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Physical Properties of Isolated Perfused Renal Tubules and Tubular Basement Membranes

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Physical Properties of Isolated Perfused Renal Tubules and Tubular Basement Membranes Physical properties of isolated perfused renal tubules and tubular basement membranes Larry W. Welling, Jared J. Grantham J Clin Invest. 1972;51(5):1063-1075. https://doi.org/10.1172/JCI106898. Research Article To study the physical properties of renal tubular basement membranes directly, the epithelial layer of single isolated perfused rabbit proximal convoluted, proximal straight, and cortical collecting tubules was removed with sodium desoxycholate. Tubular segments were perfused using micropipets. The distal end of each segment was occluded in order to simplify the measurement of transmembrane water flow. The relation between outer tubular diameter and applied transmural pressure was identical in intact tubules and their respective isolated tubular basement membranes indicating that the basement membrane determines tubular distensibility. Young's modulus for basement membranes from all tubular segments corresponded to that of tendon collagen. Membrane hydraulic conductivity was measured in two ways: (a) from the rate of transmural flow in response to an applied difference in hydrostatic pressure and,b () from the rate of transmural flow in response to a difference in colloid osmotic pressure. The hydraulic conductivity of tubular basement membranes was 300-800 times greater than that of the intact epithelial layer. Basement membrane hydraulic conductance was similar to that of peritubular and glomerular capillaries in vivo. The hydrostatic conductance of tubular basement membranes exceeded the osmotic conductance by 3-10-fold owing largely to the fact that the membranes were moderately permeable to the osmotic solute (albumin). In view of these findings we suggest that oncotic and hydrostatic pressure may play an important […] Find the latest version: https://jci.me/106898/pdf Physical Properties of Isolated Perfused Renal Tubules and Tubular Basement Membranes LARRY W. WELLING and JARED J. GRANTHAM with the technical assistance of PAMrI QUALIZZA From the Departments of Pathology and Medicine, University of Kansas Medical Center, Kansas City, Kansas 66103 A B S T R A C T To study the physical properties of renal INTRODUCTION tubular basement membranes directly, the epithelial layer In contrast to its counterpart in the renal glomerulus of single isolated perfused rabbit proximal convoluted, little is known about the structure and function of the proximal straight, and cortical collecting tubules was tubular basement membrane. From numerous morpho- removed with sodium desoxycholate. Tubular segments logical studies of normal and injured kidneys it has been were perfused using micropipets. The distal end of each inferred that the tubular basement membrane is relatively segment was occluded in order to simplify the measure- tough and probably serves principally as a structural ment of transmembrane water flow. The relation be- tween outer tubular diameter and applied transmural support upon which the tubular epithelial cells rest. Re- pressure was identical in intact tubules and their re- cently, peritubular physical factors, most notably hydro- spective isolated tubular basement membranes indicating static pressure and oncotic pressure, have been con- that the basement membrane determines tubular dis- vincingly implicated in the control of fluid absorption in tensibility. Young's modulus for basement membranes cortical nephrons in vivo (1-3). Clearly, the tubular from all tubular segments corresponded to that of tendon basement membrane is permeable to relatively small collagen. Membrane hydraulic conductivity was measured solutes and water, as the glomerular membrane, since in two ways: (a) from the rate of transmural flow in all tubular reabsorbate crosses the barrier before uptake response to an applied difference in hydrostatic pressure by the peritubular vasculature. Definitive information is and, (b) from the rate of transmural flow in response lacking, however, with regard to the hydraulic con- to a difference in colloid osmotic The pressure. hydraulic ductivity and to serum proteins of tubular conductivity of tubular basement membranes was 300- permeability basement membranes. In the present 800 times greater than that of the intact epithelial layer. study we have de- termined several physical properties of tubular basement Basement membrane hydraulic conductance was similar membranes using a direct approach. Evidence was ob- to that of peritubular and glomerular capillaries in vivo. tained to indicate that basement membranes provide The hydrostatic conductance of tubular basement mem- the principal structural support of branes exceeded the osmotic conductance by 3-10-fold renal tubules and are bar- riers across which oncotic as well as hydrostatic pres- owing largely to the fact that the membranes were mod- sure erately permeable to the osmotic solute (albumin). In may play an important role in the movement of tu- bular absorbate. view of these findings we suggest that oncotic and