The pathologic physiology of chronic Bright's disease. An exposition of the "intact nephron hypothesis".

Clinical and experimental data relating to the functional capacity of the surviving nephrons of the chronically diseased kidney for the most part support the thesis that these nephrons retain their essential functional integrity regardless of the nature of the underlying form of chronic Bright's disease. There are instances in which specific alterations of function correlate with pathologic involvement of a particular site of the nephron but these appear to represent the exceptions, and in general the more advanced the disease becomes, the less evident are the differentiating features. Studies on dogs with unilateral renal disease indicate that the functional capacity of the nephrons of the diseased kidney parallels that of the nephrons of the contralateral normal kidney. These data tend to exclude widespread intrinsic damage to the functioning nephrons by the underlying pathologic processes. From these observations, as well as from certain supporting clinical and experimental observations, it is suggested that the majority of surviving nephrons in the patient with bilateral renal disease similarly are functionally intact. Concepts of the pathologic physiology of the kidney, based on the "intact nephron hypothesis", are presented. Within the framework of this hypothesis it is concluded that (1) the diseased kidney consists of a diminished number of nephrons, most of which retain essentially normal functional abilities; (2) certain of the apparent abnormalities in function in bilateral renal disease may relate to adaptive changes imposed by the decreased nephron population and the attendant derangements in body fluids rather than to structural distortion of nephrons; (3) the over-all flexibility of the diseased kidney decreases as the number of constituent nephrons decreases; but (4) there is an orderly and predictable pattern of excretion for all substances.

Biologic and physical characteristics of the non-peptidic, non-digitalis-like natriuretic hormone.

At least three independent groups of natriuretic hormones have been isolated over the past ten years. Two, atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP), are proteins and the third is made up of digitalis-like substances (DLS). The present report concerns the isolation, substantial purification and biologic actions of an entirely different natriuretic hormone (NH) which appears to be steroidal in nature and an isomer of cortisone. The source of NH was uremic urine. Purification involved successive chromatographic steps including gel filtration and multiple HPLC runs through C-18 resins. A translucent crystal ultimately was obtained. The product was examined using mass spectroscopy with trimethylsilyl derivatization. Only one compound was identifiable. The characteristics of the molecule include: a molecular weight, 360.4; a molecular formula, C21H28O5; a steroidal nucleus; UV absorption at 220 and 290 nm; and intrinsic fluorescence. The onset of action occurs within minutes both in the rat and, as previously shown, in several in vitro systems including the frog skin, toad bladder, fibroblasts and renal tubular epithelial cells grown in culture and isolated perfused cortical collecting tubules. In contrast to DLS, NH has been previously shown not to cross react with digoxin antibodies. Moreover, when given to intact rats, it produces a profound natriuresis but little or no kaliuresis. In contrast to ANF and BNP the compound is active orally as well as intravenously. It is clearly different from cortisone, based both on its biologic and mass spectroscopic characteristics.

Effect of protein on glomerular filtration rate and prostanoid synthesis in normal and uremic rats.

Normal and uremic conscious rats that had been maintained on a low-protein diet were given oral protein or carbohydrate loads, and clearance studies were performed. Both the normal and uremic animals demonstrated a approximately 30% increase in glomerular filtration rate (GFR) in response to the protein bolus, but no significant increase in GFR was seen following the carbohydrate bolus. Similar studies were performed in uremic rats on a standard protein diet. The changes in GFR that were seen after an albumin bolus were similar but not as pronounced as those noted in the animals on the low-protein diet. Pretreatment with either aspirin or meclofenamate, cyclooxygenase inhibitors, completely blocked the protein-induced rise in GFR. The rats of glomerular production of prostaglandin (PG) E2 and 6-keto-PGF1 alpha (a stable metabolite of prostacyclin, PGI2) were determined by radioimmunoassay in a similar group of normal rats. The synthetic rates of PGE2 and 6-keto-PGF1 alpha following the protein bolus were 40 and 52% greater, respectively, than those observed following the carbohydrate load (P less than 0.005). Aspirin decreased glomerular prostanoid production in protein-treated animals by greater than 60%. Thus it appears that in the setting of protein restriction, the percent increase in GFR following a protein load is similar in both the normal and uremic rats, the increase in GFR in uremic rats is attenuated when animals were allowed to ingest a normal protein diet prior to study, and the increase in GFR seen in response to a protein load may be related to an increase in the synthesis of one or more vasodilatory glomerular prostanoids.

