Micropuncture study of the proximal and distal tubule in the dog

Stationary and Nonstationary Ion and Water Flux Interactions in Kidney Proximal Tubule: Mathematical Analysis of Isosmotic Transport by a Minimalistic Model

Our mathematical model of epithelial transport (Larsen et al. Acta Physiol. 195:171–186, 2009) is extended by equations for currents and conductance of apical SGLT2. With independent variables of the physiological parameter space, the model reproduces intracellular solute concentrations, ion and water fluxes, and electrophysiology of proximal convoluted tubule. The following were shown:

Micropuncture study of nephron function in the rhesus monkey

The function of the proximal and distal tubule was studied in the rhesus monkey during antidiuresis and during the diuresis after furosemide administration (during which extracellular fluid volume was maintained).In the proximal tubule, fluid to plasma ratios for sodium, potassium, and osmolality approximated unity. During antidiuresis, about 30% of the filtered water remained at the end of the accessible portion of this segment (92% of length). Fluid was hypotonic to plasma throughout the distal tubule. 25% of the filtered water was present in the early distal tubule. Small but significant water reabsorption (about 8% of filtered) occurred in remainder of this segment. Tubule fluid to plasma potassium concentration ratios tended to increase along the distal tubule, and the amount of potassium, relative to the amount filtered, increased from 13% in the early portion of this segment to 26% in the late portion.After furosemide was administered animals excreted about one-third of the filtered sodium and water. Despite this diuresis, electrolyte and water reabsorption along the proximal tubule did not differ from values obtained in control animals. Osmolality and sodium concentration of fluid from the distal tubule approached those of plasma. 22% of the filtered sodium (twice the control values) reached the distal tubule, whereas the fraction of filtered water remaining was only slightly increased. These findings indicate that, after the administration of this drug, inhibition of sodium reabsorption occurred in the water-impermeable segment of the nephron, rather than in the proximal tubule. After furosemide administration, all tubule fluid to plasma potassium concentration ratios in the distal tubule were equal to or greater than one, suggesting inhibition of active potassium reabsorption at or prior to this site.Fluid to plasma bicarbonate concentration ratios from the midportion of the proximal tubule were consistently less than one in normal monkeys. After acetazolamide was administered, the bicarbonate concentration of samples of tubule fluid recollected from these same sites was the same as, or higher than in plasma. This fact demonstrates the inhibition of bicarbonate reabsorption in this portion of the tubule.

Reversal of trimethoprim-induced antikaliuresis

High-dose trimethoprim-sulfamethoxazole (TMP-SMX) causes hyperkalemia, thought to result from TMP-induced blockade of amiloride-sensitive Na(+)-channels in the distal nephron. The present study was performed in anesthetized dogs to determine if increasing distal sodium delivery affects this antikaliuretic effect. In Group 1, intrarenal infusion of vehicle did not alter renal function. In Group 2, i.v. infusion of amiloride led to diuresis, natriuresis and antikaliuresis associated with a reduction of the transtubular potassium gradient (TTKG) in both kidneys. Intrarenal infusion of TMP (0.2 mg/kg/min) into the left kidney did not further alter these parameters. In groups 3 and 4, intrarenal infusion of TMP caused an ipsilateral diuresis, natriuresis, antikaliuresis and a reduction in (TTKG) without affecting the contralateral kidney. The TMP infusion was followed by furosemide (20 mg i.v.) in group 3 and acute saline loading in group 4. Despite continuous TMP infusion, both furosemide and saline loading reversed the antikaliuretic effect of TMP in the ipsilateral kidney and was associated with a similar kaliuresis, diuresis, natriuresis and decrease in urine osmolality in both kidneys. The TTKG following furosemide or saline loading increased in the ipsilateral kidney and decreased in the contralateral kidney. In all groups the systemic and renal hemodynamics remained unchanged. These results suggest that acute administration of TMP inhibits the amiloride-sensitive Na(+)-channel and K+ secretion in the distal nephron. Maneuvers that increase distal Na+ delivery can abrogate TMP's antikaliuretic effect due, in part, to an increase of the low TTKG observed with TMP.

