Plasma taurine levels in broilers with pulmonary hypertension syndrome induced by unilateral pulmonary artery occlusion

Plasma Taurine Levels in Broilers with Pulmonary Hypertension Syndrome Induced by Unilateral Pulmonary Artery Occlusion C. A. RUIZ-FERIA,*,1 K. W. BEERS,* M. T. KIDD,† and R. F. WIDEMAN, JR.* *Department of Poultry Science, University of Arkansas, Fayetteville, Arkansas 72701, and †Nutri-Quest, Inc., Chesterfield, Missouri 63017 postsurgery, the birds were euthanatized, and ventricles were weighed for calculating the right:total ventricular weight ratio (RV:TV). The RV:TV of PAC birds (>0.35) consistently was higher (P < 0.01) than that of CONTROL and SHAM birds (<0.27 and 0.25, respectively). In Experiments 1 and 2, plasma taurine was higher (P < 0.05) in PAC-ascites (380 and 370 nmol/mL) than in SHAM broilers (183 and 186 nmol/mL), whereas CONTROL (262 and 278 nmol/mL) and PAC-normal (362 and 300 nmol/ mL) broilers tended to have intermediate plasma taurine levels. In Experiment 3, PAC birds had higher (P < 0.05) plasma taurine at 8 and 12 d postsurgery when compared with presurgery levels, whereas plasma taurine was unchanged over time in CONTROL and SHAM birds. These results suggest cardiac taurine may be released into the plasma as a protective mechanism in response to the induction of pulmonary hypertension, hypoxemia, and right-side heart failure, similar to the mechanism reported for protecting cardiac muscle from ischemia in mammals.

(Key words: taurine, broiler, pulmonary hypertension, ascites, heart) 1999 Poultry Science 78:1627–1633

Taurine (2-aminoethanesulfonic acid) is a ubiquitous free amino acid in animals and is virtually absent in plants. Long referred to only as an end product of sulfur amino acid metabolism, taurine is increasingly acknowledged as a factor involved in the functional regulation of many organ systems, with appreciable evidence accruing concerning its role in excitatory processes in the central nervous system and muscle (Hayes and Sturman, 1981). Taurine forms the major component of the cytoplasmic free amino acid pool in various tissues, including the myocardium. It is neither metabolized nor incorporated into cellular protein, and, in most tissues, there is no consensus as to its precise role. Among the most important physiological actions of taurine in the heart are the modulation of sodium and calcium balance, the regu-

lation of membrane structure and function, and the regulation of intracellular osmolality (Lake, 1993; Allo et al., 1997). Taurine concentrations are higher in the heart than in any other tissue, in most mammals. Cardiac taurine levels can be lowered by gross insults, such as anoxia or myocardial infarction, and may become elevated in pathological states such as congestive heart failure (Huxtable, 1982). Low plasma taurine has been associated with dilated cardiomyopathy in cats (Pion et al., 1987, 1992a,b; Fox et al., 1993). Cats with dilated cardiomyopathy have an enlarged end-diastolic diameter as measured by echocardiography. Taurine-deficient cats with clinical signs of dilated cardiomyopathy subsequently showed clinical improvement after being fed supplemental taurine, and the enlarged end-diastolic diameter indicative of dilated cardiomiopathy was reduced after 2 wk of taurine supplementation (Pion et al., 1987). Taurine is poorly reabsorbed

Received for publication April 5, 1999. Accepted for publication July 8, 1999. 1 To whom correspondence should be addressed: cruizfe@comp. uark.edu

Abbreviation Key: CONTROL = unoperated control; PAC = pulmonary artery clamp; PHS = pulmonary hypertension syndrome; RV:TV = right:total ventricular weight ratio; SHAM = sham operated.

INTRODUCTION

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ABSTRACT Low plasma levels of taurine are associated with losses of cardiac sarcomeric proteins, leading to heart failure in mammals. Recently, it was proposed that cardiac taurine depletion serves to defend the heart against injury caused by regional ischemia in mammals. The role of taurine has not been well documented in broilers, particularly in relation to pulmonary hypertension syndrome (PHS; ascites). Three independent experiments evaluated plasma taurine in male broilers by utilizing the following treatments: unoperated controls (CONTROL; n = 10 in each experiment); sham operated (SHAM; n = 11, 12, and 10); or, unilaterally pulmonary artery clamped (PAC; n = 18, 29, and 24) that did (PAC-ascites) or did not (PAC-normal) develop ascites within 12 d postsurgery. Plasma samples were collected 9 and 11 d postsurgery in Experiments 1 and 2, respectively, and 2 d before and 4, 8, and 12 d after surgery in Experiment 3. Plasma taurine was analyzed by HPLC. Twelve days

