CELLULAR ELECTROPHYSIOLOGY OF THE HEART 



283 



fi-cm for p, indicates that discs contribute little to 

 the total internal resistance in Purkinje tissue. It 

 seems unlikely that the p-, of rat atrial myoplasm is 

 more than 100 S2-cm, so most of the measured p; must 

 be attributed to disc resistance. On this basis, the 

 average value of the disc specific resistance (Rmd) 

 was close to 6 fl-cm-, not taking into account the 

 folding of the disc membrane. Weidmann (133) has 

 obtained a similar but somewhat smaller value for Rmd 

 from measurements of the rate of diffusion of K"*- 

 along Purkinje fibers, indicating that current is 

 carried through the disc membrane largely by K"*^. 

 The resting potential could be maintained across 

 such a selectively permeable disc membrane. A sepa- 

 rate calculation by Woodbury & Crill (in 41) gives 

 further evidence that R,,,,) may be as low as i fi-cm-. 

 On the basis of a simplified representation of the 

 transmembrane potentials across the disc membranes 

 at the junction between an active and an inactive 

 cell, these investigators calculated that Rmd must be 

 of the order of i 12-cm- or less to insure efficient current 

 transmission across the disc. In other words, to insure 

 that most of the current leaving one cell through a 

 disc enters the adjacent cell and does not escape into 

 the interstitial fluid, R,„d must be i Q-cm- or less. 

 Thus three completely separate approaches yield Rmd 

 values ranging from 1-6 0-cm^. Together, these argu- 

 ments constitute strong evidence for the correctness 



of these values and the view that disc resistance is 

 low. 



It is worth pointing out that membrane resistivities 

 of I H-cm- are not unknown in cells. If disc membrane 

 area is ten times the cellular cross-sectional area, 

 then R,„d is about 10 fi-cm-. Red blood cell membranes 

 have resistivities of about i fi-cm- owing to their high 

 anion permeability. The Rm of a squid a.xon at the 

 peak of activity can be as low as 6 fi-cm- (19). Despite 

 the low R„„i disc resistance contributes substantially 

 to the total internal resistance of atrial tissue. 



The findings that the resistance of intercalated 

 discs is low and that K+ diffuses through them rapidly 

 enough to account for this low resistance suggest a 

 possible explanation of the rapid disappearance of 

 injury potentials in cardiac muscle. It is known that 

 increasing [K+]„ increases gs (9, 65, 75). It seems 

 possible that normal K+ leakage into the interdisc 

 space maintains there a [K+] high enough to reduce 

 considerably disc resistance and transdisc potential. 

 In an injured area, the membranes of the injured 

 cells are ruptured; hence they will graduallv lose their 

 K+. Concomitantly the [K+] in the interdisc space 

 common to intact and injured cells would fall. In 

 turn, this fall in [K+]o could increase disc resistance 

 and potential, which would have the effect of elec- 

 trically isolating the intact cells from the injured ones. 



REFERENCES 



1 . Adri.^n, R. H. The effect of internal and external potas- 

 sium concentration on the membrane potential of frog 

 muscle. J. Physiol. 133: 631, 1956. 



2. BozLER, E. The initiation of impulses in cardiac muscle. 

 Am. J. Physiol. 138; 273, 1943. 



3. Bradv, a. J. .\ND J. W. Woodbury. EfTects of sodium 

 and potassium on repolarization in frog ventricular 

 fibers. Ann. New York Acad. Sc. 65: 687, 1957. 



4. Br.\dy, a. J. AND J. W. Woodbury. The sodium-potassium 

 hypothesis as the basis of electrical activity in frog 

 ventricle. J. Physiol. 154: 385, i960. 



5. Brazier, M. A. B. The historical development of 

 neurophysiology. In : Handbook of Physiology. Washington : 

 American Physiological Society, 1959, sect, i, vol. I, p. i. 



6. Caesar, R., G. A. Edwards, and H. Ruska. Electron 

 microscopy of the impulse conducting system of the 

 sheep heart. Zlschr. Zellforsch. 48: 698, 1958. 



7. Caldwell, P. C. and R. D. Keynes. The effect of 

 ouabain on the efflux of sodium from a squid giant axon. 

 J. Physiol. 148; 8P, 1959. 



8. Carmeliet, E. Effets de la substitution des ions chlorure 

 sur le potentiel de membrane des fibres de Purkinje. 

 Helv. physiol. el Pharmacol, acta i 7 : C 18, 1 959. 



9. Carmeliet, E. L'influence de la concentration extracellu- 

 laire du K sur la permeabilite de la membrane des fibres 

 de Purkinje de mouton pour les ions ^'K. Helv. physiol. et 

 Pharmacol, acta 18: C 15, i960. 



10. Carmeliet, E. and L. Laccjuet. Duree de potentiel 

 d' action ventriculaire de grenouille en fonction de la 

 frequence. Influence des variations ioniques de potassium 

 et sodium. Arch, internal, physiol. 66: i, 1958. 



I ! . Del Castillo, J. and B. Katz. Production of membrane 

 potential changes in the frog's heart by inhibitory nerve 

 impulses. Nature, London 1 75 : 1 035, 1 955. 



12. Cervoni, P., T. C. West, and G. Falk. Multiple intra- 

 cellular recording from atrial and sino-atrial cells: 

 correlation with contractile tension. Proc. Soc. Exper. 

 Biol. & Med. 93 : 36, 1 956. 



13. Clark, A. J., M. G. Eggleton, P. Egoleton, R. Caddie, 

 AND C. P. Stewart. The Metabolism of the Frog's Heart. 

 Edinburgh: Oliver & Boyd, 1938. 



14. Cole, K. S. Dynamic electrical characteristics of the 

 squid axon membrane, .irch. sc. physiol. 3: 253, 1949. 



15. Cole, K. S. Beyond membrane potentials. Ann. New York 

 Acad. Sc. 65: 658, 1957. 



16. Cole, K. S., H. A. Antosiewicz, and P. Rabinowitz. 



