Thermogenesis induced by osmotic stimulation of intestines

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Infusion of various nutrients and non-nutrients into the duodenum induced dose-dependent thermogenesis in urethane-anaesthetized rats. In contrast, infusion of 0.9% NaCl, distilled water, or safflower oil had no effect on the metabolic rate. Infusion of 7.2% urea induced a small and transient increase in the metabolic rate. These results suggested that the thermogenesis was caused mainly by changes in osmolality rather than by a specific action of the different solute molecules. Intravenous infusion of 3.6% NaCl induced a significantly smaller thermogenic response. Similarly, infusion of NaCl into the hepatic portal vein or the peritoneal cavity also produced a significantly small thermogenic response. These results suggested an intestinal or mesenteric location for osmoreceptors. The osmolality of the duodeno-jejunal contents of rats was 600-800 mosmol/kg, which suggests that the intestinal osmoreceptors are stimulated after meals. Bilateral subdiaphragmatic vagotomy greatly attenuate … More d the first hour, but not the later phase, of the thermogenesis induced by 3.6% NaCl. Neither atropine pretreatment nor capsaicin desensitization had any effect on the osmotically induced thermogenesis, suggesting the involvement of non-nociceptive vagal afferents. Bilateral splanchnic denervation also had no effect, suggesting a lack of involvement of spinal afferents and sympathetic efferents to the major upper abdominal organs. Adrenal demedullation greatly attenuated the initial phase, but not the later phase, of thermogenesis. Pretreatment with the beta-blocker attenuated the thermogenesis throughout the 3 h observation period. The plasma adrenaline concentration increased significantly 20 min after osmotic stimulation but returned to the basal level after 60 min. The plasma noradrenaline concentration increased 20 min after osmotic stimulation and remained significantly elevated for 120 min. Therefore, adrenaline largely mediated the initial phase of thermogenesis, and noradrenaline was involved in the entire thermogenic response. These results show the critical involvement of the vagal afferents and sympathoadrenal system in the thermogenesis. Less