Comparative Pulmonary Damages To Rats Caused By Exposure To Nanosized SiO₂ And Microsized SiO₂

ObjectiveNanomaterial is a new type of materials possessing grain sizes on the order of a billionth of a meter resulting from the manipulation of matter at the atomic, molecular or macromolecular levels. Nanoparticles, one kind of nanomaterials, with another name of ultrafine particles are commonly defined as particles with diameter less than 100 nm. Because of their extremely tiny diameter, they manifest extremely fascinating and useful properties such as surface effect, quantum effect, volume effect and macro quantum tunnel effect etc and are used widely.Nanosized SiO₂ is one of the nanomaterials with the most output. It is widely used in almost all the fields that microsized SiO₂ has been used such as plastic engineering, biomedicine, antiseptic, food processing and so on. It means that the exposure opportunities of nanosized SiO₂ for researchers, producers, transporters, consumers , consumable handlers of nanoproducts and even the whole ecosystem will be increased greatly. The changes in physiochemical properties of nanosized SiO₂ may cause the corresponding changes in the characteristics and intensity of biological effects.Therefore, great attention should be called to potential hazards of nanosized SiC>2 to human and environment. Many toxicological studies made it clear that nanoparticles of various inertia materials could cause severer biological effects than microsized particles with the same material. The pulmonary toxicity of crystalline silica typically including quartz has been widely known. In 1997 International Agency for Research on Cancer (IARC) working group classified crystalline silica(quartz) into IARC's Group I carcinogen concluding that there was sufficient evidence for carcinogenicity in experimental animals and sufficient evidence for carcinogenicity in human, respirable silica deposits in the lung and then interacts with macrophages and epithelial cells causing cell injury and stimulation of cytokine release, both of which lead to inflammation and oxidative stress, with concomitant slowing down of clearance of particle, lung silicosis and cell mutation.There is little information regarding the toxicity of nanosized SiC>2- Some data show inconsistent pulmonary toxicities: ultrafine colloidal silica(14 nm) has greater ability to induce lung inflammation and tissue damages than fine colloidal silica particles(213nm) in mice intratracheally instilled with particles. Nanosized crystalline SiC>2 causes a milder lung fibrosis than microsized SiC>2. There is no report on comparative acute or subacute pulmonary toxicity of nanosized SiC>2 and microsized SiC>2. It's necessary to study the toxicity of nanosizd SiC>2 and then be compared with microsized SiC>2.Intratracheal instillation is a widely used procedure to deliver materials into the lungs. It has great advantages over inhalation. The database on pulmonary toxicity of various particulate materials indicates that the two modes of exposure yield qualitatively similar results for a variety of biologic endpoints, including pulmonary inflammation, fibrosis, susceptibility to infection, allergic sensitization, and lung cancer in rats after exposure to poorly soluble particles. So several key guidelines must be considered for appropriate use of intratracheal instillation so that the valuableinformation consistent with inhalation can be achieved. Lung is the most direct and important target organ of particulate matter and rats are the most sensitive species in particle toxicology study. Several biochemical constituents in bronchoalveolar lavage fluid (BALF) will change after rats are exposed to dust. The pulmonary response induced by dust can be assessed by the analysis of the cellular and biochemical profile of the BALF. So this study was designed to describe the toxicity of nanosized SiC>2 and compare it with microsized SiC>2 that have had an extensive inhalation database. The rats were intratracheally instilled with different doses of the two dusts. And then the dose-response effects were studied by determining the total white cells count, total protein, the activity of lactate dehydrogenase (LDH), malondialdehyde (MDA) and superoxide