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

Abstract

SUMMARY1. 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 (104 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 HEPES-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 104 M-cyclic GMP. 6. Injection of pure protein kinase inhibitor (1I 6 x 104 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 ouabaininsensitive Na efflux mainly by activating cyclic AMP-protein kinase rather than cyclic GMP-protein kinase.

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

“…Similar observations have been made with cyclic AMP (Bittar, Chambers & Schultz, 1976) and with cyclic GMP (Bittar & Sharp, 1979).…”

“…Lack of dependence on external Na of the response to GTP It has been reported that the response to cyclic AMP ) and cyclic GMP (Bittar & Sharp, 1979) is completely independent of external Na when lithium is used as an Na substitute. It has also been reported that the response to high K is only partly dependent on external Na (Sharp & Bittar, 1980).…”

“…At t = 30 min, by which time peak stimulation of the Na efflux develops, the magnitude of the depolarization is 5-5 + 2 mV, n = 15. In this respect, micro-injection of distilled water in itself produces a slight and almost immediate depolarization but this lasts for not more than 5 min (Bittar, Chen, Danielson, Hartmann & Tong, 1972;Bittar & Sharp, 1979). …”

Stimulation by high external potassium of the sodium efflux in barnacle muscle fibers

“…However, this interpretation assumes that these agents act in a highly specific manner and do not interfere with normal functioning of other systems that might be involved in the response to raised [K]o. In order to test the possibility that verapamil prevents the response to K depolarization by a means other than blocking the voltage-dependent Ca z + channels, the following experiment was carried out: cyclic GMP, which, in barnacle muscle fibers causes a stimulation of the ouabain-insensitive Na efflux that is dependent on external Ca 2+ but does not involve a change in membrane potential (Bittar & Sharp, 1979), was injected into fibers following external application of 10-4M verapamil. In verapamil-treated fibers injection of 10-3 M cyclic GMP caused an average stimulation of the ouabain-sensitive Na efflux of 67.2+13.2%, n=5.…”

“…The method of membrane potential measurement was essentially as described by Bittar and Sharp (1979). In the studies presented here the electrode tip was positioned ~ 1 cm below the tip of the cannula in order to avoid penetration of the sarcolemma by the electrode when contraction of the fiber occurred following exposure to elevated [K]o.…”