Effect of extracellular fluid volume expansion on maximum glucose reabsorption rate and glomerular tubular balance in single rat nephrons

Modeling proximal tubule cell homeostasis: tracking changes in luminal flow

During normal kidney function, there are routinely wide swings in proximal tubule fluid flow and proportional changes in Na(+) reabsorption across tubule epithelial cells. This "glomerulotubular balance" occurs in the absence of any substantial change in cell volume, and is thus a challenge to coordinate luminal membrane solute entry with peritubular membrane solute exit. In this work, linear optimal control theory is applied to generate a configuration of regulated transporters that could achieve this result. A previously developed model of rat proximal tubule epithelium is linearized about a physiologic reference condition; the approximate linear system is recast as a dynamical system; and a Riccati equation is solved to yield the optimal linear feedback that stabilizes Na(+) flux, cell volume, and cell pH. The first observation is that optimal feedback control is largely consigned to three physiologic variables, cell volume, cell electrical potential, and lateral intercellular hydrostatic pressure. Parameter modulation by cell volume stabilizes cell volume; parameter modulation by electrical potential or interspace pressure act to stabilize Na(+) flux and cell pH. This feedback control is utilized in a tracking problem, in which reabsorptive Na(+) flux varies over a factor of two, in order to represent a substantial excursion of glomerulotubular balance. The resulting control parameters consist of two terms, an autonomous term and a feedback term, and both terms include transporters on both luminal and peritubular cell membranes. Overall, the increase in Na(+) flux is achieved with upregulation of luminal Na(+)/H(+) exchange and Na(+)-glucose cotransport, with increased peritubular Na(+)-3HCO(3)(-) and K(+)-Cl(-) cotransport, and with increased Na(+), K(+)-ATPase activity. The configuration of activated transporters emerges as a testable hypothesis of the molecular basis for glomerulotubular balance. It is suggested that the autonomous control component at each cell membrane could represent the cytoskeletal effects of luminal flow.

Inhibition by volume expansion of phosphate uptake by the renal proximal tubule brush border membrane

Clearance studies and examination of brush border membrane (BBM) vesicle transport were performed in rats that had been volume expanded by 10% of body weight. The results were compared to those obtained in control animals. The data indicate that the phosphaturia which resulted from the expansion procedure was accompanied by an inhibition of proximal BBM phosphate uptake. The BBM uptake of proline and glucose was unchanged. Furthermore, since plasma calcium did not change, the findings are compatible with the view that the membrane transport changes resulted from alterations induced by the saline loading itself, rather than (or in addition to) any changes caused by parathyroid hormone excretion.

The influence of tubulo-glomerular feedback on the autoregulation of filtration rate in superficial and deep glomeruli

Single nephron glomerular filtration rate (SNGFR) of superficial and juxtamedullary nephrons were measured at normal and reduced perfusion pressure in the left kidney of young Sprague Dawley rats. Perfusion pressure was lowered by constricting the aorta proximal to the branching of the left renal artery. The influence of the tubulo-glomerular feedback mechanism on SNGFR was quantitated by measuring SNGFR during intact and interrupted urine flow to the macula densa region. By using a modified Hanssen technique, SNGFR was measured under free-flow conditions. In other experiments, the urine flow to the distal nephron was blocked by a micropuncture technique, which was used for collection of the tubular fluid for measuring the filtration rate. All nephron populations autoregulated SNGFR from 70-80 to 130 mmHg, which was the upper limit of this investigation, when urine flow throughout the nephron was intact. The autoregulation in this pressure range was lost when tubular fluid was prevented from reaching the distal nephron. It was shown that the influence of negative feedback on SNGFR by the macula densa mechanism at normal blood pressure is greater in deep nephrons than in superficial ones.

Lack of effect of salt intake on urinary uric acid excretion

Although it has been established that acute expansion of the extracellular fluid volume results in enhanced uric acid clearance, the effect of chronic volume expansion by a high salt diet on urinary uric acid excretion has not been examined. Eleven normal subjects were placed on a constant diet containing 10 mEq. sodium per day for 10 days, followed by 240 mEq. sodium daily for another 10 days. Measurements were performed on the final 3 days of each phase. Urinary sodium increased from 9 plus or minus 3 standard error to 221 plus or minus 9 mEq. per day (p less than 0.001), and uric acid clearance increased from 5.9 plus or minus 0.4 to 7.1 plus or minus 0.6 ml. per minute (p less than 0.01). However, serum uric acid decreased from 6.4 plus or minus 0.4 to 5.5 plus or minus 0.3 mg./dl. (p less than 0.001). Total urinary excretion of uric acid did not change (533 plus or minus 24 to 535 plus or minus 26 mg. per day). A high salt diet does not result in sustained hyperuricosuria, although it may predispose to kidney stone formation in other ways.

