ATP as a positive effector of the sodium efflux in single barnacle muscle fibers

Concerning stimulation by injected fluoroaluminate of the sodium efflux in barnacle muscle fibers

Single barnacle muscle fibers from Balanus nubilus were used primarily to examine the validity of two ideas: first, that the injection of KF stimulates the ouabain-insensitive Na+ efflux, and that this action is potentiated by adding AlCl3 (Al) in a low concentration to the solution of KF prior to injection. And second, that the injection of a KF-AlCl3 solution into ouabain-poisoned, K(+/-)-depolarized fibers elicits a stimulatory response resembling that obtained by injecting GTP. The results of this study are as follows: injection of 0.5 M KF into unpoisoned fibers causes a sustained rise in the resting Na+ efflux. However, injection of a 0.5 M KF, 10(-3) M AlCl3 solution leads to a reduced rather than an augmented response. Whereas injection of 0.5 M KF into ouabain-poisoned fibers elicits a marked stimulatory response, the injection of 0.5 M KF, 10(-3) M AlCl3 reduces the remaining Na+ efflux. Injection of KF-AlCl3 in equimolar concentrations, e.g., 0.25 M, elicits a response that is significantly larger than that obtained by injecting 0.25 M KF. A dose-response curve indicates that a 0.2 M solution of fluoroaluminate probably represents an optimal concentration. Injection of 0.3 M KF following peak stimulation by injecting 0.3 M AlCl3 completely reverses this response to Al. In sharp contrast, injection of a 0.3 M KF, 0.3 M AlCl3 mixture following peak stimulation by injecting 0.3 M AlCl3 is ineffective. Injection of KF into ouabain-poisoned, K+ depolarized fibers does not always cause sustained stimulation of the remaining Na+ efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

The reduction of the inhibitory effect of aluminum on Na+ efflux in barnacle muscles fibers by preinjecting phosphate

The object of the present study was to test the hypothesis that the pre-enrichment of single muscle fibers from the barnacle Balanus nubilus with inorganic phosphate may protect the basal Na efflux from the inhibitory effect of Al injection. This approach was adopted in the light of evidence that the preinjection of ATP fails to stop the Na efflux in unpoisoned fibers from falling following the injection of Al. The results of the experiments are as follows: (i) Preinjection of K2HPO4 into unpoisoned fibers reduces the magnitude of the inhibitory effect on the basal Na efflux of injected Al in a dose-dependent manner but fails to completely stop it from occurring. (ii) Injection of K2HPO4 following Al into unpoisoned fibers fails to arrest or reverse the inhibitory effect of injected Al. (iii) Injection of K2HPO4 in a concentration as high as 0.5 M is without effect on the course of the basal Na efflux. (iv) Injection of K2HPO4 into ouabain-poisoned fibers fails to stop Al from stimulating the ouabain-insensitive Na efflux. Injection of K2HPO4 following peak stimulation by injecting Al is also without effect. (v) Injection of K2HPO4 in a concentration as high as 0.5 M is without effect on the course of the ouabain-insensitive Na efflux. Collectively, the results obtained with unpoisoned 'hypersensitive' fibers are consistent with the view that a significant fraction of the injected inorganic phosphate binds Al3+, and hence protects the basal Na efflux from the untoward action of Al3+.

Abolition with chloramine-T of inactivation in barnacle muscle fibers results in stimulation of the ouabain-insensitive sodium efflux

The hypothesis that chloramine-T stimulates the basal Na+ efflux in barnacle fibers as the result of the entry of trigger Ca2+ into the myoplasm from the bathing medium was examined in this study. Two reasons for doing so can be given. One is that the oxidant is known to abolish inactivation in sodium and potassium channels. The other is that L-type Ca2+ channels are present in barnacle fibers, and an increase in internal free Ca2+ in these fibers is known to stimulate the Na+ efflux, particularly in ouabain-poisoned fibers. The results of the experiments are as follows: (i) Chloramine-T exerts a biphasic effect on the Na+ efflux: inhibition is followed by stimulation, the threshold concentration being 10(-5) M. This is also found to be the threshold concentration for shortening of these fibers. (ii) The kinetics of the inhibitory effect resemble those of ouabain. (iii) Ouabain is without effect on the stimulatory phase caused by chloramine-T. (iv) Application of chloramine-T after the full effect of 10(-4) M-ouabain is reached elicits solely a stimulatory response. (v) The dose-response curves for the stimulatory action of chloramine-T in unpoisoned and ouabain-poisoned fibers are alike except that the threshold concentration is less than 10(-5) M in poisoned fibers. (vi) Basal light emission from unpoisoned and ouabain-poisoned fibers loaded with the photoprotein, aequorin, some 60 min beforehand increases as soon as they are exposed to 10(-4) M chloramine-T. The response recorded in unpoisoned fibers is monophasic and usually transitory, whereas it is multiphasic and usually sustained in ouabain-poisoned fibers. (vii) The dose-response curve for chloramine-T shows a shift to the left in poisoned fibers. (viii) The magnitude of the rise in light emission depends on the external Ca2+ concentration. A rise fails to take place in the nominal absence of external Ca2+. Taken together, these results support the above hypothesis that chloramine-T causes the entry of trigger Ca2+ into the myoplasm from the outside and provide evidence that stimulation of the Na+ efflux is associated not only with this event but also with a reduced Na+ gradient resulting from inhibition of the membrane Na+/K(+)-ATPase system by the oxidant. Thus, the suggestion put forward is that this oxidant promotes reverse Na+/Ca2+ exchange and is able to exert multiple effects on membrane transport.

