Regulation of the Urine Concentration Mechanism by the Oropharyngeal Afferent Pathway in Man

It has been reported that the oropharyngeal afferent pathway behaves as a first-order factor regulating body fluid. However, the neural mechanism has not yet been clear. This study was designed to elucidate the characteristics of the neural mechanism in man.Healthy human subjects kept either an isotonic or hypertonic solution in the oral cavity so as to moisten the oropharyngeal mucosa without intake for 20 min. Plasma vasopressin (AVP) concentration, urine volume and urine and plasma osmolalities were measured before and after the application.The oropharyngeal application of isotonic saline induced no significant changes in plasma AVP concentration, urine volume and osmolality, while that of hypertonic saline induced a significant decrease and increase in urine volume and osmolality, respectively. The plasma AVP concentration increased significantly after the application of hypertonic saline. Mannitol also induced a significant decrease and increase in urine volume and osmolality, respectively. However, the urinary responses after the application of mannitol were delayed compared with those of hypertonic saline. There were no significant changes in plasma osmolality before and after each application.These results suggest that the oropharyngeal afferent pathway from the oropharynx to hypothalamic AVP-producing cells takes part in the thirst mechanism regulating body fluid in humans. The oropharyngeal afferent signals may behave as sodium-chloride-sensitive osmoreceptor.