Intraduodenal phosphatidylcholine infusion restores the lymphatic absorption of vitamin A and oleic acid in zinc-deficient rats

Abstract

Previously, the lymphatic absorption of retinol was shown to be lowered markedly in zinc-deficient (ZD) rats, in parallel with a decrease in phospholipid (PL) output. The present study investigated whether an intraduodenal infusion of PL would restore the intestinal absorption of vitamin A in ZD rats fed 3 mg of Zn/kg of diet, as compared with a zinc adequate pair fed (PF) rats fed 31 mg of Zn/kg. Both ZD and PF rats were trained for meal feeding and fed equal amounts of the respective diets twice daily for 5 weeks. Each rat with lymph cannula was infused at 3 mL/hr via a duodenal catheter with a triolein emulsion containing 5.4 μCi 3H-retinol ( 3H-ROH) and 70 nmol retinol with or without 40 μmol phosphatidylcholine (PC). The absorption of 3H-ROH was measured by collecting lymph hourly for 8 hr during lipid infusion. When infused with no PC, the absorption of 3H-ROH was significantly lower in ZD rats beginning at 2 hr. Until it plateaued at 5 hr, the retinol absorption in ZD rats occurred at 0.42 nmol/hr, which was about 50% of the rate observed in PF rats. The cumulative absorption of 3H-ROH for 8 hr was 19.0 ± 1.4% in ZD and 29.7 ± 1.6% dose in PF rats. The cumulative PL secretion was 7.5 ± 1.1 μmol in ZD and 14.2 ± 1.1 μmol in PF rats. Also, the output of PL remained significantly lower at each hourly interval in ZD than in PF rats. The hourly PL output was correlated closely with the hourly 3H-ROH absorption ( r = 0.80, P < 0.05). When PC was infused, the rate of 3H-ROH absorption and total absorption for 8 hr in ZD rats were restored completely to the PF level, with a simultaneous increase in lymphatic PL output. In both ZD and PF rats, the 3H-ROH absorption increased at 0.97 nmol/hr prior to 5 hr and reached a maximum at 3.9 nmol/hr at 5 hr. The PL outputs for 8 hr in ZD and PF rats were 14.6 and 21.6 μmol, respectively. The lymphatic output of oleic acid was correlated highly with 3H-ROH absorption ( r = 0.80, P < 0.05). These findings provide the first evidence that a limited supply of PL to the enterocyte is a primary cause of the impaired absorption of retinol and oleic acid in ZD rats. This may be due to a defect in the biliary secretion of PL into the intestinal lumen. It is postulated that, because of the lack of PL, the enterocyte of ZD rats fails to form chylomicrons, principal carriers of dietary lipids and lipid-soluble nutrients.