Evaluation of lithium clearance as a marker of proximal tubule sodium handling

Estimations of proximal tubule sodium reabsorption with the FELi method come closer to direct measurements than any other indirect method. There is little doubt that most lithium reabsorption takes place in the proximal tubules, very likely in proportion to the reabsorption of sodium and water. It is also likely that changes in proximal tubule sodium reabsorption due to changes in volume status are paralleled by changes in proximal tubule lithium reabsorption, at least in the superficial nephrons. Nonetheless, changes in FELi probably do not purely reflect changes in proximal reabsorption, since lithium is also handled beyond the proximal tubules. Acknowledged problems are lithium reabsorption in Henle's loop and in the late distal and collecting tubules. The latter occurs in the rat and the dog, but not or much less in men. Sodium restriction enhances this lithium transport considerably. It is as yet uncertain whether other conditions, such as increased vasopressin activity or lowering of renal perfusion pressure, also influence this transport. Amiloride appears to prevent this reabsorption of lithium. Therefore, this drug can be used in lithium clearance studies whenever unwanted "distal" lithium reabsorption is expected. Lithium reabsorption in Henle's loop forms a greater problem as it cannot be prevented by any drug without influencing proximal tubule reabsorption. It is estimated that about 7% of the filtered lithium (one-tenth of total lithium reabsorption) is normally taken up here, preferentially in deep nephrons. In view of studies with furosemide, this reabsorption probably varies with sodium intake, but the proportion of this variation to that of proximal tubule lithium reabsorption is obscure. This remains an uncertain factor in any circumstance where the lithium clearance method is used. In some conditions the change in FELi may be so large relative to the expected changes in proximal reabsorption, that use of FELi as marker of end-proximal solute delivery seems unjustified. Disproportionately large suppression is likely during mineralo-corticoid-induced volume expansion, and stimulation during prostaglandin synthesis inhibition and vasopressin. Based on observations in these conditions the potential range of lithium reabsorption in the loop of Henle would be 0 to 15% of filtered load. In this review attention was paid mainly to the validity of lithium clearance as a pure "proximal marker". Many of our interpretations suffer from incomplete certainty with respect to the renal effects of tested maneuvers, a problem which is acknowledged.(ABSTRACT TRUNCATED AT 400 WORDS)

Interactions among prostaglandin E2, antidiuretic hormone, and cyclic adenosine monophosphate in modulating Cl- absorption in single mouse medullary thick ascending limbs of Henle

This paper describes the inhibitory effect of prostaglandin E2 (PGE2) on antidiuretic hormone (ADH)-stimulated net Cl- absorption and spontaneous transepithelial voltage (Ve) in single medullary thick ascending limbs of Henle (TALH, thick ascending limb; mTALH, medullary segment; cTALH, cortical segment) obtained from mouse kidney. The experimental data indicate that PGE2 reduced the ADH-dependent values of net Cl- absorption (JnetCl, eq cm-2 s-1) and Ve (mV) in a dose-dependent manner; that increasing concentrations of peritubular ADH reversed the PGE2-mediated reductions in the ADH-dependent moiety of Ve in the mouse mTALH; that PGE2 had no effect on cyclic AMP-stimulated increments in Ve in the mouse mTALH; and that PGE2 had no effect on Ve in the cTALH, where Ve is unaffected either by ADH or by cyclic AMP. These effects might be obtained because of a direct competition between ADH and PGE2 for receptor binding on basolateral membranes. Alternatively, PGE2 might have reduced the affinities between ADH-receptor units and a component(s) of the series of processes leading to adenyl cyclase activation. The latter argument requires that basolateral membranes of the mouse mTALH exhibit receptor reserve, i.e., at the minimum concentration of ADH required to enhance Ve and JnetCl maximally, a fraction of basolateral membrane ADH receptors were unoccupied. According to this view, increasing peritubular ADH concentrations might reverse the PGE2-mediated reduction in ADH-dependent salt transport by increasing the number of basolateral membrane receptors occupied by ADH.

A search for a defect of proximal transport in denervated kidneys of conscious dogs

The influence of renal nerves on proximal Na+ reabsorption was studied in clearance experiments with unilaterally renal-denervated conscious dogs prepared by surgical bladder division. Two types of experiments were made : A. maximal water diuresis, and B. Total blockade of distal NaCl reabsorption with ethacrynic acid and chlorothiazide. In maximal water diuresis CH2O + CNa was used as a measure of fluid delivery to the distal nephron. At similar GFR on both sides, the proximal reabsorption estimated as GFR--(CH2O + CNa) was 38.4 +/- 5.6 ml/min for the intact and 35.9 +/- 4.2 ml/min for the denervated kidney (n = 6, difference NS). After distal tubular blockade, proximal Na+ reabsorption calculated as filtered load minus urinary excretion was 3.84 +/- 0.43 mmol/min for the intact and 3.91 +/- 0.36 mmol/min for the denervated kidney (n = 6, difference NS). The fractional reabsorption of NA+ was 64.9 +/- 1.0% for the intact and 66.9 +/- 1.1% for the denervated kidney (difference NS). In contrast to data from renal denervation studies with anaesthetized animals, the present experiments did not show any difference in proximal reabsorption between the innervated- and denervated kidney. We conclude that in absence of anaesthesia renal efferent nerves have no major effect on NaCl transport in dog proximal tubule.

