Effects of solid-phase extraction of plasma in measuring gut metabolic hormones in fasted and fed blood of lean and diet-induced obese rats.

Glucagon-like peptide-1 (GLP-1), peptide YY (3-36) [PYY(3-36)], amylin, ghrelin, insulin, and leptin are thought to act as hormonal signals from periphery to brain to control food intake. Here, we determined the effects of solid-phase extraction of plasma in measuring these hormones in blood of lean and diet-induced obese rats. Individual enzyme-linked immunoassays and a multiplex assay were used to measure active GLP-1, total PYY, active amylin, active ghrelin, insulin, leptin, and total GIP in response to (1) addition of known amounts of the peptides to lean and obese plasma, (2) a large meal in lean and obese rats, and (3) intravenous infusions of anorexigenic doses of GLP-1, PYY(3-36), amylin, and leptin in lean rats. Extraction of lean and obese plasma prior to assays produced consistent recoveries across assays for GLP-1, PYY, amylin, ghrelin, and insulin, reflecting losses inherent to the extraction procedure. Plasma extraction prior to assays generally revealed larger meal-induced changes in plasma GLP-1, PYY, amylin, ghrelin, and insulin in lean and obese rats. Plasma extraction and the multiplex assay were used to compare plasma levels of GLP-1, PYY, and amylin after a large meal with plasma levels produced by IV infusions of anorexigenic doses of GLP-1, PYY(3-36), and amylin. Infusions produced dose-dependent increases in plasma peptide levels, which were well above their postprandial levels. These results do not support the hypothesis that postprandial plasma levels of GLP-1, PYY(3-36), and amylin are sufficient to decrease food intake by an endocrine mechanism.