An hepatic osmoreceptor mechanism in the rat: electrophysiological and behavioral studies.

Rat liver was perfused with Ringer solution through the portal vein by use of a perfusion system which was designed to switch from standard Ringer solution to hypertonic or hypotonic Ringer solution. Neural responses to the osmotic change in the perfusion solutions were analyzed. They showed that two different types of osmosensitive afferent fibers exist in the hepatic vagus; one is characterized by increasing the frequency of spike discharges responding to higher osmotic pressure, while the other shows the same response to lower osmotic pressure. Behavioral changes caused by hepatic vagotomy were also observed. Though no differences could be detected in routine behavior (e.g., daily intakes of food and water, body-weight increase) between the vagotomized and the sham-operated rats, the former lost the ability to adjust urine concentration immediately in response to osmotic changes in the internal environment. These results provide evidence for the hypothesis that hepatic osmoreceptors exist in the rat.