Somatotrophin increases thyroxine-5'-monodeiodinase activity in lactating mammary tissue of the cow.
The effect of administration of bovine somatotrophin (bST) on peripheral conversion of thyroxine (T4) to tri-iodothyronine (T3) was studied in non-pregnant lactating Holstein cows. Six cows were injected daily for 5 days with 40 mg recombinantly derived bST, while six control cows received excipient alone. Blood samples were collected hourly from 08.00 to 19.00 h on a single day the week before treatment and on days 4-5 of treatment. All other tissue samples were obtained at slaughter, 20-23 h after the last injection. Administration of bST increased milk production and caused a 9% increase in hepatic DNA. Consumption of feed did not differ between control and bST-treated cows. Treatment did not alter serum concentrations of T4 or T3, although concentrations of thyroid hormones in the serum increased from 08.00 to 19.00 h. Activity of thyroxine-5'-monodeiodinase (5'-D) in liver and kidney was similarly unaffected. However, activity of 5'-D in mammary tissue increased approximately twofold in response to bST administration. We suggest that an increase in mammary conversion of T4 to the more biologically potent thyroid hormone T3 plays a role in mediating the galactopoietic response of dairy cattle to bST.
Milk Secretion and Mastitis Laboratory, USDA, Beltsville, Maryland 20705.
A V
AV
Capuco
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Keys
J J
JJ
Smith
D000818:Animals; D001835:Body Weight; D002417:Cattle; D004247:DNA / Q000096:biosynthesis; D005247:Feeding Behavior; D005260:Female; D013006:Growth Hormone / Q000494:pharmacology*; D007453:Iodide Peroxidase / Q000378:metabolism*; D007668:Kidney / Q000378:metabolism; D007774:Lactation / Q000187:drug effects*; D008099:Liver / Q000378:metabolism; D008321:Mammary Glands, Animal / Q000201:enzymology*; D009929:Organ Size; D011247:Pregnancy; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood
D014284:Triiodothyronine; D013006:Growth Hormone; D004247:DNA; D007453:Iodide Peroxidase; D013974:Thyroxine