Free Diet Selection by Broilers as Influenced by Dietary Macronutrient Ratio and Corticosterone Supplementation. 1. Diet Selection, Organ Weights, and Plasma Metabolites R. D. Malheiros,*,† V. M. B. Moraes,‡ A. Collin,§ E. Decuypere,† and J. Buyse,†,1 *CNPq, Brası´lia, Brazil; †Laboratory for Physiology of Domestic Animals, Department of Animal Production, KULeuven, Kasteelpark Arenberg, 30, 3001 Leuven, Belgium; ‡Department of Zootechny, UNESP-Jaboticabal, Brazil; and §Station de Recherches Avicoles, INRA, Nouzilly, France ABSTRACT Male broiler chickens (aged 21 d) were allowed to chose freely for 14 d between three diets in which only one specific macronutrient (protein, lipid, or carbohydrate) was isocalorically substituted for one other macronutrient, but otherwise (nearly) isocaloric and composed of the same ingredients. The three diets were low protein (LowCP; 15.81% CP; 6.56% lipid; 50.78% carbohydrate), low lipid (LowL; 19.63% CP; 3.01% lipid; 51.12% carbohydrate), and low carbohydrate (LowCHO; 19.50% CP; 7.72% lipid; 44.00% carbohydrate). The chickens either received 0, 30, or 45 mg of corticosterone (CORT) per kg diet. As a percentage of their total intake, unsupplemented chickens consumed 24.0, 71.4, and 4.6% of the LowCP, LowL, and LowCHO diets, respectively, giving a total CP, L, and CHO intake of 282, 61, and 765 g, respectively. The addition of CORT significantly changed the diet selection, as compared to the unsupplemented chickens, CORT chickens consumed a greater percentage from the LowCP (35%), less
from the LowL (55%), and again more from the LowCHO (10%) diet. On the other hand, total feed consumption, macronutrient, and ME intake were not altered significantly by CORT supplementation, probably because of the close similarity of the diets. Corticosteronesupplemented chickens manifested hyperglycemia, hyperlipidemia, and uric acidemia suggesting insulin resistance, increased lipogenesis and protein catabolism, respectively. The elevated plasma creatine kinase (CK) activities of CORT chickens are also suggestive for decreased muscle cell membrane stability. Furthermore, CORT chickens were characterized by increased proportional weights of liver, abdominal fat pad, proventriculus, and gizzard, whereas an involution of spleen and bursa was observed. In conclusion, the present results suggest that high circulating levels of CORT as in the case of stress results in metabolic alterations, which in turn, affects diet preference as a compensatory mechanism to adapt energy and nutrient metabolism.
(Key words: diet selection, corticosterone, stress, macronutrient) 2003 Poultry Science 82:123–131
on the underlying causal mechanisms that control and regulate feeding behavior and especially diet selection and preference in these species. By mainly using rodent models, there is strong evidence that the hypothalamic-hypophysial-adrenal axis (stress axis) is involved in feeding behavior with respect to quantitative as well as qualitative (e.g., diet or macronutrient preferences) aspects (Harris et al., 2000). In poultry, the hypothalamic corticotrophin-releasing factor (CRF) and the adrenal major glucocorticoid corticosterone also affect appetite (e.g., Bartov et al., 1980a,b; Buyse et al., 1987; Denbow et al., 1999) in a more or less similar direction as in mammals. Corticosterone (CORT) is an adrenal steroid released in response to various
INTRODUCTION It has been clearly shown that layer and meat-type chickens are able to select their own diet in order to meet their requirements when given feeds that allow doing so (Forbes and Shariatmadari, 1994; Covasa and Forbes, 1995; Forbes and Covasa, 1995). Dietary energy and protein are probably the major factors determining feed intake although the importance of some specific compounds, such as lysine, methionine, and calcium cannot be underestimated. Although a reasonable number of experiments on choice feeding in poultry have been performed, there is yet a paucity of information
2003 Poultry Science Association, Inc. Received for publication April 1, 2002. Accepted for publication August 8, 2002. 1 To whom correspondence should be
[email protected].
addressed:
Abbreviation Key: CORT = corticosterone; CRF = corticotrophin releasing factor; CK = creatine kinase; FFA = free fatty acid; LowCHO = low carbohydrate; LowCP = low protein; LowL = low lipid; ME:CP = energy and protein ratio.
johan.
