INSULIN-LIKE GROWTH FACTOR-I CONCENTRATION IN HOLSTEIN FEMALE CATTLE - VARIATIONS WITH AGE, STAGE OF LACTATION AND GROWTH HORMONE-RELEASING FACTOR ADMINISTRATION

Plasma insulin-like growth factor-I (IGF-I) concentrations were monitored in Holstein females through different periods of their growth, lactation and after acute or chronic growth hormone-releasing factor (GRF) administration. Plasma samples were radioimmunoassayed using a human IGF-I antibody after a 24 hr incubation in a HC1(.1N)-glycine(.2M) buffer (pH 2). In a first study, IGF-I concentrations were measured in Holstein females of different ages and(or) stages of lactation (n = 6 per group). The IGF-I concentrations in newborn calves (102.0 ± 11.3 ng/ml) markedly decreased (P<.01) in 1 mo old animals (50.2 ± 7.1 ng/ml), then increased (P<.01) to 137.0 ± 5.1 and 137.4 ± 11.0 ng/ml in 6 and 10 mo old heifers, respectively. In dairy cows, IGF-I concentrations were low 24 hr post-partum (44.7 ± 7.6 ng/ml) and then increased (P<.05) to remain stable throughout lactation (91.3 ± 4.9, 92.8 ± 12.9, 96.1 ± 7.6, 90.7 ± 8.8 ng/ml at 2, 3, 6 and 9 mo of lactation, respectively). There was a further increase (P<.05) to 113.7 ± 3.1 ng/ml during the dry period. In a second trial, blood samples were collected from lactating dairy cows every 2 hr for 24 hr following a sc injection of saline (n = 4) or human (h) GRF (1-29)NH2 (10 μg/kg BW, n = 4). The IGF-I peak concentration was reached on average 10 hr after the GRF injection and was higher (P<.01) in treated cows than in control cows (135.4 vs 86.9 ± 16.2 ng/ml). In the last trial, daily sc injections of 10 μg of hGRF(1-29)NH2 per kg BW to dairy cows (252 days of lactation) for 57 days, which increased milk production by 14% (2 kg/day), also increased (P<.01) IGF-I concentration: 127.1 ± 5.3 and 118.0 ± 1.6 vs 90.7 ± 4.7 and 96.0 ± 5.0 ng/ml on days 29 and 57 of treatment for treated (n = 9) and control (n = 8) cows, respectively. Thus, the IGF-I concentration in dairy cattle varies with age and stage of lactation, and is increased by GRF administration in lactating dairy cows. © 1990.