The influence of sampling technique on the micropuncture determination of GFR and reabsorptive characteristics of single rat proximal tubules

Flow resistance along the rat renal tubule

The Reynolds number in the renal tubule is extremely low, consistent with laminar flow. Consequently, luminal flow can be described by the Hagen-Poiseuille laminar flow equation. This equation calculates the volumetric flow rate from the axial pressure gradient and flow resistance, which is dependent on the length and diameter of each renal tubule segment. Our goal was to calculate the pressure drop along each segment of the renal tubule and to determine the points of highest resistance. When the Hagen-Poiseuille equation was used for rat superficial nephrons based on known tubule flow rates, lengths, and diameters, it was found that the maximum pressure drop occurred in two segments: the thin descending limbs of Henle and the inner medullary collecting ducts. The high resistance in the thin descending limbs is due to their small diameters. The steep pressure drop observed in the inner medullary collecting ducts is due to the convergent structure of the tubules, which channels flow into fewer and fewer tubules toward the papillary tip. For short-looped nephrons, the calculated glomerular capsular pressure matched measured values, even with the high collecting duct flow rates seen in water diuresis, provided that tubule compliance was taken into account. In long-looped nephrons, the greater length of thin limb segments is likely compensated for by a larger luminal diameter. Simulation of the effect of proximal diuretics, namely acetazolamide or type 2 sodium-glucose transporter inhibitors, predicts a substantial back pressure in Bowman's capsule, which may contribute to observed decreases in glomerular filtration rate.

Micropuncture of the kidney: a primer on techniques

Techniques to evaluate renal function at the single nephron level have been instrumental and indispensible in furthering our understanding of the mammalian kidney. Techniques that were first introduce in the 1920s, and later refined in the 1950s and 1960s, permit sophisticated interrogation of glomerular filtration and hemodynamics, and tubular epithelial transport activity. Much of what we know about the physiology and pathophysiology of the kidney has been produced or, to some degree, confirmed by renal micropuncture. While micropuncture is perhaps not as widely employed as before, it remains an essential tool for comprehensive evaluation of kidney function, particularly in this age of genetically pliable experimental models. This review aims to provide a introduction to common methodologies and approaches used to conduct micropuncture experiments. Topics covered include instrumentation and equipment, pipet fabrication techniques, animal preparation, and experimental procedures for evaluating single nephron hemodynamics and tubular function.

The effect of reducing proximal tubular fluid delivery on the rate of filtration of single nephrons

The rate of delivery of tubular fluid from the proximal tubule (PT) is thought to reset nephron filtration rate (SNGFR). In micropuncture experiments in rats we tested this hypothesis by reducing the efflux from the PT by simultaneously "double collecting" (DC) tubular fluid from the early distal tubule (DT) and from the last convolution of the PT of the same nephrons. SNGFR measured by total collection of tubular fluid was 34 +/- 3 nl/min at the DT and 34 +/- 3 nl/min at the PT (p > 0.97, n = 42). The simultaneous collection from proximal and distal sampling site was performed between these two paired measurements. It yielded an average SNGFR of 40 +/- 3 nl/min (p < 0.02). This may be due to the collection, at the distal site, of the extra amount of inulin stored between distal and proximal pipette, prior to starting the aspiration of tubular fluid. Since this error would decrease in longer collections, the difference in SNGFR between single and double collections was plotted against the duration of collections. In fact it was negatively correlated with the sampling time (p < 0.01), indicating no difference in SNGFRs for collections > 4 minutes. Reduction and complete interruption of the delivery of native proximal tubular fluid to the Macula Densa does not seem to influence the measurement of SNGFR. Filtration rate is not significantly different when measured within few minutes at the DT and PT of the same nephrons.

