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.