Ghrelin tissue distribution: comparison between gene and protein expression

Ghrelin, the natural ligand of the GH secretagogue (GHS) receptor, was originally isolated from the stomach and detected in several tissues, but a systematic study of its tissue distribution has not been performed. In the present investigation, we evaluated ghrelin gene expression (by RT-PCR technique) and ghrelin protein concentration (by enzyme immunoassay technique) in tissues obtained from control rats as well as in rats subjected to 48-h fasting. The ghrelin gene was expressed in stomach, small intestine, brain, cerebellum, pituitary, heart, pancreas, salivary gland, adrenal, ovary and testis, with maximum expression occurring in the stomach, while no significant expression was detected by standard RT-PCR in liver, lung, kidney and skeletal muscle. Ghrelin protein was detected in stomach, small intestine, brain, cerebellum, pituitary, lung, skeletal muscle pancreas, salivary gland, adrenal, ovary and testis, at concentrations ranging from 0.05 to 1.43 ng/mg of homogenate protein (the highest concentration occurred in the lung, followed by the brain). Ghrelin was not detectable in the heart, liver and kidney. Therefore, gene and protein expression were dissociated. Fasting did not produce significant changes in ghrelin gene expression, while the distribution of ghrelin between different tissues was significantly modified: protein concentration increased in the brain, cerebellum, lung and salivary gland, while it decreased in the stomach.