123
124
MALHEIROS ET AL.
stressors, and one result is the catabolism of protein tissues, such as skeletal muscles. Moreover, intramuscularly CORT injections change diet preference when chickens are freely allowed to choose between a protein concentrate or whole wheat (Covasa and Forbes, 1995), between a high- or a low-fat diet (Sahin and Forbes, 1998), or between a diet with maize oil or poultry fat (Sahin and Forbes, 1999). However, it is not always clear if the chickens make their selection based on ME or macronutrient content, ingredients used, or other dietary factors. Therefore, the present study’s aim was to determine the effect of dietary corticosterone inclusion on free diet selection of growing male broilers provided with diets formulated with the same ingredients and in which only one specific macronutrient (protein, fat, or carbohydrate) was nearly isocalorically substituted for another macronutrient. Furthermore, the impact of CORT on proportional organ weights and plasma hormone and metabolite levels and CK activities were also determined in order to establish what metabolic changes are induced by CORT and if these are related to diet preference.
MATERIALS AND METHODS Experimental Design Thirty, 1-d-old male broiler chicks (Cobb) were obtained from a commercial hatchery2 allocated to a onefloor pen with a heater, and provided water and food ad libitum. At 14 d of age, all birds were transferred to 15 battery cages, two birds per cage (40 × 50 × 35 cm). Temperature was set initially at 34 C and gradually reduced with 1 C per 1 or 2 d until 22 C was reached at 28 d of age. From Days 1 to 14, all chicks received a starter diet with 22% of CP and 2,900 kcalME/kg, and from Days 14 until 21, they received a grower diet (21% of CP and 3,050 kcalME/kg). At Day 21, after 7 d of adaptation, the diet selection experiment was started, and chickens were divided into three experimental groups. Each of the 15 cages was provided with three feeders each containing a feed (Table 1) with different macronutrient composition (lowprotein (LowCP); low-lipid (LowL) and low-carbohydrate (LowCHO) content). In addition, CORT,3 in doses of 0, 30, and 45 mg/kg of diet were mixed in all feeds, so that in total five repetitions per CORT dose were used. All birds had the free choice between these three diets from 21 to 35 d of age.
Measurements Every day of the experimental period, between 0900 and 1000 h, all birds were weighed individually, and the consumption of each diet was determined to the nearest gram. At the ages of 21, 28, and 35 d, a blood sample was collected from the wing vein with a heparinized syringe from all chickens involved. Blood samples were collected on ice and centrifuged, and plasma stored at −20 C until analyzed for glucose, CK, triglyceride, free fatty acid (FFA), uric acid, and CORT content. At 35 d of age, all birds were weighed and killed, and liver, heart, spleen, adrenals, proventriculus, gizzard, bursa, and abdominal fat pad excised and weighed to the nearest milligram.
Plasma Analysis Plasma glucose, CK, triglyceride, and uric acid concentrations were measured spectrophotometrically with an automated apparatus.4 Free fatty acid was measured with Nefa C,5 test kit, an enzymatic colorimetric test, and modified for use with the Monarch Chemistry System. Plasma corticosterone was measured using a sensitive and highly specific radioimmunoassay kit,6 with a sensitivity of 0.39 ng/mL, cross-reaction with aldosterone (0.20%), cortisol (0.40%), and deoxycorticosterone (3.30%). The intraassay variability was 3.8%. Before assay, plasma samples were heated at 80 C for 10 min to inactivate CORT-binding proteins.
