Vagal glucoreceptors in the small intestine of the cat.

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Abstract
IN anesthetized cats, the unitary activity of 78 sensory vagal neurons was recorded in nodose ganglia by means of extracellular glass microelectrodes. These neurons were stimulated by perfusion of the small intestine (duodenum and first part of jejunum) with glucose or other different carbohydrates at concentrations of 1-20 g/l. The neurons slowly adapted to stimulation and their discharge frequency was always low (1-30 Hz). The activity of these neurons depended on the particular carbohydrate used and on its concentration: the discharge frequency generally increased when the concentration rose. The neurons were of the C type (conduction velocities: 0.8-1.4 m/s; mean, 1.1 m/s). In contrast with the known neurons connected to the gastrointestinal tension receptors, they were not obviously activated by intestinal contractions or distensions. In the same way, the stimuli which produced the response of other known endings, i.e., the mucosal receptors, were not effective; these stimuli included in particular stroking of the mucosa, over-distension of the bowel, intestinal perfusion with alkaline or acid solutions. On the other hand, the use of substances other than glucose (KCl and NaCl of the same osmolarity) showed that the osmotic pressure was not directly related to the receptor activation. It is proposed to call the endings corresponding to these neurons ''glucoreceptors''. The effect of glycemia and intestinal motility were also studied. These variables acted presumably by changing the intestinal absorption rate. The functional characteristics of the glucoreceptors (in particular the short latency of their response) strongly suggested that they were located close to the intestinal epithelium. An ultrastructural study was performed in an attempt to identify the histological site of the receptors. Many non-medullated fibers were observed in the villi, especially beneath the epithelial layer. They gave complex branchings with abundant swellings. Some of them, at least, belonged to the vagal sensory component, because they were less numerous after unilateral selective sensory vagotomy. These complex endings could serve as the vagal glucoreceptors. The roles of vagal intestinal glucoreceptors are discussed. Their functional characteristics as well as the clinical and experimental data suggest that they may be involved in the regulation of different types of alimentary behavior (hunger, thirst, allesthesia) and energy balance.