Experimental Study On Pathophysiologic Effects Of Repeated +Gz Exposure On Liver And Its Protection In Rats

Part 1 Effects of repeated +Gz exposure on liver function in ratsBACKGROUND:Some clinical data show that liver dysfunction was observed in pilots. However, the reason is not clear. The present study aimed to investigate rat liver function changes in response to repeated +Gz exposures.METHODS:Ninety male Wistar rats were randomly divided into a blank control group (Group BC, n=30), a+6 Gz/5 min stress group (Group 6GS, n=30), and a+10 Gz/5min stress group (Group 10GS, n=30). Groups 6GS and 10GS were exposed to +6 Gz or+10 Gz, respectively, in an animal centrifuge. The onset rate of +Gz was 0.5 G/s; the sustained time at the peak+Gz was 5 min each time and exposures occurred 5 times, with 5-min intervals between exposures and a total exposure and unexposures time of 50 min. We assessed liver injury by measuring the portal venous flow volume, liver function index, liver tissue malondialdehyde (MDA), Na+-K+-ATPase and changes in liver histology. The above parameters were recorded at 0 h,6 h, and 24 h after repeated+Gz exposures.RESULTS:1. Liver function after repeated+Gz exposuresALT and AST values in the BC group were 46.6 ± 4.7 IU/L and 110.5 ± 7.6 IU/L, respectively. After repeated+Gz exposures, ALT and AST values in the 6GS and the 10GS groups were higher than in the BC group (P<0.01) at 0 and 6 h after exposure. However, rats in the 6GS group showed lower ALT and AST levels than those in group 10GS at 0 and 6 h post-exposure (P<0.01,6GS group versus 10GS group). All the rats displayed normal ALT and AST levels at 24 h after exposure. These results demonstrate that the degree of damage to the liver function was positively correlated with the increase in the G-value.2. Histopathological observations in the liver after repeated+Gz exposuresThe hepatic pathological injury after repeated+Gz exposures was assessed and scored according to Suzuki’s criteria.The structure of hepatic lobules and liver antrum was clear and cellular edema was not obvious in the BC group (Suzuki’s score=2.12 ± 0.35). At 0 h after exposure, the hepatic sinus cord-like structure was maintained in the 6GS group (Suzuki’s score=3.21 ± 0.13), whereas the hepatic sinus cord-like structure was less well maintained and hepatocyte edema was present in the 10GS group (Suzuki’s score=4.63 ± 0.25). At 6 h after exposure, hepatocyte edema was significantly relieved in the 10GS group (Suzuki’s score=3.53 ± 0.31;P<0.01). There were no significant differences between 0 and 6 h after exposure in the 6GS group (Suzuki’s score=3.21 ± 0.13 versus 3.24 ± 0.28; P>0.05). The hepatic histology in both the 6GS and the 10GS group was nearly normal 24 h after exposure (Suzuki’s score=2.14 ± 0.33 vs Suzuki’s score=2.13 ± 0.36).3. Portal venous flow after repeated+Gz exposuresThe normal portal venous flow in Wistar rats was 11.468 ± 0.237 ml/min. After repeated+Gz exposures in the 6GS and the 10GS groups, the velocity and flow signal in the portal vein were significantly decreased as compared to the BC group (P< 0.01) at 0 h after exposure. Meanwhile, we found that the portal vein diameter did not change significantly. However, rats in the 6GS group showed a much higher portal venous flow volume than those in the 10GS group (P<0.01). All the rats had normal portal venous flow at 6 h after repeated+Gz exposures. Therefore, as the G force increased, the portal venous blood flow was reduced significantly.4. Tissue MDA levels after repeated+Gz exposuresMDA concentrations in both the 6GS and the 10GS group increased at 0 h or 6 h after exposure. However, MDA concentrations in the 6GS group were lower than in the 10GS group at 0 h (2.89 ± 0.24 nmol/mg protein versus 3.32 ± 0.25 nmol/mg protein, P< 0.01) and 6 h (2.64 ± 0.18 nmol/mg protein versus 3.18 ± 0.19; P< 0.01) post-exposure. It is noteworthy that the 6GS and 10GS groups did not recover 24 h after exposure. We concluded, based on these data, that repeated+Gz exposures may induce lipid peroxidation in the rat liver.5. Evaluation of the Na+-K+-ATPase activityNa+-K+-ATPase activity decreased significantly after exposure in both the 6GS and the 10GS groups as compared to the BC group. The 10GS group had lower Na+-K+-ATPase activity than the 6GS group at 0 h (0.73 ± 0.05 μmolPi/mg protein/h versus 0.85 ± 0.04 μmolPi/mg protein/h, P< 0.01) and 6 h (0.78 ± 0.05 μmolPi/mg protein/h versus 0.87 ± 0.03 μmolPi/mg protein/h, P< 0.01). There were no significant differences between the 10GS and the 6GS groups 24 h after exposure.6. Ultrastructure observations in the liver mitochondria after repeated+Gz exposuresMitochondria structure was normal and matrix density was uniform in the BC (Flameng’s score=0.313±0.104) group. At 24 h after +Gz exposures, individual mitochondrion were slightly swollen and showed decreased matrix density in the 6GS (Flameng’s score=0.626±0.119) group, whereas a small number of mitochondria were swollen, exhibited low-density cell matrix and detached cristae in the 10GS (Flameng’s score=0.835±0.208) group. The Flameng’s score of the 6 GS and 10 GS groups were higher than the BC group (P<0.01). Nevertheless, there was no statistical significance between the 6 GS group and the 10 GS group (P>0.05).CONCLUSIONS:1. Short-term repeated exposure to either+6 Gz or+10 Gz reduced the portal venous flow. Blood redistribution between the liver and body surface is similar to liver ischemia reperfusion. Repeat+Gz exposures may result in liver ischemia-reperfusion injury.2. ALT and AST levels were only slightly increased and could soon revert back to normal. With an increase in the G force, additional impairment also occurred in the liver function of