Continuous determination of Na concentration in CSF during recovery from thermal dehydration in rats.

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To determine changes in Na concentration in the cerebrospinal fluid([Na]csf)during restitution from thermal dehydration, we developed a method to measure[Na]csf continuously in rats using a double barreled Na sensitive electrode. The Na sensitive membrane was prepared by immobilizing an ionexchanger(Bis 12-crown-4). The confidence limit for measuring Na concentration was <plus-minus>0.5 meq/kg H20 within physiological range. We placed the electrode in the right lateral ventricle of an anesthetized rat thermally dehydrated by about 11% of body wt. Na concentration in plasma([Na]p)was measured by a flow cell typed Na sensitive electrode placed in an extracorporeal by-pass circuit between the left carotid artery and right jugular vein. We measured changes in[Na]p and[Na]csf continuously during the infusion of 2.5 ml/ lOOg body wt of distilled water for 10 min and following 20 min after the end of infusion. [Na]p began to decrease 3 min after the start of infusion and attained the lowest value of -11.6<plus-minus>0.9 meq/kg H_2O at 6 min after the end of infusion. This level was maintained until the end of experiment. [Na]csf decreased slowly after the start of infusion and at the end of experiment the change was -5.9<plus-minus>0.5 meq/kg H_2O. To assess the change in water and/or Na ion movement between blood and CSF in dehydrated rats, the response time of[Na]esf to change in[Na]p was compared between dehydrated(D)and enhydrated rats(E). The response time in E was 5.3<plus-minus>0.6 min which was significantly larger than in D, 2.0<plus-minus>0.5 min(p<0.01). These results suggest that[Na]csf showed attenuated and delayed response against acute change in[Na]p and recovered more slowly than[Na]p during rehydration. In addition. the movement of water and/or Na between blood and CSF was accelerated during rehydration.