Search

Micropuncture studies were carried out in the rat to evaluate the in situ distensibility characteristics of the proximal and distal tubules under a variety of experimental conditions. In the first phase, we determined the response of tubular diameter (D) to changes in tubular pressure (P) induced by partially obstructing single tubules. The response observed under these conditions (i.e., when interstitial pressure is presumed to be constant) has been defined as the compliance of the tubule. Over the range of tubular pressures studied (10-35 mm Hg for the proximal tubule, 5-25 mm Hg for the distal tubule) the compliance characteristics of the proximal and distal tubule were found to be markedly different; the proximal tubular pressure-diameter relationship was linear, DeltaD/DeltaP = 0.45 mum/mm Hg, whereas the distal pressure-diameter relationship was curvilinear, DeltaD/DeltaP = c(-0.1xP+2.2). In the second phase we used the compliance data to construct a series of theoretical pressure-diameter curves that define the response of the tubule to increments in interstitial as well as intratubular pressure. These curves indicate that changes in distal diameter should provide a sensitive index of a rise in interstitial pressure under conditions in which the transtubular pressure gradient is increased by a small amount, but that proximal diameter should provide a more sensitive index of changes in interstitial pressure when the transtubular pressure gradient is increased by a large amount. In subsequent experiments in which furosemide was administered, we observed that the pressure-diameter relationships for both the proximal and distal tubule were indistinguishable from the compliance curves, a finding consistent with the interpretation that interstitial pressure was not appreciably changed from control. By contrast, when mannitol was administered, both proximal and distal tubular pressure-diameter relationships were significantly altered in a fashion consistent with a large increase in interstitial pressure. Neither with furosemide nor mannitol administration did it appear likely that significant changes in tubular compliance could account for the observed behavior of the tubule.Finally, we propose that a knowledge of tubular compliance will be useful in exploring the interrelationships between tubular and peritubular pressures, tubular anatomy, and transtubular ionic permeability. Recent studies linking changes in the geometry of lateral intercellular spaces of the tubule to changes in passive ion movement suggest that an investigation of such anatomical-functional correlates should be productive.

To determine the sites of tubular phosphate reabsorption in the nephron, microinjection studies were undertaken, utilizing isotonic electrolyte solutions, containing either 1.4 or 8.0 mM phosphate and radioactive PO(4)-(33)P and inulin-(3)H, in rats made mildly diuretic by infusion of mannitol. The injected sites were localized by the technique of latex dissection. The relation between proximal tubular length and per cent (33)P recovery for injections of 1.4 mM phosphate (physiological amounts) suggest that relatively little reabsorption of phosphate occurs in the distal 30% of the proximal tubule compared with the proximal portion of the tubule. The corresponding recoveries for proximal tubular microinjections of 8.0 mM phosphate fall along a smooth curve tending to plateau with essentially complete (33)P recovery (> 95%) beyond 50% of the tubule. Absolute reabsorption of injected phosphate for both concentrations (i.e., absolute efflux per unit tubular length in the proximal tubule) was independent of phosphate delivery, since the relationship between reabsorption and site of injection was no different for the two concentrations. Distal convoluted tubular microinjections for both phosphate concentrations showed complete recovery of (33)P from all injection sites. THE DATA INDICATE THAT: (a) no phosphate reabsorption occurs in the distal convoluted tubule or in the collecting duct, (b) phosphate efflux per unit tubular length is greater in the first one-third of the proximal tubule than in the remaining two-thirds, and (c) in the last two-thirds of the proximal tubule, absolute phosphate reabsorption is relatively small and might be limited by factors other than the amount or concentration of injected phosphate.