Effect of feeding level and nitrogen source of the diet on GH, IGF-I, 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 (PR L) receptors in mammary and liver parenchymal tissue. The heifers were fed a hay-barley diet supplemented with either urea or rapeseed meal. The treat ments were: (1) LU, low feeding level + urea; (2) LR, low feeding level + r apeseed meal. (3) HU, high feeding + urea; (4) HR, high feeding level + rap eseed meal. The average daily gains of the heifers on LU, LR, HU and HR die ts were 692, 655, 805 and 890 g. At slaughter, mammary and liver parenchyma l tissue was collected for preparation of elude membranes, and ligand-bindi ng 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 hig h-affinity binding sites for IGF-I in mammary tissue. Specific binding of I GF-I to mammary membranes was unaffected by feeding level and nitrogen sour ce. Furthermore, Scatchard analysis showed unchanged binding capacities and binding affinities of the IGF-I receptors in mammary tissue from low and h igh fed heifers. Specific binding of prolactin to mammary membranes was inc reased 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 l ower (P < 0.07) in liver tissue from heifers fed rapeseed meal as diet nitr ogen source. The present study shows that the previously reported negative effect of hig h feeding level on pubertal mammary development (Mantysaari ct al., 1995), cannot be explained by changes in binding to GH and IGF-I receptors. The im portance of the increase in prolactin binding to mammary membranes from hei fers at high feeding level and with rapeseed meal needs further investigati on.