Relationship between numbers of alpha 2- and beta 2-adrenoceptors in teat tissue and blood cells and milkability of primiparous cows.
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.
Roets
E
E
Department of Veterinary Physiology, Veterinary Faculty of the University of Ghent, Belgium.
Vandeputte-Van Messom
G
G
Burvenich
C
C
Peeters
G
G
D000818:Animals; D001792:Blood Platelets / Q000378:metabolism; D002417:Cattle / Q000502:physiology*; D005260:Female; D007774:Lactation / Q000502:physiology*; D008214:Lymphocytes / Q000378:metabolism; D008321:Mammary Glands, Animal / Q000378:metabolism*; D011247:Pregnancy; D011942:Receptors, Adrenergic, alpha / Q000032:analysis*; D011943:Receptors, Adrenergic, beta / Q000032:analysis*
D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta