Reduction of cerebral NaCl concentration can abolish mineralocorticoid escape.

The effect of lowering cerebrospinal fluid (CSF) Na concentration on renal Na excretion (UNaV) was investigated in conscious sheep undergoing mineralocorticoid escape induced by intravenous infusion of aldosterone (20 micrograms.ml-1.h-1) for 3 days. On the 3rd day of aldosterone administration, when plasma and CSF Na concentration and mean arterial blood pressure (MABP) were increased as a result of the mineralocorticoid treatment, a reduction in the CSF Na concentration was induced by infusing a Na-free solution of 300 mmol/l mannitol (1 ml/h) into a lateral cerebral ventricle. This caused significant reductions in UNaV and MABP and a significant increase in renal free water clearance (CH2O). There was no significant change in glomerular filtration rate or plasma atrial natriuretic peptide concentration, but renal lithium clearance decreased. Simultaneous intravenous infusion of vasopressin (0.03 microgram/h) and lowering of CSF Na concentration also caused significant reductions in UNaV and MABP, but CH2O did not increase. We propose that increased Na concentration of brain fluid may initiate natriuretic and pressor mechanisms contributing to the process of mineralocorticoid escape. Reduced UNaV may have been due to reduced MABP, but it is unlikely to have been due to reduced plasma vasopressin levels.