Effects of norepinephrine on renal function in chronically hypoxic rats

The sympathetic nervous system is activated in response to altitude hypoxia and activation of renal sympathetic nerves may cause vasoconstriction and fluid retention. However, renal excretion does not differ significantly between rats exposed to high altitude hypoxia and control rats. We hypothesize that renal response to norepinephrine (NE) is altered after chronic hypoxia. Female Wistar rats weighing 200 to 220 g were exposed to hypoxia in an altitude chamber (5,500 m, 380 torr) 15 hours/day for 4 weeks (HA, high altitude). Our findings showed that systemic infusion of NE (300 μg · kg^-1 · hr^-1) produced less diuresis/natriuresis in HA rats that in sea level (SL) controls. With mechanical elevation of arterial blood pressure, both SL and HA rats showed no significant difference in their response to pressure diuresis. Direct intrarenal arterial NE (10 μg · kg^{- 1} · hr^-1) administration reduced renal function more in HA rats than in SL rats. Intrarenal arterial administration of L-arginine (100 μg · kg^-1 · hr^-1) did not alter the renal action of NE in HA rats. However, with intrarenal arterial infusion of phosphoramidon (100 μg · kg^-1 · hr^{- 1}), NE increased renal response in HA rats to almost the same level as that in SL rats. These results suggest that HA rats may have either an excess renal action of antidiuretic and antinatriuretic factors or an insufficient renal action of diuretic and natriuretic factors during NE administration.