Effects of ascorbic acid on lead induced alterations of synaptic transmission and contractile features in murine dorsiflexor muscle
Introduction
Exposure to toxic heavy metals such as lead has been continuously increasing with industrialisation and humans may be exposed through different environmental sources. Heavy metals such as lead will impact various organs of the body and therefore controlling their toxicity is crucial for exposed individuals. For instance the effects of lead exposure during intrauterine life may include reduced birth weight, impaired mental development and disturbances in sensory pathways within the central nervous system Shy, 1990, Johnston and Goldstein, 1998. During the childhood period lead may cause serious neurotoxic effects and prolonged exposure to lead at this stage may result particularly in learning difficulties and modification of memory function Needleman and Gatsonis, 1990, Needleman et al., 1990, Needleman, 1994, McMichael et al., 1994, Johnston and Goldstein, 1998. Peripheral nervous system is also susceptible to toxic effects of lead exposure and this may cause significant muscle weakness in humans (Rubens et al., 2001).
Lead exposure may as well impact neuromuscular junctions and modify skeletal muscle function in various experimental models (Al Dhaheri et al., 1996). Like other heavy metals lead may exert specific effects on the neurotransmitters release or on ion movement potentially impacting neuromuscular junction and causing alteration in muscle function Hirata and Kosaka, 1993, Oortigiesen et al., 1993, Shao and Suszkiw, 1991, Struzynska and Rafalowska, 1994. Various mechanisms have been proposed to explain lead's actions on peripheral nerves and skeletal muscles Bressler and Goldstein, 1991, Zacharova et al., 1993. For instance it has been suggested that muscle diaphragm fatigue observed following lead may be the result of modification of Ca2+ release from the sacroplasmic reticulum and increased oxygen free radical formation related to cellular energetics (Smith-Blair, 2002). It seems free radical mechanism may have a potential role in mediating lead's effect on the nerves system and skeletal muscle.
A positive correlation was found between the presence of lead in blood and significant increase in malondialdehyde (MDA) concentrations and superoxide dismutase (SOD) activity (Ye et al., 1999). Effects of lead on parameters of oxidative stress in the brain from rats indicated that lead intoxication induces an oxidative stress cascade that eventually affects cellular homeostasis (Adonaylo and Oteiza, 1999). Lead was found to impact levels of glutathione-S-transferase (GST), glutathione reductase (GR), glutathione peroxidase (GPx), superoxide dismutase (SOD), and catalase, suggesting tissue specific changes of antioxidant defence components in response to lead exposure (Somashekaraiah et al., 1992). Presumably it can be postulated that sulfur-containing compounds and antioxidants may decrease metal toxicity possibly by providing a reducing power that prevents the enzymes from undergoing oxidation (Rai and Raizada, 1988).
However, despite extensive research on heavy metals toxicity relatively little attention has been paid to the modulating effects of vitamins and possible implications of nutritional status on heavy metals exposure in humans and animals. Apparently the deleterious effects of lead on skeletal muscle might be modified by ascorbic acid supplementation. Protective effects of antioxidant vitamins in heavy metals poisoning may occur either directly at the cellular level or indirectly by interfering with the intestinal absorption of heavy metals in other occasions (Pace and Iannucci, 1994).
Although ascorbic acid may exhibit some protective role in reducing potentially adverse effects from heavy metals, the clinical and experimental data, in the literature, in support of this theory are still limited. The present study was designed to study the effects of lead on skeletal muscle contraction as well as related electrophysiological parameters such as resting membrane potentials (RMPs), endplate potentials (EPPs) and miniature endplate potentials (MEPPs). The study also aimed to assess possible protective effects of ascorbic acid on lead induced modifications of skeletal muscle twitch tensions and electrophysiological parameters.
Section snippets
Animals
All the experiments were performed on C57 BL strain male mice. Animals were housed in groups of 5 in plastic cages with a controlled light and dark cycle of 12 hours each at 24–26 °C. Food and water were available ad libitum. Forty animals were divided into four groups of n = 10 each. (10 control, 10 lead treated, 10 ascorbic acid, 10 lead treated plus ascorbic acid). Lead treatment experiments were carried out by giving daily injections of 1.0 mg/kg lead acetate in 5% glucose solution i.p for
Animal weights
Mice chronically treated with lead did not differ significantly in their body weights from control groups (34.1 ± 1.3 g for control vs. 32.3 ± 1.5 g for lead treated). The dorsiflexor muscle to the body weight ratio also remained unchanged. Furthermore lead treated mice did not exhibit signs of severe lead induced neuromuscular pathology such as ataxia or splayed gait.
Lead level
Treated animals showed significantly higher lead level in their plasma (0.10 ± 0.02 in control vs. 0.76 ± 0.01 in lead treated,
Discussion
Understanding features of heavy metals poisoning and corresponding cellular and molecular mechanisms might facilitate developing treatments for specific neurotoxic conditions caused by these metals. Most experiments dealing with the effects of lead on the peripheral nervous system and skeletal muscle function did not address the modulating effects of vitamins and possible implications of nutritional status on improving heavy metals exposure. The current study was designed to investigate the
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