Plasma and cerebrospinal fluid vasopressin and osmolality in relation to thirst

Conscious dogs chronically implanted with a device for cerebrospinal fluid (CSF) sampling from the anterior 3rd ventricle were submitted to 24 h dehydration. During rehydration by drinking the total water intake (TWI) after 16 min was determined in 8 and after 90 min in 14 experiments. Samples were simultaneously drawn to determine the osmolalities (P_osm, CSF_osm) and AVP concentrations (P_VVP, CSF_AVP) of plasma and CSF. After 24 h dehydration all of these parameters were significantly elevated in comparison to euhydrated dogs investigated on 19 occasions. In 8 experiments 60% of the final TWI had been ingested within the first 16 min with no changes of P_osm, CSF_osm and CSF_AVP, but a significant decrease of P_AVP at this time. TWI per kg body weight (TWI·kg^–1) after 90 min was significantly correlated with the osmolalities and AVP levels in plasma and CSF prior to rehydration. The decreases of P_osm, CSF_osm and P_AVP, but not of CSF_AVP, were significantly correlated with TWI·kg^–1. The results indicate that P_AVP and CSF_AVP are subject to long term control by body fluid tonicity exhibiting a feedback relationship to water intake. In addition, P_AVP but not CSF_AVP seems to be under short term, possibly nonosmotic, control during water intake.Supported by the Alexander von Humboldt-Foundation