134 J. W. WOODBURY AND W . E. GRILL 



probable possibility that acetylcholine acts at the intercalated disks. This 

 possibility is ruled out by the finding that the time constant was reduced by a 

 factor of 4, indicating that the principal change was in R,n- 



A few chloride replacement experiments have been done. If chloride is 

 replaced with sulfate, the space constant increases appreciably, about 20%. 

 If acetylcholine is present in the bath, however, replacement of chloride has 

 no effect on the space constant. This is indirect confirmation of Hutter's 

 direct and Trautwein and Dudel's indirect findings that the principal action 

 of acetylchohne is to increase specifically the membrane conductance to 

 potassium. 



SUMMARY 



The spread of an intracellularly applied current in trabeculae of rat atrial 

 appendage has been mapped in two dimensions. The space constant of voltage 

 decrement is about 130 ju parallel to the fiber direction and about 65 /x 

 perpendicular to fiber direction. 



Since the space constant is considerably larger than the cell dimensions, 

 local circuit current flow is the most likely mechanism for the spread of 

 excitation in cardiac muscle. 



An approximate mathematical analysis shows that the total specific 

 membrane resistance of an intercalated disk must be 1 D. cm^ or less to ensure 

 efficient intercellular transmission by local current flow. This figure is in the 

 range of Weidmann's estimates of disk resistance from diffusion measurements 

 with42K. 



After consideration of a number of possibilities it is tentatively concluded 

 that the principal reason for the short space constant is the large membrane 

 surface area per unit area of muscle surface ; the effect is the same as that of 

 a low membrane specific resistance. The calculated disk resistance is too low 

 to have more than about a 25 % effect on the space constant. 



Acetylcholine (lO'^w/v) cuts the space constant in half and the time 

 constant to one-fourth. Thus, the action of acetylcholine is to decrease the 

 membrane resistance. Chloride replacement experiments indicate that 

 acetylcholine specifically increases potassium conductance. 



REFERENCES 



Crill, W. E. and Woodbury, J. W. (1960) Non-uniform two-dimensional electrotonic 



spread in rat atrium. Federation Proc. 19 : 1 14. 

 Draper, M. H. and Mya-Tu, M. (1959) A comparison of the conduction velocity in cardiac 



tissues of various mammals. Quart. J. Exptl. Physiol. 44 : 91-109. 

 HoDGKiN, A. L. (1937) Evidence for electrical transmission in nerve. Part 1. J. Phvsiol. 



(London) 90 : 183-210. Part II. Ibid. 90 : 211 232. 

 Hutter, O. F. (1957) Mode of action of autonomic transmitters on the heart. Brit. Med. 



Bull. 13 : 176-180. 