Statistical Analysis The data were statistically analyzed by analysis of variance (Procedure General Linear Models; SAS Institute, 1998) with dietary CORT level (0, 30, 45 mg CORT/ kg diet) as classification variable. When an overall significant (P < 0.05) effect of CORT treatment was observed, means between the supplementation levels were separated by using the Tukey test. Plasma CORT levels and CK activities were log-transformed before analysis in order to reduce nonnormality and heterogenity of variance. The nontransformed data are presented in the tables.
RESULTS Body Weight Body weight was decreased by CORT treatments over the entire experimental period (Figure 1A), and this was more pronounced for the highest dose (45 mg/kg feed). This is also reflected in increasing difference in the daily BW gain between CORT-treated and control chickens in the course of the experiment.
2
Avibel, Halle-Zoersel, Belgium. Sigma Chemicals, Ltd., St. Louis, MO. 4 Monarch Chemistry System, Instrumentation Laboratories, Zaventem, Belgium. 5 Wako Chemicals GmbH, Neuss, Germany. 6 IDS, Inc., Boldon, UK. 3
Diet Intake Cumulative feed intake during the 14-d experimental period was not significantly affected by dietary CORT
125
FREE DIET SELECTION BY BROILERS TABLE 1. Ingredients and calculated analyses of the experimental diets: Low protein (LowCP), low lipid (LowL), and low carbohydrate (LowCHO) Ingredients (%) Maize Soybean meal Soybean oil Soybean protein Starch Calcium phosphate Limestone flour DL-Methionine L-Lysine-HCL L-Threonine L-Tryptophan Sodium chloride Vitamin and mineral premix1 Celite Calculated analysis (%) Crude protein Ca P (available) Carbohydrates + starch Crude fat Crude ash Crude fiber Threonine Methionine + cystine Tryptophan Lysine Metabolizable energy (kcal/kg) ME:CP ratio
LowCP
LowL
LowCHO
59.70 14.58 4.34 4.40 8.40 2.24 1.60 0.43 0.69 0.26 0.08 0.28 0.40 2.60
57.00 14.58 0.80 8.80 11.20 2.18 1.60 0.43 0.69 0.26 0.08 0.28 0.40 1.70
59.69 14.58 5.50 8.80 — 2.16 1.60 0.43 0.69 0.26 0.08 0.28 0.40 5.53
15.81 1.16 0.45 50.78 6.56 6.32 1.88 0.86 0.91 0.29 1.35 3048 193
19.63 1.15 0.45 51.12 3.01 6.37 1.83 1.01 0.98 0.29 1.59 2804 143
19.50 1.15 0.45 44.00 7.72 6.37 1.88 1.01 0.99 0.29 1.59 3103 159
1 Provided (mg/kg of diet): vitamin E, 30; vitamin K, 2; vitamin B1, 2; vitamin B2, 4; pantothenic acid, 10; niacin, 30; biotin, 0.05; vitamin B12, 0.12; vitamin B6, 2; folic acid, 1; Fe, 90, Cu, 22; Zn, 50; Mn, 80; Co, 0.2; I, 0.54; Se, 0.2; vitamin A, 3.1; vitamin D, 0.06.
FIGURE 1. Plot A represents BW of birds by age and corticosterone (CORT) supplementation level (n = 10); Plot B, C, and D, represent the food intake (g/bird per day) of each diet (LowCP, LowL, and LowCHO) according to the CORT supplementation level (0 (B), 30 (C), and 45 (D) mg CORT/kg of diet; n = 5). LowCP = low protein; LowL = low lipid; LowCHO = low carbohydrate.
126
MALHEIROS ET AL.
