Stimulation by injected guanosine triphosphate of the sodium efflux in barnacle muscle fibres pre-exposed to aldosterone.

1. A study has been made of the response of the Na efflux to injected guanosine triphosphate (GTP) in barnacle muscle fibres pre-exposed to aldosterone. 2. (i) Injection of GTP into unpoisoned fibres causes a transitory stimulation. By contrast, injection of GTP into ouabain-poisoned fibres causes a sustained stimulation. Little or no fall in Em is recorded following GTP injection. (ii) The stimulatory response to GTP of aldosterone pre-exposed ouabain-poisoned fibres differs from that seen in unexposed, ouabain-poisoned fibres, in that its magnitude is greater and always sustained. 3. The magnitude of the response to GTP depends on external Ca and pH but not external Na. The response itself is not seen at 0 degrees C. 4. (i) Verapamil reduces the size of the response to GTP only if applied before GTP. Injection of EGTA, Fe, Zn and Co partially abolishes the residual response. (ii) Injection of MgCl2 almost completely reverses the response to GTP. KCl is ineffective. 5. These results are explained by supposing that removal of internal Mg and trace elements by GTP leads to activation of phosphoprotein phosphatase.