- Plasma concentrations of cortisol, insulin, triiodothironine, thyroxine and insulin-like growth factor-I in different breeds of small stock
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Plasma concentrations of cortisol, insulin, triiodothironine, thyroxine and
insulin-like growth factor-I in different breeds of small stock
M J Herselman & A A van Loggerenberg
Grootfontein Agricultural Development Institute, Middelburg, 5900, South Africa
email: Tino Herselman
In South Africa, wool and mohair are predominantly produced under adverse environmental conditions. However, several studies revealed an inverse relationship between fibre production potential and hardiness7,2. In this regard, it has been reported that high abortion rates observed in high fibre producing Angora goats may be linked to hypoadrenocorticism13. In another study it was reported that plasma cortisol concentrations in mutton sheep were higher when compared to that of wooled sheep during periods of nutritional and environmental stress6. The objective of this study was to further investigate hormone concentrations in small stock breeds with varying potentials for fibre production and hardiness.
Hormone concentrations were studied in female Angora, Merino, Afrino, Dorper and Namaqua Afrikaner lambs (6 per breed) over a period of one year. Lambs were kept with their dams until four months of age. From two to nine months of age animals had ad libitum access to milled lucerne hay and thereafter received a pelleted diet consisting of 60 % lucerne, 10 % maize, 20 % wheat chaff and 10 % molasses. Animals were shorn at the age of six months and body weight was recorded monthly from four months of age.
Twelve blood samples per animal were collected monthly over a 24-hour period (one sample every two hours) by venipuncture from the right jugular vein into 10 ml heparinized and evacuated glass blood collection tubes (Radem Laboratory Supplies (Pty) Ltd, Santon. S.A.). Plasma was harvested by centrifugation (2500 x g; 4°C; 20 min). Subsequently, composite plasma samples were prepared monthly for each animal, by pooling 500 μl of each of the 12 two-hourly plasma samples. These composite samples were stored at -20°C for hormone assays.
Different hormones were assayed by radio immunoassay. Commercially available kits were used for assaying insulin (Diagnostic Systems Laboratories, Webster, Texas), free T3, free T4 and IGF-I (Amersham International plc, England). Blood samples collected at any specific time were analysed in a single batch to minimize the effect of experimental error on variation among breeds.
Statistical analysis were performed using the GLM procedures of SAS 11. Data were analysed as a split-plot, repeated measures design. The model statement included, breed, age and breed x age interaction. The breed effect was tested using animal within breed as error term. Least square means were separated using the PDIFF option of SAS 11.
RESULTS & DISCUSSION
The mean hormone concentrations are shown in Table 1. The results on cortisol concentration has been reported earlier 8. The indigenous, fat-tailed, Namaqua Afrikaner displayed a consistently higher (P < 0.01) plasma cortisol concentration than all other breeds. Plasma cortisol concentration in Angora goats was lower (P < 0.05) than that of Afrino, Dorper and Namaqua Afrikaner sheep. No difference in the plasma concentration of cortisol was observed between Angora goats and Merino sheep.
Plasma insulin concentration was considerably higher in Namaqua Afrikaner sheep (Fig 1A) than all other breeds and was lowest in the high fibre producing breeds. As blood glucose is the primary regulator of both insulin release and its biosynthesis, high insulin levels observed in this study may be the result of high blood glucose levels in Namaqua Afrikaner sheep as reported earlier 6. Furthermore, these high levels of insulin found in Namaqua Afrikaner sheep is in agreement with previous studies in non-ruminants 5,10 where a greater insulin-secreting ability has been considered responsible for a greater degree of fat deposition. This is due to the effect of insulin to increase the activity of the pyruvate dehydrogenase system, thereby effecting increased fatty acid synthesis and oxidation via the Krebs cycle. From the age of 6 to 10 months, plasma insulin concentration was lower in all breeds and coincided with the cold winter months.
Table 1 Mean plasma concentration of cortisol, insulin, T3, T4 and insulin-like growth factor-I in five breeds of small ruminants between 2 and 14 months of age
Ang Mer Afr Dor Nam se Breed
Cortisol ng/ml 8.1 8.7 10.9 11.4 22.5 1.15 0.0001
Insulin μIU/ml 7.2 5.7 11.2 8.9 19.4 1.27 0.0001
T3 pg/ml 4.4 4.2 3.9 4.1 5.4 0.14 0.0001
T4 pg/ml 14.9 15.9 16.9 15.9 21.2 0.89 0.0003
IGF-I ng/ml 278 420 435 480 515 16.3 0.0001
Plasma T3 and T4 concentrations were higher in Namaqua Afrikaner sheep than all the other breeds but did not differ among the other breeds. However, the change in diet at the age of 9 months brought about significant changes in the concentration of thyroid hormones. The plasma concentration of T3 changed by 47.7, -7.3, -9.4, 1.1 and 0.1 % for Angora goats, Merino, Afrino, Dorper and Namaqua Afrikaner sheep respectively from the 5-month-period prior to, to the 5-month-period following the change in diet. It has been shown by several workers that plasma concentration of thyroid hormones correlate positively with energy intake 1,3,4,12. The large increase in T3 observed in Angora goats was possibly the result of the goats not accepting the first diet as well as the other breeds, which caused T3 concentration in Angora goats to be very low prior to the change in diet. The second diet was well accepted by all breeds. The low voluntary intake of the first diet was also reflected by changes in body weight 8 where the body weight of the Angora goat at 9 months was virtually the same as their weight at 2 months of age. From the age of 2 months until the age of 9 months, body weight increased by 6.1, 51.0, 42.5, 35.2 and 37.5 % for Angora goats, Merino, Afrino, Dorper and Namaqua Afrikaner sheep respectively compared, to increases of 58.5, 45.2, 58.0, 59.9 and 44.3 % over the next 5 months.
Figure 1 Changes in plasma concentrations of insulin (A) and insulin-like growth factor-I (B) in five breeds of small ruminants from two to fourteen months of age
The concentration of insulin-like growth factor-I (IGF-I) in Angora goats was much lower when compared to the sheep breeds (Fig 1B). Among the sheep breeds a tendency of lower IGF-I concentration in higher fibre producing breeds was apparent but this did not reach significance. All breeds showed increased concentrations of IGF-I after 8 months of age but it is not possible to ascribe this to change in diet, age or season. As it has been shown that IGF-I concentration is associated with blood glucose9, the extremely low concentration of IGF-I in Angora goats from 6 to 8 months of age was most probably caused by malnutrition during that period which would have led to low blood glucose levels6,14.
All hormone concentrations measured in this experiment were highest in the indigenous Namaqua Afrikaner sheep. Most hormone concentrations tend to be lower in high fibre producing breeds such as the Angora goat and Merino. These results illustrate that there are some differences in hormone concentrations in plasma of sheep and goats that seem to be related to hardiness of the breed. The effects of age, season and diet were, however, confounded in this experiment which prevent any definite conclusion to be draw on the physiological relevance of these differences.
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