Actions of calcium and certain multivalent cations on potassium contracture of guinea-pig's taenia coli

1. Using a calcium buffer system, the effect of severe calcium lack on the shape of K-contracture of the guinea-pig's taenia coli was studied. Under conditions of calcium lack, the initial phasic response was preferentially affected and it disappeared completely at concentrations below 10(-7)M, while the ensuing tonic response persisted, though considerably diminished in size, even at the concentration of 10(-8)M.2. In calcium-free media, various multivalent cations, which according to Frank (1962) can support the K-contracture of a skeletal twitch muscle fibre in calcium-free solution, augmented the remaining tonic response, but did not restore the phasic response, when it was eliminated in calcium free environment.3. When K-contractures were induced in normal calcium media, these cations produced, in contrast, an abolition of the phasic response together with a partial depression of the tonic phase. They also inhibited a part of the fully developed contracture. The last effect was no longer obtainable in calcium-free media.4. It is concluded that the phasic response and a part of the tonic response of taenia coli depend upon the extracellular calcium for their initiation (and also for maintenance of tension in the case of the latter) and that the rest of the tonic response draws on a store of ;bound' calcium for its evolution.

Permeability changes associated with the action potential in procaine-treated crayfish abdominal muscle fibers

Permeability changes associated with prolonged action potentials have been analyzed in procaine-treated crayfish abdominal muscle fibers. The effect of external Ca indicates that the increase in membrane conductance observed during the rising phase of the action potential is primarily due to a permeability increase for Ca. A remnant of the permeability increase may cause the succeeding plateau as shown by its high conductance and by the effect of low Mn. A delayed increase in conductance precedes the termination of the plateau phase. This is due to a delayed increase in permeability, probably for K, that is observed when depolarizing electrogenesis is eliminated. High external Ca reduces the action potential duration, the falling phase starting at a higher depolarization. These changes may be related to an earlier onset of the delayed increase in permeability, induced by a larger inside positivity in the presence of higher Ca. No "anomalous rectification" is seen in early or late I-V curves for small depolarizations. Ba may replace Ca in its role in depolarizing electrogenesis, and the first action potential induced in Ba saline has a large overshoot and a long duration. In higher Ba salines, action potentials are greatly prolonged. Long term soaking in Rb-containing or K-free saline also augments and prolongs the action potential. These changes are assumed to be related to depression of the K permeability of the membrane.