R. Greger

Stationary microperfusion study of phosphate reabsorption in proximal and distal nephron segments

SummaryMicropuncture studies demonstrate phosphate reabsorption in proximal tubules and between the late proximal and early distal convoluted tubule accessible to micropuncture. To further define the sites of phosphate reabsorption, the stationary microperfusion technique was applied to proximal and distal nephron segments. Phosphate reabsorption was evaluated in superficial loops of proximal tubules, descending segments beyond late proximal tubules accessible to micropuncture, ascending segments up to the point of micropuncture in the distal tubule, and superficial loops of distal tubules of thyroparathyroidectomized rats. Microperfusates of 1.3 or 2.6 nl (100 mmol/l mannitol, 100 mmol/l NaCl,32P-phosphate and3H-inulin) were injected and then withdrawn after contact times of 2–108 s. Phosphate recovery relative to that of inulin was determined. A steep exponential decline of phosphate recovery (R) with increasing contact time (t) was observed in the superficial proximal tubule and descending segments. The slopes of the logarithmic regressions (10logR)/t, ±SEM) were: −1.68±0.33 and −1.21±0.24 min−1 in superficial proximal tubules and descending segments respectively. In contrast, no significant decline in phosphate recoveries (−0.02±0.04 and +0.11±0.10 min−1) was apparent in the ascending segments and distal tubule. It is concluded that phosphate is reabsorbed in the proximal convoluted tubule and adjacent descending segments of the superficial nephron and that there is no significant phosphate reabsorption in distal convoluted tubules and adjacent ascending segments.

Site of renal phosphate reabsorption

SummaryUsing modified microinfusion and free flow micropuncture techniques in the same intact and acutely thyroparathyroidectomized (TPTX) Munich-Wistar rats the nephron sites for phosphate reabsorption were reinvestigated. In intact animals, 62% of filtered phosphate was reabsorbed in the proximal tubule but none in the loop of Henle, here defined as the nephron segment between the last accessible proximal and the first distal convolution. Delivery of phosphate to the superficial distal tubule significantly exceeded urinary phosphate excretion but no phosphate reabsorption could be detected in the terminal nephron by distal microinfusions of radioactively labelled phosphate (32P). In TPTX rats, proximal phosphate reabsorption was enhanced and there was marked phosphate reabsorption in the loop of Henle. Similarly,32P microinfused in the late proximal tubule was almost completely reabsorbed. Again, no phosphate tracer outflux was detected after distal microinfusion. It is concluded that phosphate reabsorption is confined to the proximal tubule and the loop of Henle.

Saturation kinetics of phosphate reabsorption in rats.

It is a well known fact that increasing plasma concentration of phosphate is followed by enhancement of phosphate clearance, a finding which has been ascribed to saturation of phosphate reab-sorption. However, although it has been possible to determine a maximal transport rate for a given experimental situation, further description of the kinetics has not been possible with the use of clearance techniques.

Renal Handling of Phosphate: Update

Phosphate is predominantely reabsorbed in the proximal tubule (1, 2) and this reabsorption is inhibited by parathyroid hormone (PTH) (3, 4). However, in both normal and TPTX animals, phosphate delivery beyond the point of micropuncture in the late proximal tubule exceeds that excreted in the urine (3–11). Furthermore, most investigators report an increase in delivery of phosphate from the proximal tubule but only a moderate phosphaturia after saline expansion in TPTX animals (3, 4, 6, 11). This blunting of the saline induced phosphaturia in TPTX animals has been interpreted as evidence for distal phosphate reabsorption. Similarly, the phosphaturia induced by acetazolamide is also blunted in TPTX animals. From these studies it was concluded that PTH, although it has effects similar to acetazolamide on the proximal tubule, also has a more distal site of action (10, 12). Since these latter conclusions were derived from micropuncture studies where only proximal tubules were punctured, the site of altered phosphate reabsorption could be in any segment beyond the point of micropuncture.

Factors Involved in the Altered Phosphate Reabsorption During Phosphate Loading in Thyroparathyroidectomized Rats

Several investigators (3,4,5,9) observed a decline of renal phosphate reabsorption after prolonged intravenous infusion of phosphate. Similarily high phosphate diets are known to produce phosphaturia and phosphate depletion enhances renal phosphate reabsorption (11,12). The decline of renal phosphate reabsorption during acute or chronic phosphate loading was demonstrated in both, intact and thyroparathyroidectomized (TPTX) rats. Irrespective of the potentially different mechanisms involved in acute and chronic phosphate loading, the kidney appears to be capable to adjust phosphate reabsorption to phosphate input even in the absence of parathyroid hormone.

Sites of Urate Transport in the Rat Nephron

The rat has been used frequently for studying renal urate excretion (1–14, 16). This is clearly correlated with the fact that the overall renal handling of urate in this animal is comparable to that in man (5). Previous clearance studies have revealed that the net reabsorption of urate in the mammalian nephron is the result of bidirectional tubular transport (17). Recently several investigators have applied different micropuncture and microanalytical methods to the rat model to further elucidate the complicated pattern of bidirectional tubular transport (1, 3, 4, 7–10, 13, 14, 16). As a result of these studies, it is generally accepted that both a secretory and a reabsorptive mechanism for urate exist in the proximal tubule. In the distal nephron very little, if any, urate is transported. However, some controversy still remains concerning the quantitative correlation of reabsorption and secretion in the proximal tubule and the quantitative importance of the post-proximal reabsorption, i.e. loop reabsorption (14).

Additivity of the phosphaturic action of parathyrin and calcitonin in the rat kidney.

Several investigations performed in man as well as in different animal species have been carried out to define the effects of parathyrin and calcitonin on renal phosphate transport.

Factors affecting urate reabsorption in the rat kidney.

1. Urate transport in the rat appears to be saturable. However, affinity of the transport system for urate is very low and transport far from saturated at physiological plasma concentrations. 2. Since increase of the nonionized fraction of uric acid by a factor of five failed to increase urate reabsorption, transport cannot be due to nonionic diffusion but rather involves ionized urate. 3. Increases in luminal flow rate markedly depress urate reabsorption in the loop of Henle, which results in wash out of medullary urate.

Outflux of 45calcium along the rat nephron.

From previous micropuncture data (1,4–6,9–15,17,19) it was postulated that calcium is reabsorbed in the proximal tubule and in addition in the distal tubule and/or collecting duct. This was concluded by comparing the amount of calcium present at the distal puncture site and that in final urine. Although this discrepancy between distal and urinary calcium load as detected by free flow micropuncture and clearance techniques might reflect distal reabsorption of calcium, it is no direct prove for that. A similar result might be obtained if there was nephron heterogeneity in regard to calcium reabsorption with the implication that deeper nephrons reabsorb calcium more avidly than superficial ones.

The postproximal site of phosphate reabsorption in presence and absence of parathyroid hormone.

From previous studies there is ample evidence that phosphate, in addition to its proximal reabsorption is reabsorbed at some postproximal site (1–3,5,6,8,9,11). In free flow micropuncture studies this reabsorption has at least partially been ascribed to the distal tubule and collecting duct system (1,5). In microinjection studies, on the other hand, no reabsorption of phosphate beyond the loop of Henle could be detected (4,10). These data, however, are methodologically not unequivocal since the phosphate load injected exceeded the amount of phosphate which ought to be expected there under conditions of free flowing tubular fluid. The free flow micropuncture data also are subject to restricted interpretation since it seems likely that nephron heterogeneity in regard to phosphate reabsorption exists between superficial cortical and deeper nephrons.