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Growth rate, tissue repair and reproductive functions are mediated by the somatotrophic axis, the growth hormone (GH) being one of its main components. GH is released in a pulsatile manner and a single measurement does not provide accurate information on the activity of the somatotrophic axis. The actions of GH on tissues are mediated by insulin-like growth factor type 1 (IGF-1), mainly released by the liver, and thus, the measurement of IGF-1 could be considered a good indicator of the activity of GH and the somatotrophic axis. Serum IGF-1 concentrations are relatively stable due to its long biological half-life without obvious diurnal rhythm. Additionally, many diseases significantly alter circulating IGF-1 concentrations, leading to potential diagnostic and prognostic uses in veterinary medicine. However, serum IGF-1 concentrations are affected by many factors, such as breed, age and sex. The present study analyzes the influence of these factors on serum IGF-1 concentra - tions in a population of 255 Andalusian horses (141 females and 114 males), divided into age groups: 1-2, 2-3, 3-4, 4-5, 5-6 and 6-12 months and 1-2, 2-4, 4-6, 6-10 and 10-14 years. The animals belonged to six different farms located in the same geographic location and were subjected to similar feeding and management protocols. Two measurements of body size were made: height at the withers (HW) and diameter of the thorax (DTx). Blood samples were taken always in the morning, in the month of July and serum IGF-1 concentrations were measured with a sandwich ELISA after dissociation of IGF-1 from its binding proteins. It was found that age and sex signifi - cantly influenced serum IGF-1 concentrations, whereas the effects of the farm and the time of blood withdrawal were not significant. Mean serum concentrations for both males and females respectively were: 246.3 and 231.0 (1-2 months), 201.9 and 194.7 (2-3 months), 174.2 and 170.4 (3-4 months), 161.7 and 155.4 (4-5 months), 166.1 and 136.9 (5-6 months), 127.2-114.5 (6-12 months), 103.3 and 89.01 (1-2 years), 104.3 and 73.41 (2-4 years), 105.4 and 64.40 (4-6 years), 53.29 and 68.27 (6-10 years) and 59.56 and 65.53 ng/ml (10-14 years). A progressive decrease in serum IGF-1 concentrations with increased age was found for both sexes. Males aged between five and 12 months and between two and six years had significantly higher serum IGF-1 concentrations than females of the same age. Coefficients of correlation between the indicators of body size (HW and DTx) and IGF-1 were -0.800 and -0.690 for the whole population of Andalusian horses, -0.860 and -0.750 for the males and -0.740 and -630 for the females. It is concluded that serum IGF-1 concentrations in Andalusian horses are reduced with ageing, male horses of determined age groups had higher IGF-1 than the females and there are negative correla - tions between body size and IGF-1 concentrations. The knowledge of the normal serum IGF-1 concentrations will help us to understand the role of the somatotrophic axis in several diseases and physiological situations and will provide information for further research on this equine breed.

A study of serum insulin-like growth factor type 1 (IGF-1) concentrations in resting untrained Andalusian horses: influence of age and gender