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2

Cobb-Vantress, Inc., Siloam Springs, AR 72761. Fort Dodge Laboratories, Inc., Fort Dodge, IA 50501. 4 Miles Inc., Shawnee Mission, KS 66201. 5 Miltex-Instruments Co., Lake Success, NY 11042. 6 Pickering Laboratories, Mountain View, CA 94043. 3

MATERIALS AND METHODS Three independent and sequential experiments were carried out. In each experiment male broiler chicks (Cobb 5002) were reared on fresh wood shavings litter in one environmental chamber (8 m2 floor space). The chicks were brooded at 32 and 30 C during Weeks 1 and 2, respectively, and, thereafter, the temperature was maintained at 24 C. Ventilation was adjusted as needed to maintain air quality. Throughout the experiment, they were fed a corn-soybean meal-based broiler ration formulated to meet or exceed the minimum NRC (1994) standards for all ingredients, including 22.7% CP, 3,059 kcal ME/kg, 1.5% arginine, and 1.43% lysine. Feed and water were provided for ad libitum consumption. A detailed description of the surgical procedure has been provided elsewhere (Wideman and Kirby, 1995, 1996; Wideman et al., 1997). Briefly, chickens between 20 and 21 d of age were anesthetized to a surgical plane with intramuscular injections of a 1:1 mixture of ketamine HCl (Ketaset3, 100 mg/mL) and Xylazine (Rompun4, 100 mg/mL). The thoracic inlet was opened, and a silver clip was positioned to fully occlude the left pulmonary artery (PAC groups, n = 18, 29, and 24 for Experiments 1, 2, and 3, respectively). Sham-operated birds (SHAM) were anesthetized, and the thoracic inlet was opened; however, the pulmonary artery remained unoccluded (n = 11, 12, and 10 for Experiments 1, 2, and 3, respectively). Surgical incisions were closed with stainless steel surgical wound clips5 and sprayed with a topical antibacterial powder. The birds were wing-banded and placed under heat lamps until they recovered from the anesthesia. Then they were returned to the environmental chamber. Chicks in the control (CONTROL) group were not anesthetized and did not undergo surgical procedures (n= 10 in each experiment). Within each experiment, all broilers were from the same hatch, reared together in the same environmental chamber, and fed the same diets. On Days 9 and 11 postsurgery (Experiments 1 and 2, respectively), blood samples (one per bird, 3 mL each) were collected from the wing vein in heparinized vacutainers. In Experiment 3, blood samples (3 mL each) were collected 2 d before and 4, 8, and 12 d after surgery. The samples were centrifuged, and the plasma was stored at −60 C until analysis. Plasma samples were analyzed for free amino acids by HPLC. Briefly, plasma was deproteinized by mixing equal volumes of plasma and sulfosalicylic acid (5% wt/vol) in a 1.5-mL microcentrifuge tube. The samples were vortexed (∼ 30 s) and centrifuged (10 min, 10,000 × g) and 20 µL of the resulting supernatant was injected in the HPLC column for amino acid analysis. Amino acids were separated by cation exchange using lithium buffers6 with ultraviolet light detection (570 nm) of individual amino acids achieved by postcolumn ninhydrin derivatization. Two days after blood sampling in Experiments 1 and 2 and 12 d after surgery in Experiment 3, broilers were weighed and killed by CO2 inhalation. Birds in the PAC groups were examined to verify clamp placement on the

