Modified ghrelin dynamics in H pylori infection were demonstrated in the present study using an experimental animal model, and reduced preproghrelin mRNA expression and ghrelin levels were clearly shown in the stomach of gerbils with H pylori colonisation, in association with a significant increase in inflammatory cell infiltration.
The amino acid sequence corresponding to the ghrelin peptide in the gerbil was 93% identical to that of humans and 96% identical to that of its mouse or rat counterpart (fig 1B), suggesting functional homogeneity of this peptide in various animals. Differences in the amino acid sequences among the four species examined were found only at position 34 or 35 (fig 1B), a rather small segment of the molecule, probably insignificant in relation to biological function.3
Indeed, studies on the structure-activity relationship of the peptide showed that the octanoyl group on Ser3
is the essential moiety for biological activity of this peptide,1
and that the N terminal fragments showing conservation of the first five amino acids exhibited full functional activity.25
Using the same experimental animal, we previously reported a significant increase in the extent of inflammatory cell infiltration and oxidative stress in H pylori
colonised gastric mucosa,15
which validated the usefulness of this model for the study of H pylori
associated gastritis. The present study also showed marked gastric inflammation induced following H pylori
inoculation (table 2), which was reflected by an increase in stomach weight (table 2).
As ghrelin levels in the whole stomach specimens with H pylori
infection were in the same range as those in controls, even after release of the peptide into the bloodstream in response to food deprivation (table 3), it would be difficult to propose that the increase in plasma ghrelin levels in gerbils with H pylori
infected gastric mucosa (fig 6) could be the result of increased gastric total ghrelin production. Increase in plasma ghrelin levels, particularly active ghrelin, may be due to enhancement of degranulation of the ghrelin producing A-like cells induced by inflammatory stimuli, such as cytokines26
or free radicals.15
As plasma ghrelin elevation has been reported to occur in association with an increase in tumour necrosis factor α,27
the increase in H pylori
associated tumour necrosis factor α production28
may play a role in the enhanced ghrelin release noted following food deprivation. These contentions are endorsed by the significant correlation between gastric MPO activity (table 1) and plasma ghrelin levels (fig 7). Even though the underlying mechanisms have still to be elucidated in detail, there may be a compensatory increase in levels of plasma ghrelin following food deprivation in H pylori
colonised gerbils, in association with a decrease in gastric ghrelin density. Such elevation of plasma ghrelin levels may attenuate the reduction in appetite and body weight in H pylori
infected gerbils until 23 weeks (table 2). As longer term H pylori
infection (72 weeks) than in the cohorts examined in the present study (23 weeks) reportedly evoked significant weight loss in gerbils,29
further extension of gastric inflammation and atrophy may nullify the effects of the compensatory increase in plasma ghrelin levels and lead to reduction of both appetite and body weight.
As the average intestinal wet weight in uninfected gerbils at four weeks after inoculation was 1540 mg, the total ghrelin content in the small intestine was computed to be less than 0.029 nmol (fig 8). In contrast, the average wet weight of the stomach in uninfected gerbils at four weeks was 632.1 mg (table 2) and the average gastric total ghrelin content was computed to be 2.168 nmol. The gastric total ghrelin content was estimated to be at least 74-fold greater than that of the small intestine. Therefore, it would be difficult to consider an extragastric source of ghrelin as the origin of the increased plasma ghrelin in H pylori colonised gerbils.
The present data in gerbils were inconsistent with the results of two recent clinical studies,10,11
the results of which were also discrepant. These discrepancies might be attributable to differences in the methods of measurement of ghrelin peptide or patient selection, or interspecies difference in the distribution of extragastric ghrelin.
In the present study, plasma total ghrelin levels were not significantly increased in H pylori colonised cohorts at 23 weeks after H pylori inoculation (fig 6). Among the H pylori colonised cohorts at 23 weeks, four of 11 gerbils showed plasma total ghrelin levels below the average value observed in controls at the corresponding time point. Among these four gerbils with H pylori infection, two showed severe gastric atrophy, with the number of ghrelin immunoreactive cells being less than 1.2%, as well as lower than average values for gastric ghrelin and preproghrelin mRNA. Severe atrophy might account for the absence of a statistically significant difference between infected and uninfected gerbils at 23 weeks. These results suggest that the ghrelin content in the stomach decreases in response to H pylori infection, and with further extension of gastric atrophy, plasma ghrelin levels also decrease. Although there does seem to be differential regulation of preproghrelin mRNA, mucosal ghrelin, and possibly secretion of active ghrelin, the molecular physiology underlying ghrelin dynamics is outside the purview of the current study. Further investigation of ghrelin dynamics based on long term H pylori infection could help to clarify these issues.