Pancreatic and thyroid hormones in rainbow trout (Salmo gairdneri): what concentration does the liver see?

Blood was sampled within 1 min from the hepatic portal vein, caudal vein, and heart (bulbus arteriosus) of individual rainbow trout (N = 26). Blood plasma levels of the pancreatic hormones, insulin, glucagon, and glucagon-like peptide (GLP) and the thyroid hormones, thyroxin and 3,5,3'-triiodo-L-thyronine, were measured by specific radioimmunoassays. Blood levels of insulin, glucagon, and GLP were significantly higher in the portal vein than in either the caudal vein or heart. Thyroid hormone concentrations did not vary between blood vessels. Based on the higher hormone titers in portal blood as compared to blood sampled from the other sites, the liver appears to be exposed to concentrations of pancreatic hormones that are three- to eightfold higher than those experienced by other tissues. There was no statistically significant difference between the caudal vein or the heart in blood concentrations of the different pancreatic hormones, although the average concentration of GLP was higher in the heart (0.22 +/- 0.03 ng/ml) than in the caudal vein (0.15 +/- 0.03 ng/ml). The molar ratio of average blood insulin/GLP concentrations was lower in the heart (3.9) than in either the hepatic portal vein (6.9) or the caudal vein (7.2), which may be favorable for potential physiological effects of GLP on the heart and gills. The results of this study imply some role for GLP in the regulation of cardiac and gill metabolism or function. The higher hormonal titers to which fish liver, as compared to other target tissues, is exposed should be taken into account when a dosage of pancreatic hormones is calculated for in vivo or in vitro experiments.