1. The chronically clipped left kidneys of two-kidney, one-clip renal hypertensive rats (duration >4 weeks) showed a 45% decrease in organ weight compared with left kidneys from age-matched normotensive Wistar rats (0.59 ± 0.09 vs 1.12 ± 0.05 g).

2. These ‘low-pressure’ kidneys exhibited at maximal vasodilatation a 40% decrease in total renal vascular resistance per unit kidney weight, a reduction in the preglomerular/postglomerular resistance ratio (filtration curve displaced markedly to the left of the control), though combined with some decrease in maximal glomerular filtration capacity.

3. Thus the vascular bed of chronically clipped low-pressure kidneys displays a ‘downward’ structural autoregulation, which by lowering of the total renal vascular resistance and the preglomerular/postglomerular resistance ratio serves to maintain blood flow and to increase filtration pressure thereby raising the filtration fraction.

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