An investigation of the action of ouabain on the sodium efflux in barnacle muscle fibres.

Measurements of radiosodium efflux were carried out on single fibres from the barnacle Balanus nubilus or B. aquila , using the micro‐injection technique. 10 ‐4 M ouabain caused an average fall of 68% of the Na efflux, the minimal effective concentration being ca. 10 ‐7 M. Internal application of 4 × 10 ‐3 M ouabain was without effect. Abolition of the ouabain‐sensitive component with ouabain, followed by raising the external K concentration to 30 mM, resulted in a marked rise in the residual Na efflux. Stimulation of the ouabain‐insensitive Na efflux was also seen after reducing the external pH, as well as after microinjecting CaCl 2 but not MgCl 2 . Uncoupling of the active transport system by K removal, followed by external application of ouabain, and subsequent acidification of the bathing medium, led to stimulation which was considerably greater than that taking place when the Na efflux was uncoupled by K removal and then stimulated by external acidification.