INHIBITION IN MOLLUSCAN HEARTS 139 



ventricle is lowered, the heart rate declines and in the absence of potassium 

 the heart stops in diastole and shows complete relaxation. If the amount of 

 potassium is increased beyond twice the normal concentration, the heart 

 undergoes a contracture. Intermediate concentrations cause acceleration and 

 an increase in amphtude. If a ventricle has been exposed to a medium con- 

 taining three or four times the normal amount of potassium and if this 

 medium is exchanged for one that contains twice the normal amount of 

 potassium, the ventricle undergoes rapid and complete relaxation and re- 

 sumes beating with supranormal amplitude a few minutes later. No slowing 

 or relaxation results if the ventricle is returned to normal medium after 

 exposure to a medium containing twice the normal potassium concentration. 

 Acetylcholine and potassium act antagonistically so that a ventricle is more 

 sensitive to the drug if the potassium concentration of the medium is lowered 

 and less sensitive if the potassium concentration is raised (see Fig. 1). 



The behavior of the Mytilus ventricle is quite different. In this case acetyl- 

 choline causes acceleration of the heartbeat and leads to systolic arrest. The 

 same effect is achieved if the ventricle is subjected to a medium which con- 

 tains no potassium (see Fig. 2). Higher than normal amounts of potassium 

 cause a slowing and relaxation of the heart muscle. As in the case of Mya 

 and Protothaca the inside of the ventricle is more sensitive to acetylcholine 

 than the outside, the difference in sensitivity is, however, not as great and 

 does not exceed a ratio of 200. 



It is interesting in this regard that according to Welsh and Slocombe (1952) 

 anodal current results in decreased heart rate and relaxation in the case of 

 Venus, where acetylcholine is inhibitory, and that the same inhibition by 

 anodal current was observed by Jullien and Marduel (1938a) in the heart of 

 Mytilus where acetylcholine is excitatory. 



The comparison of the effects of acetylchohne and potassium on the two 

 types of lamellibranch hearts offer challenging problems. What is the reason 

 for the opposite actions on the two types of heart? Following the popular 

 concepts that acetylcholine excites neurogenic and inhibits myogenic hearts, 

 one could indeed assume that in the case of Mytilus acetylcholine acts on the 

 ganglion cells of the heart rather than on the heart muscle. There are, how- 

 ever, objections to this. First of all, we do not know whether there are nerve 

 cells present in the hearts of Mya and Protothaca; secondly, we know that the 

 heart of Ostrea which contains ganglion cells is inhibited by acetylcholine 

 and the same is true for the cephalopod heart. In addition, we would assume 

 that if acetylcholine acted on the ganglion cells of the heart it would prim- 

 arily affect the frequency of the heartbeat whereas it actually affects the 

 contraction process much more than the pacemaker rhythm. 



The almost instant effect of changes in potassium ion concentration indi- 

 cates that this ion affects the membrane potential of the cardiac muscle fibers. 

 It is interesting, however, that, at least in Protothaca the immediate effect 