hy- drostatic pressure may play an important role in the METHODS movement of tubular absorbate from the epithelial com- Female New Zealand white rabbits weighing 2-3 kg were partment into the renal interstitium. maintained on water and regular rabbit chow before the experiments. They were lightly anesthetized with i.v. pento- Abstracts of portions of this work were published by the barbital, the left kidney was removed, and several thin American Society of Nephrology, November 1970, and in saggital sections of renal cortex were obtained. We used 1971 Clin. Res. 19: 553. only kidneys free of obvious disease. For dissection of in- Received for publication 23 August 1971 and in revised dividual renal tubular segments the cortical slices were form 19 November 1971. immersed in either rabbit serum (Kam Laboratories, Inc., The Journal of Clinical Investigation Volume 51 1972 1063 FIGURE 1 Arrangement for perfusing renal tubules. Grandview, Mo.) (RS) 1 or in a standard medium (SM) could be determined repeatedly to within + 0.3 ,.z Tubule containing NaCl, 115 mM; KCl, 5 mar; NaHCO3, 25 mM; length was measured using a reticule in the microscope Na acetate, 10 mM; NaH2PO4, 1.2 mM; MgSO4, 1.2 mM; ocular. Because longitudinal stretching of the tubule de- CaCl, 1.0 mMI; 5% calf serum (Kam Laboratories, Inc.); creased tubule diameter we measured diameter when the and dextrose, 5.5 mm. The commercial rabbit serum protein distance between the holding pipets was 10% less than the was 67% albumin and 33% globulin by electrophoretic measured length of the tubule segment. In several experi- analysis. The total protein concentration was 62 g/liter. All ments the tubule diameter at a specific point was measured media were equilibrated with 95% 02 and 5 % C02 and using an Image Splitting Measuring Eyepiece (Vickers the osmolality adjusted to 290±5 mOsm/kg. Dissection of Instruments, Inc., Malden, Mass.). Average deviation using nephron segments was performed under 10-90 X magnifica- this instrument was +0.3 A. tion using fine-tipped forceps with the media maintained The method of tubular perfusion is shown in Fig. 1. The at 4°C. Three anatomically distinct tubule portions were perfusion pipet assembly is on the right. The tubules were studied: the proximal convoluted (PCT), the proximal secured by applying suction to the lumen of the outer pipet straight (PST), and cortical collecting tubules (CCT). so that a concentric inner pipet, filled with perfusate, could Proximal convoluted tubules were obtained only from the be advanced 50-150 A into the tubule lumen. After the midzone of the cortex. Proximal straight and collecting initial delivery of perfusate had caused the tubule lumen tubules were obtained from the mid- and juxta-medullary to open, suction to the outer pipet was markedly increased zones of the cortex. The length of individual tubule frag- in order to impact the tubule between the outer and per- ments ranged from 0.05 to 0.25 cm. We avoided crushing fusion pipets and prevent leakage retrograde at the per- or stretching the tubules during dissection. Tubules with f used end. In all experiments the perfusate was identical holes, an opulescent appearance, or swollen cells were re- to the SM except for the exclusion of calf serum and jected. Dissection time never exceeded 40 min. Single neph- the addition of 'H20 (0.2 mCi/ml, International Chemical ron fragments were transferred in fluid to a thermostati- and Nuclear Corporation, Irvine, Calif.). The 'H20 was cally controlled chamber containing 0.5 ml of SM and added in order to determine the rate of flow of perfusate observed directly through an ultrathin coverslip in the floor out of the pipet. of the chamber using an inverted microscope at 50 to All intact tubules were perfused at 28°C or 37°C by a 1,000 X. Fluid in the chamber was oxygenated and vigor- gravity flow apparatus in which the perfusion pipet was ously stirred by bubbling with 95% 02 and 5% C02 and connected by plastic tubing to a movable fluid reservoir. By was maintained at 28° or 37°C. A silicone antifoaming positioning the reservoir above or below the level of the agent (Antifoam A Spray, Dow Corning Corp., Midland, pipet tip hydrostatic pressures of -10 to + 90 cm H20 Mich.) was lightly sprayed onto the surface of the bath. could be applied. Zero pressure was determined to within The specimens were photographed with a Polaroid or a 0.1 cm H20 before the tubule was attached. After impac- 35 mm camera. In most experiments Polaroid photographs tion on the perfusion pipet, the tubule segment was per- of the same portion of the tubule or isolated basement fused for several minutes at a pressure slightly greater membrane were taken at 600 X to determine outer tubule than that necessary to maintain a patent lumen. The distal diameter. By averaging the diameters measured at 10 equally end of the tubule was then crimped and occluded by spaced points on these photographs the tubule diameter applying suction to a second pipet, as shown on the left in 1 Fig. 1. Studies were begun after the occluded tubule had Abbreviations used in this paper: A, outer tubular sur- remained at low applied pressure for an additional 5-10 min.
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