Ghrelin O -acyltransferase (GOAT) attaches octanoate to proghrelin, which is processed to ghrelin, an octanoylated peptide hormone that stimulates release of growth hormone (GH) from pituitary cells. Elimination of the gene encoding ghrelin or its receptor produces only mild phenotypes in mice. Thus, the essential function of ghrelin is obscure. Here, we eliminate the Goat gene in mice, thereby eliminating all octanoylated ghrelin from blood. On normal or high fat diets, Goat −/− mice grew and maintained the same weights as wild-type (WT) littermates. When subjected to 60% calorie restriction, WT and Goat −/− mice both lost 30% of body weight and 75% of body fat within 4 days. In both lines, fasting blood glucose initially declined equally. After 4 days, glucose stabilized in WT mice at 58–76 mg/dL. In Goat −/− mice, glucose continued to decline, reaching 12–36 mg/dL on day 7. At this point, WT mice showed normal physical activity, whereas Goat −/− mice were moribund. GH rose progressively in calorie-restricted WT mice and less in Goat −/− mice. Infusion of either ghrelin or GH normalized blood glucose in Goat −/− mice and prevented death. Thus, an essential function of ghrelin in mice is elevation of GH levels during severe calorie restriction, thereby preserving blood glucose and preventing death.

Ghrelin, an octanoylated peptide hormone produced in the stomach, rises dramatically in mouse plasma during chronic severe calorie deprivation, an event that is essential to maintain life. The mechanism for this increase is not understood. Here, we study the control of ghrelin secretion in tissue culture cells derived from mice bearing ghrelinomas induced by a tissue-specific SV40 T-antigen transgene. We found that the ghrelin-secreting cells express high levels of mRNA encoding β 1 -adrenergic receptors. Addition of norepinephrine or epinephrine to the culture medium stimulated ghrelin secretion, and this effect was blocked by atenolol, a selective β 1 -adrenergic antagonist. When WT mice were treated with reserpine to deplete adrenergic neurotransmitters from sympathetic neurons, the fasting-induced increase in plasma ghrelin was blocked. Inhibition was also seen following atenolol administration. We conclude that ghrelin secretion during fasting is induced by adrenergic agents released by sympathetic neurons and acting directly on β 1 receptors on the ghrelinsecreting cells of the stomach.

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