Carl W. Gottschalk

Effect of renal sympathetic nerve stimulation on proximal water and sodium reabsorption.

The renal responses to sympathetic nerve stimulation were studied in saline-expanded rats. The left kidney was partially denervated by crushing the left greater splanchnic nerve. Then the distal portion of the nerve was stimulated with square wave pulses of 0.5 ms duration, voltage twice threshold, and 1 or 2 Hz frequency while monitoring the compound action potential. Fibers with conduction speeds of 13-17 m-s-1 and of 0.7-1 m-s-1 were identified. Only stimulation of the latter appeared to produce changes in renal Na and water excretion. Whole kidney and individual nephron studies were performed alternating control and nerve stimulation periods. Nerve stimulation produced approximately a 25% reduction of the left kidney urine volume and sodium excretion. Glomerular filtration rate and renal plasma flow remained unchanged. Right kidney Na and water excretion, glomerular filtration rate, and renal plasma flow remained constant. In the left kidney, during nerve stimulation, the tubular fluid to plasma inulin concentration ratio increased significantly in the late proximal tubule. We conclude that the antidiuresis and antinatriuresis seen during sympathetic nerve stimulation were caused by increased sodium and water reabsorption in the proximal tubule, probably mediated by the stimulation of slowly conducting unmyelinated fibers. These responses appeared to be unrelated to systemic or intrarenal hemodynamic changes.

Pathogenesis of acute renal failure following temporary renal ischemia in the rat.

In this study, we characterized the sequence of several intrarenal events and evaluated their relative importance in the pathogenesis of unilateral oliguric acute renal failure induced experimentally in rats by complete occlusion of a renal artery for 1 hour. Kidneys were studied prior to occlusion and 1–3 hours and 22–26 hours after release of the temporary occlusion. Renal blood flow measured by an electromagnetic flow transducer was reduced to 40-50percent of control during both postocclusion periods. Flow of tubular fluid was markedly reduced, and the damaged kidneys were oliguric. Proximal and distal convolutions were filled with fluid and dilated 1–3 hours after occlusion; their pressures were greatly heterogeneous and were elevated, on the average, to 31 and 16 mm Hg, respectively. Glomerular capillary pressure at this time was normal or slightly increased. Histological sections showed extensive tubular obstruction. We conclude that initially the oliguria is primarily due to intraluminal obstruction in the absence of predominant increases in preglomerular vascular resistance. Observations at 22–26 hours after occlusion indicated acute tubular necrosis. Moreover, the combined involvement of preglomerular vasoconstriction, persisting tubular obstruction, and passive backflow of tubular fluid appeared to be important in the maintenance of the oliguria. Glomerular capillary, proximal intratubular, and peritubular capillary hydrostatic pressures were reduced below control values. After acute volume expansion, the reduced pressures and renal blood flow were reversed, yet the experimental kidneys remained oliguric. Thus, it is clear that tubular obstruction is a significant factor responsible for both the genesis and the maintenance of oliguria in this experimental model of ischemia-induced acute renal failure.

Pathophysiology of Experimental Glomerulonephritis in Rats

Micropuncture, clearance, immunofluorescence and light microscopy techniques were used to study kidney structure and single nephron function in rats with autologous immune complex nephritis (AICN), a membranous glomerulonephritis developing over 5 to 20 mo, in the more acute and proliferative glomerular basement membrane (GBM) nephritis and in controls. Both models are known to have clinical counterparts in human disease. Kidney functional abnormalities correlated with the degree of architectural derangement. In both AICN and anti-GBM nephritis filtration fraction fell in direct proportion to the fall in glomerular filtration rate (GFR), renal plasma flow being unchanged. Fractional electrolyte excretion increased as the GFR fell. Despite marked heterogeneity of single nephron filtration rate (SNGFR) (AICN, 5-93 nl/min; anti-GBM, 0-50 nl/min) and of proximal tubular hydrostatic pressure (4-48 mm Hg), each nephron showed almost complete glomerulotubular balance, absolute reabsorption to the late proximal convolution varying directly with filtration rate. In addition SNGFR could be related both to proximal intratubular hydrostatic pressure and to calculated glomerular capillary pressure (Pg), being lowest in those nephrons with the highest intratubular pressure. Nephrons with very high filtration rates did not apparently reach filtration equilibrium. Mean SNGFR was significantly lower in the anti-GBM group, while calculated Pg was the same in both. This probably reflects the acute and diffuse involvement of the anti-GBM lesion with different filtration characteristics from the more chronic AICN disease. Tubular damage was more marked in AICN, and extraction of p-aminohippurate was reduced in this group.

Nephron Stop-Flow Pressure Response to Obstruction for 24 Hours in the Rat Kidney

Complete ureteral ligation of 24-h duration significantly reduced stop-flow and estimated glomerular capillary pressures in nephrons accessible to micropuncture in obstructed kidneys. In kidneys without ureteral obstruction, a similar response occurred in single tubules blocked for 24 h without affecting nearby unblocked tubules. Thus, the response to tubular obstruction occurs on an individual nephron basis and results from constriction of individual afferent arterioles. The mechanism leading to the response is unknown, but a feedback mechanism operating through the juxtaglomerular apparatus of individual nephrons is an attractive possibility.

