Chemical changes and influences of rapeseed
antinutritional factors on lamb physiology
and performance
2. Plasma substances and activity of the thyroid
S.N.M. Mandiki
a,*, N. Mabon
b, G. Derycke
a, J.L. Bister
a,
J.-P. Wathelet
b, R. Paquay
a, M. Marlier
baFaculteÂs Universitaires Notre-Dame de la Paix, Laboratoire de Physiologie Animale, Rue de Bruxelles 61,
B-5000 Namur, Belgium
bFaculte Universitaire des Sciences agronomiques, Unite de Chimie GeÂneÂrale et organique,
Passage des deÂporteÂs 2, B-5030 Gembloux, Belgium
Received 7 July 1998; received in revised form 19 January 1999; accepted 15 February 1999
Abstract
In order to study the effects of a high level of rapeseed meal in the diet on endocrine function and thyroid activity, sixty six Texel, Suffolk or crossbred lambs allocated to three groups were fedad libitumwith concentrates containing 0% (Control) or 25% of rapeseed meal obtained with either Samourai (Samourai) or Honk (Honk) rapeseed varieties. Lambs were weaned at 888 days of age and 245 kg of live weight and were slaughtered when their fattening state was estimated to be satisfactory. Blood samples were taken fortnightly for the determinations of hormones, glucose and transaminases (GOT and GPT). To evaluate the capacity of hormone production, the thyroid glands were collected at slaughter and the in-vitro release of thyroid hormones was recorded.
The plasma concentrations in triiodothyronine (T3) and thyroxine (T4) decreased (P<0.05) with
the age of lambs and were lower (P<0.05) in the Samourai and Honk groups than in the Control. Moreover, the capacity of T3and T4production by the thyroid gland shown by the in-vitro T3and
T4accumulation in the thyroid tissues was higher (P<0.05) in the Control than in the two rapeseed
groups. In contrast, the concentrate with rapeseed meal had no effect on the plasma concentrations in GH, cortisol and insulin nor on other substances (glucose, GOT and GPT).
These results indicate that the ingestion of rapeseed meal induces a low functional disorder of the thyroid without affecting the animal performance of lambs, as we reported previously. This is because the thyroid hormones do not appear to play a major role in the processes of growth and
81 (1999) 93±103
*Corresponding author. Tel.: 0032-8172-4377; fax: 0032 8172 4420
E-mail address: robert.mandiki@fundp.ac.be (S.N.M. Mandiki)
fattening, these having been successively taken under the control on priority by GH and insulin.
#1999 Elsevier Science B.V. All rights reserved.
Keywords: Rapeseed meal; Sheep-lamb; Animal physiology; Thyroid
1. Introduction
The effects of rapeseed meal on the release of thyroid hormones as well as their plasma concentrations have been widely studied, particularly in monogastric animals for whom a decrease in T4level was observed (Vermorel et al., 1987; Busato et al., 1991; Spiegel
et al., 1993; Mawson et al., 1994).
In the case of ruminants, data are still variable and conflicting, suggesting the absence of effects (Hill, 1991; Mawson et al, 1994; Vincent et al., 1988) or the reduction in the level of T4(Papas et al., 1979). Little is known concerning other blood subtances.
Various experiments have been also made to study the effects of glucosinolates and their derivatives on the activity of thyroid gland. It is known that thiocyanates may decrease the trapping of iodides and the accumulation of iodine ions in the follicular cells (Ruckebush et al., 1991), whereas goitrin inhibits the synthesis of thyroid hormones (Mawson et al., 1994; Gutzwiller, 1996). An increase in the ratio between monoiodotyrosine (MIT) and diiodotyrosine (DIT) (Theriez et al., 1971; Mawson et al., 1994) and in the production of TSH following an injection of thyroid-releasing hormone (TRH) (Ahlin et al., 1994) have been reported after the ingestion of rapeseed meal.
In addition to the general objectives described in the first part of this work (Derycke et al., 1999), we have considered it useful to study the effects of the incorporation of rapeseed meal in the diet on the thyroid activity as well as on other main hormones inducing metabolic actions (GH, cortisol, insulin), on transaminases as indicators of hepatic activity and other substances such as glucose.
2. Materials and methods
2.1. Animals and management
The general description of animals and management was given in the first part of this series (Derycke et al., 1999).
In summary, sixty six Texel, Suffolk or crossbred lambs were allocated to three groups according to their sex, body weight (8.92.8 kg) and age (288 days of age). Lambs receivedad libitum water and hay plus concentrates containing 0% (Control) or 25% of rapeseed meal either from theSamourai(15mmol/g dry matter (DM) of
glucosinol-ates in the free-fat meal, 1.95mmol/g in the complete concentrate) or the Honk (33
and 4.22mmol/g respectively) variety. The start of rapeseed incorporation was taken as
In addition to the evaluation of animal performance, blood samples were taken fortnightly by venipuncture for the determination of hormones (T3, thyroxine - T4, GH,
cortisol and insulin), glucose and transaminases (glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT). Samples were immediately centrifuged (2000gfor 15 min) and plasma was stored atÿ208C until assayed. The thyroid gland were collected, weighed and processed immediately after slaughtering.