dismatase (SOD) in BALF. Another groups of rats were instilled intratracheally with this two dusts and the time-response effects were studied by determining the total white cells count, total protein and lung histopathology. The ultimate aim was to establish a screening model of pulmonary toxicology for nanomatenals and to be the experimental supports for the dose selection of the following inhalation study and the elucidation of the possible effect mechanismMethodsPart I Dose-response Study: Forty-two Sprague Dawley (SD) rats were randomly divided into seven groups by weight. The rats were instilled intratracheally with lml suspension of 0.1 mg/ml, 0.2 mg/ml, or 0.4 mg/ml nanosized SiO2 or microsized SiO2 respectively. One group was instilled with 1 ml physiological saline as control. At 3d after exposure, total white cells count, total protein and the activity of LDH, MDA and SOD in BALF were determined and the acute pulmonary toxicity of the two SiO2 dusts was compared.Part II Time-response Study: Seventy-two SD rats were randomly divided into twelve groups by weight. The rats were instilled intratracheally with 0.5ml suspension of 0.6 mg/ml nanosized SiO2 or microsized SiO2 respectively. One group was instilled with 0.5 ml physiological saline as control. At 3, 7,14,28d after exposure, total white cells counts and total protein in BALF were determined and the histopathological changes in left lungs were observed. The subacute pulmonary damages of the two SiO2 dusts were compared.ResultsPart I The recovery of BALF did not differ significantly among the dust-exposedgroups and control groups. The total white cells number (16.0 X106) and the activityof LDH (914.29 U/L) in the group of nanosized SiO2 at dose of 0.4 mg weresignificantly increased compared with those in the control group(7.2 X106, 85.71 U/L,respectively;PO.05 or PO.01) and microsized SiO2 (9.1 X106, 476.19 U/L,respectively) No significant differences were observed in total protein among theseven groups. The activity of MDA in group of microsized SiO2 at dose of 0.2 mg(2.500 nmol/L) increased significantly compared with that in the control group(0.633 nmol/L, PO.05 ) . The activity of SOD in group of nanosized SiO2 at dose of0.2 mg and 0.4 mg,and microsized SiC>2 at dose of 0.2 mg (4.133 U/ml,4.011 U/ml, 3.929 U/ml, respectively) were decreased signicicantly compared with that in control group (6.088 U/ml, PO.01). The activity of SOD in group of nanosized SiC>2 at dose of 0.4 mg was lower than that in group of microsized SiC>2 at dose of 0.4 mg.Part II The recovery of BALF did not differ significantly among the dust-exposed groups and control groups except that the microsized SiC^-exposed group at 7d post-exposure was lower than the control group.( PO.05). Nanosized SiC>2 caused significant increase at 3, 7, 14d post- exposure (16.0X106, 11.1 X106, 12.2 X106) compared with saline (4.7 X106, 5.3 X106, 5.6 X106, respectively, PO.05 or P<0.01) in the total numbers of white cells and at 3d post-exposure compared with microsized SiO2 (5.7 X106, P<0.01). The total protein was raised by nanosized SiCb at 14, 28d post-exposure (0.41 g/L, 0.41 g/L) compared with those done by saline (0.16 g/L, 0.19 g/L) and microsized SiO2(0.21 g/L,0.24 g/L .PO.05 or PO.01). The lungs from control animals showed normal bronchiolar and alveolar architectures. Nanosized SiC>2-exposed rats showed marked infiltration with monocytes, macrophages and neutrophils in alveolar space, inconsistent congestion and great influx of neutrophils, lymphocyte and eosinophils around the blood vessel. An increased cells number in interstitial space and enlarged macrophages and neutrophils were observed besides the inconsistent thickening of the alveolar wall in the left exposure time. Microsized SiC>2 caused similar but less severity response compared with nanosized SiC>2. Van Gieson's -stained sections showed no significant fibrosis in these two dusts at 28d post-exposure.Conclusion1. Nanosized SiC>2 can cause severer acute pulmonary damages in rats than microsized SiC>2 can do.2. The pulmonary load threshold of nanosized SiC>2 may be lower than that ofmicrosized Si(>2.3. The degree of fibrosis may have no significant difference between nanosized Si and microsized SiC>2.