Relationship between cerebrospinal fluid pressure and plasmatic ADH

In a healthy human being, the extracellular volume is kept constant by home ostatic systems. One of these is represented by the antidiuretic hormone (A DH). ADH release is modulated by osmoreceptors and baroreceptors which resp ond to an increase in osmolality of extracellular fluid and a decrease in b lood volume, respectively. In previous studies we investigated the existenc e of additional structures sensitive to plasma volume modifications. We fou nd evidence of the presence of such receptors in the inner ear, with nervou s connections to supraoptic and paraventricular nuclei. However, the possib ility that the cerebral ventricle wall contained stretch sensors could not be excluded. To test our hypothesis, we studied 19 rats divided into three groups: Group 1 (n = 7), Group 2 (n = 7) and Group 3 (control group n = 5). In each rat, under total anaesthesia, a femoral cannula was inserted into the left artery and a 22 gauge stainless steel cannula was implanted into t he left cerebral ventricle. In the first group an isotonic fluid, similar t o the animal's cerebrospinal fluid (CSF), was infused intracerebroventricul arly (ICV) at a rate of 0.6 mu l min(-1) continuously for 6 h. In the secon d group, under the same conditions, CSF was aspirated; the third group was used as the control. In all animals, plasma modifications of ADH (pADH), os molality (pOSM), Na+ (pNa(+)) and K+ (pK(+)) were evaluated before and afte r the experimental procedures. Mean arterial pressure (MAP) and heart rate (HR) were recorded throughout the experiment. At the end of the experiment no significant changes in pNa(+), pK(+), MAP and HR were observed. Plasma o smolality was significantly lower in Group 2 before and during the experime ntal procedure, since we deliberately expanded the volume in animals of Gro up 2 to partially suppress ADH, in order to evaluate its modifications. Pla sma ADH fell in the first experimental group (-37.4% +/- 6.3 SEM) after the ventricular pressure had been increased, and rose in the second (+ 47.3% /- 14.7 SEM) after ventricular decompression. These changes were statistica lly significant in comparison with those occurring in control subjects (- 0 .9 +/- 18.9 SEM; P = 0.07 and P = 0.03, respectively). These results sugges t the presence of additional volume receptors probably located in the cereb ral ventricles, capable of controlling ADH. The importance of these recepto rs in physiological situations of plasma volume contraction or expansion re mains to be established. (C) 1999 Academic Press.