Progress in isolation and purification of an inhibitor of sodium transport obtained from dog urine.

Among the potential modulators of transtubular sodium transport is the putative natriuretic hormone. Widespread efforts are underway to isolate this substance in pure form. The present studies describe a series of experiments directed to this goal. Urine samples in the amount of 150 liters were taken from normal, mineralocorticoid hormone "escape" dogs and were chromatographed through Sephadex G-25. The active fraction of the eluate (that is, the fraction containing the inhibitor of sodium transport) was then subjected to high pressure liquid chromatography (HPLC) in four consecutive steps using three different resins. Approximately 5% of the each column eluate was diverted by use of a stream-splitting apparatus, and the fluorescent pattern was measured and recorded, in most instances following the addition of fluorescamine. Based on the respective fluorescent patterns obtained from the eluates of the successive chromatographic steps, the residual portion (95%) of each eluate was divided into fractions, and each fraction was bioassayed. In each instance only the biologically active fraction was subjected to further purification. In the first step involving HPLC and a cation exchange resin, six fractions were obtained. Only one was active. When this fraction was subjected to reverse-phase chromatography, seven new fractions emerged. Again only one was active. When it was chromatographed using a second cation exchange resin, two fluorometrically detectable peaks, termed N ad H, were identifiable. N exhibited spontaneous fluorescence; H exhibited fluorescamine-dependent fluorescence. In the final step, N and H were separated and bio-assayed individually. N was inactive; H proved to be a potent inhibitor of sodium transport. Accumulation of H in sufficient quantity will determine whether it is a single compound and will permit analysis of its chemical nature.

Functional profile of the isolated uremic nephron. Impaired water permeability and adenylate cyclase responsiveness of the cortical collecting tubule to vasopressin.

Resistance of the chronically diseased kidney to vasopressin has been proposed as a possible explanation for the urinary concentrating defect of uremia. The present studies examined the water permeability and adenylate cyclase responsiveness of isolated cortical collecting tubules (CCT) from remnant kidneys of uremic rabbits to vasopressin. In the absence of vasopressin the CCTs of both normal and uremic rabbits were impermeable to water. At the same osmotic gradient, addition of a supramaximal concentration of vasopressin to the peritubular bathing medium led to a significantly lower net water flux per unit length (and per unit luminal surface area) in uremic CCTs than in normal CCTs. Transepithelial osmotic water permeability coefficient, P(f), was 0.0232 +/-0.0043 cm/s in normal CCTs and 0.0059+/-0.001 cm/s in uremic CCTs (P < 0.001). The impaired vasopressin responsiveness of the uremic CCTs was observed whether normal or uremic serum was present in the bath. Basal adenylate cyclase activity per microgram protein was comparable in normal and uremic CCTs. Stimulation by NaF led to equivalent levels of activity in both, whereas vasopressin-stimulated activity was 50% lower in the uremic than in the normal CCTs (P < 0.025). The cyclic AMP analogue, 8-bromo cyclic AMP, produced an increase in the P(f) of normal CCTs closely comparable to that observed with vasopressin. In contrast, the P(f) of uremic CCTs was only minimally increased by this analogue and was not further stimulated by theophylline. These studies demonstrate an impaired responsiveness of the uremic CCT to vasopressin. This functional defect appears to be a result, at least in part, of a blunted responsiveness of adenylate cyclase to vasopressin. The data further suggest that an additional defect in the cellular response to vasopressin may exist, involving a step (or steps) subsequent to the formation of cyclic AMP.A unifying concept of the urinary concentrating defect of uremia is proposed which incorporates a number of hitherto unexplained observations on the concentrating and diluting functions of the diseased kidney.

