Identification of immunoreactive plasma and stomach ghrelin, and expression of stomach ghrelin mRNA in the bullfrog, Rana catesbeiana

In this study, we established a radioimmunoassay (RIA) specific for ghrelin from the bullfrog Rana catesbeiana using a novel antibody raised against the C-terminal amino acid sequence of bullfrog ghrelin [13-28]. We also examined the distribution of ghrelin-producing cells in the stomachs of bullfrogs using this antibody and a cRNA probe specific for the bullfrog ghrelin gene. Ghrelin levels in plasma and stomach extracts were approximately 150 fmol/ml and 83-135 fmol/mg wet tissue, respectively. Reverse-phase high performance liquid chromatographic analysis, combined with bullfrog ghrelin RIA, revealed that ghrelin immunoreactivity in the stomach was composed of non-acylated ghrelin (des-acyl ghrelin) and several acylated forms of ghrelin bearing different fatty acid modifications, which could induce increases in intracellular Ca²⁺ in cells expressing the rat GH secretagogue receptor. In the stomach, the major storage form was acylated ghrelin. In bullfrog plasma, however, the majority of ghrelin immunoreactivity was des-acyl ghrelin and C-terminal fragments of frog ghrelin. Acylated ghrelin forms comprised only minor peaks. Ghrelin-immunopositive and ghrelin mRNA-expressing cells were observed within the mucosal layer of the stomach. Following starvation, significant increases in plasma ghrelin levels and stomach ghrelin mRNA levels were observed as early as 10 days after starvation. These results indicate that ghrelin is present in the stomach and plasma of the bullfrog, which can be detected with our novel antibody. Interestingly, the primary storage form of ghrelin in the stomach differed from the circulating form dominating in the plasma. Furthermore, increases in ghrelin levels in plasma and mRNA levels in the stomach after starvation suggest the possible involvement of ghrelin in energy homeostasis in the bullfrog.