TIME-COURSE OF MILK ACCUMULATION-INDUCED OPENING OF MAMMARY TIGHT JUNCTIONS, AND BLOOD CLEARANCE OF MILK COMPONENTS

Eight cows in early lactation were used to study the effect of milk ac cumulation on the state of mammary tight junctions and to examine a-la ctalbumin as an indicator of tight junction permeability in vivo. Duri ng three successive periods, the cows were milked twice (4 days), once (6 days), and twice daily (4 days). Plasma lactose, oc-lactalbumin, a nd milk sodium concentrations were used as indicators of tight junctio n permeability. Furthermore, four cows were used to study the clearanc e of lactose and a-lactalbumin from the blood. Milk yield during once- daily milking decreased by 15.4% (P < 0.001). All indicators of mammar y tight junction patency increased (P < 0.05) transiently during once- daily milking and indicated that tight junctions opened after similar to 18 h. Plasma alpha-lactalbumin and lactose were highly correlated ( r = 0.82, P < 0.001), indicating the suitability of plasma alpha-lacta lbumin as an indicator of tight junction status in vivo. Clearance of alpha-lactalbumin and lactose from the blood was best described by a b iexponential model. Elimination half-lives for lactose and alpha-lacta lbumin were 44 and 40 min, respectively. This study showed that milk s tasis during early established lactation induces tight junctions to sw itch to a leaky state after similar to 18 h and to revert to the close d state shortly after milking.