2.2. Methods
2.2.1. Blood substances
Plasma hormone concentrations were quantified by radio-immunoassay (RIA). For each hormone, plasma concentration was determined for duplicate samples in one assay; the intra-coefficient of variation and the sensitivity are presented in Table 1.
Glucose was determined by spectrophotometry and GOT and GPT by colorimetry using Sigma Diagnostic kits.
2.2.2. Thyroid activity
Just after collection at the time of slaughter, the thyroids were immersed in a physiological solution. After being cleaned out of adhesive tissues, they were sliced using a Thomas slicer (slices of 7mm depth and 10±20 mg). Thereafter, these slices underwent
a culture challenge using a 5 ml multiwell tissue culture plate. The medium culture M199 Gibco was used after being modified as follows : 982 mg of M199 Gibco, 220 mg of Na HCO3, 50ml of gentamycine, 100ml of KI (5%) and 100 ml of distilled H2O.
Placed in 2 ml of medium culture, thyroid slices were incubated in a culture chamber at 398C containing 5% CO2 and relative humidity of 80%. For each animal, and each
thyroid, 6 slices were processed in separate wells. Attempts at stimulation were made by bovine TSH at variable doses (100 or 1000 mg/ml) and inhibition by methimazole at 400 mM and propyl-thio-uracile (PTU) at 10 and 40 mM.
For each slice, samples of medium (100ml) were collected after 1, 3, 5, 12 and 24 h of
culture process. They were stored atÿ208C until RIA analyses for T3and T4hormones.
2.2.3. Statistical analysis
Data were analysed using GLM procedures (Statistical Analysis System, 1985). A mixed model analysis of variance was used to determine the effect of diet, lamb within diet, age of lambs and thier interactions on each parameter. Diet and age of lambs were considered as fixed effects, whereas ram within breed was considered as a random effect (Dagnelie, 1975). Contrasts were used to compare group means when the main or interaction effects were significant, (P< 0.05).
Table 1
Intra-coefficient of variation (cv%) and sensitivity of plasma hormones
T3 T4 GH Cortisol Insulin
Intra-cv (%) 4 3.9 3 5 5.8
3. Results
3.1. Blood substances
Plasma T3 and T4concentrations (Fig. 1) decreased significantly (P< 0.05) with the
age of lambs and were higher (P< 0.05) in the Control than in the Samourai and Honk groups. The plasma cortisol concentrations (Fig. 1) showed quite large fluctuations over the time of the experiment and values did not differ significantly among groups.
Plasma GH concentrations (Fig. 2) decreased (P< 0.05) with the age of lambs but values did not differ significantly among groups. The plasma insulin concentrations (Fig. 2) increased significantly (P< 0.05) with the age of lambs and values were higher (P< 0.05) in the Control than in the two rapeseed groups. The plasma glucose concentrations (Fig. 2) varied slightly with the age and values were comparable among groups.
The GOT values (Fig. 3) varied greatly among animals and within the same group; they decreased (P< 0.05) with the age of lambs but did not differ among groups. For the GPT (Fig. 3), values increased significantly (P< 0.05) with the age of lambs but did not vary among groups.
3.2. Thyroid activity
Fig. 4 shows the changes in T3and T4concentrations over time of thyroid tissues from
the Control in challenge with TSH, methimazole or PTU. It is worth noting that the liberation of T3 and T4 by the thyroid slices may not be stimulated/inhibited by TSH,
PTU or methimazole. The profiles of liberation showed that after one hour of stimulation or inhibition, the greatest proportion of hormones contained in the thyroid tissues has been passed into the culture solution. This seemed to reflect the amount of the hormone available in the follicles without any synthesis occurring.
Since there was not response to the stimulation/inhibition challenge, the average of the six replicates from each thyroid gland and animal were used to study the effect of dietary treatments on the rate of hormone release during in vitro incubation as shown in Fig. 5. The in vitro concentration of T3was significantly lower in the Samourai (P< 0.05) and
Honk (P< 0.01) than in the Control. The T4values were comparable for the Honk and
Samourai but significantly lower (P< 0.05) for these two rapeseed groups than for the Control.
4. Discussion
Fig. 1. Changes in plasma T4, T3and Cortisol concentrations in the Control, Samourai and Hong groups over
the time of the experiment. Day 0 = start of rapeseed incorporation.
end of the experiment apart from a small quantity of hay (0.4 kg/day). Moreover, the rapeseed meal was the only protein supplement for the two experimental concentrates.