Role of tubular obstruction in acute renal failure due to gentamicin

Gentamicin sulfate was administered by intraperitoneal injection to male Sprague-Dawley rats in a dose of 100 to 120 mg/kg/day for 4 to 5 days to induce severe nephrotoxicity. In comparison to controls, inulin clearance was markedly decreased (2.87 +/- 0.31 vs. 8.65 +/- 0.31 ml/min/kg, P less than 0.001) as was urinary osmolality (462 +/- 36 vs. 1196 +/- 46, P less than 0.001). Surface tubules appeared heterogeneous. Some were plugged by whitish debris, whereas others were markedly dilated (I.D. = 41.5 +/- 2 mu). All other tubules were moderately dilated (I.D. = 28.8 vs. 20 mu). The microinfusion of tubules with cellular debris with an isotonic "equilibrium" solution resulted in a rise in intratubular pressure to as high as 60 to 80 mm Hg, compared with 13 to 15 mm Hg in normal rats. In better functioning nephrons, free-flow pressure (FFP) was increased significantly (16.0 +/- 0.5 vs. 10.2 +/- 0.1 mm Hg, P less than 0.001). Paired measurements of single nephron glomerular filtration rate (SNGFR) in these nephrons, made while monitoring intratubular pressure (ITP), revealed a rise in SNGFR when ITP was lowered from the initially high level to 10 mm Hg. Comparable changes in SNGFR were induced in normal rats by varying ITP from 10 to 15 mm Hg. The data suggest that in severe gentamicin nephrotoxicity, many cortical nephrons may be contributing very little to excretory function, presumably because of intratubular obstruction. The less impaired nephrons have reduced SNGFR, due in part to increased free-flow pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Internephron heterogeneity of filtration fraction and disparity between protein- and hematocrit-derived values

The variability of single nephron filtration fraction (SNFF) and the relationship between filtration fractions determined simultaneously from 125I albumin (SNFFALB) and hematocrit (SNFFHCT) values were assessed by multiple samplings, 5 to 8 per kidney, in normal, hydropenic, Munich-Wistar rats. Single nephron filtration rate (SNGFR) measurements served as an independent marker of internephron heterogeneity. Whether derived as SNFFALB or SNFFHCT, a wide degree of internephron variability in SNFF, comparable to that found for SNGFR (P greater than 0.2) was observed in all kidneys. Hematocrit-derived SNFF values, however, were some 80% higher than those obtained with 125I albumin (SNFFHCT 0.447 +/- SEM 0.016 versus SNFFALB 0.258 +/- 0.009, P less than 0.001). The disparity in results obtained by the two methods disappears if the efferent arteriolar hematocrit (Hctg) is assigned a value 17 +/- SEM 1.4% higher than the corresponding femoral arterial hematocrit and might well be a reflection of the perfusion of superficial glomeruli with erythrocyte rich blood produced by the "plasma-skimming" phenomenon. The data suggest that multiple estimates of SNGFR and SNFF in a given rat kidney are needed to provide representative mean values for glomerular dynamic studies.

Effect of isotonic volume expansion on glomerular filtration rate and renal hemodynamics in the developing rat kidney

Young rats (20-24 days) and adult rats (40-45 days) were studied during hydropenia and volume expansion with regard to glomerular filtration rate (GFR) and the determinants of GFR. During hydropenia, GFR and renal blood flow (RBF) were significantly lower in younger than in adult rats both in absolute terms and when related to bodyweight. Equivalent degrees of volume expansion (6% of b.wt.) resulted in a much more pronounced increase in GFR and RBF in younger than in older rats. This suggests that the high renal vascular resistance in hydropenic young rats is primarily due to vasoconstriction. The relationship between the filtration rate of superficial nephrons and the total GFR was the same in hydropenic and volume expanded rats in both age groups. The tubular stop flow pressure, the calculated hydrostatic glomerular capillary pressure and ultrafiltration pressure in the afferent part of the glomerular capillaries was slightly lower in hydropenic young rats than in hydropenic adult rats. The pressures did not rise after volume expansion. It is concluded that the marked increase in GFR in volume expanded young rats is mainly due to increased renal plasma flow.

Autoregulation of superficial nephron function in the alloperfused dog kidney

Isolated dog kidneys were each pump-perfused by another dog during 4 experimental periods at perfusion pressures (PP) of 21, 17, 13, and 8 kPa, resp. (i.e. 160, 130, 94, and 60 mm Hg). At the 3 highest PP values, the total kidney renal blood flow (RBF) and glomerular filtration rate (GFR) were perfectly autoregulated while at the lowest value both values were significantly lowered. No significant difference was observed between the single nephron GFR (SNGFR) of periods 1 and 2; in period 3 (PP = 13 kPa) a lower value was observed (P less than 0.05). Free flow pressure in proximal convolution (FFP), stop-flow pressure (SFP), and peritubular capillary pressure (PCP) were not different in period 2 than in period 1, but were significantly lower in period 3 (P = 0.02--0.05). Effective filtration pressure (EFP) was the highest in period 1, decreasing significantly with decreasing PP. Filtration pressure equilibrium was observed in period 4 at PP 8 kPa. Total blood flow resistance (RT) fell with decreasing PP, the drop being due to a steep decline in afferent resistance (RA). Efferent resistance (RE) increased as PP decreased. Ultrafiltration coefficient (Kf) rose with declining PP both within and outside the autoregulatory range. The results indicate that the lower limit of autoregulation is higher in superficial nephrons than in the whole kidney.

