112 THE BIOLOGY OF MARINE ANIMALS 



Much recent work is concerned with the participation of chemical 

 mediators in the neural regulation of cardiac activity. Acetylcholine inhi- 

 bits the hearts of certain gastropods, lamellibranchs and cephalopods, 

 sometimes in very low concentration (10~ 12 in Mercenaria (= Venus) (Fig. 

 3.16)). A similar inhibitory effect is shown by pilocarpine in the lamelli- 

 branch heart (Anomia), and by nicotine (Mercenaria, Ostrea, Sepia). 

 Within the body acetylcholine is quickly hydrolysed and rendered 

 ineffective by acetylcholine esterase, the activity of which is blocked by 

 certain tertiary ammonium compounds, e.g. eserine (physostigmine) and 

 prostigmine. Eserine sensitizes the heart of Mercenaria to acetylcholine, 

 resulting in enhanced inhibition when the latter drug is applied. This 

 effect, however, is not apparent in the cephalopod heart. 



If we accept the hypothesis that acetylcholine may be a normal chemical 

 transmitter produced by inhibitory fibres in the molluscan heart, other 



Fig. 3.16. Effect of Acetylcholine on the Heart of Mercenaria mercenaria 

 Concentrations: 1: 2 x 10- 12 ; 2: 2 x lO" 11 ; 3: 2 x lO" 10 . (From Wait (129).) 



direct evidence should be forthcoming. Acetylcholine, by assay, has been 

 demonstrated in hearts of gastropods, lamellibranchs and cephalopods, and 

 acetylcholine esterase in the first two groups. Acetylcholine content of the 

 myocardium is rather low among lamellibranchs and opisthobranch 

 gastropods, but higher among prosobranchs. Cholinesterase activity is 

 proportional to acetylcholine levels. 



Eserine prolongs cardiac inhibition produced by stimulation of the 

 visceral ganglion in Mercenaria, and the perfusate from an animal subjected 

 to stimulation of the visceral ganglion causes inhibition of a second, test 

 heart. There is no comparable evidence for acetylcholine participation in 

 cardio-regulation of cephalopods. In these animals eserine fails to potenti- 

 ate nervous inhibition caused by stimulation of the visceral ganglion, and 

 tests for acetylcholine in the perfusate are negative. 



Adrenaline and noradrenaline have an acceleratory effect on various 

 molluscan hearts (Aplysia, Ostrea, Loligo). Ergotamine excites the hearts 

 of Mercenaria and Loligo, and abolishes or reduces the stimulatory effect of 

 adrenaline. Tyramine resembles adrenaline in causing acceleration of the 

 cephalopod heart, but only in high concentrations. Recent studies show 

 that some molluscan hearts are excited by serotonin (5-hydroxytrypt- 

 amine), e.g. Mercenaria, Aplysia, Octopus. Tyramine, noradrenaline, 

 serotonin occur in appreciable quantities in the posterior salivary glands 

 of some octopods, and serotonin has been found in ganglia of Mercenaria 



