ABSTRACT
Electrical activity from the heart of Modiolus demissus was recorded simultaneously by sucrose-gap and microelectrode techniques, confirming the validity of the extracellular method of assessing the shape of the action potential.
Hearts beat in Na-free SW (Tris) but with loss of the plateau and a reduced rise time. Mechanical contractions did not persist, however, for more than 2 h in the absence of sodium, and the viability of the preparation declined rapidly after 3 h.
Lithium substitution (for sodium) also failed to support the plateau and tetro- dotoxin had little effect on the spike.
Hearts also beat in Ca-free SW. Here, the spike component and contractile activity were lost at nearly the same time although slowly rising plateau-like potentials persisted for over 6 h.
EGTA potentiated the effects of Ca-free SW, and manganese had calcium-anta-gonizing effects producing longer plateaus.
Spikes and plateau phases could be selectively initiated from hearts made quiescent in Na-free and Ca-free SW by readmitting either ion individually.
In Na-free SW spike amplitude and rate of rise were increased by raising calcium concentration. This effect was self-limiting at high levels of calcium, however, and the spike was reduced.
In divalent-free solutions (Ca- and Mg-free SW) the membrane was depolarized to near zero potential. In this situation the membrane permeability to sodium exceeded that to potassium, and the membrane behaved as a sodium electrode.
It is concluded that the action potential in Modiolus heart is dependent primarily on calcium for the spike component and on sodium for the amplitude of the plateau.
Calcium is also important in the control of plateau duration and membrane permeability by reason of its stabilizing properties.
A possible correlation exists between the occurrence of the plateau, calcium sensitivity and habitat of three bivalve species. The hearts of Modiolus and of the oyster have plateaus and will beat in Ca-free SW, and these bivalves live in estuarine habitats, whereas Mytilus lives in a subtidal environment and its heart is sensitive to low calcium and has no plateau component.
