Local generation of triiodothyronine by the mammary gland as a source of measurable quantities of the hormone in milk.
Simple diffusion or facilitated diffusion, by a carrier system represented by binding sites on plasma membranes, are thought to be responsible for thyroid hormones (TH) uptake by tissues. The concept implies the existence of an equilibrium ratio between plasma and cellular hormone content. Studies of TH presence in milk have shown that the passage of TH through mammary gland differs between T4, rT3, and T3, and only T3 exists in the milk in measurable amounts (without definite relationship between the levels of T3 in milk and the blood plasma). The present experiments have revealed that the whole milk or its cellular components (namely macrophages, lymphocytes and granulocytes) possess deiodinating enzyme system converting T4 into T3. The milk from animals displaying mastitis show lower T3 levels than controls. Of different enzyme systems, the outer ring deiodinase (5'D-II) dominates in mammary secretion. It plays an important role in galactopoiesis yielding biologically potent T3. The presence of TH deiodinating system(s) in mammary gland is thought to lower T4 and T3 in milk, already in small concentrations due to a poor permeability of the blood-mammary gland barrier for these compounds. On the other hand measurable amount of T3 in milk is due to a local enzyme mechanism generating T3 from T4. It has been concluded that the ultimate THs level in milk depends on the rate of transport of the hormones, their degradation and simultaneous generation of T3 from T4 in mammocytes.
Slebodziński
A B
AB
Polish Academy of Sciences, Institute of Agrotechnology and Veterinary Sciences, Department of Developmental Endocrinology, Poznan.
Brzezińska-Slebodzińska
E
E
D000818:Animals; D005260:Female; D008321:Mammary Glands, Animal / Q000166:cytology / Q000378:metabolism*; D008892:Milk / Q000032:analysis*; D013970:Thyronines / Q000378:metabolism; D014284:Triiodothyronine / Q000032:analysis* / Q000378:metabolism*
D013970:Thyronines; D014284:Triiodothyronine