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PHYSIOLOGY, ENDOCRINOLOGY, AND REPRODUCTION |
* College of Veterinary Medicine, Huazhong Agricultural University, Wuhan, China; College of Veterinary Medicine, China Agricultural University, Beijing, China; and College of Animal Science, Hebei Agricultural University, Baoding, China
1 Corresponding author: lishaowen0611{at}yahoo.com.cn
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: sex • fast-growing broiler • ventricular vulnerability • serum enzyme activity • sudden death syndrome
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The pathogenesis of SDS in broilers is poorly understood, but a growing body of evidence suggests that it may be related to the high predisposition of fast-growing broilers to cardiac arrhythmia (Ononiwu et al., 1979; Grashorn, 1994; Olkowski et al., 1997; Olkowski and Classen, 1997; Korte et al., 1999), and death from SDS in broilers is associated with the catastrophic cardiac arrhythmia of ventricular fibrillation (VF; Olkowski and Classen, 1997). Studies in laboratory animals suggest that stress may induce ventricular arrhythmia and sudden cardiac death (Sgoifo et al., 1999; Lampert et al., 2002; Rubart and Zipes, 2005). The increased susceptibility of the myocardium to arrhythmia in some fast-growing chickens may be associated with specific pathophysiological changes in the myocardium, causing a high risk of acute heart failure when further compromised by stress (Olkowski, 2007). Greenlees et al. (1989) used the ventricular vulnerability (VV) threshold induced by electrical stimulation to evaluate VV and studied the relationship between growth rate and VV or SDS. Li et al. (2002) had set up the methods for evaluating the VV of chicks by measuring the VF thresholds induced by electrical stimulation (VFT-ES) and by injection of KCl in the wing vein, first in China, by which the VV between broilers and layers was compared and the effect of earlier food restriction on the VV of broilers was studied (Li et al., 2004a,b).
The broilers with SDS had high serum activities of lactate dehydrogenase (LDH), aspartate aminotransferase (AST), and creatine kinase (CK), and these were used previously as indicators for a clinical diagnosis of circulatory disturbance in humans (Bowes et al. 1989; Itoh, 1997; Imaeda, 1999). The difference in such enzymatic activities in serum could be related to the sensitivity of broilers to SDS (Li et al., 2004b).
Male broilers (MB) are far more susceptible to SDS, accounting for 70 to 80% of total SDS mortality compared with female broilers (FB; Olkowski and Classen, 1995; Maxwell and Robertson, 1998; Korte et al., 1999). The present study was designed to compare the VV, serum enzyme activities, and electrolyte levels of broilers of different sexes and primarily to clarify the inner mechanism of the different sensitivities to SDS of MB and FB.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The present study used 25-d-old commercial broiler chickens (Avian x Avian, provided by Beijing Zhengda Ltd., Beijing, China). All the birds were obtained from 1 d of age and reared in one pen. They were fed a commercial feed provided by Beijing Zhengda Ltd. Feed and water were supplied ad libitum. The experiment plan was approved by the Animal Care Committee of China Agricultural University. The sex of broilers was determined after the experiments.
Detection of Serum Enzyme Activity and Electrolyte Levels
Blood samples were taken from broilers via wing vein puncture and immediately injected into tubes. Each blood sample was centrifuged at 3,000 x g for 10 min and the serum was collected. Serum enzyme activities and electrolyte levels were detected. The activities of AST, LDH, and CK were detected by a 7150 automatic biochemical analyzer, Hitachi, Tokyo, Japan, and the concentrations of potassium, sodium, and chloride were detected by direct electrode tests as described previously (Li et al., 2004b).
Detection of VFT-ES
The experimental animals were bound at the operating table and were linked to the electrode conductor according to the standard method (Olkowski et al., 1997). A mixed anesthetic solution [5.0% (g/vol) pentobarbital sodium, 40% (vol/vol) propylene glycol, and 10.5% (vol/vol) ethanol] was injected (0.5 mL/kg) into the wing vein (Qiao, 1994). The thoracic cavity of the anesthetized broilers was opened from a site about 1 cm under the left sternum. The blood vessel and nerve must be escaped. The heart was exposed after opening the pericardium. The stimulation electrodes of the physiological multiuse equipment were placed in front one-third of the left ventricle. A continuing square wave and double-pulse electrical stimulation method was used, and the stimulation indexes indicated that the timing of the electrical pulses was 1 s, the last 2 periods were 2 and 5 ms, the frequency was 80 Hz, and the wave width was 1 ms. The original voltage was 1 V and the stimulation period was 15 s. The voltage was increased by 1 V every 5 s. A standard lead II electrocardiograph monitored the physiological multiuse equipment until the VF wave (Figure 1
) was seen. The VFT-ES is the minimum voltage that can induce VF (Greenlees et al., 1989; Li et al., 2002).