FIGURE 2. Percentage of feed intake of each diet by male broiler chickens as a function of the level of corticosterone (CORT) supplemented in the diets (n = 5). Small letters indicate significant (P < 0.05) differences among percentages of intake of each diet within each corticosterone level. Capital letters indicate significant (P < 0.05) differences among percentages of intake of a diet between the corticosterone levels. LowCP = low protein; LowL = low lipid; LowCHO = low carbohydrate.
supplementation. Chickens fed the 30 mg/kg CORT consumed 7% more food (1,613 ± 8 g/bird), compared to the control birds (1,508 ± 10 g/bird), whereas the chickens receiving the highest CORT dose, consumed less food (1,411 ± 6 g/bird) compared to both groups. This corresponded with a mean cumulative CORT in-
take per bird of 48 and 63 mg for the 30 and 45 mg/kg diets. This intake over the experimental period resulted in a mean feed conversion of 1.87, 4.47, and 5.14 for the 0, 30, and 45 mg dose, respectively. Control birds consumed 24% of LowCP and 71.4 and 4.6% of LowL and LowCHO, respectively, of their total
FIGURE 3. Macronutrient and ME intake (inset) during the period of 22 to 35 d of age according to supplementation level of corticosterone in the diets (0, 30, and 45 mg/kg of diet; n = 5). LowCP = low protein; LowL = low lipid; LowCHO = low carbohydrate.
FREE DIET SELECTION BY BROILERS
intake over the experimental period (Figure 2). However, the addition of CORT to the diet clearly changed the diet selection. Indeed, from their total intake, chickens fed the 30 mg CORT/kg consumed 35, 55, and 10% from diets LowCP, LowL, and LowCHO, respectively. For chickens treated with the highest dose of CORT the diet selection was 35, 54, and 11%. The control chickens made their diet selection within 2 d after the three experimental diets were presented (Figures 1B, C, and D). When diets were supplemented with 30 mg CORT/kg, it took about 7 d until a clear choice between the LowCP and LowL diet was made. On the other hand, chickens fed the 45 mg CORT/kg discriminated within a couple of days between the LowCP and LowL diet, although the difference in intake between both diets was not as pronounced as observed for the control chickens. The cumulative intake (g/bird per 14 d) of each macronutrient as well as ME intake was not significantly different between the control and CORT chickens except for the somewhat lower protein and carbohydrate intake of 45 mg CORT compared to the unsupplemented chickens (Figure 3). The LowL diet provided the largest part of each macronutrient and ME intake (Figure 4). However, due to the CORT-induced change in diet preference, a greater percentage of protein intake came from the LowCP and LowCHO diet at the expense of the LowL diet. In the control birds, more than half of total fat intake was provided by the LowL diet, whereas this was reduced to about one-third for the CORT chickens. Similarly,
127
control chickens obtained more than 70% of their carbohydrate from the LowL diet, but this percentage was reduced in CORT chickens to about 57%.
Blood Metabolites The results for plasma CORT, glucose, triglycerides, uric acid, FFA, and CK levels are summarized in Table 2. As expected, plasma CORT levels were increased following the diet supplementation with 30 and 45 mg/ kg feed of CORT, showing the effectiveness of the treatment. However, the highest dose of CORT was not associated with a supplementary increase in plasma CORT levels. Corticosterone treatments resulted in increased blood glucose levels although these were not yet significant after 1 wk, but after 2 wk the highest dose resulted in significantly increased levels (P < 0.05). A significant effect of CORT treatment at both ages was observed on plasma triglyceride levels. At 28 d the values rose up to 60 mg/dL when compared with control birds having 24.8 mg/dL. At 35 d a significant difference between 0 and 45 mg/kg of diet was observed. Both doses of CORT affected significantly the plasma uric acid profile. At 28 d, levels of uric acid were for both doses of CORT around fourfold the levels of control birds. The same was measured at 35 d. The free fatty acid levels showed a significant difference between 0 mg and 30 mg of CORT in the diet
FIGURE 4. Percentage of total macronutrient, and energy intake between 22 and 35 d of age as provided by each diet (LowCP, LowL, and LowCHO) as a function of the level of corticosterone supplemented in the diets (0, 30, and 45 mg/kg). LowCP = low protein; LowL = low lipid; LowCHO = low carbohydrate.
