Gubbiotti et al. 2007. Heifer Mastitis Conference, Gent (Belgium), June 24-26. Pages 132-133. Growth hormone, insulin- like growth factor and prolactin during bovine E. coli mastitis A. Gubbiotti 1 , V. Van Merris 2 , H. Dosogne 2 , R.M. Bruckmaier 3 , J. Blum 3 , G. Bertoni 1 , E. Trevisi 1 , C. Burvenich 2 1 Institute of Zootechnics, Faculty of Agriculture, Università Cattolica del Sacro Cuore, Piacenza, Italy 2 Department of Physiology and Biochemistry, Faculty of Veterinary Medicine, Merelbeke, Belgium 3 Division of Nutritional Pathology, Faculty of Veterinary Medicine, University of Berne, Berne, Switzerland * Corresponding author: christian.burvenich@ugent.be Introduction: Prolactin (PRL) and Growth hormone (GH) play an important role in hormonal regulation of lactation. PRL, released in plasma in high concentrations at calving, acts on differentiation of mammary gland epithelial cells and contributes to lactogenesis step. GH is necessary for the maintenance of lactation and its plasma levels are positively correlated with milk yield (3). The importance of GH, PRL and IGF-1 in the immune system has already been recognized (6). The GH receptor have been identified on polymorphonucleate leukocytes (PMN), and it augments the differentiation of bone marrow derived PMN and enhances a number of immune responses in humans (1, 4). Our aim was to check the hypothesis that naturally high GH levels could enhance resistance against severity of E. coli mastitis through stimulation of both proliferation and maturation of PMN. Materials and Methods: Twelve healthy early postpartum Holstein Friesian cows were used. Clinical mastitis was induced in all cows infusing 10 ml pyrogen-free 0.9% NaCl with 1000 cfu E. coli strain P4 into the front and rear left udder quarter, immediately after the morning milking at 8 a.m. Blood samples were collected from the external jugular vein into Vacutainer tubes (Lithium heparin) at: -96, -24, 0, +2, +4, +6, +8, +10, +12, +18, +24, +48, +72, +72, +144 and +216 hours from challenge. The number of leukocytes in whole blood was determined and the immature neutrophils were identified as metamyelocytes and myelocytes and band neutrophil. Plasma was stocked in several aliquots at -20°C for GH, IGF-I and PRL quantification and for glucose and non esterified fatty acids (NEFA), this last one using a method previously described by Bertoni et al. (2). Quarter milk production, rectal temperature, heart rate and rumen motility were registered at the times of blood sampling. Data were analyzed with a two way ANOVA, including group and time in the model, using the statistical analysis program package Statistix (Analytical Software, Tallahassee, FL, USA). The cows were retrospectively divided in 2 groups, moderate (M, 9 cows) and severe (S, 3 cows) responding. The qualifying criteria for severity was a milk production higher (M) or lower (S) than 50% of the pre-challenge milk production at 2 days after challenge, according to Heyneman et al. (5). Differences between S and M groups were evaluated using the Kruskal-Wallis test, because variances were not equal in the two groups according to a Bartlett’s test. Results: According to the criteria used to partition the cows, milk production was recovered in M cows at day +3 post challenge, but remained low in S cows until +9 days after challenge. Rectal temperature increased monophasically in M and biphasically in S cows with peaks of 40.5°C at +10 h (S and M cows) and 39°C at +24 h (only in S cows) after challenge. Before challenge, plasma NEFA levels were slightly higher in M (0.23±0.04 mmol/l) than in S cows (0.14±0.06), but during E. coli challenge NEFA grew more in M than in S cows (P<0.001). Plasma glucose levels were similar in M and S cows (about 3.5 mmol/l) before and after challenge. Before challenge, plasma GH levels were consistently higher in M than in S cows (P<0.001), while plasma IGF-I and PRL levels were not significantly different between the two groups. Whereas the general trend of plasma GH was similar in M and S cows, average values remained higher in M cows throughout the experiment (P<0.001). Fluctuation of plasma IGF-I