The coincidental effects of dehydration and rehydration on plasma and cerebrospinal fluid angiotensin II levels in unrestrained steers.

Changes in plasma (pAII) and cerebrospinal fluid (csf AII) immunoreactive angiotensin II have been measured in 6 unrestrained steers during dehydration and rehydration. Prior to dehydration, plasma osmolarity (pOsm) was 301.3 +/- 0.62 mOsm/l (mean +/- S.E.M.) and on dehydration progressive elevation of pOsm developed so that after 4 days it was 338.5 +/- 3.00 mOsm/l (P less than 0.001). Packed cell volume was elevated during dehydration from 39.9 +/- 0.64% to 44.7 +/- 1.24% (P less than 0.001). Radioimmunoassay (RIA) of pAII indicated a similar pattern of progressive elevation during dehydration when after 4 days pAII had risen from 54.7 +/- 11.6 fM/ml to 177.6 +/- 18.4 fM/ml (P less than 0.001). RIA of csf AII, however, indicated no complementary pattern of change during dehydration, the only significant alteration from basal levels of 166.0 +/- 13.2 fM/ml occurred after 2 days when csf AII was reduced to 83.1 +/- 14.5 fM/ml (P less than 0.01). Antiserum specificity tests indicated that some of the csf AII could have been due to metabolites of AII, and high performance liquid chromatographic separation of angiotensin peptides in csf suggested that the major contributants were AII and AIII. These analogues which may have central neural effects were not consistently elevated during dehydration of cattle. It is concluded therefore, that the role of angiotensin in central neural mechanisms of fluid balance in dehydrated animals depends primarily on the systemic renin-angiotensin system and not an endogenous brain renin-angiotensin system.