Search

Proximal convoluted renal tubules (PCT) are difficult to dissect for in vitro study, particularly in newborn animals, and are identified with certainty only when obtained in continuity with their parent glomeruli. We show here that the supravital colloidal dye trypan blue preferentially stains early PCT segments of rabbit in vivo and greatly facilitates their isolation. We also show that trypan blue administration does not adversely affect animal growth rate, kidney weight, or several general measures of kidney function. Using isolated PCT from newborn to adult rabbits, control (unstained) and trypan blue-stained tubules also are found not to differ in lateral membrane surface density, in their ability to maintain normal cell volume or to respond to hypotonic osmotic challenge, and in their swelling rate in isotonic medium after ouabain treatment. We conclude that trypan blue has no adverse effects on PCT cell function while greatly facilitating their isolation for physiological studies.

To measure simultaneously the concentration profiles of bicarbonate, chloride and inulin along the length of the superficial proximal convoluted tubule, free-flow micropuncture measurements were made sequentially from the end-proximal tubule to Bowman's space in 10 tubules of hydropenic Munich-Wistar rats. Bicarbonate and volume reabsorption were 354 +/- 21 pmol X mm-1 X min-1 and 5.9 +/- 0.4 nl X mm-1 X min-1 in the first millimeter and fell progressively in the remaining 3.8 mm of tubule, averaging 83 +/- 4 pmol X mm-1 X min-1 and 2.3 +/- 0.5 nl X mm-1 X min-1, respectively. The values in the initial millimeter represents a high transport capacity since they exceed rates that have been observed when comparable or even higher mean luminal substrate concentrations were presented to the late proximal tubule. In contrast, chloride reabsorption was only 206 +/- 55 peq X mm-1 X min-1 in the first millimeter compared with a mean of 306 +/- 22 peq X mm-1 X min-1 in the rest of the tubule. In conclusion, there is substantial axial transport heterogeneity, with bicarbonate and water reabsorption higher but chloride reabsorption lower in the early compared with the late superficial proximal convoluted tubule.

The microanatomy of a unique type of juxtamedullary nephrons has been studied in the rabbit kidney by means of corrosion casts, scanning electron microscopy and the air cast method. The nephrons described in this paper are located in the outer stripe of the outer medulla and are the only nephrons that are not arranged radially within the kidney. They differ from other juxtamedullary nephrons in the morphology and course followed by the proximal tubule and by the close relationship that they establish with the arcuate vessels. The glomerulus of these nephrons is supplied by a short afferent arteriole that arises directly from the arcuate artery. Because of the special characteristics of these nephrons we have named them 'arcuate nephrons'. The morphology, spatial relationships and vascularization of the arcuate nephrons suggest that these nephrons differ functionally from the other juxtamedullary nephrons. Possible developmental factors responsible for the special microanatomy of the arcuate nephrons are analyzed.

Effects of parathyroid hormone (PTH), calcitonin (CT), vasopressin (VP), and glucagon (GL) on adenylate cyclase activity and cyclic AMP (cAMP) levels in isolated cortical thick ascending limbs of Henle's loop (CTAL) of the rat kidney were examined. PTH, CT, and VP each stimulated adenylate cyclase of this nephron segment in a dose-dependent manner. Stimulation of the enzyme activity was greatest with a maximal dose of PTH and least with VP. With maximal doses, the effects of PTH and CT were not additive; whether or not the effects of maximal doses of VP and PTH or CT were additive was not clear. All three hormones increase cAMP in intact CTAL ina dose-dependent manner. Maximal doses of PTH, CT, VP, and GL resulted in comparable rises in cell cAMP, and there was no additive effect. These data suggest that PTH and CT may stimulate the same adenylate cyclase moieties, whereas VP may stimulate distinct enzyme moieties, and that these three peptide hormones as well as GL definitely act on the same cell group in rat CTAL. Thus, it is possible that these hormones may induce qualitatively similar effects on CTAL functions if such effects are mediated by cAMP.