Renal Tubular Permeability during Increased Intrarenal Pressure

Renal tubular permeability was studied by microinjection techniques during increased intrarenal pressure in anesthetized diuretic rats. Intrarenal pressure, as evidenced by intratubular pressure (ITP), was increased by elevation of ureteral pressure, partial renal venous constriction, or massive saline diuresis. Various combinations of radioactive inulin, creatinine, mannitol, sucrose, and iothalamate in isotonic saline were microinjected into superficial proximal and distal convolutions, and recovery of the isotopes was measured in the urine. Inulin was completely recovered in the urine from the injected kidney at both normal and elevated ITP. Creatinine, mannitol, sucrose, and iothalamate were also completely recovered at normal ITP, but recoveries were significantly lower, averaging 73, 85, 89, and 85%, respectively, after early proximal injection when proximal ITP was increased to 30+/-2 mm Hg by elevation of ureteral pressure. Since transit time is prolonged under these conditions, mannitol recovery was also studied during aortic constriction, which prolongs transit time but lowers ITP. Recovery was complete. A significant loss of mannitol was observed during massive saline diuresis, which shortens transit time but increases ITP. During renal venous constriction producing a proximal ITP of 30+/-2 mm Hg, mannitol recovery was significantly less than 100% even after microinjection into distal convolutions, but the loss was greater injection at more proximal puncture sites. Mannitol recovery was complete during elevation of ureteral pressure in the contralateral kidney. These studies demonstrate a change in the permeability characteristics of all major segments of the renal tubule during elevation of intrarenal pressure. This change is rapidly reversible and does not appear to be due to a humoral factor which gains access to the general circulation.

Urate-2-14C transport in the rat nephron

Intrarenal transport of urate-2-(14)C was studied in anesthetized rats using the microinjection technic. During saline diuresis, small volumes of urate-2-(14)C (0.24-0.48 mM) and inulin-(3)H were injected into surface proximal and distal convoluted tubules, and ureteral urine was collected serially. Total (74-96%) and direct (57-84%) urate recovery increased significantly the more distal the puncture site. Delayed recovery (+/-20%) remained approximately the same regardless of localization of the microinjection. After proximal injections, total and direct recoveries of urate-2-(14)C were significantly higher in rats treated with probenecid, pyrazinoate, or PAH than during saline diuresis alone, while the excretion rates were comparable after distal injection. Delayed recovery was not altered by drug administration. The decreased proximal reabsorption of urate is presumably due to an effect of the drugs on the luminal membrane of the nephron. For perfusion at high urate concentrations, nonradioactive urate was added to the injectate (0.89-1.78 mM). Urate-2-(14)C recovery was almost complete and there was no delayed excretion, demonstrating saturation kinetics. These findings are compatible with a carrier-mediated mechanism for urate transport probably located at the luminal border of the proximal tubular epithelium. No definitive evidence for urate secretion was found in these studies.

History of the Science of Dialysis

Thomas Graham (1805-1869), who is credited with seminal work on the nature of the diffusion of gases and of osmotic forces in fluids, can properly be called the father of modern dialysis. His apparatus to study the behavior of biological fluids through a semipermeable membrane clearly presaged the artificial kidney in clinical use today. In 1913, John Abel and coworkers reported the first application of the principles of diffusion to remove substances from the blood of living animals. Unaware of Abels work, Georg Haas (1886-1971) performed the first human dialysis in the German town of Giessen in 1924. But it was not until 1945 that Willem Johan Kolff, working under extremely difficult wartime conditions in The Netherlands, achieved the first clinically successful hemodialysis in a human patient.

Micropuncture Study of Inulin Absorption in the Rat Kidney

By means of a microinjection technique, inulin-carboxyl-C14 or inulin-methoxy-H3 was injected into single proximal tubules of the rat at various urine flow rates. Urine collected separately from the two kidneys showed negligible amounts of inulin activity on the noninjected side, thus demonstrating directly that there is no significant reabsorption of inulin by the renal tubular epithelium under these conditions.

Evidence That the Mammalian Nephron Functions as a Countercurrent Multiplier System

Fluid collected by micropuncture from the bend of the loop of Henle in the concentrating hamster kidney had the same osmotic pressure as fluid from a collecting duct at the same level, while that from the distal convolution was more dilute. This indicates that the tubular fluid is first concentrated, then diluted, before its final concentration.

Osmotic Concentration and Dilution in the Mammalian Nephron

The micropuncture evidence relating to the location and the mechanism for concentration and dilution of the urine is reviewed. As required by the countercurrent hypothesis for urine concentration, these data demonstrate that in the presence of antidiuretic hormone, the tubular urine is first concentrated in the descending limb of the loop of Henle and then diluted in the ascending limb of the loop before its final concentration in the collecting ducts. The loop of Henle is believed to function as a countercurrent multiplier system, and the vasa recta as countercurrent diffusion exchangers. Additional data are required during water diuresis before the course of events in this condition is established.