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The experimental birds were bound at the operating table after weighing and were linked to the electrode conductor according to the standard method (Olkowski et al., 1997). After the birds were anesthetized, a 5% KCl solution was administrated i.v. at a speed of 1 mL/min into the wing vein. A standard lead II electrocardiograph was continuously monitored until the VF wave was seen. The VFT-KCl is equal to the minimum solution volume inducing VF divided by the BW (Li et al., 2002).
Statistical Analysis
Values are expressed as the mean ± SD. Statistical significance was tested by using Student’s t-test, and differences were considered significant when P < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The VFT-ES of MB (7.2 ± 1.2V) was significantly lower than that of FB (8.3 ± 0.7 V, P < 0.01; Figure 2), and the VFT-KCl of MB (2.26 ± 0.75 mL/kg) was significantly lower than that of FB (3.21 ± 1.76 mL/kg, P < 0.05; Figure 3). The results indicated that the sensitivities of MB to VF induced by electrical stimulation and by i.v. injection of KCl were higher than those of FB, suggesting that the VV of MB was higher than that of FB.
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The serum LDH activity in MB [817.16 ± 164.04 units of enzymatic activity (HU)/L] was significantly higher than that in FB (606.55 ± 92.81 HU/L, P < 0.01; Figure 4) and the activity of serum CK in MB (1,974.14 ± 536.9 HU/L) was significantly higher than that in FB (1,226.11 ± 396.8 HU/L, P < 0.01; Figure 5). However, no significant difference in the serum AST activity was observed between MB (229.29 ± 17.64 HU/L) and FB (227.67 ± 16.29 HU/L; Figure 6).
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There were no significant differences between MB and FB (P > 0.05) in serum concentrations of potassium, sodium, and chloride (Table 1).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In our study, the VFT-ES and VFT-KCl of MB were both significantly lower than those of FB, indicating that the sensitivity of MB to VF induced by electrical stimulation and by i.v. injection of KCl were both higher than those of FB. The results indicated that the VV of MB was higher than that of FB, which was consistent with previous studies showing that the incidence of SDS was higher in MB than in FB (Olkowski and Classen, 1995; Maxwell and Robertson, 1998; Korte et al., 1999). This is the first report investigating the difference in VV between MB and FB.
Enzymes such as LDH, CK, and AST can be released from an injured myocardium into the blood, resulting in a higher activity of enzymes in the serum. Thus, the activity of these enzymes can reflect the extent of cardiac injury (Wang, 1994). Previous studies have suggested that the activities of LDH, CK, and AST in serum are related to the incidence of SDS in broilers (Bowes et al. 1989; Imaeda, 1999; Li et al., 2004b). In our study, the results of serum enzyme activities between broilers of different sexes showed that the LDH and CK activities in MB were significantly higher than those in FB (P < 0.01), consistent with the results for LDH in previous studies (Bowes et al. 1989). Whether the increase in serum activities of LDH and CK was related to the higher sensitivity of MB to SDS should be researched further. However, there was no significant difference in AST activity between MB and FB, in disagreement with the research by Bowes et al. (1989). The reason may be the different detection method used.
The results for serum electrolyte levels showed that there were no significant differences between broilers of different sexes in the concentrations of potassium, sodium, and chloride. These results were in accordance with previous studies (Bowes et al. 1989; Imaeda, 1999; Li et al., 2004b).
In conclusion, the results suggested that MB were more sensitive to injury of the myocardium by stress, and the myocardial function of MB was lower than that of FB, which might have resulted in a difference between MB and FB in sensitivity to SDS.
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ACKNOWLEDGMENTS |
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Received for publication November 26, 2007. Accepted for publication March 13, 2008.
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REFERENCES |
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