EFFECT OF MILKING FREQUENCY AND SOMATOTROPIN ON THE ACTIVITY OF PLASMINOGEN-ACTIVATOR, PLASMINOGEN, AND PLASMIN IN BOVINE-MILK

Six pairs of identical twin cows during late lactation (213 d) were us ed to study the effect of milking frequency (twice vs. once daily) and bST during once daily milking on the activity of plasminogen activato r, plasminogen, and plasmin in milk. Less frequent milking increased t he activity of plasminogen, plasmin, and plasminogen activator in milk . The ratio of plasminogen to plasmin, a measure that is independent o f milk volume, decreased during less frequent milking, suggesting that at least part of the increase in activity of plasmin was due to the a ccelerated conversion of plasminogen to plasmin. Changes in the activi ty of plasminogen and plasmin in milk were positively correlated with increases in the concentrations of milk BSA and plasma lactose, both o f which are indicators of disruption of tight junctions between mammar y epithelial cells, indicating that paracellular leakage may have cont ributed to increased protease activity in milk during less frequent mi lking. No correlation existed between changes in plasminogen activator and indicators of tight junction disruption, suggesting that increase d activity of plasminogen activator in milk was not due to leakage acr oss the mammary epithelium, but rather to increased local production i n the mammary gland. Administration of bST during once daily milking d id not significantly affect milk protease activity.