Micropuncture studies of sodium transport in the remnant kidney of the dog. The effect of graded volume expansion

Proximal and distal tubule micropuncture studies were performed to examine the response to graded extracellular volume (ECV) expansion in 10 normal dogs (stage I), 11 dogs with a unilateral remnant kidney (stage II), and 7 dogs with a remnant kidney after removal of the contralateral kidney (stage III). Before ECV expansion in stage III, there was a suggestive reduction in proximal tubule as well as loop fractional reabsorption of sodium. After ECV expansion to 3% body weight proximal tubule reabsorption was depressed in all groups of animals, while little further inhibition was observed in this segment with additional expansion to 10% body weight. In contrast, the fraction of filtered sodium remaining in the distal tubule rose progressively in all three groups after graded ECV expansion, suggesting that the graded natriuretic response found in the final urine was largely due to a similar response in the loop of Henle rather than that in the proximal tubule. The distal tubule response of the remnant kidney in both stages II and III was greater than that in stage I. These data indicate that although enhanced sodium excretion per nephron in chronic renal failure may be related to uremia, its exaggerated response to ECV expansion is due, at least in part, to certain as yet unidentified intrarenal factors consequent to reduction in functioning renal mass.

Proximal tubular function in dogs with thoracic caval constriction

The effect of saline infusion on proximal sodium reabsorption was compared in normal dogs and in dogs with acute or chronic partial thoracic vena cava obstruction. After acute vena cava obstruction, proximal fractional sodium reabsorption rose by 74%. During continued caval obstruction, saline loading strikingly reduced proximal reabsorption but sodium excretion remained minimal. In chronic caval dogs, saline loading reduced proximal fractional sodium reabsorption by 31% but sodium excretion in the micropunctured kidney was only 41 muEq/min. After saline infusion in normal dogs, proximal fractional sodium reabsorption fell 39% while unilateral sodium excretion rose to 584 muEq/min. Nephron filtration rate was also measured before and after saline loading in normal and chronic caval dogs in both repunctured and fresh tubules. There was a marked increase in nephron filtration rate in repunctured tubules and no change in freshly punctured tubules in both groups. The effect of saline loading on nephron filtration rate in normal and chronic caval dogs was similar, therefore, whether repunctured or fresh nephrons were considered.We conclude that saline infusion depresses proximal sodium reabsorption in acute and chronic TVC dogs. Since saline loading markedly increases distal delivery without a concomitant natriuresis, enhanced distal reabsorption must play a major role in the sodium retention exhibited by chronic caval dogs. Redistribution of filtrate does not appear to be a factor in this sodium retention.

The relative contributions of reabsorptive rate and redistributed nephron filtration rate to changes in proximal tubular fractional reabsorption during acute saline infusion and aortic constriction in the rat