Editorial review. Recent formulations of the urinary concentrating mechanism: a status report

The status of the purely passive mode of solute concentration as of 1979 appears to be similar to that of the original countercurrent hypothesis 10 years ago. The passive mode concept has advanced our understanding of the concentrating process by qualitatively incorporating the permeability characteristics of tubule segments and the lack of an active transport process in the thin loop of Henle into a mechanism which has attractive economy and explanatory value. But in the final analysis some assumptions are not borne out by experimental findings (for example, the high urea concentration of fluid in the rat and hamster end-descending limb; the likelihood of net transepithelial addition of sodium chloride to the Psammomys descending limb; the removal of sodium chloride from the hamster ascending limb against an apparent electrochemical gradient under certain circumstances; and the osmotic lag between vasa recta blood and interstitium in the rat). Furthermore, when the known permeability and transport characteristics of the renal tubule are incorporated into a mathematic model of the passive operating mode, numerical simulations fail to establish a progressively hyperosmotic inner medulla. This does not rule out the applicability of the more general model (Eq. 1), particularly if evidence for some form of active transport in the inner medulla, heretofore lacking, is forthcoming.

Effect of amiloride on the renal response to saline expansion in new-born dogs

1. The renal effects of amiloride were studied in twenty-six new-born and nine adult dogs, with and without saline expansion. 2. Without saline expansion, amiloride inhibited more sodium reabsorption (normalized to GFR) in puppies than in adults (5.95 vs. 2.18 muequiv/ml. GFR, P less than 0.01). Amiloride inhibited sodium reabsorption more in saline expanded than in nonexpanded puppies (9.39 vs. 5.95 muequiv/ml. GFR, P less than 0.01) but there was no difference between expanded and non-expanded adults. 3. Saline expansion by itself increased fractional sodium excretion (CNa/GFR) more in the adult than in the puppy (0.071 vs. 0.019, P less than 0.01). During amiloride inhibition, saline expansion increased CNa/GFR to 0.045 in the puppy (compared to 0.019 in the nonamiloride inhibited puppy) but saline expansion increased CNa/GFR to the same degree in the amiloride as in the non-amiloride inhibited adult. 4. In all puppies amiloride inhibited more sodium reabsorption than potassium secretion and there was poor correlation between the degree of inhibition of sodium reabsorption and potassium secretion for both the puppy (r = 0.14) and the adult (r = 0.05). 5. Assuming that amiloride acts by inhibiting sodium reabsorption and potassium secretion in the late distal and cortical collecting tubules, these results support the conclusion that in these regions of the nephron of the new-born dog (a) a greater fraction of the filtered sodium is reabsorbed than in the adult and (b) increased fractional sodium reabsorption is responsible, in part, for the attenuated natriuretic response to saline expansion.

Effects of acetazolamide on proximal tubule C1, Na, and HCO3 transport in normal and acidotic dogs during distal blockade

It has been suggested that the establishment of a tubular fluid to plasma chloride gradient in the late proximal tubule by the reabsorption of bicarbonate (and other anions) in the early proximal tubule is responsible for a significant part of sodium chloride and water reabsorption in the proximal tubule. In the present study the effects of acetazolamide on proximal tubule water and electrolyte excretion were examined in 6 normal dogs and 10 chronic ammonium chloride-loaded dogs during distal blockade produced by ethacrynic acid and chlorothiazide administration. During distal blockade control urine/plasma osmolality and urine/plasma sodium were close to unity in all experiments. Urine/plasma chloride and urine/plasma bicarbonate were 1.21+/-0.02 and 0.75+/-0.07 in normal and 1.24+/-0.01 and 0.04+/-0.01 in acidotic dogs, respectively. After the administration of acetazolamide (20 mg/kg i.v.), there was a significant increase in urine flow, absolute and fractional excretion of sodium, bicarbonate, and chloride in all animals. Associated with these effects, urine/plasma osmolality and urine/plasma sodium remained unchanged but urine/plasma chloride decreased significantly to 1.15+/-0.01 in normal and to 1.19+/-0.01 in acidotic dogs. In acidotic dogs there was a significant correlation between the increase in bicarbonate, sodium, or chloride excretion after acetazolamide and the plasma bicarbonate level (range 6.8-12.5 meq/liter). These data demonstrate a significant effect of acetazolamide on bicarbonate, sodium, and chloride reabsorption in the proximal tubule even in the face of severe acidosis. Moreover, the data suggest that the decrease in chloride reabsorption (and accompanying sodium) after acetazolamide is related to the decrease in bicarbonate reabsorption and the associated decrease in the transtubular chloride gradient.

