Calcium spike in the longitudinal muscle of the lobworm, Tylorrynchus heterochaetus, (Nereidae).

In crustacean muscle, earthworm longitudinal muscle, leech longitudinal muscle and barnacle muscle the major role in the production of the action potential is played by divalent cations rather than sodium ions, which are believed to be most important in many other excitable cells (Fatt & Ginsborg, 1958; Fatt & Katz, 1953; Werman, McCann & Grundfest, 1961; Hagiwara, Chichibu & Naka, 1964; Hagiwara & Takahashi, 1967; Hidaka, Ito, Kuriyama & Tashiro, 1969; Ito, Kuriyama & Tashiro, 1970; Higuchi, 1967). In the longitudinal muscle of the earthworm, Ito, Kuriyama & Tashiro (1970) concluded from the results obtained from the effects of divalent cations on the spike that Ca, Sr and Ba could generate the spike in sodium-free solution, but not Mg. Co and Mn blocked the generation of the spike elicited in the presence of Ca, Sr or Ba. The amplitude of the spike was proportionally increased with increased external concentrations of Ca, Sr and Ba. They concluded that Ca in normal Ringer solution played an important role in determining the membrane potential in normal solution and also in the production of the action potential as a current carrier during the active state of the membrane. The present experiments were intended to investigate the properties of the muscle membrane in the Nereidae during the active state and the main factors involved in determining the membrane potential during the resting state. The experimental results were compared with the results obtained from the earthworm which were carried out under similar experimental conditions. The results led to the conclusion that the membrane potential was mainly determined by the ionic concentration gradient and permeability of K across the membrane and that Na had less influence on the resting membrane potential than that in the longitudinal muscle of the earthworm. The spike was mainly generated by the inward movement of Ca. However, in the physiological solution, Na+ (at least in part) is also important for the generation of the spike.