Ionic requirements for non synaptic electrogenesis in the muscle fibres of a lepidopterous insect

Study was made of the influence of external sodium, calcium and magnesium concentrations on the directly elicited response of the segmental muscle fibres of the larva of Galleria mellonella, as recorded with intracellular microelectrodes. The graded response caused by an outward current pulse was converted into an all-or-none spike in the presence of 10 mM tetraethylammonium. The amplitude of these spikes increased with increasing external Ca concentration but not with increasing Na or Mg concentration. The spikes were not influenced by tetrodotoxin (3 × 10−5 M) but were suppressed by Mn ions (20 mM). After replacement of TEA-containing saline with an isotonic CaCl2 solution, spikes were still produced in the latter. It appears that only calcium ions act as charge carriers of inward current at the non-synaptic membrane.