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by the proximal renal tubules in the rat and is readily excreted by the kidneys. Consequently, urine usually contains taurine concentrations equal to or greater than that of plasma (Hayes and Sturman, 1981). Little is known regarding the role of taurine in broiler chickens. Elevating dietary taurine reportedly promotes growth, but with inconsistent results (Monson, 1969; Dunnington et al., 1996). Taurine has been studied in relation to sudden death syndrome, leading to the conclusion that taurine either does not change the incidence of sudden death syndrome (Blair et al., 1991; Jacob et al., 1991) or that reductions in sudden death syndrome mortality are marginal (Campbell and Classen, 1989). In the latter experiment, no attempt was made to differentiate between mortality caused by sudden death syndrome or ascites. There are no reports regarding an association between taurine and the pathophysiological progression leading to pulmonary hypertension syndrome (PHS; ascites). The pathophysiological progression leading to PHS in broilers can be initiated by a variety of conditions that increase pulmonary vascular resistance or attenuate flowdependent pulmonary vasodilation during the rapid increments in cardiac output associated with fast growth and exposure to cold temperature (Wideman and Bottje, 1993). During normal cardiac function, the right ventricle generates a low pulmonary arterial pressure that is sufficient to overcome pulmonary vascular resistance while propelling the requisite cardiac output through the blood vessels of the lungs. The unilateral pulmonary artery occlusion technique has been used as a model for directly increasing the pulmonary vascular resistance and forcing the right ventricle to propel the entire cardiac output through the unoccluded lung (Wideman and Kirby, 1995). Broilers with a fully occluded left pulmonary artery develop pulmonary hypertension, as demonstrated by increased right:total ventricular weight ratios (RV:TV) (right ventricular hypertrophy) and by increasingly negative electrocardiogram lead II S wave amplitudes (generalized ventricular dilation and hypertrophy). Susceptible broilers then proceed through a characteristic pathophysiological progression to terminal PHS, including systemic hypoxemia, vascular congestion, and the onset of rightside congestive heart failure (Wideman et al., 1996a,b, 1997; Wideman and Kirby, 1995, 1996). The hypoxic conditions generated during the progression leading to ascites, and the changes in heart structure at the onset of ascites may initiate alterations in the metabolism and utilization of taurine in broilers. To test this hypothesis, three experiments were carried out to evaluate plasma taurine profiles after one pulmonary artery was chronically occluded to induce pulmonary hypertension (Experiments 1 and 2) or before and 4, 8, and 12 d after surgery (Experiment 3).

PLASMA TAURINE AND ASCITES IN BROILERS

RESULTS None of the broilers in the CONTROL or SHAM groups developed ascites, whereas 39% (7/18), 28% (8/29), and 50% (12/24) of the broilers in the PAC groups developed ascites by the day of necropsy in Experiments 1, 2, and 3, respectively. In all three experiments, body weights were reduced and RV:TV were elevated in the PAC groups when compared with the CONTROL and SHAM groups (Figures 1 and 2). Birds in the PAC-ascites group had a more pronounced reduction in weight than did birds in the PAC-normal group in Experiments 1 and 2, but not in Experiment 3. The SHAM birds had a lower

FIGURE 1. Body weight at necropsy of male broiler chickens with a chronic pulmonary artery clamp that developed (PAC-ascites) or did not develop ascites (PAC-normal) by the time of necropsy, sham-operated birds (SHAM), or unoperated controls (CONTROL) in three independent experiments. a–dBars with different letters denote values within the experiment that are different (P < 0.05).

FIGURE 2. Right to total ventricular weight ratio (RV:TV) of male broiler chickens with a chronic pulmonary artery clamp that developed (PAC-ascites) or did not develop ascites (PAC-normal) by the time of necropsy, sham-operated birds (SHAM), or unoperated controls (CONTROL) in three independent experiments. a–cBars with different letters denote values within the experiment that are different (P < 0.05).

body weight than did CONTROL birds in Experiment 1, but not in Experiments 2 and 3. Birds in the PAC-ascites group in Experiment 2 had a higher RV:TV than PACnormal birds, but, in Experiments 1 and 3, the RV:TV did not differ between PAC-ascites and PAC-normal birds. In Experiments 1 and 2, PAC-ascites birds had higher plasma taurine levels than did SHAM birds, whereas plasma taurine levels were intermediate in CONTROL and PAC-normal birds (Figure 3). At 8 d after surgery, PAC-ascites birds had higher taurine levels than did CONTROL birds, whereas SHAM and PAC-normal birds had intermediate values (Figure 3). Plasma taurine levels before and following the pulmonary artery clamp surgery in Experiment 3 are shown in Figure 4. Taurine levels did not differ among treatment groups for blood samples obtained before surgery or 4 and 12 d after surgery in Experiment 3. In the time series analysis, only three groups were considered (CONTROL, SHAM, and PAC) because all PAC-ascites birds died between the third and fourth sampling periods (between 8 and 12 d). No changes in taurine levels over the four sampling periods were observed within the SHAM group. Taurine levels in CONTROL birds at 4 and 12 d after surgery were not different from taurine levels before surgery, but, at 8 d after surgery, taurine levels were lower compared with the other sampling periods. The PAC birds had higher levels of taurine at 8 and 12 d after surgery compared with taurine levels before surgery, and, at 4 d postsurgery, taurine levels were intermediate between presurgery and 8 and 12 d after surgery (Figure 4). The association between right ventricular hypertrophy (RV:TV) and plasma taurine is shown in Table 1. When each experiment was considered separately, the highest correlation was ob-

