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Membrane currents are investigated under voltage-clamp conditions in crab muscle fibre. 2. Step depolarizations elicit an initial composite current followed by a late outward current. 3. One of the components of the initial current is inward. It is sensitive to the external calcium concentration and inhibited by manganese ions, it can be carried also by strontium ions; thus it is expected to be a calcium current. 4. In TEA solution this calcium current appears alone, it reverses when the membrane polarization is carried beyond an internal potential of +30 or +35 mV. Such a low equilibrium potential for calcium ions can be explained either by a low selectivity of the calcium channel or by a local accumulation of calcium ions. 5. Calcium conductance shows voltage- and time dependence. 6. The late outward current corresponds to a potassium current and is inhibited by TEA ions. Its activation exhibits voltage- and time dependence. 7. The activation curve of the late potassium current is shifted in a depolarizing direction by addition of manganese ions. A similar shift produced by increasing [Ca]o or decreasing [Ca]i has been described on other preparations. It is then supposed that the electrical field of the membrane is modified by the gradient of double cations.

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