Effects of various ions on the resting and active membrane of the somatic muscle of the earthworm.

The membrane potential of the longitudinal muscle of the earthworm is only — 35 mV. and this value is much lower than those of the Loligo giant fibres (— 60 mV.), frog striated muscle ( — 90 mV.) and mammalian smooth muscle ( — 50 mV.). The difference in the membrane potential might suggest a specificity in the ionic distribution or ionic permeability in the longitudinal muscle of the earthworm different from that of other excitable cells. The mechanism of spike generation in Loligo giant fibres and frog skeletal muscle was investigated in detail by Hodgkin & Katz (1948), Hodgkin, Huxley & Katz (1952) and Hodgkin & Huxley (1952a, b). Their results supported the conclusion that the upstroke of the spikes was due to the inward movement of sodium ions. In contrast, in the frog spinal ganglion (Koketsu, Cerf & Nishi, 1959; Nishi, Soeda & Koketsu, 1965), crustacean muscle (Fatt & Katz, 1953; Fatt & Ginsborg, 1958) and barnacle muscle (Hagiwara, Chichibu & Naka, 1964) spikes could be generated in sodium-free solution (sodium was substituted by hydrazine, barium strontium or tetra-ethyl ammonium). Furthermore, in barnacle muscle, calcium-dependent spikes were clearly demonstrated by a most skilful technique; and tetrodotoxin, known to block any increase in the sodium conductance in the frog nerve fibre and muscle during the active state, had no effect on spike generation in the barnacle muscle (Narahashi, Moore & Scott, 1964; Hagiwara & Nakajima, 1965). As described in the previous paper, tetrodotoxin did not influence either the spike amplitude or the numbers of train discharges in the longitudinal muscle of the earthworm, generated either spontaneously or in response to electrical stimulation (Hidaka, Ito & Kuriyama, 1969). The present experiments were intended to investigate the effects of various ions on the resting and active states of the membrane. The results led to the conclusion that the spikes are generated by the inward movement of calcium ion, and the low membrane potential is due to relatively high sodium permeability and low potassium permeability. METHODS