THE EFFECT OF CHRONIC AND ACUTE CHANGES IN PLASMA COMPOSITION ON VASOPRESSIN SECRETION AND CEREBROSPINAL-FLUID IN THE RAT

The effects of chronic and acute changes in plasma composition on the osmolality and sodium concentration of cerebrospinal fluid and plasma vasopressin (AVP) concentration have been examined. Chronic elevation of plasma osmolality in three strains of genetically AVP-deficient rats (Brattleboro and New Zealand hypertensive and normotensive Brattleboro) was associated with increased cerebrospinal fluid osmolality by comparison with AVP-replete controls (Long Evans and New Zealand genetically hypertensive and normotensive rats). The linear correlation between plasma and cerebrospinal fluid osmolality did not reflect a similar relationship between plasma and cerebrospinal fluid sodium concentration. Hypertensive animals exhibited a threefold higher plasma AVP concentration in association with significantly elevated cerebrospinal fluid osmolality by comparison with normotensive controls. Although ip hypertonic saline injection elicited parallel increases in plasma and cerebrospinal fluid osmolality and sodium concentration in both hypertensive and normotensive rats, only in the normotensives did this result in an increase in plasma AVP concentration. These results indicate that cerebrospinal fluid is subject to modest chronic and acute changes in osmolality and sodium concentration which may contribute to the osmotic control of AVP secretion. The disturbed control of vasopressin secretion in hypertensive rats may in part be related to the abnormal cerebrospinal fluid composition in these animals.