128
MALHEIROS ET AL. TABLE 2. Plasma metabolite levels according to dietary corticosterone (CORT) supplementation (0, 30, and 45 mg/kg diet) and age of birds CORT levels (mg/kg diet) Age CORT (ng/mL) Glucose (mg/dL) Triglycerides (mg/dL) Uric acid (mg/dL) Free fatty acids (mmol/L) Creatine kinase (IU/L)
21 28 35 21 28 35 21 28 35 21 28 35 21 28 35 21 28 35
0 25.6 16.4 12.7 269 255 266 53.9 24.8 26.2 7.77 3.84 3.25 0.704 0.467 0.322 759 1,209 1,291
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
1
2.6 (30) 6.4b (10) 3.5b (10) 3 (30) 10 (10) 5b (10) 3.9 (30) 3.3b (10) 4.8b (10) 0.47 (29) 0.41b (10) 0.31b (10) 0.032 (30) 0.056 (10) 0.029b (10) 42 (30) 237 (10) 134 (10)
30
45
F
— 89.3 ± 11.5a (10) 69.7 ± 6.0a (10) — 326 ± 31 (10) 293 ± 23ab (10) — 64.0 ± 11.7a (10) 56.8 ± 7.6ab (10) — 12.06 ± 0.70a (10) 10.04 ± 0.94a (10) — 0.534 ± 0.062 (10) 0.446 ± 0.031a (10) — 2,693 ± 654 (9) 1,956 ± 352 (10)
— 92.8 ± 9.2a (10) 64.1 ± 4.7a (10) — 312 ± 10 (10) 373 ± 45a (10) — 60.0 ± 7.6a (10) 95.2 ± 24.4a (10) — 11.02 ± 0.81a (10) 10.96 ± 0.90a (9) — 0.435 ± 0.048 (10) 0.352 ± 0.021ab (10) — 2,431 ± 598 (10) 2,117 ± 254 (9)
21.66** 41.69** — 2.66ns 4.39* — 6.78** 5.28* — 45.75** 30.66** — 0.84ns 5.62** — 2.33ns 2.77ns
a,b Means ± SEM similar row with different superscripts are significantly different (P < 0.05). 1Numbers of measurements. ns P > 0.05; *P < 0.05; **P < 0.01.
at Day 35, but not between 45 mg of CORT and both other treatments. Independent of age, CORT supplementation tended to increase CK in the blood, but these differences were not significant (P > 0.05). The CK variability was very high, with a coefficient of variation of 77 and 46% at 28 and 35 d of age, respectively.
Proportional Weights of Organs Proportional weights of liver, abdominal fat pad, spleen, adrenals, heart, proventriculus, gizzard, and cloacal bursa are summarized in Table 3. The proportional weights of adrenals and heart were not affected by both levels of CORT supplementation. In contrast, the proportional weights of liver, fat pad, proventriculus, and gizzard were significantly increased, whereas those of spleen and bursa were decreased by CORT treatments.
