Effect of Feeding Level and Nitrogen Source of the Diet on GH, IGF-T, and Prolactin Receptors in Mammary and Liver Tissue from Pre-pubertal Heifers
Pre-pubertal (86-220 kg BW) Finnish Ayrshire heifers were used to study the effect of feeding level and nitrogen source on GH, IGF-I, and prolactin (PRL) receptors in mammary and liver parenchymal tissue. The heifers were fed a hay-barley diet supplemented with either urea or rapeseed meal. The treatments were: (1) LU, low feeding level+urea; (2) LR, low feeding level+rapeseed meal; (3) HU, high feeding+urea; (4) HR, high feeding level+rapeseed meal. The average daily gains of the heifers on LU, LR, HU and HR diets were 692, 655, 805 and 890 g. At slaughter, mammary and liver parenchymal tissue was collected for preparation of crude membranes, and ligand-binding assays were performed. Specific binding of bGH to mammary membranes could not be detected. Linear Scatchard plots were obtained showing the presence of a single class of high-affinity binding sites for IGF-I in mammary tissue. Specific binding of IGF-I to mammary membranes was unaffected by feeding level and nitrogen source. Furthermore, Scatchard analysis showed unchanged binding capacities and binding affinities of the IGF-I receptors in mammary tissue from low and high fed heifers. Specific binding of prolactin to mammary membranes was increased by high feeding level (P<0.02) and by rapeseed meal (P<0.01). No interaction between feeding level and nitrogen source was identified. Hepatic binding of bGH, IGF-I and prolactin was unaffected by feeding level and nitrogen source. However, specific binding of prolactin tended to be lower (P<0.07) in liver tissue from heifers fed rapeseed meal as diet nitrogen source. The present study shows that the previously reported negative effect of high feeding level on pubertal mammary development (Mäntysaari et al., 1995), cannot be explained by changes in binding to GH and IGF-I receptors. The importance of the increase in prolactin binding to mammary membranes from heifers at high feeding level and with rapeseed meal needs further investigation.