¹ From the Laboratory of Neurophysiology, Department of Neurology, and the Department of Anatomy, College of Physicians and Surgeons, Columbia University, New York 10032, and the Marine Biological Laboratory, Woods Hole, Massachusetts 02543.

Dr. Kurtz's present address is the Departamento de Farmacologia, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil.

Depolarization thresholds for eliciting tension and Ca electrogenesis have been compared in isolated crayfish muscle fibers. Just-detectable tensions and Ca spikes induced after treatment with procaine were elicited with intracellularly applied depolarizing currents of fixed duration. Both thresholds were found to increase in a similar manner in fibers exposed to increased concentrations of Ca in the bathing solution or addition of other divalent cations (Mg, Mn, Ni). However, antagonistic effects between divalent cations were also demonstrated. Substitution of increasing amounts of NaSCN for NaCl in the standard saline produced a progressive decrease in both thresholds. The correlation in the change in thresholds for the two processes supports the hypothesis that a change in membrane Ca conductance is an integral step in excitation-contraction coupling.

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