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pulmonary artery. For broilers of all groups, the heart was removed, dissected, and weighed for calculation of the RV:TV as an index of pulmonary hypertension (Burton et al., 1968). Broilers were diagnosed as having PHS only when they exhibited abdominal fluid accumulation. Data sets were analyzed as a one-way ANOVA using the General Linear Models procedure of SAS威 (SAS Institute, 1982), and the Student-Newman-Keuls’ multiple range test was used to separate means that differed (P < 0.05). Pearson Correlation Coefficients and linear regression analysis were performed using SigmaStat威 (Jandel Scientific, 1994). For statistical analysis, the PAC group was divided in two groups: birds that had not developed ascites by the time of necropsy (PAC-normal), and birds that had developed ascites by the time of necropsy (PACascites). In Experiment 3, plasma taurine levels over time were analyzed with a Time Series Analysis procedure using SigmaStat威. In this case, the PAC group was analyzed as a whole because all of the PAC-ascites birds died after Day 8 but before the last sampling at Day 12.

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DISCUSSION

tained using all of the values of Experiment 1 (R = 0.425), and the lowest correlation was obtained with the data of Experiment 2 (R = 0.311). When the data of all three experiments were pooled and analyzed by group, the correlations obtained were low and not significant (P ≥ 0.20).

FIGURE 4. Plasma taurine levels (nanomoles per milliliter) of male broiler chickens with a chronic pulmonary artery clamp (PAC), shamoperated birds (SHAM), or unoperated controls (CONTROL) 2 d before surgery and 4, 8, and 12 d after surgery. Asterisk (*) designates difference (P < 0.05) compared with the sample obtained 2 d before surgery within an experimental group.

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FIGURE 3. Plasma taurine levels (nanomoles per milliliter) of male broiler chickens with a chronic pulmonary artery clamp that developed (PAC-ascites) or did not develop ascites (PAC-normal) by the time of necropsy, sham-operated birds (SHAM), or unoperated controls (CONTROL) at 9, 11, or 8 d after surgery (Experiment 1, 2, and 3, respectively) in three independent experiments. a,bBars with different letters denote values within the experiment that are different (P < 0.05).

The pulmonary artery clamp model used to induce ascites experimentally was effective as reported elsewhere (Wideman and Kirby, 1995, 1996; Wideman et al., 1997). The incidence of ascites ranged from 28 to 50% by 11 to 13 d postsurgery in the PAC groups across three experiments compared with no spontaneous ascites in the CONTROL and SHAM birds, demonstrating that the ascites was induced by the pulmonary occlusion and not by the surgical procedure per se. The first cases of ascites appeared as soon as 8 d after the PAC surgery. Following pulmonary artery occlusion, the entire cardiac output is forced to pass through only one lung, necessitating increases in the pulmonary artery pressure as well as the rate at which blood flows past the gas exchange regions in the lungs. The rapid blood transit time creates a diffusionperfusion mismatch, leading to poor blood oxygenation (hypoxemia), right ventricular hypertrophy (pulmonary hypertension), and, finally, ascites (Wideman and Bottje, 1993; Wideman et al., 1996a, 1997). The reduced growth performance of susceptible broilers presumably reflects the systemic hypoxemia (low oxygen content of arterial blood) that characteristically arises during the pathophysiological progression leading from the onset of pulmonary hypertension to terminal ascites. Susceptible broilers have compromised cardio-pulmonary systems that are incapable of delivering oxygen in quantities sufficient to support maximal growth when pulmonary artery occlusion forces the entire cardiac output to flow through only one lung (Wideman and Kirby, 1995, 1996; Wideman et al., 1998). Although not all of the PAC birds developed ascitic fluid accumulation in the abdominal cavity by the time of necropsy (11 to 13 d after surgery), there was a significant increase in the RV:TV in all PAC birds when compared with CONTROL and SHAM birds. The increased RV:TV directly reflects the onset of pulmonary hypertension, demonstrating the extra work performed by the right ventricle to produce the increased pressure needed to overcome the restricted lung vasculature (Burton et al., 1968). The right ventricle is designed to function as a lowpressure pump, and, under normal conditions, a pulmonary artery pressure of 20 mm Hg is sufficient to propel the entire cardiac output through the lungs. During the progression leading to PHS, the pulmonary arterial pressure exceeds 30 mm Hg, resulting in right ventricular hypertrophy. These observations are consistent with previous reports that pulmonary hypertension, as reflected by elevated RV:TV in susceptible broilers, develops rapidly when the unilateral pulmonary artery occlusion technique is used to directly increase pulmonary vascular resistance (Wideman and Kirby, 1995, 1996; Wideman et al., 1997). Our results also suggest that important differences exist in the susceptibility of individual broilers to ascites. Had the birds been allowed to live longer, a higher ascites incidence would be expected, with relatively few birds remaining resistant to PHS. Fast-growing birds may remain highly resistant to pulmonary hypertension as

