ION MOVEMENTS DURING VAGUS INHIBITION OF THE HEART 119 



04 



03 



02 



01 



Fig. 3. Each block represents the amount of labelled potassium taken up by a tor- 

 toise sinus venosus during successive 5 min exposures to Ringer's solution 

 containing 2-7 niM labelled K. Ordinate scale, milli-equivalents labelled K 

 taken up per min/kg wet weight tissue. During the fourth and ninth period 

 acetycholine 10"^g/ml was added to the ^-K Ringer's solution. 



The effects of rubidium on excitable tissues resembles that of potassium. 

 In two experiments in which the outflow of ^-Rb into Ringer's solution was 

 examined acetylcholine produced an approximately twofold increase in the 

 rate of outflow. As the half life of ^-Rb is conveniently long (18-6 days) this 

 isotope could perhaps be used with economy in experiments of this kind. 



The striking increase in the rate of movement of potassium under the 

 influence of acetylcholine raises the question of how specific the perme- 

 ability change is. In particular, it seemed interesting to examine the move- 

 ment of chloride as there is strong electrophysiological evidence that synaptic 

 inhibition of some tissues is brought about by an increase in permeability 

 to that anion (Coombs et ai. 1955; Boistel and Fatt, 1958). 



Here a difficulty is met. The specific activity of ^^Cl is so low in the relation 

 to the amount of chloride in the cells and the bulk of sensitive tissue available 

 that experiments with this isotope can be made only under limited conditions. 

 We have therefore turned to the more active isotope of bromine. Figure 4 

 shows the procedure adopted in most outflow experiments with ^'Br. The first 

 part of the experiment was conducted at C in order to achieve a better 



