Abstract
1. Simultaneous determination of the single nephron glomerular filtration rate (s.n.g.f.r.) and the amount of [14C]ferrocyanide contained per unit length (Fe/mm) of the proximal tubule makes it possible to calculate the tubular radius and flow velocity along this structure. These measurements were made in normal rats, chronically salt-loaded rats, and rats rendered hypotensive by controlled haemorrhage or aortic constriction. 2. In normal rats, Fe/mm remains constant along the proximal tubule, indicating a gradual decrease in tubular radius and constant flow velocity. 3. In salt-loaded rats, with high s.n.g.f.r.s, Fe/mm also remains constant along the proximal tubule, but at a higher level than in normal rats. This indicates larger tubular radii, but the same evolutionary pattern for tubular radius and flow velocity along the proximal tubule, as in normal rats. 4. In rats with controlled haemorrhage, Fe/mm values are low but rise slightly along the proximal tubule. Tubular radius is reduced, and again, gradually decreases along the length. Considering the very low s.n.g.f.r.s, this implies that a large drop in reabsorption along the proximal tubule accompanies the reduction in s.n.g.f.r. 5. In rats with aortic construction, Fe/mm exhibits a large increase along the proximal tubule, suggesting a huge Fe concentration along the tubule, which in turn would indicate that water reabsorption may not diminish in proportion to the reduction in s.n.g.f.r. In some cases, this disruption of the glomerulo-tubular balance led to the cessation of tubular flow in the proximal tubule. 6. In all cases, Fe/mm and the calculated radius in the initial portion of the proximal tubule were correlated to the s.n.g.f.r., suggesting passive adaptation of tubular radius to fluid delivery.