Renal responses to acute volume expansion in young spontaneously hypertensive rats.

The renal responses to acute volume expansion (VE) were measured from intact and denervated kidneys in 5 week old spontaneously hypertensive rats (SHR). Urine flow and sodium excretion were measured before, during and after VE from innervated and denervated kidneys in anesthetized (Inactin--0.1 g/kg, ip) age and sex matched SHR and normotensive Wistar Kyoto (WKY) rats. Mean arterial pressure was 23 mm Hg higher in SHR than in WKY. During VE the increment in urinary flow rate and sodium excretion from both innervated and denervated kidneys were greater in SHR than WKY. In another group of SHR, renal perfusion pressure was maintained at a level similar to the arterial pressure in the WKY group (84 mm Hg). When renal perfusion pressure was controlled at the lower level in the SHR there was no longer the increase in diuresis and natriuresis in response to acute VE. Examining differences within a strain, the SHR demonstrate a greater increase in diuresis and natriuresis in response to VE in the absence of renal nerves, unlike the WKY. In conclusion, these results suggest that there is a greater diuresis and natriuresis in the young SHR due to increased renal perfusion pressure and the renal nerves produce a greater retention of water and sodium in SHR compared to WKY.