Stress down regulates milk yield in cows by plasmin induced beta-casein product that blocks K+ channels on the apical membranes

Stress and stress related hormones such as glucocorticoids inhibit lactatio n in cows. In the present study we propose a novel mechanism connecting str ess with plasminogen-plasmin system (PPS) (an enzymatic mechanism in milk, which leads to the breakdown of the major milk protein casein). We show tha t stress activates the PPS leading to an increase in plasmin activity, and that a distinct plas min-induced beta-casein breakdown product (fraction 1- 28) is a potent blocker of potassium channels in mammary epithelia apical m embranes. The reduction in milk production due to dehydration stress or glu cocorticoid (dexamethsone) was correlated with the activities of plasmin an d channel blocking activity in the milk of the tested cows. The notion that the axis Stress-PPS-beta-casein fraction 1-28 is responsible fur the reduc tion in milk yield is supported by the results of experiments showing that injecting solution composed of casein digest enriched with beta-casein frac tion 1-28 to the udder lumen leads to a transient reduction in milk product ion. Furthermore, injecting a pure beta-casein fraction 1-28 to the udder l umen of goat's lead also to a transient reduction in milk production with k inetics that was similar to the kinetics observed in cows. (C) 2000 Elsevie r Science Inc. All rights reserved.