The influence of injected cyclic AMP protein kinase catalytic subunit on the sodium efflux in barnacle muscle fibres.

1. A study has been made of the response of the Na efflux in single barnacle muscle fibres to the injection of cyclic AMP protein kinase catalytic subunit (CSU).2. (i) Injected CSU stimulates the Na efflux in unpoisoned and ouabain-poisoned fibres. (ii) Stimulation in both situations is always sustained, and (iii) stimulation of the ouabain-insensitive Na efflux is observed with as low a concentration as 10(-7) M-CSU.3. (i) The response to injected CSU depends on [Ca](o) but is unaffected by prior external application of verapamil. (ii) An effect fails to occur in fibres cooled to 0 degrees C.4. (i) Injection of 5'-guanylylimidodiphosphate (Gpp(NH)p) following peak stimulation by CSU causes a further sustained stimulatory response. (ii) Injection of CSU following peak stimulation by Gpp(NH)p causes a further sustained stimulatory response. (iii) Injection of undialysed cholera toxin following CSU causes little or no effect, whereas injection of dialysed cholera toxin causes a stimulatory effect. Injection of CSU following undialysed cholera toxin causes little or no effect, whereas its injection after dialysed cholera toxin causes a fairly appreciable effect.5. (i) The response to CSU is reduced by prior injection of protein kinase type 2 regulatory subunit (R(II)) but not when R(II) is injected following peak response to CSU. (ii) Injection of protein kinase inhibitor partially reverses the response to CSU.6. (i) Injection of Mg(2+) fully reverses the response to CSU. (ii) Fe and Zn cause only partial reversal.7. (i) External imipramine but not trifluoperazine reduces the response to CSU. (ii) Diphenylhydantoin is ineffective.8. These results support the view that CSU behaves as a modulator of the ouabain-insensitive Na efflux and that Mg(2+), Fe and Zn are inhibitors of the catalytic reaction.