Sensitivity of the sodium efflux in single barnacle muscle fibers to the microinjection of aluminum

Aluminium-induced biphasic effect

Increased bioavailability of aluminium has raised concerns about the toxic effect of aluminium. The cholinotoxic effect of aluminium is already well established. The biological response of an organism following exposure to a chemical may be biphasic. Although aluminium-induced biphasic change has been reported in diverse organ systems, the biphasic effect on cholinergic system has received less attention. In vitro and in vivo studies have demonstrated an aluminium-induced biphasic effect on the marker enzyme of cholinergic system, acetylcholinesterase. The biphasic effect of aluminium on the acetylcholinesterase enzyme activity may be due to the direct neurotoxic effect of the metal and the level of aluminium accumulated. Among various hypotheses, peroxidation-induced changes in the structure of membrane following aluminium accumulation seems to explain the biphasic effect of aluminium on acetylcholinesterase activity.

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.

The cellular toxicity of aluminium

Aluminium is a serious environmental toxicant and is inimical to biota. Omnipresent, it is linked with a number of disorders in man including Alzheimer's disease, Parkinson's dementia and osteomalacia. Evidence supporting aluminium as an aetiological agent in such disorders is not conclusive and suffers principally from a lack of consensus with respect to aluminium's toxic mode of action. Obligatory to the elucidation of toxic mechanisms is an understanding of the biological availability of aluminium. This describes the fate of and response to aluminium in any biological system and is thus an important influence of the toxicity of aluminium. A general theme in much aluminium toxicity is an accelerated cell death. Herein mechanisms are described to account for cell death from both acute and chronic aluminium challenges. Aluminium associations with both extracellular surfaces and intracellular ligands are implicated. The cellular response to aluminium is found to be biphasic having both stimulatory and inhibitory components. In either case the disruption of second messenger systems is observed and GTPase cycles are potential target sites. Specific ligands for aluminium at these sites are unknown though are likely to be proteins upon which oxygen-based functional groups are orientated to give exceptionally strong binding with the free aluminium ion.

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.

Parenteral aluminum compounds produce a local toxic myopathy in rats: importance of the anion

Aluminum lactate, injected in rats, produced skeletal muscle necrosis of diaphragm and abdominal wall subjacent to peritoneal surfaces. Deeper muscle cells (distal from inoculum) were less severely affected. Ultrastructural studies of diaphragm revealed inoculum coating collagen fibrils, aggregating next to muscle basal lamina and localized within phagocytes. Aluminum lactate penetrated lymphatic vessels and caused reactive changes on the pleural as well as peritoneal surfaces of diaphragm. In contrast, injection of aluminum citrate did not produce myopathy. Also, mixtures of aluminum lactate with aluminum citrate, sodium citrate, or another chelating agent failed to produce myopathy. Therefore, the regional myopathy produced by the lactate salt provides a model for in vivo cytotoxicity of aluminum in which anionic binding is a critical determinant.

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.

An investigation of the sensitivity of the ouabain-insensitive sodium efflux in single barnacle muscle fibers to pentachlorophenol

The aim of the present work was to explore the possibility that pentachlorophenol (PCP) influences the behavior of the resting Na efflux in single muscle fibers from the barnacle, Balanus nubilus. It is shown here that PCP causes a transitory rise in the Na efflux in both unpoisoned and ouabain-poisoned fibers and that the response is dose-dependent, the minimal effective concentration in ouabain treated fibers being less than 10(-6) M. The efficacy of PCP is significantly greater than that of 2,3,4-trichlorophenol. 2,3-Dichlorophenol is ineffective. This is also the case with phenol. The magnitude of the response to PCP is a function of external pH. Lowering pHe increases the response. The response has an absolute requirement for external Ca2+ and is a sigmoidal function of external Ca2+ concentration. Since treatment of these fibers with PCP in high concentration leads to prompt contraction, experiments were designed to determine whether the observed rise in ouabain-insensitive Na efflux is due to a fall in myoplasmic pCa and whether trigger Ca2+ originates from the bathing medium. The results obtained show that prior injection of ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid (EGTA) or 1,2-bis(2-aminophenoxyethane-N,N,N',N'-tetraacetic acid (BAPTA) leads to a drastic reduction in the response to PCP. They also show that prior external application of verapamil or devapamil stops the response to PCP from occurring. Both Cd2+ and Co2+ are also effective but only temporarily. Last, the effects of ryanodine and 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) were tested, since the former is known to block the sarcoplasmic reticulum Ca2+ release channel, and the latter to impair the action of agents known to release Ca2+ from internal depots. Both ryanodine and TMB-8 are found to reduce the response to PCP. Taken together, these observations support the hypothesis that PCP stimulates the ouabain-insensitive Na efflux by increasing the internal free Ca2+ and that the increase in internal Ca2+ is due to the entry of trigger Ca2+ from the outside via Ca2+ channels, as well as release of Ca2+ by the sarcoplasmic reticulum via its channel. They also indicate that the efficacy of PCP depends on the 5 Cl atoms present in its aromatic ring and pHe.

Protection by GTP from the effects of aluminum on the sodium efflux in barnacle muscle fibers

The idea that guanine nucleotides act as chelators of Al3+ and that Al interrupts the mechanism by which GTP or Gpp(NH)p stimulates the Na efflux in single muscle fibers from the barnacle Balanus nubilus has been tested. As a rule, injection of GTP or Gpp(NH)p into unpoisoned and ouabain-poisoned fibers produces a rise in the 22Na efflux that is usually transitory in nature. Fibers preinjected with GTP show a fall in the Na efflux following the injection of AlCl3 in an equimolar concentration. If, however, the concentration of Al for injection is halved, then GTP is found to be fully protective. Fibers preinjected with AlCl3 show little or no response to the injection of GTP. This is also the case with ouabain-poisoned fibers. Ouabain-poisoned fibers preinjected with GTP also show little or no response to the injection of AlCl3. The stimulatory response to the injection of AlCl3 into fibers preinjected with 0.5 M GTP is dose-dependent. A graded response is also found when 0.5 M AlCl3 is injected into fibers preinjected with GTP in varying concentrations. Gpp(NH)p is fully protective against the inhibitory effect of Al injection in unpoisoned fibers. Further, Gpp(NH)p abolishes the biphasic effect of Al injection on the ouabain-insensitive Na efflux. To strengthen the argument that GTP acts as a chelator of Al, a solution mixture of 0.5 M GTP/0.5 M AlCl3 (pH 1-2) was injected into unpoisoned fibers. This is found to lead to a smaller fall in the resting Na efflux than that obtained by injecting AlCl3 alone or injecting AlCl3 after GTP. It is thus quite clear that the barnacle muscle fiber is a useful preparation for studies of this type.