Renal nerves and renal responses to head-up tilt in dogs

Experiments were performed in anesthetized dogs to compare the effects of acute and chronic unilateral renal denervation on the renal responses to head-up tilt and to assess denervation hypersensitivity to infused norepinephrine (NE). Responses of the denervated kidney were compared with those of the contralateral innervated kidney in each animal. With acute denervation, 40 min of 45 degrees head-up tilt decreased urine flow (V) 37%, absolute sodium excretion (UNaV) 53%, and fractional sodium excretion (FENa+) 44% in the innervated kidneys, but no decreases occurred in the denervated kidneys. NE infusion (125 ng X kg-1 X min-1) increased arterial pressure by 11 mmHg and increased V, UNaV, and FENa+ in both kidneys. In the chronically denervated animals (2-4 wk prior to experiment) tilt decreased V by 32%, UNaV by 44%, and FENa+ by 21% in the innervated kidneys, but again no changes occurred in the denervated kidneys. NE infusion in this group also increased arterial pressure approximately 11 mmHg and caused V, UNaV, and FENa+ to increase in the innervated kidneys but decrease in the denervated kidneys. These results demonstrate that the renal responses to tilt are abolished by both acute and chronic renal denervation even though the chronically denervated kidney is hypersensitive to NE-stimulated fluid reabsorption. Therefore endogenous plasma NE levels must not increase enough during tilt such that this hypersensitivity phenomenon can compensate for chronic ablation of the renal nerves.