Age-dependent alterations in the structurally determined vascular resistance, pre- to postglomerular resistance ratio and glomerular filtration capacity in kidneys, as studied in aging normotensive rats and spontaneously hypertensive rats.

Maximally dilated renal vascular beds of 13-month-old NCR and SHR were compared to explore how aging respectively longstanding primary hypertension structurally alters total renal resistance, pre/postglomerular resistance ratio and maximal glomerular filtration capacity, as measured per unit kidney weight. According to comparisons of 1.5- and 3.5-month-old NCR and SHR (Folkow et al. 1977), a structurally increased pre/postglomerular resistance ratio rapidly resets the renal "longterm barostat function" in SHR to match the 30-40% pressure rise, thereby increasing total renal resistance 15-20%, while filtration capacity is unaltered so far. In NCR aging to 13 months hardly alters arterial pressure, but increases total renal resistance 10-15%, mainly affecting postglomerular vessels, while filtration capacity is reduced 25%. 13-month-old SHR show an additional 15% pressure rise and--relative to agematched NCR--a further 35% reduction of filtration capacity with a 30-35% increase of total renal resistance, which mainly affects the postglomerular vessels as the resistance ratio is now barely above that in NCR. Thus, advancing SHR hypertension seems to start a renal vicious circle, because accentuated reductions of filtration capacity are parallelled by structural postglomerular resistance increases apparently to maintain GFR by raised filtration pressure which, however, accelerates glomerular deterioration.