Relationship between milkability and adrenoceptor concentrations in teat tissue in primiparous cows

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.