DISCUSSION We were unable to detect any significant changes in total feed intake or calculated intakes of each macronutrient between control and CORT-supplemented chickens. It must be recognized that there were very large differences within each treatment as some birdpairs made quite deviating choices. This variability was also identified by Covasa and Forbes (1995) and may be linked to variability in individual needs and influenced by behavioral factors, such as adaptation, learning capabilities, and social interactions. Nevertheless, as a percentage of total intake, CORT-supplemented chickens ate significantly more of the LowCP diet and less of the LowL diet, and although the LowCHO diet was only marginally appreciated, CORT chickens increased its consumption by a factor of two. Hence, the contribution of the LowL diet to the ingestion of each macronutrient
TABLE 3. Effect of corticosterone (CORT) on proportional weights of some organs of 35-d-old chickens CORT level mg/kg diet) Organs
0
Liver (g/kg) Fat Pad (g/kg) Spleen (mg/kg) Adrenals (mg/kg) Heart (g/kg) Proventriculus (g/kg) Gizzard (g/kg) Bursa (mg/kg)
± ± ± ± ± ± ± ±
2.61 0.781 85 10 0.64 0.39 1.85 254
30 b
0.12 0.71b 3a 0.001 0.03 0.02b 0.05b 20a
3.58 2.795 73 12 0.72 0.66 2.93 80
± ± ± ± ± ± ± ±
45 ab
0.10 1.84a 7ab 0.001 0.04 0.04a 0.06a 13b
4.57 2.708 59 9 0.66 0.70 3.00 51
± ± ± ± ± ± ± ±
F a
0.10 1.72a 4b 0.001 0.03 0.04a 0.06a 6b
11.12** 56.57** 4.80* 2.93ns 1.53ns 31.27** 64.18** 58.85**
Means ± SEM (n = 9 to 10) within a row with different superscripts are significantly different (P < 0.05). P > 0.05; *P < 0.05; **P < 0.01.
a,b ns
FREE DIET SELECTION BY BROILERS
diminished with CORT treatment (Figure 4), although total macronutrient did not change significantly (Figure 3). The CORT chickens also needed much more time to establish a clear choice between the diets. Calculations of the ME:CP ratio of the self-composed alimentation, based on mean intake of each experimental diet and multiplied with its ME and CP content, revealed no significant treatment differences in ME:CP ratio, which were 153, 159, and 159 kcal ME per percentage of protein, respectively from 0, 30, and 45 mg/kg of CORT in the diets. This observation contrasts with the findings of Covasa and Forbes (1995) who noticed an increase in ME:CP ratio of the chosen alimentation of their CORT-injected chickens, mainly due to a higher wheat intake. These authors suggested that CORT birds are able to detect metabolic changes and attempt to redress them by changing their diet preference. The question that now can be raised is what parts of the responses as changes in body weight gain, body composition, protein turnover, and metabolic rate are responsible for the CORT-induced changes in feed intake and choice. Covasa and Forbes (1995) observed that CORT-injected chickens consumed more protein, but this came mainly from a higher wheat intake, which provides the least protein on a weight basis. Similarly, our CORT birds also consumed more of that diet containing the lowest CP concentration but less of the diet LowL with the lowest energy content. Our diets were not so much different in macronutrient and ME levels, so that it was nearly impossible for CORT chickens to greatly elevate their macronutrient and energy intake. This might be a critical aspect of the employed diets. On the other hand, the CORT chickens made a clear preference for diet LowCP and because of the strong similarities of the three diets, we can be sure that this preference is purely based on the energy and macronutrient content of the diets and not on other factors, such as different ingredients, taste, and smell. We were unable to observe a significant effect of dietary CORT supplementation on food intake, although 30 mg/kg of CORT slightly increased (+7%) and 45 mg/ kg CORT slightly decreased (−6.4%) feed consumption compared to the control birds. Literature data are not consistent about the effects of CORT on appetite. Some authors report a significant increase in food consumption (Nagra and Meyer, 1963; Bartov et al., 1980ab), whereas others did not find any effect (Buyse et al., 1987; Davison et al., 1983). These discrepancies between studies may be due to dose, route (intramuscularly or oral), and duration of CORT administration, age, and genotype of the bird. As known in mammals, it was demonstrated that the CRF, administered centrally, strongly inhibits food intake in chickens (Furuse et al., 1997; Denbow et al., 1999). Hence, the observed increase in food consumption in some studies due to CORT treatment may be the result of the CORT-induced suppression of the anorectic CRF activity. However, the control and regulation of food
129
