138 ERNST FLOREY AND HARRIET MERWIN 



the inhibitory nerve supply and Mytolon which prevents the acetylchohne 

 action blocks the action of the inhibitory nerves. Although Welsh (1957) 

 believes that 5HT is the accelerating transmitter in Venus, there is little 

 evidence for this other than that this compound occurs in the ganglia of this 

 animal and that the heart is rather sensitive to it. It should be mentioned that 

 Bumpus and Page (1955) found the Venus heart to be five to ten times more 

 sensitive to N : N-dimethyl-5-hydroxy-tryptamine than to 5HT. 



The transmitter substances of the cardiac nerves of more than 100,000 

 species of molluscs have not been established. There is evidence, however, 

 that in some species acetylchohne is not the inhibitory transmitter to the 

 heart. Of these species the most striking examples are provided by those 

 lamelhbranchs in which acetylcholine stimulates the heart and causes systolic 

 contracture (JuUien and Vincent, 1938; Pilgrim, 1954). while stimulation of 

 the visceral ganglion, which gives rise to the cardiac nerves, produces typical 

 inhibition (Diederichs, 1935). 



Variation in the ionic composition of the perfusion medium has different 

 effects on the hearts of different molluscs. In his Comparative Animal Physio- 

 logy Prosser (1950) summarizes the data in the literature and states that 

 moUuscan hearts are not very sensitive to changes in potassium ion concentra- 

 tion but may be slightly accelerated by increases, and may be stopped in 

 systole by a great excess of potassium. Increase in calcium ion concentration 

 inhibits the pacemaker. In the absence of calcium ions the heart of some 

 molluscs stops in diastole, while that of others stops in systole after consider- 

 able acceleration. Magnesium has the same actions as calcium. 



We have recently concerned ourselves with a comparison of the actions of 

 acetylcholine and of potassium on the isolated ventricles of lamelhbranchs in 

 which acetylcholine causes cardiac inhibition and of lamelhbranchs in which 

 acetylcholine causes cardiac acceleration and systolic contracture. Of the 

 former group we selected Protothaca staminea and Mya arenaria, of the 

 latter Mytilus califomianus as representative species and experimental 

 animals. The heart of Mya and Protothaca is inhibited by acetylcholine con- 

 centrations as low as lO^i'-g/ml. Complete diastolic arrest with maximum 

 relaxation can be achieved with concentrations of 10'^ g/ml. This is true for 

 unfilled ventricles which are simply suspended in a bath, as well as for 

 ventricles which are filled with perfusing medium and to which acetylcholine 

 is applied to the inside of the heart. It is rather surprising that the outside of 

 filled ventricles is relatively insensitive to acetylcholine. The ratio between 

 inside and outside sensitivity can be as high as 30,000! Since unfilled ventricles 

 are extremely sensitive to acetylcholine applied to the outside we may assume 

 that filhng leads to a rotation of acetylcholine-sensitive sites towards the 

 inside of the ventricle. This may well mean that not all of the muscle fiber 

 surface is sensitive to this compound. 



If the amount of potassium in the fluid bathing the isolated Protothaca 



