Changes in plasmin, plasminogen and plasminogen activator activities in milk of late lactating ewes: Effects of bovine somatotropin (bST) treatment

The aim of the research was to evaluate the effect of bST on activities of plasmin (Pi), plasminogen (Pg) and plasminogen activator (PA) in milk of ewes in late lactation and at drying off. Forty lactating ewes (about 210 days post-partum) were randomly assigned to one of two treatments: control and treated with slow release bST (120 mg/injection). Slow release bST was administered for two injection cycles of 21 days each (at 0 and 21 days). Ewes were milked twice a day from 0 to 35 days, once a day from 35 to 41 days of the trial and then dried off. Milk yield and composition (fat, protein, lactose, somatic cell counts) were measured weekly. Plasmin, Pg and PA activities were measured in individual milk samples collected during the second cycle of treatment, at 21, 28, 35, 38, 41 days and 3 days after cessation of milking, Least squares analysis of Variance was performed, the model included milking frequency and bST treatment as main effects. A significant increase of milk yield (0.59 vs 0.49 kg/day, P < 0.01) was observed in bST-treated ewes. Somatotropin reduced milk fat and protein contents (8.48 vs 8.59%; and 6. 11 vs 6.26%, P < 0.01). Milk lactose content and somatic cell counts did not differ between treatments. Plasmin and PA activities increased (14.5 vs 22.8 U/ml, P < 0.05; and 161.5 vs 485.5 U/ml, P < 0.001) and the Pg/P1 ratio decreased (1.56 vs 0.73, P < 0.01) in both control and treated ewes with less frequent milking. P1 and PA activities were not affected by bST treatment. Plasminogen activity and Pg/P1 ratio were lower in bST-treated ewes (17.5 vs 21.2 U/ml, P < 0.01; and 1.2 vs 0.9, P < 0.05). The lower Pg activity observed in bST-treated ewes might be due to a lower influx of Pg from the blood, thus, bST might preserve the integrity of tight junctions in late lactation. Published by Elsevier Science B.V.