intake in avian species is multifactorial (Denbow, 1994), and other factors besides CRF may be involved, since the effects of CORT on appetite are not consistent. In chickens, it is believed that glucose can exert a satiety effect within the liver, albeit less pronounced in meat-type compared to layer-type chickens (Denbow, 1994). High circulating plasma glucose levels as observed in the present study and by others (Davison et al., 1983) are not linked with a depressed appetite but rather to the contrary. These observations corroborate the findings of Shurlock and Forbes (1981) that glucoseinduced satiety is only mediated by the liver and presumably not at central sites. This hyperglycemia is very likely to be the consequence of enhanced gluconeogenesis from amino acids, resulting from an increased protein catabolism as reflected in the markedly elevated uric acid levels and increased glycogenolysis (Carsia and Harvey, 2000). Furthermore, a state of insulin resistance leading to a decreased uptake of glucose by peripheral tissues is also a causative mechanism (Dupont et al., 1999). The latter investigators clearly demonstrated that CORT impaired insulin signaling in chicken liver and muscle although by different mechanisms. Finally, it was also demonstrated that CORT increases the in vivo intestinal uptake of glucose (Nasir et al., 1999) It is believed that reductions in intracellular glucose concentrations as well as the elevated protein catabolism are responsible for growth retardation in CORTtreated animals. The markedly increased proportional liver weight due to CORT therapy is in accordance with most other studies (Bartov et al., 1980a; Davison et al., 1983; Buyse et al., 1987) and is likely to be induced by lipid accumulation. All reported studies are in agreement with respect to the increased fat deposition in CORT-treated chickens. In conjunction with the observed liver steatosis and elevated plasma triglycerides, an enhanced hepatic lipogenesis can be inferred. Early studies of Nagra and Meyer (1963) indeed showed that CORT redirects glucose carbon to lipid preferably and less to protein synthesis, indicating that the biosynthesis of fatty acids is promoted by glucocorticoids. The effects of CORT on circulating FFA levels are not equivocal. A pronounced increase in plasma FFA levels was measured by continuous infusion of CORT for 14 or 28 d in laying hens (John et al., 1987), whereas we only observed significant increase in the 30 mg/kg CORTtreated group after 14 d. It is well recognized that plasma CK activities are a reliable biomarker for increased myopathy due to impaired cell membrane integrity and disruption of intracellular Ca2+ homeostasis (Mitchell et al., 1999; Sandercock et al., 2001). These authors also clearly showed that stress situations, such as acute heat stress, transport, or halothane anesthesia, are characterized by elevated plasma CK levels, and are very likely to increase plasma CORT levels as well. Our observations corroborate the positive relationship between plasma CORT
130
MALHEIROS ET AL.
levels (indicator of stress) and plasma CK activities and suggest even a direct effect of this glucocorticoid on muscle cell functioning leading to enzyme efflux. The interindividual variability in plasma CK activities was very high even within the same treatment group. This phenomenon is, however, not exceptional as we have observed this in other studies as well (Buyse and Malheiros, personal communication) and has probably a genetic origin. High CORT levels induced involution of lymphoid organs as spleen and bursa of Fabricius as observed also by many others (Gross et al., 1980; Covasa and Forbes, 1995; Davison et al., 1983). The absence of an effect of CORT on proportional adrenal weight is in accordance with the observations of Davison et al. (1983) and Covasa and Forbes (1995) but not with the studies of Gross et al. (1980) who observed a reduced size of the adrenals in CORT-supplemented chickens. Furthermore, Davison et al. (1983) did not observe any change in adrenal cholesterol concentration, suggesting that the exogenous CORT load does not affect endogenous CORT production. On the other hand, the metabolic clearance rate of CORT from the circulation must be enhanced in the 45 mg/kg BW, CORT chickens as they consumed about 30% more CORT than the 30 mg/kg BW CORT chickens, but without any differences in plasma CORT levels. Our CORT chickens were characterized by increased proportional weights of the proventriculus and gizzard. We believe that the higher proportional weights of both stomachs are not a direct effect of the imposed CORT treatment but rather a consequence of the CORT-induced growth retardation. Indeed, the stomachs are early maturing glands as reflected in their low (
