Antagonistic action of Ca and Mg on the neuromuscular transmission and excitation in a molluscan muscle (Radula protractor)

1. In the radula protractor ofRapana thomasiana, decrease in the external concentration of Mg increased the rate of rise and the amplitude of the action potential. 2. When the ratio of Ca and Mg in the fluid was kept constant, the variation in the concentration of these cations did not produce any appreciable effect on the rate of rise and the amplitude of the spike potential. The slow component of the potential and contraction were reduced as both cations were increased. 3. The effect of 5-HT on the radula muscle was examined. At lower concentrations, up to 10−5 M, 5-HT had no apparent effect on the membrane potential and mechanical tension, though the excitability was definitely enhanced. With higher doses over 10−5 M, the membrane depolarized and rhythmical electrical firing and mechanical oscillations were induced. 4. The amplitude of the junctional potential elicited in curarized muscle varied directly with Ca concentration and inversely with Mg concentration. 5. Membrane potential of the muscle was not affected by the change of Mg concentration. 6. The sensitivity of the muscle fiber to the depolarizing action of ACh was virtually unaffected by changing the concentration of Ca or Mg. 7. These results suggest that the concentration of Ca has direct effects and Mg has inverse effects on the amount of transmitter release at the nerve ending.