Interrelationships between circulating concentrations of the insulin-like growth factors (IGF-I and IGF-II) were investigated in 235 blood samples taken from 145 healthy beef or dairy calves, bulls and cows of different breeds and ages. Autoradiography of Western ligand blots indicated different IGF binding protein (IGFBP) profiles between sera from different categories of cattle. Each IGF radioimmunoassay was validated by determining the effects of IGFBPs, ligand and contraligand, as well as serial dilution and comparison with results obtained after molecular sieve chromatography in acid. In female cattle mean values for IGF-I varied from 5.1 nmol/1 in postparturient Holstein cows to 18.5-20.5 nmol/1 in growing beef heifers, while mean IGF-II concentrations ranged from 30.0 nmol/1 in the cows to 14.7-15.7 nmol/1 in the beef heifer calves. In male cattle mean serum IGF-I ranged widely from 8.2 nmol/1 in 1-day-old Holstein calves to 67.4 nmol/1 in 16-month-old Simmental-type bulls. Mean IGF-II concentrations decreased from 22.9 nmol/1 in 1-day-old Holstein bull calves to 11.9 nmol/1 in 12-month-old beef bulls. Thus, total molar IGF concentrations were fairly stable in female cattle (24.7-35.1 nmol/1) but extended from 27.3 nmol/1 to 81.8 nmol/1 in the male cattle. The tendency for a reciprocal relationship between serum concentrations of these growth factors was most obvious in the periparturient cows.
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