Chylomicron vitamin a uptake in the lactating mammary gland: possible relationship to retinoids in breast milk and breast

We have conducted studies to determine whether vitamin A (VA), present as retinyl esters (RE) in chylomicra (CM), is taken up into the mammary tissue of lactating rats and whether VA uptake depends on active lipolysis. Previously, we reported that the concentration of VA in the milk of lactating rats varies with dietary VA intake, even when plasma retinol is unaffected. We have now tested the hypothesis that CM, through lipolysis, deliver newly absorbed VA directly to the breast, in proportion to CM VA contents. ³H-RE-labeled CM were prepared in donor rats and administered i.v. to lactating recipients. ³H-RE-labeled CM disappeared rapidly from the plasma and accumulated (2-4 min max.) in the mammary glands, followed by a decline. Compartmental analysis (SAAM program) predicted that ∼41% of CM-³H marginated to the mammary glands during lipolysis and a total of 6.2% transferred to mammary tissue. Active lipolysis was necessary for uptake, as evidenced by little uptake in post-lactating rats or after release of endothelium-bound lipoprotein lipase. When the dose of CM HE was varied over a 25-fold range (holding CM triglyceride constant), the content of ³H in the breast varied directly with the administered CM RE dose, suggesting first-order assimilation of CM-RE during remnant formation in situ. We speculate that CM metabolism may contribute significantly to breast and hence, milk, VA content. Additionally, these data predict that supplementation with VA will produce a transient, dose-dependent, increase in milk VA, as well as increase hepatic VA reserves. This mechanism of uptake also could be of general importance for other iipophilic substances.