Fatty acid synthesis in lambs in vivo as affected by nutrition and hormone administration

Abstract

Fatty acid (FA) synthesis was studied in lambs in vivo using the incorporation of tritium from tritiated water into FA under a number of experimental conditions. Expt 1 investigated the use of a high dose of tritiated water, with slaughter of the sheep after 4 h, and showed that sampling of tail fat was sucient as an index of the activity in carcass fat. Calculations of FA synthesised demonstrated the relative amounts contributed by carcass and offal. Extrapolation to a time of 24 h, assuming linearity of incorporation of tritium with time, gave results that were similar to what would be expected from the gains in body weight. Restriction of feed intake reduced the amount of FA synthesis.

Expt 2 demonstrated that incorporation of tritium into FA was almost linear over 24 h after an initial delay (mixing time) and there was probably no redistribution of tritiated FA between tissues. The increased time for incorporation had 3 major advantages: the dose of tritiated water could be reduced to more manageable levels, the error due to mixing time was reduced, and the measurement of synthesis was integrated over 24 h.

Expt 3 showed that increasing feed intake above 24 g/kg liveweight, which was close to maintenance, resulted in progressive increases in FA synthesis, measured by the technique of Expt 2 over 24 h. Incorporation of tritium into FA and non-saponifiable lipids, including cholesterol, in liver increased with feed intake in a sigmoid curve, with a maximum rate of change around an intake of 30 g/kg. This reflected the release of FA via lipoproteins into the circulation. The specific activity of the FA in the liver was similar to that of the ruminal FA and suggested that there was no actual synthesis in the liver.

Expt 4 investigated the effects of growth hormone (GH), oestradiol, andff Triton WR1339 administration to lambs on FA synthesis following short-term administration of GH at 0·3 mg/lamb·day, oestradiol (120 µg on Day 1 and 3), or Triton WR1399. Growth hormone caused a significant reduction in FA synthesis/g FA (P < 0·05) and in FA synthesis/10⁶ cells in subcutaneous fat (P < 0·01) after 4 days of administration. There was a reduction in cell diameter and number of fat cells/g FA (P < 0·05). Oestradiol showed a tendency to cause increased FA synthesis; there was a significant effect of treatment (P < 0·01) in omental fat. FA synthesis was significantly increased in the group of lambs given oestradiol by comparison with the group given GH (P < 0·01), whereas Triton WR1399 had no effect on FA synthesis in adipose tissue, substantiating that liver and plasma lipoproteins contribute only low amounts of FA to subcutaneous fat.

Expt 5 further investigated the effects of administration of GH for 1 or 4 days to lambs on high-grain pellets consumed at high intakes (57 g/kg liveweight). GH had no effect, irrespective of how the results were expressed, but rates of FA synthesis were >10-fold higher than those in Expt 4. In both trials, plasma levels of GH were raised and plasma levels of insulin-like growth factor-1 indicated that GH was active.

These results showed that FA synthesis can be measured in vivo in lambs effectively, and the method should be of more practical use than alternative methods in studies of agents affecting the level of carcass fat. The effects of GH were not as marked as those found in previous studies in pigs because of the short-term administration of GH, but even so, the excess intake of high-grain pellets appeared to prevent any effect of GH.