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

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Abstract
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.