Ghrelin treatment reverses the reduction in weight gain and body fat in gastrectomised mice.

BACKGROUND AND AIMS: The gastric hormone ghrelin has been reported to stimulate food intake, increase weight gain, and cause obesity but its precise physiological role remains unclear. We investigated the long term effects of gastrectomy evoked ghrelin deficiency and of daily ghrelin injections on daily food intake, body weight, fat mass, lean body mass, and bone mass in mice. METHODS: Ghrelin was given by subcutaneous injections (12 nmol/mouse once daily) for eight weeks to young female mice subjected to gastrectomy or sham operation one week previously. RESULTS: Gastrectomy reduced plasma concentrations of total ghrelin (octanoylated and des-octanoylated) and active (octanoylated) ghrelin by approximately 80%. Immediately after injection of ghrelin, the plasma concentration was supraphysiological and was still elevated 16 hours later. Daily food intake was not affected by either gastrectomy or ghrelin treatment. The effect of ghrelin on meal initiation was not studied. At the end point of the study, mean body weight was 15% lower in gastrectomised mice than in sham operated mice (p<0.001); daily ghrelin injections for eight weeks partially prevented this weight loss. In sham operated mice, ghrelin had no effect on body weight. The weight of fat was reduced in gastrectomised mice (-30%; p<0.01). This effect was reversed by ghrelin, enhancing the weight of fat in sham operated mice also (+20%; p<0.05). Gastrectomy reduced lean body mass (-10%; p<0.01) and bone mass (-20%; p<0.001) compared with sham operated mice. Ghrelin replacement prevented the gastrectomy induced decrease in lean body mass but did not affect bone. In sham operated mice, ghrelin affected neither of these two parameters. CONCLUSIONS: Ghrelin replacement partially reversed the gastrectomy induced reduction in body weight, lean body mass, and body fat but not in bone mass. In sham operated mice, ghrelin only increased fat mass. Our results suggest that ghrelin is mainly concerned with the control of fat metabolism and that ghrelin replacement therapy may alleviate the weight loss associated with gastrectomy.

Histamine and histidine decarboxylase are hallmark features of ECL cells but not G cells in rat stomach.

The oxyntic mucosa of the rat stomach is rich in ECL cells which produce and secrete histamine in response to gastrin. Histamine and the histamine-forming enzyme histidine decarboxylase (HDC) have been claimed to occur also in the gastrin-secreting G cells in the antrum. In the present study, we used a panel of five HDC antisera and one histamine antiserum to investigate whether histamine and HDC are exclusive to the ECL cells. By immunocytochemistry, we could show that the ECL cells were stained with the histamine antiserum and all five HDC antisera. The G cells, however, were not stained with the histamine antiserum, but with three of the five HDC antisera. Thus, histamine and HDC coexist in the ECL cells (oxyntic mucosa) but not in G cells (antral mucosa). Western blot analysis revealed a typical pattern of HDC-immunoreactive bands (74, 63 and 54 kDa) in oxyntic mucosa extracts with all five antisera. In antral extracts, immunoreactive bands were detected with three of the five HDC antisera (same as above); the pattern of immunoreactivity differed from that in oxyntic mucosa. Food intake of fasted rats or treatment with the proton pump inhibitor omeprazole raised the HDC activity and the HDC protein content of the oxyntic mucosa but not of the antral mucosa; the HDC activity in the antrum was barely detectable. We suggest that the HDC-like immunoreactivity in the antrum represents a cross-reaction with non-HDC proteins and conclude that histamine and HDC are hallmark features of ECL cells but not of G cells.

Rat stomach ECL cells: mode of activation of histidine decarboxylase.

Histidine decarboxylase (HDC) occurs in ECL cells in the oxyntic mucosa of rat stomach. It is activated by gastrin. Refeeding of fasted rats or treatment with the proton pump inhibitor omeprazole promptly raised the serum gastrin concentration and consequently the HDC activity and the HDC protein content of the oxyntic mucosa. The food- and omeprazole-induced increase in HDC mRNA expression in the oxyntic mucosa was modest by comparison. Blockade of translation (cycloheximide) but not transcription (actinomycin D) prevented the postprandial rise in HDC activity. The half-life of HDC activity (after blockade of translation) was 94 min in omeprazole-treated rats and 55 min in fasted controls. The rate of enzyme synthesis was estimated to be 15 times higher in omeprazole-treated rats than in fasted controls. Inhibition of histamine uptake into ECL-cell granules by reserpine, a blocker of the vesicular monoamine transporter type-2, lowered the HDC activity and prevented the gastrin-induced HDC activation. We suggest that HDC activation reflects enhanced transcription, translation and/or posttranslational enzyme activation as well as stabilization, and that a high cytosolic histamine concentration suppresses HDC activation.

A-like cells in the rat stomach contain ghrelin and do not operate under gastrin control.

Ghrelin is a 28 a.a. gastric peptide, recently identified as a natural ligand of the growth hormone secretagogue receptor (orphan receptor distinct from the receptor for growth hormone releasing hormone). In the present study, radioimmunoassay demonstrated ghrelin-like material in the rat oxyntic mucosa with moderate amounts also in antrum and duodenum. Small amounts were found in the distal intestines and pancreas. Northern blot analysis revealed abundant ghrelin mRNA in the oxyntic mucosa. Immunocytochemistry demonstrated ghrelin-immunoreactivity in endocrine-like cells in the oxyntic mucosa. Such cells occurred in low numbers also in the antrum and duodenum. The rat oxyntic mucosa is rich in endocrine (chromogranin A/pancreastatin-immunoreactive) cells, such as the histamine-rich ECL cells (65-75% of the endocrine cells), the A-like cells (20-25%) and the D cells (somatostatin cells) (10%). The ghrelin-immunoreactive (IR) cells contained pancreastatin but differed from ECL cells and D cells by being devoid of histamine-forming enzyme (ECL cell constituent) and somatostatin (D cell constituent). Hence, ghrelin seems to occur in the A-like cells. The ghrelin-IR cells in the antrum were distinct from the gastrin cells, the serotonin-containing enterochromaffin cells and the D cells. Conceivably, ghrelin cells in the antrum and distally in the intestines also belong to the A-like cell population. The concentration of ghrelin in the circulation was lowered by about 80% following the surgical removal of the acid-producing part of the stomach in line with the view that the oxyntic mucosa is the major source of ghrelin. The serum ghrelin concentration was higher in fasted rats than in fed rats; it was reduced upon re-feeding and seemed unaffected by 1-week treatment with the proton pump inhibitor omeprazole, resulting in elevated serum gastrin concentration. Infusion of gastrin-17 for 2 days failed to raise the serum ghrelin concentration. Omeprazole treatment for 10 weeks raised the level of HDC mRNA but not that of ghrelin mRNA or somatostatin mRNA in the oxyntic mucosa. Hence, unlike the ECL cells, ghrelin-containing A-like cells do not seem to operate under gastrin control.