The action of sodium is similar to that of the 

 potassium, but response is less pronounced. Small 

 excesses of calcium cause negative chronotropic 

 and positive tonotropic effects, and magnesium 

 acts in a way similar to that of calcium, i.e., 

 produces negative chronotropic effect and causes 

 diastolic arrest of the heart. Lack of magnesium 

 results in a systolic arrest (JuUien and Morin, 

 1931b; Jullien, 1936a). 



Among the effects of various drugs the most 

 interesting is that of acetylcholine, a chemical 

 agent in neuromuscular transmission which de- 

 presses heart action of oysters and other mollusks 

 (Jullien, 1935c; Jullien and Vincent, 1938; Jullien, 

 Vincent, Vuillet, and Bouchet, 1939; Prosser and 

 Prosser, 1938; Prosser, 1940, 1942; and Wait, 

 1943) and is particularly effective on the heart 

 of the clam {Mercenaria mercenaria) . Prosser 

 (1940) has shovm that inhibition of the heart of 

 this species can be obtained with a concentration 

 as low as 10"'^ Recent investigations by Pilgrim 

 (1954) and Greenberg and Windsor (1962) showed 

 that in the hearts of many bivalves acetylcholine 

 produces a "combination response", depressing 

 the cardiac activity in low concentrations and 

 exciting it at high concentrations. The authors 

 used ventricle strip preparations of the hearts of 

 40 American (in the Greenberg and Windsor ex- 

 periments) and 8 New Zealand species (in Pil- 

 grim's tests). Preparations which remained qui- 

 escent when first set up attained regular rhythm 

 in 2 to 3 hours, a condition which was also observed 

 in tests made in the Woods Hole laboratory on C. 

 mrginica. In Greenberg's and Windsor's experi- 

 ments the quiescent preparations were induced 

 to beat with 10"' to 10'^ molar concentrations of 

 5-hydroxytryptamine. 



There exists great variability in the responses of 

 different bivalve species to acetylcholine. In some 

 of them only the depressing effect of the drug was 

 recorded. This group includes oysters (C. virgin ica 

 and C. gigas), several clams of the family Veneridae 

 {Mercenaria mercenaria, Tapes philipinarum, Saxi- 

 domus giganteus, and others), Mya arenaria, 

 Entoderma saxicola, and Prododesmus macro- 

 schisma. The excitor effect was demonstrated for 

 Mytilus calijornianus and M. canaliculus (in Pil- 

 grim's tests), thus confirming previous observa- 

 tions on Mytilidae by JulHen and Vincent (1938). 

 In Pectinidae, Matridae, Carditidae, and other 

 families, both types of responses were recorded. 

 The following explanation of the "combina- 



tion response", i.e., depression in low concentra- 

 tion and excitation in high concentration, was sug- 

 gested by Pilgrim (1954): the low concentration 

 tends to inhibit pacemaker activity; at high con- 

 centration, while the pacemaker is inhibited, the 

 drug acts directly on the nmscle causing a steady 

 contraction. Further research is needed to cor- 

 roborate tliis hypothesis. 



Greenberg and Windsor (1962) remark that "a 

 reasonable mode of acetylcholine action on bi- 

 valve hearts should involve either two separate 

 sites of action or two modes of attachment to the 

 same site at high and low concentrations". 



Sensitivity of bivalve hearts to acetylcholine 

 varies in different species. The most sensitive 

 ones, reported by Pilgrim, are Dosinia, Amphi- 

 derma, and Mercenaria mercenaria. Oysters are 

 less responsive to the drug. Jullien (1935c) re- 

 ported that in C. angulata the frequency and the 

 amplitude "f heart beat are decreased in a con- 

 centration of 10"^ with diastolic arrest following 

 at two times 10"^ to two times 10"'* concentration. 

 In New Zealand species, Ostrea hejferdi, the 

 cardiac activity is depressed with a diastolic arrest 

 at concentrations varying from 10"' to 10"^ 

 (Pilo;rini, 1954). In C. virginica the decrease in 

 the frequency and amplitude of isolated heart was 

 apparent at concentration 10"^ (fig. 229) and the 

 effect persisted for several minutes after the 

 preparation was flushed with fresh sea water 

 (second line). The effect of tlie drug can be 

 noticed even in extremely low concentrations of 

 10"** and 10"'. Under normal conditions the 

 iiearts of bivalves contain little acetylcholine 

 (Jullien and Vincent, 1938), but the heart of the 

 gastropod Murex is very rich in this compound. 



Eserine causes periodical alterations in tlie 

 amplitude of heart beat and slight increase in the 

 rate of beating (fig. 230). The significance of the 



I 



i 



Fr S W. 



FiiiURE 229. — Effect of acetylcholine in the concentration 

 10-5 on the beat of isolated heart of C. virginica. ACh— 

 acetylcholine added; Fr.S.W.— perfusion chamber 

 fluslied with fresh sea water. Temperature 23.7° C. 

 Time interval, 5 seconds. 



252 



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