The absolute rate of reabsorption by superficial rat proximal tubules was measured by the in situ microperfusion technique under conditions of hydropenia, infusion of saline, and infusion of saline plus aortic constriction sufficient to decrease whole kidney filtration rate below hydropenic levels. Fractional reabsorption was measured in adjacent filtering nephrons by collecting and recollecting tubular fluid from late proximal convolutions during each experimental condition. During hydropenia, the absolute rate of proximal tubular reabsorption averaged 3.56 +/-0.60 nl/min per mm and late proximal tubular fractional reabsorption averaged 0.56 +/-0.10. From these two measurements and measurements of tubule length to the site of micropuncture, a value for filtration rate was calculated for filtering nephrons. During hydropenia this value averaged 32.9 +/-7.1 nl/min. Saline infusion increased sodium excretion to 5.5% of the filtered load as the absolute rate of proximal tubular reabsorption decreased 38% and fractional reabsorption decreased 45%. Calculated superficial nephron filtration rate increased 21% which on the average was identical with the simultaneously measured increase in whole kidney filtration rate. Similar results were obtained in a separate group of animals by the technique of total collection of late proximal tubular fluid. Aortic constriction during saline infusion decreased whole kidney and calculated nephron filtration rate to the same degree and to values lower than those during hydropenia. Fractional reabsorption increased but not to hydropenic values. The persistent natriuresis during aortic constriction was associated with a continued depression of the absolute rate of proximal tubular reabsorption which was sufficient to maintain an increased delivery of filtrate out of the proximal tubule despite the fall in nephron filtration rate. These results indicate that depressed fractional reabsorption in the proximal tubule during acute saline infusion is due predominantly to a decrease in absolute reabsorptive rate and to a lesser extent to an increase in superficial nephron filtration rate which is proportional to the increase in whole kidney filtration. Continued natriuresis when filtration rate is decreased during saline infusion can be accounted for entirely by the persistent large reduction in the absolute rate of proximal tubular reabsorption.

ostnatal development of renal function: micropuncture and clearance studies in the dog

Postnatal renal development was studied in dogs between 2 and 77 days. Single, superficial nephrons were evaluated by micropuncture, concurrently with measurements of total renal function and morphometric analyses in the same animals. Glomerular filtration rate for the entire kidney increased linearly from 0.13 ml/min per g kidney weight at 2 days to 0.91 at 77 days. Extraction of p-aminohippurate increased from about 20 to 80%, and renal plasma flow per g kidney weight, measured as Cpah/Epah, increased threefold during the same period. Filtration fraction increased to the mature value during the first half of the postnatal period studied. The clearance of urea per unit of renal mass increased with age, whereas the fraction of filtered urea reabsorbed declined during the early part of the postnatal period. The pattern of fractional urea reabsorption may be due mainly to increased medullary recycling of urea and to a rise in the reabsorption of water from the medullary collecting duct. Urine osmolality was higher than plasma from birth onward and rose with age. Osmolal equality of collecting duct fluid and medullary interstitium reflected mature vasopressin (ADH)-induced water permeability. The rise in urinary concentration was predominantly due to increasing medullary sequestration of urea. Glomerular filtration rate of the superficial nephron increased from 3.2 nl/min at 21 days, when subcapsular nephrons were uniformly patent, to 23.1 at 77 days. Despite this rise in filtered load, fractional reabsorption of sodium and water in superficial proximal tubules was constant and at the mature level from the onset of intratubular perfusion. Changes in arterial plasma protein concentration, in filtration fraction, and in the hydrostatic pressure gradient between proximal tubule and peritubular capillary may interact to maintain glomerulotubular balance. The data, together with results of an accompanying morphological study, demonstrate a sequence of coordinated changes during postnatal renal maturation.

Effect of saline infusions on intrarenal distribution of glomerular filtrate and proximal reabsorption in the dog

The effect of acute extracellular volume expansion with saline on the intrarenal distribution of glomerular filtrate, was studied in dogs utilizing micropuncture techniques in which samples were obtained by both recollection and from new tubules. Recollection was examined in seven dogs during continuous hydropenia and in five dogs during continuous saline diuresis. Recollection was associated with an increase in nephron flow rate of 8% during hydropenia and 27% during saline diuresis. In addition, during continuous saline diuresis, shortened transit times and lowered intratubular pressures were recorded in previously punctured tubules. Despite increased tubular flow, fractional reabsorption was unchanged. Nephron glomerular filtration rates (gfr) were measured during hydropenia and then after acute volume expansion in 10 dogs. In the repunctured tubules gfr rose 38% more than total glomerular filtration rate (GFR). In contrast, when new tubules were punctured during volume expansion, nephron gfr and total GFR changed proportionately. The disproportionate rise in nephron gfr after volume expansion noted with the recollection technique appears to be artifactual when contrasted to micropuncture of new tubules. With acute volume expansion, fractional reabsorption decreased 15% in recollected samples and 16% in newly sampled tubules. Increased nephron gfr cannot account for the fall in fractional reabsorption. It is concluded that in dogs, saline diuresis is not associated with redistribution of filtrate from deep to superficial nephrons, and that the fall in proximal fractional reabsorption is caused by diminished absolute reabsorption.