These studies were conducted to determine whether
ghrelin, a 28-amino
acid peptide produced mainly by the stomach, was involved in
tryptophan-mediated appetite stimulation in swine. In experiment 1, 36 crossbred (Long WhitexLarge White) barrows were used in a 2x3 factorial design to determine the effects of food intake (ad libitum versus limit fed) and
tryptophan level (0.12%, 0.19% and 0.26%) on growth performance as well as
ghrelin expression, plasma
insulin,
ghrelin and
leptin levels. Ad libitum fed pigs gained more weight, but had poorer feed conversion than limit fed pigs.
Weight gain, food intake and feed conversion all improved with increased ingestion of dietary
tryptophan. Ad libitum feeding increased plasma
insulin. Plasma
insulin was unaffected by the level of dietary
tryptophan. However, plasma
leptin was significantly lower in pigs fed 0.19%
tryptophan compared to those fed 0.12%
tryptophan. Plasma
ghrelin levels and
ghrelin mRNA level in gastric fundus and duodenun was significantly higher in pigs fed 0.19% and 0.26%
tryptophan diet compared with pigs fed 0.12%. In the second experiment, 18 crossbred barrows were divided into three treatments involving oral infusion of saline,
tryptophan (40mg/kg BW) or
5-hydroxytryptophan (40mg/kg BW). Plasma
ghrelin levels at 20, 40 and 60min after infusion of
tryptophan were higher than after saline and
5-hydroxytryptophan infusion,
5-hydroxytryptophan infusion induced lower food intake than saline infusion, and
tryptophan infusion increased food intake 2, 8 and 24h after infusion. In conclusion, oral
tryptophan ingestion increased
ghrelin expression in gastric fundus and plasma
ghrelin level.