Effects of Divalent Cations On Spike Generation in the Longitudinal Somatic Muscle of the Earthworm Available to Purchase

Effects of various divalent cations on the spike-generation mechanism in the longitudinal muscle of the earthworm, Pheretima communissima, under sodium-free conditions were observed.

1. In the sodium-free solution the membrane was hyperpolarized from −35 mV. to −55 mV. and the input resistance of the membrane increased from 32 MΩ to 52 MΩ. The spike amplitude was greater in sodium-free solution than in Ringer solution.

2. The amplitude of the spike elicited by intracellular depolarizing currents in sodium-free solution was proportional to the external calcium concentration. The slope of the overshoot potential against tenfold change of the calcium concentration was 26 mV. Similar results could be observed with strontium.

3. The threshold membrane potentials for spike generation varied under various concentrations of calcium. Strontium had nearly the same effect as calcium on the threshold membrane potential.

4. The maximum rates of rise of the spike under various damped membrane potentials were measured in sodium-free solution. The maximum rates of rise of the spike showed sigmoidal curve against the changes of the membrane potential. The half-value of the inactivation curve was estimated to be −32 mV.

5. It was concluded that the spike was elicited by the inward movement of calcium ions, presumably not only in sodium-free solution but also in normal Ringer solution.

6. Strontium and barium ions also carried current during the active state of the membrane.

7. Barium, however, inhibited the potassium conductance of the membrane during the resting and active states, i.e. depolarized the membrane, increased the input resistance of the membrane and prolonged the falling phase of the spike.

8. Magnesium of itself could not elicit the spike in sodium-free solution but prevented spike generation by elevating the threshold membrane potential in the presence of calcium. In the presence of 10 mM Mg, the overshoot potentials recorded in the various concentrations of calcium were not affected, but the maximum rate of rise of the spike was reduced.

9. Cobalt and manganese elevated the threshold membrane potential without changing the resting membrane potential and prevented spike generation. Cobalt was more effective than manganese in preventing spike generations. Cobalt and manganese reduced the amplitude of the overshoot potential and the maximum rate of rise of the spike.