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The behaviour of membrane currents in a giant muscle fibre of a certain barnacle Balanus nubilus, Darwin, was studied by using voltage clamp technique after treating fibres with a Ca-chelating internal solution.2. Membrane currents can be classified into an early transient current and a late outward current.3. The early transient current can be considered a sum of an inward and an outward component of different properties.4. A conditioning depolarization suppresses the inward component but not the outward component.5. Procaine suppresses the outward component and Co ions suppress the inward component.6. Changes in the external Ca ion concentration alter the inward component but not the outward component.7. The inward component is considered to be carried by Ca ions and the outward component mainly by K ions.8. The outward component of the early current is similar to the current expected from the resting membrane conductance.9. It is concluded that the early conductance increase of the barnacle muscle fibre membrane occurs only to Ca ions but not to K ions, although the membrane potential at the peak of spike is determined by both ions.
Hagiwara et al. (1969) studied this question.