Tissue distribution of glucagon, glucagonlike immunoreactivity, and insulin in the rat

To show that glucagon, glucagonlike immunoreactivity (GLI), and insulin are synthetized by organs other than the pancreas and the gastrointestinal tract, different rat tissue acid-ethanol extracts were obtained and analyzed by immunoassay using specific antisera. Significant amounts of glucagon were found in the gastrointestinal tract (44.77 +/- 5.4 ng), salivary glands (1.50 +/- 0.17 ng), thymus (2.80 +/- 0.46 ng), thyroid (0.25 +/- 0.02 ng), and adrenal glands (0.25 +/- 0.06 ng). Whereas GLI appeared in the gut mucosa, adrenal and salivary glands, genuine insulin was detected only in the pancreas. Aliquots of the tissue extracts, fractionated on Bio Gel P 30 columns, gave a 3,500 mol wt immunoreactive (30 K) peak that behaved as pancreatic glucagon on acrylamide gel electrophoresis and displaced 125I-labeled glucagon previously bound to its hepatic receptors. Arginine, epinephrine, and low glucose concentrations stimulated glucagon release from parotid, thymus, and thyroid. Active glucagon biosynthesis by these organs was established by the incorporation of L-[3H]tryptophan into a 3,500 mol wt polypeptide with specific immune reaction with 30 K antiserum. These results suggest that different rat tissues can contribute to the circulating levels of glucagon and GLI and therefore to metabolic homeostasis.