Triiodothyronine (T3), insulin and characteristics of 5'-monodeiodinase (5'-MD) in mare's milk from parturition to 21 days post-partum.

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It is generally accepted that hormones and tissue growth factors are supplied from mother to neonate via mammary secretion. Among the protein hormones, insulin and prolactin are considered as the most important milk components for neonates. The significance of the thyroid hormones, namely triiodothyronine (T3) generated locally by 5'-monodeiodinase (5'-MD) in the mammary tissues, for the mammary gland itself and for suckling neonates is still under consideration. In the present study the activity of the 5'-MD and the concentrations of T3 and insulin in mare's colostrum and milk during the first 21 days of lactation were measured. Post partum, T3 increased to its highest concentration around day 4 (1.14+/-0.08 nmol/L), then progressively decreased until day 7, reaching a relatively stable concentration of 0.71+/-0.06 nmol/L (overall mean for days 7-21). The colostral insulin concentration, highest on the day of parturition (401.0 +/-24.9 microU/mL), decreased to a nadir value (25.0+/-3.4 microU/mL) on day 5, after which it tended to increase. The mare's milk showed the presence of PTU-sensitive (type I) and PTU-insensitive (type II) 5'-monodeiodinases (5'-MD). Contrary to the classical type II 5'-MD, the mare's milk isoenzyme was inhibited non-competitively by aurothioglucose. A significant relationship (r=0.962, P < 0.01 ) between T3 concentration and 5'-MD activity, from the I st to the 6th lactational day was found, which may indicate a dependence of T3 concentration on the milk 5'-MD activity. The presence of 5'-MD of type II suggests that intra-mammary T3 generation may play a paracrine role supporting lactogenesis. Estimating that 1.8 microg of colostral T3 (0.456 microg/L) is consumed daily by a suckling foal, the T3 hormone action within the intestinal tract cannot be ruled out. This is the first paper to provide evidence of T3 and insulin concentrations, and of T4-5'-monodeiodinases activity in colostrum and milk of the mare.