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In the present study newly calved cows were tentatively classified as moderate and severe responders to experimentally induced Escherichia coli mastitis based upon the reactive oxygen species (ROS)-generating capacity of their blood neutrophils before infection. The groups differed in blood and milk composition prior to infection. This initial classification was supported by the corresponding variation in clinical symptoms and in the changes in milk production and composition measured during mastitis. Responses of newly calved cows to Esch. coli challenge varied from mild to severe symptoms of inflammation in infected glands and differed in the intensity of systemic disturbances and general illness. Losses in milk yield and compositional changes were most pronounced in inflamed glands and in severe responders. In inflamed glands milk yield and composition did not return to preinfection level in either moderate or severe responders. The yields of lactose, a-lactalbumin, casein and fat followed the same pattern as milk yield. It is concluded that the severe and long lasting systemic disturbances observed in severe responders can be ascribed to absorption of endotoxin from infected glands into circulation, indicating the important role of endotoxin in the pathology of coliform mastitis in periparturient cows. Evaluation of the ROS-generating capacity of blood neutrophils and blood and milk composition before infection might help to predict the cow's sensitivity to Esch. coli mastitis.The great variation in illness and pathological responses (Hill et al. 1979;Vandeputte-Van Messom et al. 1988) observed in individual cows between calving and peak lactation has been associated with the phagocytic and bactericidal properties of the polymorphonuclear leucocytes (PMN) and macrophages. The efficiency of PMN against bacterial invasion largely depends upon their rate of diapedesis into the milk at the early stages of infection (Hill et al. 1979), as well as on the capacity of neutrophils to generate reactive oxygen species (ROS) (Heyneman et al. 1990). In newly calved cows Heyneman et al. (1990) were able to demonstrate a significant correlation between the ability of blood neutrophils to produce ROS and the suppression of milk and a-lactalbumin production following inoculation of Escherichia coli bacteria into the udder. In order to obtain more insight into the

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Milking characteristics were measured on 14 cows at morning milkings. After collection of blood, cows were slaughtered and teats were removed. beta 2-Adrenoceptors on teat tissue and lymphocyte membranes were identified by binding of [3H]dihydroalprenolol. For the determination of alpha 2-adrenoceptors on teat tissue and blood platelet membranes, [3H]rauwolscine was used. In teat tissue and blood cells, except for the amount of milk collected during the 3rd min, all other milking parameters were highly correlated with the ratio of beta 2-adrenoceptors to alpha 2-adrenoceptors. Good milkability appears to be associated with low ratio of beta 2-adrenoceptors to alpha 2-adrenoceptors densities in teat tissue, and a high ratio of beta 2- to alpha 2-adrenoceptors densities in blood cells. We postulated earlier that a low ratio of beta 2- to alpha 2-adrenoceptors in teat tissue of fast milking cows probably reflects changes of mainly prejunctional adrenoceptors. From our results on blood cells, we are now hypothesizing that in teat tissue prejunctional and extra junctional alpha 2- and beta 2-adrenoceptors might have been identified together, whereas alpha 2- and beta 2-adrenoceptors identified on blood platelets and lymphocytes, respectively, are reflecting by preference extra junctional adrenoceptors are activated primarily by circulating epinephrine. The high ratio of beta 2- to alpha 2-adrenoceptors observed in blood cells of fast milking cows might indicate a decreased vasomotor tone in the teat and an increased dilatation of the teat sphincter in these animals.

Milking characteristics were measured on 19 primiparous cows of the Red Pied breed at morning milkings. Measurements included maximum and average rate of flow, yield of milk through the 1st, 2nd, and 3rd min, amount of milk through the first 2 and first 3 min of milking, and milking time. Subsequently cows were slaughtered and teats immediately removed. Adrenoceptors on membranes isolated from teat tissue were identified by radiolabeled antagonists: [3H]dihydroalprenolol for beta 2-adrenoceptors, [3H]rauwolscine for alpha 2-adrenoceptors, and [3H]prazosin for alpha 1-adrenoceptors. Measurements of milking characteristics were highly repeatable within cow. Cows showed five distinct and significantly different milk flow patterns, which were characteristic for fast (type I, II), relatively fast (type III), and slow (type IV, V) milking cows. Covariance analysis for the five different types revealed an average regression between alpha 2-adrenoceptor density and milking rate 1st min as well as between beta 2:alpha 2-adrenoceptor ratios and milking rate 2nd min and milking time. A hypothesis is presented to explain these observations. During milking, tone of efferent sympathetic nerves in the teat is low. This phenomenon is more pronounced in fast milking cows. The typical presynaptic adrenoceptor pattern in the teat of fast milkers (low beta 2:alpha 2-adrenoceptor ratio) results in a decline of norepinephrine release by feedback mechanisms.