Despite all these conditions, no differences were observed in the animal performance and carcass quality, apart from a slight inrease in the quality of perirenal fat. In contrast, the Honk which was the richest in glucosinolates, induced an hyperthyroidism and a change in the histological structure of the thyroid.
4.1. Thyroid hormones and activity
The results described in the current paper show that the morphological changes in the thyroid that were due to the ingestion of a large amount of rapeseed meal are connected with a physiological change. The plasma concentrations in thyroid hormones were decreased significantly by the ingestion of rapeseed meal to the same extent for the two varieties. Moreover, the storage of hormones in the thyroid was significantly less in the lambs which received rapeseed meal than in the controls as evidenced by the in-vitro culture of thyroid tissues. It is worth noting that the Samourai rapeseed meal induced an important disturbance in thyroid physiology despite its lack of effect on the histological structure in relation to a weaker capacity to retain glucosinolates as compared to the Honk. This can perhaps be explained by the differences in the distribution of glucosinolates between the two rapeseed meals and by their metabolization pathways into the animal. More details concerning this aspect will be discussed in the third part of this work.
Fig. 3. Changes in plasma GOT and GPT concentrations in the Control, Samourai and Honk groups over the time of the experiment. Day 0 = start of rapeseed incorporation.
The results are in agreement with previous findings for non-ruminant animals (Mawson et al., 1994). Indeed, it is well known that glucosinolates, especially their derivatives, as thiocyanates, induce a decrease in the trapping of blood iodine ions, an increase in MIT and a low DIT, T3content and T4and therefore an elevation of the ratio MIT/DIT in the
lamb (Theriez et al., 1971; Ruckebush et al., 1991). These modifications result in lesser accumulation of T3and T4in the thyroid and lesser secretion of these hormones into the
blood. But, few data are available in the literature in the case of ruminants. In agreement with results in the current study, some authors (Hill, 1991) think that young ruminants are more sensitive to glucosinolates than adults because they have more important requirements in thyroid hormones and because they are less able to metabolize the glucosinolate derivatives. Vincent et al. (1988) reported that in the adult ewe, rapeseed did not induce a decrease in plasma concentrations of thyroid hormones, whereas Papas et al. (1979) observed a reduction in the concentration of T4, but not of T3, in the dairy
cow.
Fig. 4. Changes in T3and T4concentrations over the time of stimulation (TSH 100 or 1000 mg/ml) or inhibition
4.2. Other plasma substances
Apart from the secretion of thyroid hormones, the results of the current study show that the incorporation of a high propotion of rapeseed meal in the diet had no negative effect on hormones secreted by the main endocrine glands implicated in the metabolic process (GH by the pituitary, cortisol by the adrenal).
In the case of insulin, higher values were observed in the controls than in lambs which received rapeseed meal, but this difference was observed from the beginning of blood sampling, and it did not seem to be necessarily attributed to the ingestion of rapeseed. In addition to this, the activity of the liver was not altered since the concentrations in transaminases were similar throughout the three groups of lambs. Also, as the level of glucose in the blood, the major source of energy in the young ruminant, was comparable among the three groups, its is easier to understand why animal performance was in no way modified by the disturbance of the thyroid (Derycke et al., 1999).
Fig. 5. Changes in mean T3and T4production over the time of incubation of thyroid tissue from Control,
Samourai and Honk.
In his review, Hill (1991) suggested that in the case of hyperthyroidism due to glucosinolates, ``a strong compensatory mechanism maintains as long as possible adequate hormone concentrations in the presence of substances affecting the metabolism of the thyroid gland''. As confirmed by the results of the current study, we may assume that the growth and fattening mechanisms in young animals are due first of all to the GH, whose emission decreased progessively as the lambs grew, and thereafter to insulin, whose emission increased with the age. It seems likely that the disturbance of the thyroid would only have damaging effects if the animals had been reared in unfavorable conditions, necessitating a greater reactivity of the animal organisms. This was not the case at the time of our experiment.
In summary, these results indicate that the ingestion of rapeseed meal induces a low functional disorder of the thyroid without affecting the animal performance of lambs, as we reported previously. This is because the thyroid hormones do not appear to play a major role in the processes of growth and fattening, these having been successively taken under the control on priority by GH and insulin.
Acknowledgements
This study has received financial support from the General Office of Research and Development of the Belgian Agricultural Ministery and from the General Direction of Technologies, Research and Energy of the Ministery of `Region Wallonne' in Belgium. The authors are grateful to M.A. Bouckoms-Vandermeir and P. Ghys of the laboratory of Animal Physiology of Namur and to M. Hardenne of the Department of General and Organic Chemistry of Gembloux for their technical assistance.
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