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PLASMA TAURINE AND ASCITES IN BROILERS TABLE 1. Linear regression equations, Pearson correlation coefficients (R), and probability values (P) for relationships between right:total ventricular weight ratios (RV:TV) vs plasma taurine levels by experiment or using pooled values of all three experiments for each treatment group Data set 1

Experiment 1 Experiment 2 Experiment 3 Experiments pooled2 CONTROL birds pooled3 SHAM birds pooled4 PAC birds pooled5 PAC-normal birds pooled6 PAC-ascites birds pooled7

n

Regression equation

39 51 44 134 30 33 71 44 27

RV:TV RV:TV RV:TV RV:TV RV:TV RV:TV RV:TV RV:TV RV:TV

= = = = = = = = =

0.000269 taurine + 0.233 0.000179 taurine + 0.283 0.000259 taurine + 0.230 0.000227 taurine + 0.252 0.0000327 taurine + 0.227 0.0000839 taurine + 0.232 0.0000433 taurine + 0.364 0.0000202 taurine + 0.367 0.0000995 taurine + 0.370

R

P

0.425 0.311 0.409 0.370 0.127 0.150 0.105 0.052 0.248

0.0070 0.0262 0.0059 0.0001 0.5047 0.4033 0.3829 0.7390 0.2130

1

All birds in the particular experiment were used to generate the equation. All birds in the three experiments were pooled to generate the equation. 3 All CONTROL (unoperated control) birds in the three experiments were pooled to generate the equation. 4 All SHAM (sham operated) birds in the three experiments were pooled to generate the equation. 5 All PAC (pulmonary artery clamp) birds in the three experiments were pooled to generate the equation. 6 All PAC-normal (PAC birds that did not develop ascites by the time of necropsy) in the three experiments were pooled to generate the equation. 7 All PAC-ascites (PAC birds that developed ascites by the time of necropsy) in the three experiments were pooled to generate the equation. 2

sion, timing of the blood sampling is an important consideration. When the experimental groups were analyzed separately, the correlations were lower, and the regression equations were nonsignificant. This result was expected because, within the separate groups, the range in RV:TV and plasma taurine levels is reduced, and no association between the two factors can be detected. Nathan and Crass (1982) reported that isolated Purkinje fibers, which contain the highest levels of taurine in heart tissue, had a 50% decrease in taurine content after 4 h of ischemia induced by coronary occlusion. Cellular hypoxia, the principal manifestation of ischemia, triggered the efflux of intracellular taurine into the extracellular compartment. Poor blood oxygenation caused by the diffusion-perfusion mismatch in birds developing PHS may elicit a response similar to that of ischemic mammalian cardiac tissue. Although heart taurine was not measured in the present study, it can be hypothesized that the increased plasma levels of taurine in birds developing PHS may arise, at least in part, from taurine losses of cardiac tissue receiving poorly oxygenated blood. Allo et al. (1997) studied infarct size and areas at risk in isolated hearts of control and taurine-depleted rats subjected to experimental hypoxia by a 45-min ligation of the left descending coronary artery. The taurine-depleted hearts exhibited a 57% reduction in the infarct size-to-risk area ratio, leading to the conclusion that taurine depletion rendered the heart resistant to injury caused by regional ischemia. Therefore, the increased levels in plasma taurine in PAC birds may be related to a similar mechanism of protection against the progressive hypoxemia accompanying the development of PHS. On the other hand, taurine plays an important role in cardiac function and contractility. Franconi et al. (1982) reported that taurine exerts a positive or negative inotropic effect (increase or decrease in contractility), depending on the ionized calcium (Ca++) concentration in the external medium. When external Ca++ was 0.9 mM, taurine in-

