Vasopressin in plasma and cerebrospinal fluid of hydrated and dehydrated steers.

Levels of arginine-vasopressin (AVP), the antidiuretic hormone, have been concurrently measured by radioimmunoassay (RIA) in plasma (p) and cerebrospinal fluid (CSF) from conscious, unrestrained steers. During 4 days of dehydration, plasma osmolarity (posm) rose progressively from control hydrated levels of 301.3 +/- 0.632 mosm/1(mean +/- SE) to 338.5 +/- 3.090 mosm/1(p less than 0.001). Packed cell volume also rose from 39.9 +/- 0.64% to 44.7 +/- 1.24% (p less than 0.001). Associated with these changes was a progressive increase in pAVP from control levels of 1.3 +/- 0.19 microU/ml to 16.9 +/- 1.88 microU/ml(p less than 0.001) after 4 days without water. Log pAVP was linearly related to posm (r = 0.82, p less than 0.001). Mean level of CSF-AVP in control animals was 5.4 +/- 0.97 microU/ml. During dehydration, CSF-AVP levels also rose, becoming significantly greater than control levels (p less than 0.01) after 3 days without water and further increasing to reach 15.2 +/- 1.83 microU/ml after 4 days without water (p less than 0.001). Log CSF-AVP could also be linearly related to posm (r = 0.62, p less than 0.01). Termination of dehydration by restoration of ad libitum water supply was accompanied by return of pAVP and CSF-AVP to pre-dehydration levels. Regression analysis of concurrent levels of pAVP an CSF-AVP indicates that CSF-AVP levels are linearly correlated to pAVP levels in hydrated and dehydrated animals (r = 0.70, p less than 0.001). These experiments suggest that neurosecretory neurons secreting AVP at sites accessible to CSF respond to similar stimuli during dehydration as those neurons secreting AVP into blood.