Myoepithelial cell contraction participates in mammary epithelial cell exfoliation in cow milk

A varying proportion of mammary epithelial cells (MEC) is exfoliated through shedding from the mammaryepithelium into milk, which might influence MEC number in the udder and thus milk yield. We hypothesizedthat myoepithelial cell contraction induced by oxytocin (OT) release at milking might stimulate MECexfoliation through mechanical forces. Therefore, to investigate the role of myoepithelial cell contraction inMEC exfoliation, we inhibited or induced myoepithelial cell contraction by injecting the OT receptor antagonistAtosiban (Ato) or exogenous OT, respectively. Eight multiparous cows were assigned to 2 treatments during2 milkings according to a crossover experimental design of 2 periods: (1) Control + OT; and (2) Ato + OT.For the Control + OT treatment, cows were first milked to recover standard milk and then received an i.v.injection of OT (5 IU) to collect residual milk through a second milking. For the Ato + OT treatment, cowswere injected with Ato (50 μg/kg of BW), milked to recover the cisternal milk, and then received 5 IU ofOT before a milking to collect the alveolar milk. For each type of collected milk, MEC were purified usingan immuno-magnetic method and the percentage of apoptotic MEC was assessed by flow cytometry. Tightjunction opening was assessed by plasma lactose concentration determination. Data were analyzed using anANOVA procedure. Inhibiting myoepithelial cell contraction by Ato injection induced a decrease in MECnumber in milk (P<0.01). In contrast, OT injection after standard milking increased the concentration ofMEC in milk (P<0.05) and decreased the percentage of apoptotic MEC in milk (P<0.01) while OT injectionduring Ato + OT treatment increased the number of MEC in milk (P<0.01). During the milking after the Atoinjection, plasma lactose concentration was on average lower than during standard milking whereas in bothtreatments, OT injections increased the lactose concentration in plasma (P