Contribution of macrophages to plasmin activity in ewe bulk milk

Abstract

A total of 225 bulk sheep milk samples were collected throughout lactation to assess the contribution of macrophages to the regulation of the plasmin/plasminogen system. Samples were analyzed for composition, milk renneting parameters, and for activities of plasmin (PL), plasminogen (PG) and plasminogen activators (PA). Isolation of macrophages from milk was performed using a magnetic positive separation; separated cells were lysed and activity of urokinase-PA was determined. PL activity in milk decreased during lactation (P < 0.001). The reduction in plasmin activity recorded in the mid and late lactation milk matched with the increase in PG/PL ratio (P < 0.001). The activity of PA increased throughout lactation (P < 0.001), the highest value being recorded in the late lactation milk. The amount of isolated and concentrated macrophages was higher in early and mid lactation milk than in late lactation milk (P < 0.01). Stage of lactation did not influence the activity of u-PA detected in isolated macrophages. The activity of u-PA associated with macrophages was lower than total PA activity detected in milk. Our results lend support to the hypothesis that in ewe bulk milk from healthy flocks macrophages only slightly contributed to the activation of plasmin/plasminogen system.