New nephron anlages appear in the renal cortex up to the 4th postnatal day (PD). The last anlages to be formed develop into functional nephrons by PD 10, and the cortex appears mature at PD 12 after formation of the cortex corticis. The renal medulla develops by the longitudinal growth of loops of Henle and collecting ducts. The immature medulla cannot be divided into different zones and corresponds structurally to the later inner stripe of the outer zone. The inner zone is formed by PD 8, and the outer stripe of the outer zone by PD 12. The renal medulla is mature at PD 21. From the start of its development, the renal proximal tubule consists of the pars convoluta and pars recta. In both parts the formation of the brush border is accompanied by the simultaneous appearance of brush border enzymes (alkaline phosphatase, gamma-glutamyltranspeptidase, dipeptidylaminopeptidase IV) and lysosomal enzymes (acid phosphatase, acid beta-galactosidase, N-acetylglucosaminidase, dipeptidylaminopeptidase II) over the full length of the proximal tubule. During the course of proximal tubule maturation, however, the lysosomal enzyme activities decline in the pass convoluta (with constant brush border enzyme activities), while the brush border enzyme activities increase in the pars recta (with constant lysosomal enzyme activities). The two parts further differ in that they exhibit different lysosomal patterns from the outset, the pars convoluta containing numerous large, highly enzyme-active lysosomes arranged in groups, and the pars recta containing only a few very small lysosomes with low enzyme activity. Thus, even in the newborn rat, the lysosomal pattern of the pars recta already corresponds to that of the mature S3 segment. The S1 and S2 segments of the pars convoluta first differentiate between PD 10 and 21, as the groups of large lysosomes are progressively broken up and the extent of the lysosomal apparatus is diminished, this proceeding in a retrograde direction from the end of the immature pars convoluta.

The kidney is an extremely heterogeneous organ, with morphological, physiological, and metabolic changes occurring from segment to segment along each nephron. To determine the heterogeneity that might exist within discrete anatomical segments of rabbit nephron, we developed a technique for making quantitative enzyme assays in serial samples, about 100 micron long, along identified segments of the nephron. Results for three enzymes in proximal convoluted and straight tubules show that adenylate kinase, an enzyme of high-energy phosphate metabolism, gradually decreases along the S1 and S2 segments of the proximal tubule, with no abrupt changes. Fructose bisphosphatase, a gluconeogenic enzyme, is high along the major portion of the proximal tubule but plummets along the final millimeter of S3. Conversely, phosphofructokinase, a glycolytic enzyme, is very low along the proximal tubule but increases sharply within the final millimeter. These data underscore the biochemical heterogeneity of the nephron, illustrating the enzyme levels may change markedly even within anatomically defined regions. They also suggest the importance of further studies of this type and demonstrate a practical means for such studies.

We have evaluated the functional and morphological changes that occur in juxtamedullary proximal convoluted tubules before and after the completion of nephrogenesis at approximately 3 wk of age. Tubules were dissected in connection with glomeruli in the deep juxtamedullary zone, crimped at both ends, and induced to swell in isotonic medium by sequential exposure to ouabain, reduced bath protein concentration, and collagenase. Basolateral membrane surface areas were measured morphometrically in paired tubules if possible. The swelling rates and basolateral areas were the same in tubules from 2-7- and 14-17-day-old rabbits (during nephrogenesis), had increased in 28-42-day-old rabbits (after nephrogenesis), and increased further in adults. Because the surface areas and swelling rates increased proportionally, the nephrons "matured" only after nephrogenesis was complete and then only in basolateral surface area. The intrinsic membrane properties of hydraulic conductivity and ion permeability apparently remained constant.