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long as their pulmonary vascular capacity increases in synchrony with growth (Wideman, 1997). Plasma taurine levels at 9 and 11 d postsurgery in Experiments 1 and 2, respectively, were higher in PACascites birds than in SHAM birds, and PAC-ascites birds had significantly higher levels of taurine than did CONTROL birds in Experiment 3 (at 8 d postsurgery). The PAC birds, in general consistently had an elevated level of taurine when compared with CONTROL birds. Plasma taurine levels in PAC-normal birds were 38.4, 20.9, and 57.5% higher than those in birds in the CONTROL group in Experiments 1, 2, and 3, respectively, whereas plasma taurine in PAC-ascites birds were 45, 49, and 95.6% higher than those in the CONTROL birds. These results clearly indicate an association between PHS and plasma taurine levels. In cats, low plasma taurine levels are associated with myocardial failure leading to dilated cardiomyopathy. Taurine supplementation restores plasma taurine levels and normal cardiac function in cats (Pion et al., 1987, 1992a). The elevated plasma taurine levels in chickens developing PHS in the present experiments, as opposed to low plasma taurine levels in cardiomyopathic cats, suggests either that cardiac failure in ascites is unrelated to dilated cardiomyopathy or that taurine metabolism is much different in the bird. The correlations between RV:TV and plasma taurine levels (Table 1) are moderate and may be influenced by the time at which the plasma samples were taken after the PAC challenge. Samples were taken 9 d after surgery in Experiment 1, and the best R value (0.425) was obtained, but, when the samples were taken 11 d after surgery (Exp. 3), the R value obtained was lower, even though the number of birds sampled was larger. At 8 d after surgery in Experiment 2, the R value was lower than that obtained at Experiment 1 but higher than that obtained in Experiment 3. These temporal relationships suggest that, to improve the association between plasma taurine levels and the magnitude of pulmonary hyperten-

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subjected to hypoxemia, and additional studies will be required to determine the contribution of heart taurine in this process. These experiments also demonstrate that increases in plasma taurine can be detected before the onset of ascites in broilers. Plasma taurine is easily assayed using available HPLC methodology. Consequently, taurine may serve as a screening tool for detecting and culling birds susceptible for developing ascites in genetic selection programs designed to improve the resistance of broilers to ascites.

ACKNOWLEDGMENTS This research was supported by Nutri-Quest, Inc. and by BARD project No. US-2736-96. Ciro A. Ruiz-Feria is a Ph.D. candidate supported by a scholarship from the National Council of Science and Technology (CONACYT) of Mexico.

