 

 inside a tightly fitting tube into which a length of the muscle 

 was drawn. Transmembrane potentials were measured just 

 outside the tube with two microelectrodes, one inside and one 

 just outside the cell. A.' effects of small, 20 msec depolarizing 

 and hyperpolarizing currents applied at various times through- 

 out the action potential. B: effects of large, 70 msec polarizing 

 currents. Post-hyperpolarization excitation occurs in the low- 

 est record. Note that in both A and B the potential changes 

 produced by current flow are concave toward the zero current 

 potential curve. [From Cranefield & Hoffmann (28).] 



-100 



0.2 04 0.6 



TIME (Sec) 



FIG. 22. Transmembrane potential (E), relative slope 

 conductance (G/G,) and stable equilibrium potential (fi(.q) 

 during one cycle of a spontaneously beating excised Purkinje 

 strand. 6 was measured with an intracellular electrode; G/G, 

 was estimated from the potential changes produced at the 

 recording electrode by current pulses applied via a nearby 

 intracellular electrode. These curves are after Weidmann 

 (129). £cq = S -|- (Cm/G)S was calculated from values of £ 

 and G/Gr graphed here and values of Gr and Cm obtained by 

 Weidmann (126). 



applied by extracellular electrodes. Although the 

 resultant changes in S cannot be quantitated in 

 terms of slope conductances, these changes do serve 

 to characterize the membrane during activity. In 

 figure 2 1.4 the current pulse was 20 msec long and 

 the final change in £ was 20 mv or more; in figure 

 ■21 B the current was stronger and lasted 70 msec. 

 Four aspects of the records in figure 2 1 should be 

 mentioned, a) Equal currents change S by about 

 the same amount at all times during repolarization. 

 /)) Depolarizing pulses change S less than equal 

 hyperpolarizing pulses — i.e., the membrane is a 

 rectifier which has less resistance to outward than to 

 inward current flow. Hutter & Noble (75) found the 

 opposite rectification in cells in Na+-free media, c) 

 The curve of S in time during current flow is always 

 concave toward the 8 when no current is flowing — i.e., 

 AS is of the form (i — e""''') rather than e"'''. There 

 is one exception, the inflection in S following the 

 break of current in the lower record in figure 21^. 

 d) Statement c is true even for hyperpolarizations 

 which carry S more negative than Sr (lower record 

 in fig. 2 1^4 and lower two records in fig. 21/?). 



Weidmann's results were similar to those shown in 

 figure 2 1 , but the current was applied through an 

 intracellular electrode so that membrane slope 

 conductance could be estimated. Figure 22 shows 8 

 and G plotted as functions of time throughout a 

 single cardiac cycle. G reaches a high value during 



