Sensitivity of the segmental renal arterioles to angiotensin II in the aging rat.

With advancing age the old rat kidney becomes tonically vasoconstricted by endogenous angiotensin II (ANGII) (C. Baylis. Am. J. Kid. Dis., (1993) 842). The present study was designed to investigate the sensitivity of the cortical glomerular microvasculature of the old rat kidney (19-22 months of age) to exogenous ANGII, using the in vivo micropuncture technique. In the baseline state, glomerular blood pressure (P(GC)) in old male rate was higher compared to young rats (4-5 months of age). During exogenous ANGII infusion (40 ng/kg/min), a significant rise in arterial blood pressure and renal vasoconstriction occurred in both young and old rats. In young rats, the ANGII induced fall in renal plasma flow (RPF) and glomerular plasma flow (QA) was accompanied by a rise in PGC and thus the glomerular hydrostatic pressure gradient, with little change in Kf. Therefore, the glomerular filtration rate (GFR) and single nephron GFR (SNGFR) were unchanged by ANGII infusion in young rate. In old rats, RPF and QA fell, a rise occurred in PGC and also a fall was seen in the glomerular capillary ultrafiltration coefficient (Kf), thus GFR and SNGFR fell significantly. The magnitude of the pressor and renal vasoconstriction response to ANGII were not affected by age; of interest, ANGII increased preglomerular and efferent arteriolar resistance (RA, RE) and PGC by similar accounts in young and old rats. SNGFR was reduced in old rats, due to the marked ANGII-induced decline in Kf. Neither absolute nor fractional proximal reabsorbtion were affected by ANGII infusion in either young or old rats. by 19-22 months of age, old rats had much more injured glomeruli than young rats. These data demonstrate that the afferent and efferent arterioles had similar sensitivity to exogenous pressor dose of ANGII in both young and old rats, but Kf was more sensitive to ANGII in old rats leading to a significant fall in SNGFR.