α-Retinol Is Distributed through Serum Retinol-Binding Protein-Independent Mechanisms in the Lactating Sow-Nursing Piglet Dyad

Abstract

α-Retinol (αR) is a structural isomer of retinol [vitamin A (VA)] that does not bind to serum retinol-binding protein (RBP). In this study,α-retinyl acetate (αRA) was synthesized and given orally (35μmol) to VA-deficient lactating sows (n= 11) to assess its potential to trace RBP-independent retinol transport and tissue uptake. TheαRA dose primarily appeared in sow serum as 4α-retinyl esters (αRE) with peak serum totalαR concentrations (the sum of the alcohol and ester forms) detected at 2 h (70 ± 23 nmol/L, mean ± SEM) postdose. From 0 to 40 h postdose, the percentage of serum totalαR in the alcohol form did not increase. RapidαR uptake into sow milk was observed with peak concentrations (371 ± 83 nmol/L) at 7.5 h postdose, consistent with the uptake ofαRE from chylomicra. A high percentage of theαRA dose (62 ± 15%, mean ± SD) was present in the livers of sows (n= 6) killed 22–28 d postdose. Approximately 15–26% of the sowαRA dose was transferred to the livers of the nursing piglets (n= 17) after 3 d. In piglets and sows, a similar percentage of hepatic totalαR was detected in the ester form as that of hepatic total retinol. Taken together, these data suggest that an oral dose ofαRA effectively traces the uptake, esterification, chylomicron transport, and hepatic storage of retinol and may be useful for deciphering the role of RBP-independent delivery of retinol to other tissues.