Segments of superficial and juxtamedullary proximal convoluted tubules of the rabbit were perfused in vitro to examine the mechanisms responsible for net volume reabsorption. The very early postglomerular segments were not studied. Fluid reabsorptive rates and transepithelial potential differences were compared under various conditions: (a) with perfusate that simulated glomerular filtrate; (b) with perfusate that lacked glucose, amino acids, and acetate and that had HCO(3) and Cl concentrations of 5 and 140 mM, respectively; (c) with perfusate that lacked glucose, amino acids, and acetate but with 20 meq of NaHCO(3) replaced with 20 meq of Na cyclamate; (d) with the same perfusate as in b but in the presence of ouabain in the bath; (e) with ultrafiltrate of rabbit serum titrated with HCl to final HCO(3) and Cl concentrations of 2 and 134 mM, respectively. Tubules were perfused with this titrated ultrafiltrate at 37 degrees C, 21 degrees C, and in the presence of 0.1 mM ouabain in the bath. Bath fluid in all experiments was regular rabbit serum. Under conditions a and b superficial proximal convoluted tubule (SFPCT) and juxtamedullary proximal convoluted tubule (JMPCT) behaved similarly with the exception that SFPCT exhibited a lumen-positive and JMPCT a lumen-negative electrical potential under condition b. However, under condition c SFPCT failed to exhibit net volume reabsorption, whereas reabsorption in JMPCT continued unchanged. Ouabain did not affect volume reabsorption in SFPCT under condition d, whereas neither ouabain nor hypothermia affected SFPCT under condition e. In contrast, ouabain and hypothermia totally inhibited volume reabsorption in JMPCT under conditions d and e. These studies document heterogeneous mechanisms responsible for volume reabsorption in the major portions of SFPCT and JMPCT with passive forces predominating in SFPCT and active forces in JMPCT.

The kidneys from 6 domestic fowl were fixed in situ by perfusion from the left ventricle. In the bird there are two types of nephrons. One reptilian-type without Henle's loop and medullary tissue, and one mammalian-type with Henle's loop lying in medullary tissue. Serial sections from kidney tissue embedded in plexiglass or in paraffin were used to study the architecture of eight reconstructed reptilian-type nephrons from different cortical levels. All reconstructed nephrons had four major bends, but particularly in the subcapsular nephrons additional bends parallel to the kidney surface were found. There was no loop of Henle, but before entering the collecting duct the distal tubule usually had a very thin-walled segment. No proximal convoluted part was found in the reptilian-type nephrons. The length of the tubules varied between 3,000 microns and 6,000 microns. In the distal tubule a macula densa segment was found in all nephrons of the reptilian and mammalian type. The capillary network between the inter- and intralobular veins was composed of increasingly larger capillaries towards the intralobular vein. Segments of the distal tubule were indented into these capillaries and completely surrounded by them. In the nephrons of the mammalian type the proximal tubule was found to be convoluted as is usual for mammalian species.

Superficial proximal and distal tubules of both kidneys of pentobarbital-anesthetized cats, infused with 2.5% (w/v) polyfructosan in Ringer solution at 0.45 ml/min, were micropunctured. Whole one-kidney GFR average 4.1 ml/min. SNGFR averaged 15 nl/min in 51 determinations and was proportional to whole-kidney GFR in individual cats. Absolute fluid reabsorption along the length of the accessible portion of the proximal tubule was 1.6 nl/min.mm. The length of the whole proximal tubule was 6 mm as measured from the glomerulum to the descending thin limb of Henle's loop. The length of the distal tubule was 3.5 mm from the macula densa to the first branching of the collecting duct. Proximal fluid was isoosmolar with plasma whereas fluid entering the distal tubule was markedly hyposmolar. Late distal tubular fluid samples were also isosmolar. The cat appears to be well suited for micropuncture experiments in terms of freedom from respiratory movements and the presence of surface distal tubules.

Inosine 5'-diphosphatase (IDPase) activity was demonstrated cytochemically in the endoplasmic reticulum of rat kidney proximal tubule cells in tissue fixed by perfusion with glutaraldehyde--formaldehyde. Incubation for IDPase activity at pH 7.2 was performed with and without 0.5 mM levamisole, a potent inhibitor of alkaline phosphatase (AlkPase) (M Borgers, J Histochem Cytochem 21:812, 1973). Levamisole treatment of sections eliminated all reaction product in the brush border, but did not affect the IDPase activity the endoplasmic reticulum (ER). The ER appears as a basilar-luminal-oriented transcellular structure, suggesting a possible cellular transport route. This study supports and extends earlier observations made by others that suggest a transport role for the ER in these cells. It also emphasizes the value of thick section cytochemistry.