The effect of indomethacin on kidney function: indomethacin and furosemide antagonism

The effect of furosemide and indomethacin on renal function was investigated in anesthetized dogs. 1. On infusing 0.05 mg/kg/min furosemide directly into the left renal artery, about 18% of the filtered water and 19% of the filtered sodium was excreted in both non-hydrated and hydrated dogs. 2. Under the effect of 0.1 mg/kg/min indomethacin administered intravenously renal blood flow decreased markedly, glomerular filtration rate remained unchanged, the filtration fraction increased. 3. Indomethacin induced a decrease in water and sodium rejection and water and sodium excretion in both the hydrated and non-hydrated animals. 4. Indomethacin inhibited the diuretic effect of furosemide in the non-hydrated animals. This effect was much weaker in the hydrated group. These data suggest that in the anesthetized dog, endogenous prostaglandins may serve to maintain renal blood flow but not glomerular filtration rate. It is concluded that the indomethacin-induced inhibition of the furosemide effect is the consequence of hemodynamic changes occurring in the kidney. The data do not support the direct physiological role of prostaglandins in regulating tubular function.

Studies on the pathophysiology of acute renal failure. II. A histochemical study of the proximal tubule of the rat following administration of mercuric chloride

Acute renal failure was induced in male rats by the subcutaneous injectioon of 4 mg HgC12 per kg body weight. Enzyme activities of the proximal tubule were studied histochemically at six time intervals from 15 min to 24 h. The enzyme studied were alkaline phosphatase, 5'-nucleotidase, acid phosphatase, alpha-glycerophosphate dehydrogenase (NAD-independent), malic dehydrogenase, succinic dehydrogenase, latic dehydrogenase, glucose-6-phosphate dehydrogenase and glucose-6-phosphatase. Decreases in activity were observed for alkaline phosphatase and 5'-nucleotidase after 15 min. Acid phosphatase was decreased after 30 min. These three enzymes returned to control levels after 3 h, but malic dehydrogenase and alpha-glycerophosphate dehydrogenase were decreased at this time interval. Succinic dehydrogenase was first decreased after 6 h. The earliest morphological changes detectable by light microscopy were observed in pars recta tubules in the medullary rays after 6 h, a time when all enzymes studied showed widespread decreased activity throughout the proximal tubule. After 24 h, the pars convoluta appeared morphologically normal but the pars recta was necrotic and exhibited calcification, whereas enzyme activity was decreased (absent in some cases) in both pars convoluta and pars recta. These results support the hypothesis that Hg++, when given in a sublethal dose, is associated with early histochemical changes in the brush border of the proximal tubule, which may be related to early changes in sodium reabsorption and to the subsequent development of acute renal failure. The observation that changes in plasma membrane-associated enzymes occur early and prior to alterations in enzymes of mitochondria and the endoplasmic reticulum suggests that Hg++ interacts initially with the plasma membrane.

Tubular chloride transport and the mode of action of some diuretics

The renal diluting segment (thick ascending limb of Henle's loop) reabsorbs sodium chloride in excess of water and is responsible for dilution of the urine as well as reabsorption of a large fraction of the salt present in the glomerular ultrafiltrate. In this segment, there is active reabsorption of chloride which causes the voltage to be positive in the tubule lumen. Most, if not all, of the sodium transport is passive, driven by the voltage. Three major diuretic drugs (mersalyl, furosemide and ethacrynic acid) act in the lumen of the diluting segment to inhibit active chloride transport, not sodium transport as previously believed. This specific action on chloride transport may explain how these drugs are able to inhibit salt transport in the kidney while having so little effect on the electrolyte transport processes elsewhere in the body.

