Effects of muscarinic blockade on insulin secretion and on glucose-induced thermogenesis in lean and obese human subjects

To determine whether hyperinsulinaemia of human obesity is dependent on the activity of the parasympathetic nervous system, and whether activation of the parasympathetic nervous system plays a role in glucose-induced thermogenesis, the metabolic effect of a continuous intravenous glucose infusion [44.4 mumol kg-1 body weight (bw) min-1] with or without atropine infusion was assessed in 11 obese patients and 10 lean controls. Compared with lean controls, obese patients had increased basal and glucose-stimulated plasma insulin and C-peptide concentrations and increased plasma glucose concentrations during glucose infusion. Glucose oxidation during i.v. glucose was lower in obese patients than in lean controls. Glucose-induced thermogenesis was similar in obese patients and in lean controls. Atropine infusion did not affect basal plasma glucose, insulin or free fatty acid concentrations nor glucose-stimulated plasma glucose, insulin, C-peptide, glucagon or free fatty acid concentrations in both groups of subjects. Glucose and lipid oxidation rates and glucose-induced thermogenesis were also unaffected by atropine administration. It is concluded that (1) glucose-stimulated hyperinsulinaemia in human obesity is not dependent on a hyperactivity of the parasympathetic nervous system, which indicates that human obesity is different from most animal models of obesity; (2) glucose-induced thermogenesis is similar in obese and lean subjects when a similar load of glucose is administered; (3) inhibition of the parasympathetic nervous system does not affect the thermic effect of i.v. glucose.