REFERENCES Allo, S. N., L. Bagby, and S. W. Schaffer, 1997. Taurine depletion, a novel mechanism for cardioprotection from regional ischemia. Am. J. Phys. 273:H1956–H1961. Blair, R., J. P Jacob, and E. E. Gardiner, 1991. Lack of an effect of taurine supplementation on the incidence of sudden death syndrome in broiler chickens. Poultry Sci. 70:554–560. Burton, R. R., E. L. Besch, and A. H. Smith, 1968. Effect of chronic hypoxia on the pulmonary arterial blood pressure of the chicken. Am. J. Physiol. 191:309–324. Campbell, G. L., and H. L. Classen, 1989. Effect of dietary taurine supplementation on sudden death syndrome in broiler chickens. Can. J. Anim. Sci. 69:509–512. Dunnington, E. A., P. B. Siegel, and K. K. Stewart, 1996. Effects of dietary taurine on growth and Escherichia coli resistance in chickens. Poultry Sci. 75:1330–1333. Fox, P. R., E. A. Trautwein, K. C. Hayes, B. R. Bond, D. D. Sisson, and N. S. Moise, 1993. Comparison of taurine, αtocopherol, retinol, selenium, and total triglycerides and cholesterol concentration in cats with cardiac disease and in healthy cats. Am. J. Vet. Res. 54:563–569. Franconi, F., A. Giotti, S. Manzini, F. Martini, I. Stendardi, and L. Zilletti, 1982. Observations on the effects of taurine at arterial and cardiac levels. Adv. Exp. Med. Biol. 139:181–190. Hayes, K. C., and J. A. Sturman, 1981. Taurine in metabolism. Ann. Rev. Nutr. 1:401–425. Huxtable, R. J., 1982. Taurine and the heart. Adv. Exp. Med. Biol. 139:161–163. Jacob, J. P., R. Blair, L. E. Hart, and E. E. Gardiner, 1991. The effect of taurine transport antagonists on cardiac taurine concentration and the incidence of sudden death syndrome in male broiler chickens. Poultry Sci. 70:561–567. Jandel Scientific, 1994. SigmaStat威 Statistical Software User’s Manual. Jandel Scientific Software, San Rafael, CA. Kirby, K. Y., N. B. Anthony, J. D. Hughes, Jr., R. W. McNew, J. D. Kirby, and R. F. Wideman, Jr., 1999. Electrocardiographic and genetic evaluation of giant jungle fowl, broilers, and their reciprocal crosses following unilateral bronchus occlusion. Poultry Sci. 78:125–134. Lake, N., 1992. Effects of taurine deficiency on arrhytmogenesis and excitation-contraction coupling in cardiac tissue. Adv. Exp. Med. Biol. 315:173–179. Lake, N., 1993. Loss of cardiac myofibrils: Mechanisms of contractile deficits induced by taurine deficiency. Am. J. Phys. 264:H1323–H1326. Monson, W. J., 1969. Evidence that taurine may be one of the elusive unidentified factors. Poultry Sci. 48:2069–2074.

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creased cardiac contractility, but when the Ca++ concentration was increased to 2.7 mM, high levels of taurine exerted a negative inotropic effect. Lake (1992) compared taurine-depleted rats with controls and reported small deficits in sensitivity to Ca++ and in its recirculation through the sarcoplasmic reticulum in heart, but the magnitude of this change did not appear adequate to account for the ≥40% loss in maximal Ca++ activated force generation observed in taurine-depleted muscles. Taurine depletion also decreased the content of cardiac contractile protein, confirming the loss of myofibrils observed in electron micrographs and accounting for the force deficits observed in taurine-depleted hearts (Lake, 1993). Losses of heart taurine triggered by cellular hypoxia in PHS birds also potentially may lead to the loss of contractile myofibrils and, consequently, to force deficits. Kirby et al. (1999) reported that ascitic birds generated more electrical current and took longer to contract and relax than nonascitic birds. Decreases in heart rate (bradycardia) previously have been associated with the onset of ascites in broilers (Roush et al., 1996, 1997; Olkouski et al., 1997, Wideman et al., 1998). These events are associated with the longer cardiac cycle required by a dilated right ventricle to complete filling and then propel the requisite stroke volume through a relatively high pulmonary vascular resistance (Wideman et al., 1996b). In addition, force deficits caused by taurine depletion could contribute to the changes in heart rate and electrocardiographic wave amplitudes in broilers developing PHS. As shown in Figure 4, taurine levels were increased after 8 d postsurgery, suggesting that hypoxemia had to persist for a week before taurine was released in amounts large enough to exceed renal clearance rates and elevate plasma levels of taurine. Alternatively, in birds subjected to hypoxia, other biochemical mechanisms, such as changes in sulfur amino acid metabolism, may play a role, resulting in elevated plasma taurine. The large variability in plasma taurine concentration (large standard errors) may reflect the variability in susceptibility to ascites among individuals. Similarly, some susceptible birds in the CONTROL and SHAM groups may have started to develop PHS spontaneously, contributing to the variability of plasma taurine within the experimental groups. Overall, the results of these experiments suggest that systemic hypoxemia in broilers facing PHS may trigger the release of taurine from cardiac tissue (detected as increased taurine levels in plasma), presumably as a mechanism to protect cardiac muscle from hypoxemic deterioration (Allo et al., 1997). However, during the pathophysiological progression leading to ascites, the sustained hypoxemia may cause the concentration of taurine in cardiac muscle to fall below that required for proper cardiac function. If so, then contractile deficits may occur because of myofibril losses and reduced sensitivity to Ca++, factors that would contribute to the cardiac dilation, congestive heart failure, bradycardia, and irregular electrical function of the heart, which are also consistently associated with the onset of ascites. Other factors may influence the elevation of plasma taurine in birds

PLASMA TAURINE AND ASCITES IN BROILERS

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