Previous investigations have suggested that there is a functional heterogeneity along the length of the proximal convoluted tubule. This study was designed to confirm and extend these suggestions by examining the intrinsic absorption of fluid and the effect of parathyroid hormone (PTH) on net fluid absorption in isolated, anatomically defined segments of rabbit superficial proximal tubules. The EPCT (early proximal convoluted tubule), LPCT (late proximal convoluted tubule), and PR (pars recta) segments were studied under controlled conditions by the isolated perfused tubule technique. In 23 EPCT, base-line fluid absorption was 1.38 +/- 0.04 (SE), a rate significantly higher than those of 11 LPCT (0.62 +/- 0.02; P less than 0.001) and 12 PR (0.52 +/- 0.03 nl mm-1 min-1, P less than 0.001) segments. In 10 EPCT, mean control fluid absorption was 1.31 +/- 0.04 nl mm-1 min-1; addition of PTH resulted in a decrease to 0.95 +/- 0.05 nl mm-1 min-1 (P less than 0.001); and, after removal of PTH, fluid absorption increased (P less than 0.001). Parathyroid hormone had no effect on either seven LPCT segments or six PR segments. These results demonstrate differences in intrinsic capacity to absorb fluid by anatomically defined segments of the rabbit proximal tubule. This functional heterogeneity is further supported by the observed differential response to PTH by the various anatomic segments of the proximal tubule.

Conflicting data exist regarding the ability of the rat proximal convoluted tubule to maintain a transepithelial gradient for CO2 and the effects of carbonic anhydrase on CO2 permeability. The present in vivo microperfusion experiments were designed to assess the ability of the rat proximal tubule to sustain a CO2 gradient between tubule lumen and peritubular blood. Tubules were perfused at rates ranging from 10 to 100 nl/min with isotonic sodium chloride containing no CO2. Peritubular capillary and intraluminal PCO2 was measured during microperfusion with PCO2 microelectrodes to allow determination of the transepithelial CO2 gradient. The mean PCO2 measured in peritubular capillaries of control rats was 60.6 +/- 1.9 mmHg. Since the perfusion solution initially contained no CO2, a gradient of 60 mmHg was imposed across the tubule epithelium. Intraluminal PCO2 rapidly approached that of the surrounding capillaries. At a tubule perfusion rate of 20 nl/min, the gradient between lumen and blood decreased to 0.9 mmHg, a value not significantly greater than zero. The calculated CO2 permeability coefficient (KCO2) was 3.69 X 10(-5) cm2/s. Addition of either 10(-4) M acetazolamide or benzolamide did not prolong the rapid dissipation of the imposed CO2 gradient. The KCO2 during carbonic anhydrase inhibition was not significantly different from control values. It is concluded that the rat proximal tubule does not present a physiologically significant diffusion barrier to CO2 either in the presence or absence of carbonic anhydrase activity. The previously demonstrated acid disequilibrium pH in the proximal tubule during inhibition of carbonic anhydrase represents an intraluminal accumulation of carbonic acid rather than of carbon dioxide gas.

A new method to measure time dependent (t) volume (V) changes in proximal straight tubules (PST) is described. V is calculated from diameter (d) measurements for which a video camera and an integrating circuit are used. A tubular image of high optical contrast is recorded with the TV camera such that the scan lines run crosswise to the tubule. The video signal is analyzed by a special processor which adds 225 tubular diameters of each TV frame and feeds this analog signal to a pen recorder. The fractional error in d measurements is 10(-3). Diameter changes of less than 0.05 micron can be detected, as compared to the usual error of a single measurement of about 0.4 micron. Pcbos, the osmotic water permeability of the contraluminal cell membrane was measured by setting up osmotic steps across it in less than 0.1 s and following the ensuing delta d/delta t. The time delay between solution change and the linear part of the osmotic response was 0.51 +/- 0.05 s. Pcbos was found to be 50.4 (+/- 8.7) X 10(-4) cm3.cm-2 of basement membrane area .s-1.osmolar-1.

