Initiation of Ca-spikes from an insect muscle fiber immersed in a low pH saline solution containing carboxylic anions.

Skeletal muscle fibers of beetle larvae, $\text{Xylotrupes}$$\text{dichotomus}$ L., which were inexcitable in the standard saline solution at pH 7.4 became capable of generating all-or-none spikes when a certain amount of acetate was added to a low pH saline solution. The optimum condition for the spike initiation was the presence of 20–30 mM acetate at pH 6. The spikes thus elicited were pure Ca-spikes as they were dependent on the external Ca⁺⁺ concentration but were not inhibited either by removal of the external Na⁺ or by application of tetrodotoxin. This facilitation of the Ca-spike generation was probably due to a combined action of acetate and of the low pH upon the muscle membrane.