Reducing brain sodium concentration prevents post-prandial and dehydration-induced natriuresis in sheep

Renal Na excretion during the 24 h following feeding was studied in sheep. A pronounced natriuresis occurred 3.5-5.5 h after feeding. Na excretion then fell to low levels in animals allowed to drink water, but was significantly elevated above this level in water-deprived sheep for most of the remaining period. Both the post-prandial and dehydration-induced natriuresis were prevented by intracerebroventricular (icv) infusions of low Na concentration 0.3 mol l-1 mannitol at 1 ml h-1, and a water diuresis also occurred. These effects were not caused by icv infusion of artificial cerebrospinal fluid (Na concentration = 150 mmol l-1). As a result, there was a much greater increase in plasma Na concentration and osmolality in the sheep given icv mannitol. Intravenous infusion of vasopressin prevented the water diuresis induced by icv mannitol, but the inhibition of natriuresis was still observed and plasma Na concentration increased by 8 mmol l-1 over 24 h compared with an increase of 3 mmol l-1 in dehydrated sheep infused icv with artificial cerebrospinal fluid. The results show that the ambient Na concentration in the brain plays an important role in the normal homeostatic regulation of Na balance by the kidney in sheep.