the rats. The results showed that this damage should be functional and reversible.3. Oxidative damage might be engaged in the pathophysiologic process during liver ischemia. After repeated +Gz exposures, the blood and nutrient substance supplied to the liver were reduced. Exposed group is associated with oxidative stress-induced cell injury, as reflected in the higher MDA levels. In addition, the MDA levels increase as the G value increase.4. Repeated +Gz exposures were associated with transient depression of the liver metabolism, as indicated by a decrease in the Na+-K+-ATPase activity. The main role of the Na+-K+-ATPase is to maintain the structure and function of mitochondria. When the activities of the Na+-K+-ATPase decline, the structure of mitochondria is likely to change.5. Histopathological observation also exhibited moderate liver injury in the exposed group than in the blank control group as reflected in liver cell edema.6. Morphologically mitochondria became swelling and matrix density decreased at 24 h after +Gz exposures. injury, as reflected in the higher MDA levels. In addition, the MDA levels increase as the G value increase.4. Repeated +Gz exposures were associated with transient depression of the liver metabolism, as indicated by a decrease in the Na+-K+-ATPase activity. The main role of the Na+-K+-ATPase is to maintain the structure and function of mitochondria. When the activities of the Na+-K+-ATPase decline, the structure of mitochondria is likely to change.5. Histopathological observation also exhibited moderate liver injury in the exposed group than in the blank control group as reflected in liver cell edema.6. Morphologically mitochondria became swelling and matrix density decreased at 24 h after +Gz exposures.Part 2 Low G preconditioning reduces liver injury induced by high+Gz exposures in ratsOBJECTIVE:To investigate the effect of repeated lower+Gz exposure on liver injury induced by high+Gz exposure in rats.METHODS:Sixty male Wister rats, weighed between 250 and 300 g, were randomly divided into blank control group (BC), low G preconditioning group (LG) and +10 Gz/5 min group (n=20 in each group). Each rat was placed inside a 15 cm × 5 cm × 3 cm cylindrical plastic restraint device which was mounted in the centrifuge arm with the head of the rat facing the axis of the centrifuge for+Gz orientation. In+10 Gz/5min group, the rats were exposed to+10 Gz lasting for 5 min as reported elsewhere. For low G preconditioning group, the rats were exposed to+4 Gz/5 min every day for 3 days before+10 Gz/5 min exposure. The onset/offset rate of+Gz was set at+1 G/s. The rats in blank control were mounted on the arms of centrifuge, but were free from acceleration. The blood specimens and liver tissue were harvested at 0 h and 6 h after+10 Gz/5 min exposure. Liver function was analyzed by serum alanine transaminase (ALT) and aspartate aminotransferase (AST), and liver injury was further assessed by histopathological observation. Malondialdehyde (MDA), superoxide dismutase (SOD) and Na+-K+-ATPase were determined in hepatic tissue.RESULTS:1. Low G preconditioning reduced hepatocellular damageALT and AST in Group LG or Group 10G were higher than those in BC group (P< 0.05) at 0 h after exposure, respectively. However, the rats in LG group showed lower ALT and AST level than those in 10G group at 0 h after exposure (P<0.05). Group LG displayed normal ALT and AST level at 6 h after exposure (Fig.1). These results demonstrated that low G preconditioning had a protective effect on liver function in rats after high G stress.No significant injury was found in blank control group (Suzuki’s score=2.12 ± 0.13). At 0 h after exposure, there was disorderly hepatic sinus cord-like structure associated with hepatocytes edema in 10GS group (Suzuki’s score=3.23 ± 0.37). In sharp contrast, there was regular liver lobule structure in LG group (Suzuki’s score=2.28 ± 0.16). At 6 h after exposure, hepatocytes edema became lightened, and liver lobule structure was arranged orderly in 10GS group (Suzuki’s score=2.53 ± 0.25; P< 0.01). There was no significant difference between 0 and 6 h after exposure in LG group (Suzuki’s score=2.28 ± 0.16 versus 2.31 ±0.14; P<0.01).2. Low G preconditioning protected hepatocytes from damage of oxidative stressMDA level in liver tissue of Group LG or Group 10G was higher than that of blank control group at 0 h after exposure (P<0.05). However, MDA level in liver tissue in LG group was lower than that in 10G group at 0 h after exposure. There was no significant difference between MDA level in liver tissue of LG and 10G at 6 h after exposure (P>0.05). Compared with blank control group, liver tissue SOD level in LG or 10G group reduced significantly at 0 h after exposure (P<0.05). Compared to 10G group, SOD level was higher in LG group at 0 h after exposure (P<0.05, LG versus 10G). There was no significant difference between LG and 10G group at 6 h after exposure (P> 0.05). Therefore, low G preconditioning could reduce oxidative stress injury induced by high+Gz exposure in rats.3. Low G preconditioning improved hepatic energy metabolismThe Na+-K+-ATPase activity in the liver tissue of LG and 10G group was decreased compared to blank control group. The Na+-K+-ATPase activity in LG group was higher than that in 10G group at 0 h after exposure (P< 0.05, LG versus 10G). The difference between group LG and group 10G at 6 h after exposure was not significant (P> 0.05).CONCLUSIONS:It was suggested that repeated low+Gz exposure shown protective effect on liver injury induced by high+Gz exposure in rats. The precise mechanism includes decrease of oxidative stress, preservation of hepatic energy metabolism and improvement of cellular morphology.