Citrate as an aluminum chelator and positive effector of the sodium efflux in single barnacle muscle fibers

The injection of citrate produces considerably greater stimulation of the Na efflux in ouabain-poisoned fibers (from the barnacle Balanus nubilus) than in unpoisoned fibers. When injected in excess together with Al into unpoisoned fibers it is without effect. Its injection is also without effect on the decline in the Na efflux elicited by injecting Al beforehand. Citrate injection into ouabain-poisoned fibers following peak stimulation by injecting Al produces a further rise which is a function of the Al concentration. Al injection after peak stimulation of the ouabain-insensitive Na efflux by citrate is usually without significant effect. Injection of aspartate into poisoned fibers causes a small rise in the remaining Na efflux and fails to prevent the response to Al injection from occurring. Taken together, these observations are in keeping with the view that citrate is not only a powerful chelator of Al but also a powerful activator of reverse Na+/Ca2+ exchange in ouabain-poisoned fibers, presumably because of its ability to raise myoplasmic pMg.

Lack of effect on the sodium efflux of the microinjection of D-Ins(1,4,5)P3 into ouabain-poisoned barnacle muscle-fibers

A study has been carried out using relatively intact mature muscle fibers from the barnacle Balanus nubilus to see whether D-Ins(1,4,5)P3 stimulates the ouabain-insensitive Na efflux following its microinjection and whether this is accompanied by a contraction of the fiber. Part of the impetus for a study of this type came from the on-going debate between Vergara, Rojas and co-workers and Lea and co-workers, the former group holding the view that skinned barnacle fibers and skeletal fibers in general are responsive to this isomer. The evidence brought forward indicates that the injection of D-Ins(1,4,5)P3 in concentrations in the range of 10(-2) M to 10(-6) M into cannulated unskinned fibers pretreated with ouabain fails to increase the residual efflux, and additionally fails to elicit a contraction. A similar picture emerges with the use of non-hydrolyzable DL-Ins(1,4,5)P3[S]3, analog following its injection in a concentration of 0.5 microM. Higher concentrations of the analog were unavailable for use. This work also involved verification of the idea that an effect might be obtainable in depolarized fibers. However, doubling or tripling K0+ and injection of the isomer or the analog simultaneously failed to support this idea. Since nothing is known as to whether D-Ins(1,4,5)P3 influences the behavior of the Na(+)-Ca2+ exchanger when operating in the reverse mode, experiments were done to check this possibility. ATPNa2 which is though to activate Na(+)-Ca2+ exchange was injected prior to the isomer or the analog but no significant results were obtained. A similar line of reasoning was followed, that of activating the L-type Ca2+ channel by injecting GTPNa2 which in addition is known to activate adenylate cyclase. Again, neither the isomer nor the analog were effective. Thus, the only conclusion possible is that in relatively intact, mature barnacle fibers there is no coupling between the phosphoinositide signalling pathway and two other key systems, viz. the Na(+)-Ca2+ exchanger when operating in the reverse mode and the L-type Ca2+ channel. Equally clear is that for some unknown reason the ouabain-insensitive Na efflux and the contractile apparatus are insensitive to D-Ins(1,4,5)P3[S]3.

The ability and inability of ATP to stop aluminum from reducing the sodium efflux in unpoisoned barnacle muscle fibers

A study has been made in single barnacle muscle fibers with the object of determining whether ATP is able to protect the resting Na efflux from the effects of injected aluminum (Al) and whether Al is able to reduce or abolish the stimulatory action of ATP on the efflux. The results of the experiments show that neither ATPMg nor ATPNa2 preinjection stops Al from reducing the basal Na efflux in unpoisoned fibers which undergo a large fall (hypersensitive fibers). Preinjection of Al into such fibers reduces or abolishes the stimulatory response of the Na efflux to ATP injection. In less hypersensitive fibers, however, ATPMg is protective. This is also true of ATPNa2 preinjection in both classes of fibers showing stimulation. Injection of a mixture of AlCl3-ATPNa2 into unpoisoned fibers causes less inhibition than AlCl3 injection. The hypothesis that both ATPMg and ATPNa2 are protective is also supported by the results obtained with ouabain-poisoned fibers: (i) Al injection after ATP fails to reverse the stimulatory response to ATP, while ATP injection after Al exerts only a small or no effect. (ii) Mg2+ injection fails to reverse the stimulatory response to Al injection in poisoned fibers. And (iii) Anti-proteolysis agents e.g. leupeptin and pepstatin, upon preinjection, do not alter the kinetic results obtained by injecting Al into unpoisoned and ouabain-poisoned fibers.