Stimulation by injected guanosine triphosphate of the sodium efflux in barnacle muscle fibres.

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Abstract
The mechanism by which injected GTPNa2 stimulates the ouabain-insensitive Na efflux in single muscle fibers from the barnacle, B. nubilus, was studied. Injection of GTPNa2 causes a stimulatory response which is usually transitory and almost completely reversed by injecting MgCl2 (but not KCl). Injected 5''-guanylylimidodiphosphate, Gpp(NH)p, mimics this action of GTP but the reversal seen with injected Mg2+ is less pronounced. Pre-treatment of these fibers with verapamil reduces the size of the stimulatory response to GTP and Gpp(NH)p. Pre-injection of protein kinase inhibitor (PKI) or regulatory subunits reduces the response as well. Pre-treatment with imipramine or trifluoperazine reduces the response to injected GTP; in combination with verapamil, a greater reduction in response is seen. Injection of EDTA leads to a stimulatory response which is transitory. This response is largely abolished by verapamil. Injection of cholera toxin leads to a sustained stimulatory rather than a transitory response. GTP or Gpp(NH)p when injected following peak stimulation by cholera toxin leads to a moderate sustained stimulation. The stimulatory response to injected GTPNa2 probably is the result of activation of Ca2+ channels and of increased availability of GTPMg. These 2 conditions bring about activation of adenylate cyclase and hence activation of cAMP-protein kinase by newly formed cAMP.