Functional profile of the isolated uremic nephron. Role of compensatory hypertrophy in the control of fluid reabsorption by the proximal straight tubule.

An in vitro approach to the study of single nephron function in uremia has been employed in evaluating the control of fluid reabsorption by the renal superficial proximal straight tubule (PST). Isolated segments of PSTs from the remnant kidneys of uremic rabbits (stage III) were perfused in vitro and their rate of fluid reabsorption compared with normal PSTs and with PSTs derived from the remnant kidneys of nonuremic rabbits (stage II). All segments were exposed to a peritubular bathing medium of both normal and uremic rabbit serum thereby permitting a differentiation to be made between adaptations in function which are intrinsic to the tubular epithelium and those which are dependent upon a uremic milieu.Compared with normal and stage II PSTs, there was significant hypertrophy of the stage III tubules as evidenced by an increase in length and internal diameter, and a twofold increase in the dry weight per unit length. Fluid reabsorption per unit length of tubule was 70% greater in stage III than in normal and stage II PSTs, and was closely correlated with the increase in dry weight. Substitutions between normal and uremic rabbit serum in the peritubular bathing medium did not affect fluid reabsorption significantly in any of the three groups of PSTs. Perfusion of the tubules with an ultrafiltrate of normal vs. uremic serum likewise failed to influence the rate of net fluid reabsorption. It has previously been observed that net fluid secretion may occur in nonperfused or stop-flow perfused normal rabbit PSTs exposed to human uremic serum. Additional studies were thus performed on normal and stage III PSTs to evaluate whether net secretion occurs in the presence of rabbit uremic serum. No evidence for net secretion was found. These studies demonstrate that fluid reabsorption is greatly increased in the superficial PST of the uremic remnant kidney and that this functional adaptation is closely correlated with compensatory hypertrophy of the segment. Humoral factors in the peritubular environment do not appear to be important mediators of the enhanced fluid reabsorption.

Phosphate control and 25-hydroxycholecalciferol administration in preventing experimental renal osteodystrophy in the dog.

Previous studies from this laboratory demonstrated that secondary hyperparathyroidism in dogs with chronic renal disease may occur, at least in part, as a consequence of the need for progressive adaptation in renal phosphorus (P) excretion that occurs as glomerular filtration rate falls. However, the studies were of relatively short duration. Moreover, no information emerged regarding a potential role of calcium malabsorption in the pathogenesis of secondary hyperparathyroidism. The short duration of the protocol did not lend itself to the study of the effect of P control or the administration of vitamin D in the pathogenesis of renal osteodystrophy. In the present studies, 14 dogs with experimental chronic renal disease were studied serially for a period of 2 yr. Each animal was studied first with two normal kidneys on an intake of P of 1,200 mg/day. Then, renal insufficiency was produced by 5/6 nephrectomy. The dogs then were divided into three groups. In group I, 1,200 mg/day P intake was administered for the full 2 yr. In group II, P intake was reduced from the initial 1,200 mg/day, in proportion to the measured fall in glomerular filtration rate, in an effort to obviate the renal adaptation in P excretion. In group III, "proportional reduction" of P intake also was employed; but in addition, 20 mug of 25(OH)D(3) were administered orally three times a week. In group I, parathyroid hormone (PTH) levels rose throughout the 2-yr period reaching a final concentration of 557+/-70 U (normal 10-60). In group II, values for PTH remained normal throughout the 1st yr, increased modestly between the 12th and the 18th mo, but then did not rise after the 18th mo. In group III, no elevation of PTH levels was observed at any time; however, these animals were hypercalcemic. Histomorphologic analyses of the ribs of these dogs were performed serially throughout the 2-yr period. A linear relationship was obtained between the osteoclastic resorption surface and the concentration of circulating immunoreactive PTH. The osteoid volume was greater in group I animals when compared to those in group II. None of the morphologic abnormalities associated with renal osteodystrophy were observed in the animals in the third group.