Toluene is one of the most abused inhaled volatile solvents produced and used by the industrial sector worldwide [27]. The industrial employee suffers from occupational diseases due to the exposure to chemical and/or physical aggressors such as noise. Moreover, combined exposure to multiple risk factors often occurs in the workplace. The interaction of physical agents and chemical substances, even at low doses, can have additive or synergistic effects on health. In this study, we evaluated the effect of toluene and/or noise exposure on the murine hematological and biochemical parameters.
Our hematological results showed significant increase in platelet count in the groups of rats exposed to noise N and co-exposed to noise and toluene NT, with no significant change in the group of rats treated with Olea europaea L. This increase in the platelet count caused platelet aggregates formation which led to vascular, venous or arterial obstruction observed in heart histology.
Toukh et al. (28] reported that hypercoagulability is correlated with the increase of the plasmatic level of cortisol and corticosterone resulting from construction noise exposure. The stress created during noise exposure increases the level of blood catecholamines, which consequently stimulates thrombosis formation and increases the possibility of myocardial infarction [29, 30]. Olea europaea L. leaf extract has a remarkable impact in maintaining the platelet count. This extract is found to be a rich source of oleuropein and other phenolic compounds which potentially could have anti-platelet effects. Several in vivo and in vitro studies have strongly suggested that phenolic compounds of Olea europaea L. leaf and oil extract may protect against platelet aggregation and its complication. [31, 32, 33, 34].
We also found an increase in the number of white blood cells and a decrease in red blood cell count in the groups of rats exposed to noise N, toluene T and co-exposed to noise and toluene NT. This increase was normalized in the groups of rats treated with Olea europaea L.
CK is an enzyme mainly present in muscles and involved in energy storage through a mechanism called creatine phosphorylation. On the other hand, the LDH is mainly found in plasma. It is an indicator of cell energy balance disorder, leading to accumulation of LDH and subsequent cell death [35]. In this study, we recorded a significant increase of CK and LDH activities in the group of rats exposed to toluene, to noise as well as the group of co-exposed rats that probably reflect a cardiomyopathy.
We also noted a significant increase in triglycerides and cholesterol levels in the exposed groups. In this case, the simultaneous or isolated exposure to noise and toluene can cause plasmatic disturbances of lipid parameters caused by hypercholesterolemia. These lipid changes may be at the origin of an atherosclerosis characterized by the abnormal accumulation of lipids in arteries’ walls, which consequently leads to blood vessel obstruction and an increase in cardiovascular risk.
However, the modification of biochemical parameters was absent in rats receiving Olea europaea L. leaf extract. Olea europaea L. treatment was able to protect rats against the damage produced by exposure to noise and toluene. Vezza et al. [36] reported that Olea europaea L. extract exerts beneficial effects in high fat diet (HFD)-induced obesity in mice, which was associated to an improvement in plasma and tissue metabolic profile and vascular dysfunction by reducing oxidative stress and improving the endothelial function by its anti-oxidant and anti-platelet aggregation properties in the vascular wall.
The effects of noise exposure and cardiovascular disorders were shown in several studies. Occupational noise has been associated with important cardiovascular health problems, particularly hypertension, and vascular oxidative stress [37, 38].
In the current study we evaluated cardiac oxidative stress markers of rats exposed to noise and /or toluene. The study showed a significant increase in lipid peroxidation in the heart of rats exposed to noise. These results can be explained by the increase of reactive oxygen species (ROS) production due to noise exposure [39, 40]. On the other hand, our results showed a significant increase in catalase and SOD activities of rats exposed to 85 dB (A) sound level. These findings corroborate those of Samson et al. [41] and Gannouni et al. [21].
The increased activity of superoxide dismutase is therefore an indication that the antioxidant machinery of the heart is activated in response to excessive production of free radicals which is a common denominator in many heart diseases [42].
Moreover, our results showed a significant increase in lipid peroxidation in rats exposed to toluene or co-exposed to noise and toluene. These results are consistent with those previously reported by Kamel and Shehata [43]. Lipid peroxidation increase is the most commonly measured effect of oxidative stress in humans after exposure to a toxic substance [44], and previous studies have shown that toluene induces ROS production during its metabolism [45]. Another human study also showed an increase in malondialdehyde levels in the serum of people working with paint thinner [46].
Our study demonstrated a significant decrease in catalase with an increase in superoxide dismutase in the groups of rats exposed to toluene and co-exposed to noise and toluene. Generally, oxidative stress is considered to be one of the important mechanisms of volatile organic compound’s toxicity which implicate an imbalance between ROS formation and antioxidant defense system. In fact, organic solvents express their toxicity through ROS-induced cell damage [47]. In particular, toluene inhalation was described to significantly increase blood and tissue MDA levels, and to decrease antioxidant enzyme activities [48].
Otherwise, Montes et al. [ 49] reported that sub-chronic toluene exposure increased ROS production and affected several antioxidant levels in the hippocampus and prefrontal cortex in rats.
Our results also showed that antioxidant enzyme levels and lipid peroxidation in heart tissues of rats receiving Olea europea L. leaves extract (N + OLE, T + OLE, NT + OLE) have not been modified when compared to the control. Indeed, Olea europea L. leaves contain polyphenolic compounds (hydroxytyrosol, oleuropein, secoiridoids, flavonoids and triterpenes) that may protect cells against oxidative stress [ 50, 51, 52]. The antioxidant activity of Olea europea L. leaves extract is directly correlated to its richness with bioactive compounds that have shown a powerful ability to attenuate oxidative stress. Our results are concordant with previous studies reporting the antioxidant effect of Olea europea L. in rats [53, 54, 55]. Cardiovascular physiopathology is indeed closely related to oxidative stress and inflammation [56, 57, 58]. Therefore, the use of natural products, especially from Olea europea L. may be an alternative strategy in preventing the disease.
Histological observations essentially illustrated the results of the hematological parameters and antioxidant status. In fact, histological analysis showed that noise exposure induced structural and functional alterations in rat’s myocardium. Histological heart tissue of animals exposed to noise 85 dB(A) showed blood clots in the arteries.
Various studies investigated the correlation between noise, blood pressure, and myocardial damage [59]. A recent longitudinal study has shown that occupational noise exposure increases the risk of coronary artery disease [60]. Gan et al. [61] confirmed also that intense occupational noise exposure is associated with the presence of coronary heart disease.
Another cross-sectional study suggested an association between mean noise > 65 dB(A) and the occurrence of coronary heart disease [62]. This artery obstruction was absent in animals protected with Olea europea L. leaves extract. The beneficial effects of this extract on cardiovascular functions have also been previously reported on endothelial cells cultured in vitro [63] and in an experimental model of hypertension [64]. Our histopathological findings reflect our biochemical results. In fact, rats exposed to 300 ppm toluene have numerous inflammatory cells which were corrected after Olea europea L. leaves extract treatment. On the other hand, the myocardium of rats co-exposed to noise and toluene presents blocked arteries, blood clots and numerous inflammatory cells. However, we recorded no histological modification in the group of rats receiving the Olea europea L. extract compared to the sham group.