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.
Evidence of growth hormone effect on plasma leptin in diet-induced obesity and diet-resistant rats
