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Journal of Dairy Science Vol. 73 No. 6 1494-1499
© 1990 by American Dairy Science Association ®
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Effect of Somatotropin on the Plasminogen and Plasmin System in the Mammary Gland: Proposed Mechanism of Action for Somatotropin on the Mammary Gland

I. Politis 1, E. Block 1, and J. D. Turner 1

1 Department of Animal Science, Macdonald College of McGill University, 21,111 Lakeshore Road, Ste. Anne de Bellevue, Quebec, Canada H9X 1C0

Somatotropin exerts a powerful galactopoietic effect when it is administered to dairy cattle. The mechanism by which somatotropin increases milk yield is currently unknown. This study describes the relationship between long-term exogenous somatotropin injections, milk plasmin concentrations, and milk yield. Plasmin is a serine-protease involved in rodent mammary gland involution. Thirty-five control cows were shown to increase milk plasmin concentrations and decrease milk yield as their lactation period advanced past the 2nd mo. In 42 somatotropin injected cows, milk plasmin was maintained at low concentrations, and milking performance was enhanced. Cessation of somatotropin injections at drying off led to a rapid elevation of milk plasmin to control values. Our hypothesis is that somatotropin suppresses plasmin production within the mammary gland, thereby suppressing involution and allowing a persistence of milk production. During a normal lactation, gradually increasing amounts of plasmin within the mammary gland results in the gradual involution process.

Key Words: somatotropin • plasmin • involution

Submitted on July 24, 1989
Accepted on December 28, 1989




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