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

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Abstract
The ouabain-insensitive Na efflux in barnacle [Balanus nubilus and B. aquila] muscle fibers is promptly stimulated by injection of cyclic[c]GMP. The minimal effective injected concentration is about 10-7 M. This effect of cGMP can not be mimicked by injecting 5''-GMP. External application of ouabain (10-4 M) to fibers not pretreated with ouabain during the stimulatory response to cGMP causes some inhibition of the Na efflux indicating that cGMP does not cause appreciable inhibition of the Na:K pump. The magnitude of the stimulatory response to injected cGMP depends on the external Ca2+ concentration and pHe [external pH] but not on the Na+, K+ or Mg2+ concentration. It depends on pHi [internal pH], since acidification of HCO3-containing ASW [artificial seawater] leads to a greater enhancement of the response to cGMP than is observed with acidified HEPES[N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid]-ASW. Stabilization of myoplasmic pCa by injecting 100 mM-EGTA [ethylene glycol-bis(B-aminoethylether) N,N,N'',N'' tetraacetic acid] before or after cGMP fails to alter the magnitude of the response to the nucleotide. Enrichment of the fiber with Mg2+ at the time of injection of cGMP leads to a reduced response. No change in response is seen when the internal free Mg concentration is suddenly reduced by injecting 0.05 M-pyrophosphate with cGMP. Injection of cGMP-dependent protein kinase stimulatory modulator before cGMP fails to enhance the response to the nucleotide. The same is true of phosphodiesterase inhibitor protein. Preinjection of 10-2 M papaverine enhances the response to a subsequent injection of 10-3 M cGMP. Injection of pure protein kinase inhibitor (1.6 .times. 10-4 M) before 10-3 M cGMP reduces the response to the nucleotide. Injected cGMP stimulates the ouabain-insensitive Na efflux mainly by activating cAMP-protein kinase rather than ther than cGMP-protein kinase.