Mesonephric kidneys of the mid-gestation fetus and metanephric kidneys of the adult are known to differ in many gross features but to be qualitatively similar in their cellular morphology. In the present studies, quantitative morphometric and in vitro microperfusion procedures were used to examine further the physical and transport properties of mesonephric and metanephric (S2) proximal tubule segments from rabbit. In perfused mesonephric proximal tubules the volume transport rate was 0.91 +/- 0.08 nl/min per mm tubule length, 2.7 +/- 0.2 nl/min per mm2 basement membrane (outer tubule) surface area, and 0.19 +/- 0.03 nl/min per mm2 basolateral cell membrane surface areas. For comparison, the S2 portion of metanephric proximal tubules is reported to transport 0.33 +/- 0.02 to 0.53 +/- 0.01 nl/min per mm length or, based on the present findings, 2.8 +/- 0.2 to 4.4 +/- 0.8 nl/min per mm2 basement membrane area and 0.16 +/- 0.02 to 0.25 +/- 0.07 nl/min per mm2 basolateral membrane area. Thus, proximal mesonephric and S2 metanephric segments exhibit approximately the same transport per unit area of basement membrane and per unit area of the basolateral cell membranes across which active transport presumably occurs.

We modified and improved enzyme digestion and density gradient separation procedures to obtain fractions of proximal and distal renal tubules with high yield and viability. Kidneys from two anesthetized adult Wistar rats were flushed with Krebs-Henseleit buffer (KHB) and then perfused in situ with recirculated KHB containing collagenase and hyaluronidase at 125 mmHg. Cortices were excised, minced, and incubated in KHB containing enzymes for 35 min at 37 degrees C. Dissociated tubules were removed at 10-min intervals, rinsed, and placed in KHB containing 10% calf serum, vitamins, and amino acids at 4 degrees C. Separation was achieved by suspending the tissue in 45% isosmotic Percoll layered over an undiluted Percoll cushion and centrifuging. Proximal tubules sedimented near the cushion. Distal segments were isolated in the uppermost bands of a second 35% Percoll separation. Viability was greater than 95% as measured by lactate dehydrogenase leakage and quantitated by oxygen consumption and ATP content. Basal oxygen consumption was greater than 33 nmol O2 X min-1 X mg protein-1 in all fractions and was stimulated by succinate and inhibited by amiloride and ouabain. Basal ATP content averaged 9.7 nmol/mg ATP. An average 3.3-fold separation for the proximal fraction and 24.5-fold separation for the distal fraction was assessed by the enrichment of six specific enzyme markers, with several of the markers indicating separations up to 32-fold. Isolated tubules also displayed functional responses to parathyroid hormone and vasopressin. Distal, but not proximal, segments demonstrated significantly increased adenosine 3',5'-cyclic monophosphate formation with vasopressin.

Tubular organization and vascular-tubular relations were studied by double injection in canine kidneys. Blood vessels were injected via the artery after perfusion fixation. Tubules were injected by micropipettes inserted into the urinary spaces of selected glomeruli in cleared slices. One hundred proximal convoluted tubules, 16 Henle loops, and 5 distal convoluted tubules were defined. Only subcapsular proximal convolutions were perfused by efferent vessels arising from the same glomerulus (43 of 55). In midcortex, proximal convolutions were generally perfused over less than half their length by the parent efferent (21 of 31). Here tubules entirely perfused by the parent efferent were rare (2 of 31). No inner cortical proximal convolutions were perfused by the efferent from the same glomerulus (0 of 14). Henle's loops were found to be perfused by the efferents of many glomeruli regardless of the cortical position of the parent glomerulus. Distal convolutions shared the perfusion of proximal convolutions of the same glomerulus. Thus, each nephron is apparently functionally dependent on efferent blood from glomeruli of many other nephrons. New synoptic diagrams of canine renal organization are presented.

