RESUMO
Aluminum is a toxic metal whose complex aquatic chemistry, mechanisms of toxicity and trophic transfer are not fully understood. The only isotope of Al suitable for tracing experiments in organisms-(26)Al-is a rare, costly radioisotope with a low emission energy, making its use difficult. Gallium shares a similar chemistry with Al and was therefore investigated as a potential substitute for Al for use in aquatic organisms. The freshwater snail, Lymnaea stagnalis was exposed to either Al or Ga (0.0135 mM) under identical conditions for up to 40 days. Behavioural toxicity, metal accumulation in the tissues, and sub-cellular partitioning of the metals were determined. Al was more toxic than Ga and accumulated to significantly higher levels in the soft tissues (P < 0.05). The proportion of Al in the digestive gland (DG; detoxificatory organ) relative to other tissues was significantly lower than that of Ga (P < 0.05) from day 14 onwards. There were also differences in the proportions of Al and Ga associated with heat stable proteins (HSPs) in the digestive gland, with significantly more HSP present in the DGs of snails exposed to Al, but significantly less Al than Ga associated with the HSP per unit mass protein present. From this evidence, we conclude that Ga may be of limited use as a tracer for Al in animal systems.
Assuntos
Alumínio/metabolismo , Gálio/metabolismo , Indicadores e Reagentes/metabolismo , Coloração e Rotulagem , Alumínio/toxicidade , Animais , Comportamento Animal/efeitos dos fármacos , Feminino , Água Doce , Gálio/toxicidade , Indicadores e Reagentes/química , Lymnaea/anatomia & histologia , Lymnaea/efeitos dos fármacos , Lymnaea/metabolismo , Óvulo/efeitos dos fármacos , Óvulo/fisiologia , Coloração e Rotulagem/instrumentação , Coloração e Rotulagem/métodos , Frações Subcelulares/metabolismo , Distribuição TecidualRESUMO
Silicon (Si) ameliorates aluminum (Al) toxicity to a range of organisms, but in almost all cases this is due to ex vivo Si-Al interactions forming inert hydroxyaluminosilicates (HAS). We hypothesized a Si-specific intracellular mechanism for Al detoxification in aquatic snails, involving regulation of orthosilicic acid [Si(OH)4]. However, the possibility of ex vivo formation and uptake of soluble HAS could not be ruled out Here we provide unequivocal evidence for Si-Al interaction in vivo, including their intracellular colocalization. In snails preloaded with Si(0H)4, behavioral toxicity in response to subsequent exposure to Al was abolished. Similarly, recovery from Al-induced toxicity was faster when Si(OH)4 was provided, together with rapid loss of Al from the major detoxificatory organ (digestive gland). Temporal separation of Al and Si exposure excluded the possibility of their interaction ex vivo. Elemental mapping using analytical transmission electron microscopy revealed nanometre-scale colocalization of Si and Al within excretory granules in the digestive gland, consistent with recruitment of Si(OH)4, followed by high-affinity Al binding to form particles similarto allophane, an amorphous HAS. Given the environmental abundance of both elements, we anticipate this to be a widespread phenomenon, providing a cellular defense against the profoundly toxic Al(III) ion.
Assuntos
Alumínio/toxicidade , Lymnaea/efeitos dos fármacos , Ácido Silícico/metabolismo , Poluentes Químicos da Água/toxicidade , Animais , Sistema Digestório/metabolismo , Sistema Digestório/ultraestrutura , Lymnaea/metabolismo , Lymnaea/ultraestrutura , Lisossomos/metabolismo , Lisossomos/ultraestrutura , Microscopia Eletrônica de TransmissãoRESUMO
Silicon is essential for some plants, diatoms, and sponges but, in higher animals, its endogenous regulation has not been demonstrated. Silicate ions may be natural ligands for aluminum and here we show that, in the freshwater snail (Lymnaea stagnalis), intracellular silicon seems specifically up-regulated in response to sublethal aluminum exposure. X-ray microanalysis showed that exposure of snails to low levels of aluminum led to its accumulation in lysosomal granules, accompanied by marked up-regulation of silicon. Increased lysosomal levels of silicon were a specific response to aluminum because cadmium and zinc had no such effect. Furthermore, intra-lysosomal sulfur from metallothionein and other sulfur-containing ligands was increased after exposure to cadmium and zinc but not aluminum. To ensure that these findings indicated a specific in vivo response, and not ex vivo formation of hydroxy-aluminosilicates (HAS) from added aluminum (555 microg/liter) and water-borne silicon (43 microg/liter), two further studies were undertaken. In a ligand competition assay the lability of aluminum (527 microg/liter) was completely unaffected by the presence of silicon (46 microg/liter), suggesting the absence of HAS. In addition, exogenous silicon (6.5 mg/liter), added to the water column to promote formation of HAS, caused a decrease in lysosomal aluminum accumulation, showing that uptake of HAS would not explain the loading of aluminum into lysosomal granules. These findings, and arguments on the stability, lability, and kinetics of aluminum-silicate interactions, suggest that a silicon-specific mechanism exists for the in vivo detoxification of aluminum, which provides regulatory evidence of silicon in a multicellular organism.