Mammary type I deiodinase is dependent on the suckling stimulus: Differential role of norepinephrine and prolactin

Mammary deiodinase type I(M-D1) is present only during lactation and exhibi ts a clear direct correlation with lactation intensity (size of litters). T he present work shows that M-D1 is suckling dependent and that intervals be tween suckling periods no longer than 12 h are essential to maintain this a ctivity. Moreover, we find that with only 15 min of resuckling in 12-h nons uckled mothers, the 50% decrease in both M-D1 messenger RNA and enzymatic a ctivity could be restored to control values. This restorative effect by suc kling may involve pre- and posttranscriptional mechanisms in which norepine phrine and PRL play important roles. Norepinephrine elicits a potent stimul atory effect on M-D1 messenger RNA and enzyme activities, whereas PRL only increases M-D1 activity and may modulate the enzyme response to norepinephr ine. Oxytocin and GH had no effect. These data suggest that the adrenergic nervous system and PRL could directly participate in mammary energetic expe nditure, regulating the local T-3 supply.