Disorders of proximal nephron function

The proximal nephron is responsible for reabsorbing 80 to 99 percent of several filtered solutes, including amino acids, glucose and bicarbonate. Separate, high-affinity sodium co-transport mechanisms are used. Increasing luminal concentration of each of these solutes stimulates its active transcellular reabsorption until there is saturation. Slightly less than half of the filtered chloride is reabsorbed, partly by passive mechanisms that are linked to the reabsorption of organic solutes and bicarbonate, as well as by less well defined independent cellular and/or paracellular mechanisms that appear to be sensitive to transepithelial osmotic pressure gradients. Proximal tubule reabsorption is isosmotic and isonatric, and about 50 to 60 percent of the filtered sodium and water in reabsorbed. Disorders or proximal nephron function include conditions in which luminal, cellular and/or peritubular factors affecting reabsorption are altered. Clinical disorders caused by modification of the luminal reabsorptive determinants include conditions in which tubular flow rate is increased or luminal composition is altered, as when non-reabsorbable solutes (mannitol) are filtered or when reabsorbable solutes (glucose) are filtered in concentrations exceeding their tubular transport capacity. Other disorders occur due to loss of affinity or capacity of the cellular active transport systems for specific solutes, such as amino acids (renal aminoacidurias), glucose (renal glycosurias) and bicarbonate (proximal renal tubular acidosis), or for all solutes (Fanconi syndrome). Finally, disorders due to changes in the peritubular factors affecting reabsorption include states of altered peritubular Starling forces or pH, which modify sodium chloride or sodium bicarbonate reabsorption, respectively.

Adaptive changes of juxtamedullary glomerular filtration in the remnant kidney

The participation of surviving juxtamedullary nephrons in the adaptive changes of glomerular filtration that occur in response to loss of functioning nephron mass was examined by direct micropuncture of the rat renal papilla. The solitary remnant kidney (RK) in rats with an 85% reduction of renal mass demonstrated strikingly elevated values for single nephron glomerular filtration rate (SNGFR) in both superficial (46.1 +/- 3.2 nl/min) and juxtamedullary (73.5 +/- 6.1 nl/min) nephrons in comparison to respective values observed in normal hydrophenic rats (superficial SNGFR = 15.0 +/- 1.9 nl/min, P less than 0.001, and juxtamedullary SNGFR = 30.2 +/- 3.2 nl/min, P less than 0.001). In RK rats, the proximal portions of both superficial and juxtamedullary nephrons exhibited a marked increase in absolute fluid reabsorption as well as a markedly enhanced delivery of fluid to more distal portions of the nephron. These observations indicate that similar, not preferential, functional adaptations in glomerular filtration occur concomitantly in both superficial and juxtamedullary nephrons consequent to reduction of renal mass.

Relationship between serum albumin concentration and tubular reabsorption of glucose in renal disease

Micropuncture studies in rats and dogs have provided evidence for a cause-and-effect relationship between peritubular protein concentration and proximal tubular reabsorption (PTR). If this effect is obtained in man, hypoalbuminemia in nephrotic syndrome should lead to a fall in PTR. Sodium excretion, however, is very low in nephrotic patients; but this sodium retention may be due to distal over-reabsorption. Glucose may be used as a marker of PTR. Because of the linkage between glucose and sodium, glucose reabsorption is expected to be suppressed when PTR of sodium is suppressed. Glucose titration studies were performed in 21 patients with chronic glomerulonephritis without renal failure divided in three groups: I (six patients) with serum albumin greater than 3 g/100 ml; II (five patients) with serum albumin of 2 to 3 g/100 ml; and III (10 patients with edema and nephrotic syndrome) with serum albumin less than 2 g/100 ml. The minimum threshold for glucose decreased in nephrotic patients (group III), and its fall was related directly to hypoalbuminemia. The splay of titration curve was markedly increased in group III when compared to the titration curves of patients without nephrotic syndrome (groups I and II). The splay point was 0.78 in group I, 0.52 in group II, and 0.37 in group III. These data provide evidence that glucose reabsorption is decreased in nephrotic syndrome and are consistent with a fall in PTR in nephrotic syndrome.

