Crayfish muscle fiber: ionic requirements for depolarizing synaptic electrogenesis

Abstract

Presence of sodium in the bathing medium is not essential for the electrically excitable depolarizing electrogenesis of crayfish muscle fibers, production of action potentials being dependent on calcium. The depolarizing electrogenesis of the excitatory synaptic membrane component does require sodium, however, and this ion cannot be replaced by lithium as it can in spike electrogenesis of many cells. Ionophoretic applications of glutamate, which in the presence of sodium depolarize the cell by activating the excitatory synaptic membrane, are without effect in the absence of sodium. Not only is there no depolarization, but the membrane conductance also remains unchanged. Thus, in the absence of inward movement of sodium across the synaptic membrane there is also no outward movement of potassium. Accordingly, it seems that increased conductance for potassium is not an independent process in the synaptic membrane, whereas it is independent of sodium activation in spike electrogenesis. Chloride activation is independent, however; increase in conductance and the electrogenesis of the inhibitory synaptic component are not affected by the absence of sodium. Implications of these findings regarding the structure of differently excitable membrane components are discussed.