Stimulation by cyclic GMP of sodium efflux in barnacle muscle fibres.

1. The ouabain‐insensitive Na efflux in barnacle muscle fibres is promptly stimulated by injection of cyclic GMP. The minimal effective injected concentration is found to be about 10(‐7) M. This effect of cyclic GMP could not be mimicked by injecting 5'‐GMP. 2. External application of ouabain (10(‐4) M) to fibres not pretreated with ouabain during the stimulatory response to cyclic GMP causes some inhibition of the Na efflux indicating that cyclic GMP does not cause appreciable inhibition of the Na:K pump. 3. The magnitude of the stimulatory response to injected cyclic GMP depends on the external Ca2+ concentration, as well as pHe but not on the Na+, K+ or Mg2+ concentration. It also depends on pHi, since acidification of HCO3‐containing ASW leads to a greater enhancement of the response to cyclic GMP than is observed with acidified HERPES‐ASW. 4. Stabilization of myoplasmic pCa by injecting 100 mM‐EGTA before or after cyclic GMP fails to alter the magnitude of the response to the nucleotide. Enrichment of the fibre with Mg2+ at the time of injection of cyclic GMP leads to a reduced response. No change in response, however, is seen when the internal free Mg concentration is suddenly reduced by injecting 0.05 M‐pyrophosphate with cyclic GMP. 5. Injection of cyclic GMP‐dependent protein kinase stimulatory modulator before cyclic GMP fails to enhance the response to the nucleotide. The same is true of the phosphodiesterase inhibitor protein. However pre‐injection of 10(‐2) M‐papaverine enhances the response to a subsequent injection of 10(‐3) M‐cyclic GMP. 6. Injection of pure protein kinase inhibitor (1.6 x 10(‐4) M) before 10(‐3) M‐cyclic GMP reduces the response to the nucleotide. 7. The argument is put forward that injected cyclic GMP stimulates the ouabain‐insensitive Na efflux mainly by activating cyclic AMP‐protein kinase rather than cyclic GMP‐proton kinase.