ION MOVEMENTS DURING VAGUS INHIBITION OF THE HEART 



121 



ever, to examine simultaneously the effect of acetylcholine on the outflow of 

 42K and ^eci as the half-life of the two isotopes differs greatly. The rate 

 constants applying to the outflow of each isotope from a tortoise sinus venosus 

 loaded in 120 mM ^-K ^^Cl are shown in Fig. 5. As can be seen acetylcholine 

 produced its invariable effect on the rate of movement of potassium but the 

 rate of outflow of chloride showed no obvious response. 



OI5 



OIO 



005 



30 

 Time Cmin) 



Fig. 5. Effect of acetylcholine on loss of -^-K and ^^Cl from a tortoise sinus 



venosus into Ringer's solution. Abscissa, time from removal of preparation from 



a load solution whose principal constituent was 120mM ''-K 36(^1. Ordinate, 



rate coefficient for loss of the isotopes. Vertical bars ± s.e. 



How far do these results allow a conclusion as to the specificity of the 

 permeability increase produced by acetylcholine? It might be supposed, for 

 instance, that the fibres of the sinus venosus are normally much more per- 

 meable to chloride than to potassium; an opening of channels equally 

 available to both ions might then be expected to produce a relatively greater 

 effect on the movement of potassium than on the movement of chloride. 

 This explanation for the absence of a detectable increase in chloride perme- 

 ability, however, is not in accord with available evidence. Determinations of 

 the relative contribution of chloride to the membrane conductance of mam- 

 malian heart muscle has shown that chloride is less permeant than is potas- 

 sium at the normal resting potential (Hutter and Noble. 1959). A similar 

 result has been obtained for frog auricle by Mr. W. E. Crill, University of 

 Washington, Seattle. Crill (personal communication) has also shown that 

 the contribution of chloride to the membrane conductance becomes vanish- 



