GROWTH-HORMONE CONCENTRATION AND DISAPPEARANCE RATE, INSULIN-LIKE GROWTH-FACTOR-I AND GROWTH-FACTOR-II AND INSULIN LEVELS IN IRON-DEFICIENT VEAL CALVES

In calves with severe iron (Fe) deficiency, insulin-like growth factor (IGF)-I levels and IGF-I responses to exogenous growth hormone (GH) are reduced, while insulin-dependent glucose utilization is enhanced. Blood plasma concentrations of immunoreactive insulin (IRI), IGF-I, IGF-II and GH, and the half-life of blood plasma GH [after an i.v. injection of recombinant bovine GH (rbGH; 100 mu g rbGH/kg body weight)] were measured in 20 calves at body weights between 160 and 190 kg. Calves were fed milk replacers containing 50 or 10 mg Fe/kg (groups Fe50 and Fe10, respectively). Daily weight gain and feed utilization were similar in both groups. Group Fe10 developed mild Fe deficiency anemia and blood plasma urea-nitrogen concentrations were higher (p < 0.05) than in group Fe50. IGF-I and IGF-II concentrations did not vary consistently over a 10-hour period and were not significantly influenced by different Fe intakes. The IRI concentration increased transiently (p < 0.05) after feed intake, but the total response was (not significantly) smaller in Fe-deficient calves. Plasma GH concentration changed episodically and was similar in both groups. Loss of GH from the circulation after i.v. rbGH injection, estimated by biexponential analysis, during the distribution or a phase (first 16 min) was similar in both groups, but during the beta phase was shorter (p < 0.05) in group Fe10 than in group Fe50 (29.9 and 34.2 min, respectively). The increased disappearance rate of GH, seen even in mild Fe deficiency, may contribute to reduced GH levels and IGF-I responses to GH in severe Fe deficiency.