INCREASED Na CONCENTRATION IN CSF IS INVOLVED IN MAINTAINING ARTERIAL PRESSURE IN DEHYDRATED RATS.

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To analyze the mechanism of the greater decrease in mean arterial pressure (MAP) at a given decrease in [Na]csf (DELTAMAP/DELTA[Na]csf) in dehydrated rats, we measured [Na]csf with a Na sensitive microelectrode in the lateral ventricle (LV) and MAP during infusion (INF) of hypertonic arterial cerebrospinal fluid (ACSF) (-400 mOsm) into the LV of rats with and without sino-aortic denervation (SAD). [Na]csf in both groups increased significantly -1min after the start of INF and attained the maximal increases of 16.4<plus-minus>2.4 meq/kg H_2O in innervated rats and 20.5<plus-minus>1.3 meq/kg H_2 in SAD rats (n=10) at the end of INF.There was no significant difference in DELTA[Na]csf between the two groups throughout the experiment. On the other hand, MAP in SAD rats increased to a significantly higher level than in innervated rats from 6.0 to 17.0 min after the start of INF (p<0.05) and the increase at the end of INF was 19.0<plus-minus>2.3 mmHg and 7.7<plus-minus>1.4 mmHg in the SAD and innervated rats, respectively (p<0.01). Changes in MAP in both groups were highly correlated with DELTA[Na]csf throughout the experiment, but the slope was steeper by two-folds in SAD rats (r=0.97, p<0.001) than that in innervated rats (r=0.97, p<0.001) and was identical to that in dehydrated rats (r=0.96, p<0.001).To cralifiy the mechanism of the decrease in MAP during hypotonic INF into the LV of dehydrated rats, [Na]csf, MAP,CVP (mmHg), and cardiac output (CO,ml/min 100g) were measured in dehydrated rats when [Na]csf was gradually reduced to the pre-dehydrated level. Total vascular conductance (TVC, (ml/min 100g) /mmHg) was calculated as TVC=CO/ (MAP-CVP). MAP and CO decreased and TVC increased as [Na]csf decreased with high correlations (r^2=0.94-0.96, p<0.001).The increase in [Na]csf is important to maintain arterial pressure by causing systemic vasoconstriction and preventing the decrease in CO during dehydration.