Rabbit proximal straight tubules rapidly swell to a maximum volume when abruptly immersed into hypotonic medium. However, in a second, slower phase, termed volume regulatory decrease (VRD), tubules shrink toward their basal volume due to the efflux of K, accompanying anion and water. In the present study, we investigated the nature of the anion during hypotonic volume regulation. We removed Cl and/or the HCO3 buffer to assess their relative importance in VRD, and we used furosemide, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), and barium to investigate the nature of the transmembrane anion pathway in VRD. Isosmotic replacement of peritubular Cl with gluconate had no effect on either tubule volume in isotonic medium or the initial osmometric swelling response in hypotonic medium. However, the VRD in such Cl-depleted tubules was significantly inhibited. Control tubules regulated 81% below their maximal volume in dilute medium. By contrast, Cl-depleted tubules regulated only 39%. This inhibitory effect could not be attributed to the absence of peritubular Cl or to the presence of gluconate. The absence of the HCO3 buffer or the presence of SITS (0.5 mM) had no inhibitory effect on the rate or extent of VRD. Furosemide alone (1 mM) also had no inhibitory effect on VRD. However, whereas barium alone delays VRD, addition of furosemide to barium-treated tubules further slowed their maximal rate of fluid efflux and delayed VRD even more.(ABSTRACT TRUNCATED AT 250 WORDS)

Segments of proximal convoluted (PCT) and proximal straight (PST) tubules of minipigs and normal-sized pigs were microdissected (without collagenase treatment) and incubated (30 min, 37 degrees C, pH 7.4) in Ringer solution (under O2) containing [3H]PAH (3.10(-5) M) or [14C]urate (9.10(-5) M) and, in inhibitor studies, probenecid, pyrazinoic acid (PZA), urate or PAH, all at 1 mM. In both strains the uptake of [3H]PAH expressed as means T/M ratio (cpm per ml tissue water/cpm per ml incubation medium) was significantly higher (P less than 0.001) in PCT than in PST. T/M was 34.7 +/- 5.6 (SE) in 34 PCT and 2.4 +/- 0.3 in 24 PST of eight minipigs. In two normal-sized pigs the T/M was 14.1 +/- 3.6 in 15 PCT and 1.4 +/- 0.1 in six PST. Similar results were obtained for [14C]urate. In eight minipigs the T/M was 4.9 +/- 0.5 in 24 PCT and 2 +/- 0.2 in 25 PST. In normal-sized pigs the T/M was 3.8 +/- 0.3 in 35 PCT (five pigs) and 1.9 +/- 0.4 in eight PST (two pigs). In inhibitor studies urate significantly depressed the uptake of [3H]PAH, and unlabeled PAH depressed the uptake of [14C]urate. PZA significantly inhibited the uptake of [14C]urate but not that of [3H]PAH, whereas probenecid had a strong inhibitory effect on the uptake of both compounds. These results suggest that [14C]urate and [3H]PAH are transported by a transport system located mainly in the proximal convoluted tubule. These findings are in contrast to the findings obtained in rabbits in which the transport system of PAH and urate is mainly located in the proximal part of the pars recta.

Silicone rubber injections of methyl salicylate-cleared rat kidneys were performed. In 50 of 56 injections of superficial nephrons with their accompanying blood supply, the efferent vessel and early proximal tubule were closely approximated. In 18 of 21 tubular injections filling through the pars recta, the proximal tubule folded upon itself with early and late proximal segments, in close contact, located over their parent glomerulus, and the midproximal segments separate and located over their parent interlobular artery. The distribution of blood was serially through the early-late proximal region above the glomerulus via a long unbranched efferent vessel, via branches over the capsular surface, via capillaries down through the midproximal region, then into the interlobular vein. The observed anatomical pattern of the superficial nephron appears to permit direct functional interactions between the juxtaposed early and late proximal tubule, and in turn may effect midproximal function via the distribution of blood (modified by early proximal) from the efferent vessel to midproximal convolutions. In addition, the relationship between specific segments of the proximal tubule and specific portions of the postglomerular peritubular blood supply may be important in determining the distribution of peritubular physical forces to these nephrons.

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.

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.