Effects of extracellular volume expansion on the tubular reabsorption of glucose. A microinjection study

Clearance and intratubular injections of (14C) glucose were performed in glucose loaded rats, during control (C) and extracellular fluid volume expansion (VE) to 10% of body weight. VE resulted in a significant decrease in hematocrit from 47.50 +/- 1.06 to 38.80 +/- 1.14% and plasma protein from 6.23 +/- 0.25 to 4.13 +/- 0.21 gm/100 ml. Glomerular filtration rate (GFR) increased by 51% from 1.06 +/- 0.07 to 1.60 +/- 0.35 ml/min. Fractional excretion of sodium increased significantly from 0.42 +/- 0.07 to 12.58 +/- 1.25%. Maximal glucose reabsorption (TmG) was unchanged from 3.47 +/- 0.42 to 3.29 +/- 0.41 mg/min. However, TmG/GFR decreased significantly from 3.14 +/- 0.22 to 1.94 +/- 0.21 mg/ml GFR. As compared to C, VE resulted in a significant increase in (14C) glucose recovery after injection into the early and late proximal tubules, from 63 +/- 3 to 81 +/- 2% to 88 +/- 1% respectively. After distal tubular injections (14C) glucose recovery was complete in both C and VE; early distal injection 97 +/- 1 vs 98 +/- 1%, late distal injection 98 +/- 1 vs 99 +/- 1%. These results indicate an inhibitory effect of VE on fractional glucose reabsorption in the superficial nephron. There is no evidence for glucose reabsorption in the superficial distal nephron during C and VE.

Effect of mannitol on glomerular ultrafiltration in the hydropenic rat

The effect of mannitol upon glomerular ultrafiltration was examined in hydropenic Munich-Wistar rats. Superficial nephron filtration rate (sngfr) rose from 32.0+/-0.9 nl/min/g kidney wt to 42.0+/-1.6 (P < 0.001) in eight rats. Hydrostatic pressure gradients acting across the glomerular capillary (DeltaP) were measured in glomerular capillaries and Bowman's space with a servo-nulling device, systemic (piA) and efferent arteriolar oncotic pressures (piE) were determined by microprotein analysis. These data were applied to a computer-based mathematical model of glomerular ultrafiltration to determine the profile of effective filtration pressure (EFP = DeltaP - pi) and total glomerular permeability (L(p)A) in both states. Filtration equilibrium obtained in hydropenia (L(p)A >/= 0.099+/-0.006 nl/s/g kidney wt/mm Hg) and sngfr rose because EFP increased from a maximum value of 4.2+/-1.1 to 12.8+/-0.5 mm Hg after mannitol (P <0.01). This increase was due to both increased nephron plasma flow and decreased piA. Computer analysis of these data revealed that more than half (>58%) of this increase was due to decreased piA, consequent to dilution of protein. Since EFP was disequilibrated after mannitol, L(p)A could be calculated accurately (0.065 +/- 0.003 nl/s/g kidney wt/mm Hg) and was significantly lower than the minimum estimate in hydropenia.Therefore, sngfr does increase with mannitol and this increase is not wholly dependent upon an increase in nephron plasma flow since the major factor increasing EFP was decreased piA.

The effect of glucose on urinary cation excretion during chronic extracellular volume expansion in normal man

Both glucose administration and extracellular volume expansion augment urinary calcium and magnesium excretion. While volume expansion also augments sodium excretion, glucose induces an antinatriuresis. To examine the interrelationships of volume expansion and of glucose administration on sodium, calcium, and magnesium excretion, the effects of glucose were evaluated during clearance studies in the same subjects before and after chronic extracellular volume expansion produced by desoxycorticosterone acetate (DOCA) and a normal dietary sodium intake. The augmentation of U(Ca)V and U(Mg)V by glucose was simply additive to the increments in divalent cation excretion caused by "escape" from the sodium-retaining effects of DOCA. Glucose administration reduced U(Na)V, an effect exaggerated after DOCA escape and associated with reductions in volume/glomerular filtration rate (V/GFR) and C(Na) + C(H2O)/GFR, suggesting augmented proximal tubular reabsorption. Before glucose, U(Na) was inversely correlated with U(G), and after glucose administration C(Na)/GFR was inversely correlated with T(G)/GFR. We propose that the availability of glucose in the proximal tubule stimulates Na reabsorption while delaying development of a chloride diffusion potential, thereby inhibiting tubular reabsorption of Ca and Mg.