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

Abstract

1. A study has been made of the mechanism by which injected disodium GTP stimulates the ouabain‐insensitive Na efflux in single muscle fibres from the barnacle, Balanus nubilus. 2. Injection of GTPNa2 causes a stimulatory response which is usually transitory and almost completely reversed by injecting MgCl2 (but not KCl). 3. Injected 5'‐guanylylimidodiphosphate, Gpp(NH)p, mimics this action of GTP but the reversal seen with injected Mg2+ is less pronounced. 4. (i) Pre‐treatment of these fibres with verapamil reduces the size of the stimulatory response to GTP and Gpp(NH)p. (ii) Pre‐injection of protein kinase inhibitor (PKI) or regulatory subunits reduces the response as well. (iii) Pre‐treatment with imipramine or trifluoperazine reduces the response to injected GTP; in combination with verapamil, a greater reduction in response is seen. 5. Injection of EDTA leads to a stimulatory response which is transitory. This response is largely abolished by verapamil. 6. 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. 7. These results support the view that the stimulatory response to injected GTPNa2 is the result of activation of Ca2+ channels and of increased availability of GTPMg and that these two conditions bring about activation of adenylate cyclase and hence activation of cyclic AMP‐protein kinase by newly formed cyclic AMP.

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

“…external application oft yM-DPH following peak st imulati iplon h v CSU wasco et-ely ineffe fftive in the three fibres tested DISCUSSION The results described in this paper clearly show that the ouabain-insensitive Na efflux is sensitive to the injection of pure catalytic subunits (CSU) and that the characteristic response is one of sustained stimulation. Kinetics of this type are not unexpected, since they are also seen following the injection of cholera toxin (Bittar & Nwoga, 1982a) which is known to produce persistent activation of membrane adenylate cyclase, and hence raised cyclic AMP levels (Gill, 1977). However, in contrast to CSU and cholera toxin, injected cyclic AMP is found to consistently cause only transitory stimulation ofthe Na efflux (Bittar et al, 1976 (Levitan & Adams, 1981).…”

“…There also is evidence that the injection of cholera toxin into ouabain-poisoned fibres causes a sustained stimulation of the Na efflux and that subsequent injection of GTP or Gpp(NH)p leads to a further sustained response (Bittar & Nwoga, 1982a; Bittar, E. E. & Nwoga, J., unpublished data). It thus seemed of interest to find out whether similar kinetic results are obtained by injecting GTP and Gpp(NH)p before or after CSU.…”

“…The cause of this sustained response on applying 10-3 M-ouabain is not clear. Since this is also what happens when Gpp(NH)p is injected following the onset of sustained stimulation of the ouabain-insensitive Na efflux by injected cholera toxin (Bittar & Nwoga, 1982a), it is tempting to suggest that ouabain acts by reducing myoplasmic pCa. This is in line with the accepted view that cardiotonic steroids raise the intracellular level of free Ca.…”