E. Hadas, W. Koven, D. Sklan, A. Tandler
The beneficial effect of dietary lecithin fed to larvae of marine teleosts has been demonstrated in a number of species. Phosphatidylcholine (PC), the dominant phospholipid in most tissues and commercial lecithins, has been suggested as the main growth factor in this feed additive. However, the physiological mechanism involved in the growth promoting effect of PC is unclear. Two separate studies were conducted to determine the effect of dietary PC on the kinetics of absorption and assimilation of labeled (14C) oleic acid, in its free fatty acid form, from the digestive tract of 28 day old gilthead seabream larvae.
In the first study 50 larvae (28 days post hatch) were placed in each of two groups of twenty-four 900 ml beakers. Larvae from each group were fed, in excess and under illumination, one of two microdiets (MD) for 15h followed by 8h of food deprivation in darkness. The diets were labeled equally with (14C) oleic acid, but different in whether they included or not a 2.5% (w/w) equal mixture of analytical grade PC18:0 and PC 18:2n-6. During the 24 study, larvae from 6 beakers from the non-PC and PC MD treatments were sampled at 1, 8, 14 and 24h after start feeding. Larval total lipid was extracted from each of the samples followed by lipid class separation on TLC. The lipid class bands of polar lipids, cholesterol, free fatty acids, triacylglycerol and cholesterol esters were identified under UV light and then scraped into scintillation vials together with scintillation cocktail and the radioactivity read (after calculation for quench) in a beta scintillation counter. In the second study 24 larvae (28 days post hatch) were stocked in each of two groups of twelve 900 ml beakers and similarly fed the same (14C) oleic acid MD treatments as in the first study. At the end of the second feeding trial, the digestive tracts (DT) were carefully dissected out and then both the larval body and the digestive tracts were separately solubilized (Soluene 350) in scintillation vials. Scintillation cocktail was added to the vials and the radioactivity read as in the first study.
The first study suggested that dietary PC enhances the incorporation of the labeled free fatty acid into the larval tissues. Despite similar ingestion rates, significantly (p<0.05) higher total radioactivity (5024 dpm/larva) in the PC larvae as compared to the control larvae (3701 dpm/larva), was evident by 14 h after start feeding. In addition, markedly higher (p<0.05) radioactivity was found, after 24 h, in the polar lipid fraction (1447 dpm/larva) compared to the control larvae (737 dom/larva). Moreover, these levels of radioactivity in the polar lipid of PC-fed larvae represented a significantly(p<0.05) higher fraction (46%) of the total radioactivity in the extracted lipid fraction than the control treatment (27%). Furthermore, the second study showed that a significantly (p<0.05) higher percentage (44%) of the overall radioactivity consumed in the control larvae remained in their DT compared to the radioactivity in the DT of PC larvae (22%). This suggests that dietary PC may be contributing to lipoprotein production resulting in more efficient transport of the labeled dietary fatty acids from the enterocytes lining the DT to the larval tissues.
(National Center for Mariculture, Israel Oceanographic and Limnological Research Ltd., P.O.B. 1212, Eilat 88112, Israel)