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

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 concentrations 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 significantly 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 correlations 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.