Characteristics of the relationship between the flow rate of tubular fluid and potassium transport in the distal tubule of the rat

The flow rate of tubular fluid has been suggested as one of several factors which may influence potassium transport in the distal convoluted tubule of the kidney. In the present micropuncture studies, the relationship between the flow rate of distal tubular fluid and potassium transport was examined in four groups of rats. Three groups of rats (I, II, and IV) were fed normal rat chow before study whereas one group (III) was fed chow containing 10% KCl. Group II received 10-20 mug/kg per h of d-aldosterone throughout the study. Distal tubular potassium transport in groups I, II, and III was examined before and after an increase in the flow rate of distal tubular fluid as induced by the infusion of an isotonic saline-bicarbonate solution equivalent to 10% of body weight. In group IV, distal tubular potassium transport was examined before and after enhancement of the flow rate by the infusion of hypertonic (15%) mannitol. In all four groups, distal tubular potassium secretion increased as the flow rate of tubular fluid increased. The nature of the relationship between distal tubular potassium transport and tubular fluid flow rate, however, was influenced by the extent to which the tubular fluid to plasma potassium ratio in the late distal tubule varied as the flow rate increased. As the flow rate was increased this ratio decreased significantly and to a comparable extent in groups I and II. In groups III and IV, on the other hand, this ratio was essentially identical during hydropenia and after the increase in the flow rate of tubular fluid. As a result, the increment in the amount of potassium present at the late distal tubule, which occurred as the flow rate increased, was significantly greater in groups III and IV than in groups I and II. The contrast in the relationship between the flow rate of distal tubular fluid and potassium transport which were observed, probably reflects differences in the net driving force for cell to lumen potassium movement. Seemingly, the net driving force for potassium movement was maintained, as the flow rate of tubular fluid increased, by chronic potassium loading in group III and by hypertonic mannitol infusion in group IV. In groups I and II, the net driving force for potassium movement decreased as the flow rate of tubular fluid increased. However, the net driving force did not decrease in proportion to the increase in flow rate since potassium secretion was increased by increments in flow rate in these groups as well. We conclude that our results are consistent with the view that the flow rate of tubular fluid is a factor which can affect distal tubular potassium transport. However, the nature of the relationship between the flow rate of tubular fluid and potassium transport appears to depend upon the degree to which the driving force for cell to lumen potassium movement changes as the flow rate is varied.

Sodium-independent active potassium reabsorption in proximal tubule of the dog

Prior studies of proximal tubule reabsorption have failed to distinguish conclusively between a separate active K+ transport system and K+ movement linked to Na+ reabsorption. To attempt to dissociate movement of K+ from Na+ and Ca++, recollection micropuncture experiments were performed in proximal tubules of intact and thyroparathyroidectomized (TPTX) dogs under two different conditions known to inhibit Na+ reabsorption: saline expansion to 5% body wt, and 5 mg/kg acetazolamide. A control hydropenic group was also studied. Tubular concentrations of K+, Na+, and Ca++ were measured by electron probe analysis. During initial collections, mean+/-SEM tubular fluid/plasma (TF/P)(K+) was 1.07 +/-0.05, 1.05+/-0.05, and 1.00+/-0.03 in intact hydropenic (n = 7), saline (n = 6), and acetazolamide (n = 8) groups; fractional reabsorption (FR) of K+ in proximal tubules was 0.35, 0.39, and 0.31 respectively. After saline, (TF/P)(Inul in) fell from 1.81 to 1.34 (P < 0.01); (TF/P)(K+), (TF/P)(Na+), and tubular fluid/ultrafiltrate, (TF/UF)(Ca++) did not change, so that FR of all three ions fell proportionately. After acetazolamide, however, despite a 24% inhibition of FR of Na+ and Ca++, (TF/P)(K+) fell to 0.85+/-0.04 (P < 0.005) so that FR of K+ was unchanged at 0.34. In three corresponding groups of TPTX dogs, similar results were obtained. Acetazolamide (n = 7) inhibited FR of Na+ and Ca++ by 41%, but (TF/P)(K+) fell from 1.03+/-0.03 to 0.89+/-0.04 (P < 0.005) so that FR of K+ was unchanged (0.36-0.34).A separate uphill transport system for K+ in proximal tubules is therefore unmasked by acetazolamide, a drug which selectively inhibits Na+ (and Ca++) reabsorption. Saline, on the other hand, inhibits net reabsorption of all three ions, probably by increasing passive backflux via intercellular channels.

Effect of extracellular volume expansion upon sodium reabsorption in the distal nephron of dogs

Micropuncture studies have disclosed that extracellular fluid (ECF) volume expansion inhibits sodium reabsorption in the proximal tubule. The diuresis that ensues represents only a portion of the increment in sodium and water escaping proximal reabsorption, since a large and variable fraction of the increment is reabsorbed distally. In certain experimental models proximal reabsorption may be depressed by ECF volume expansion, yet only a negligible amount of sodium appears in the final urine. This suggests that saline diuresis is the consequence of depressed distal sodium reabsorption. Previous clearance and micropuncture studies have not conclusively proven this. Eight dogs were studied repeatedly: in some studies glomerular filtration rate and distal delivery were increased markedly without sodium administration; in others comparably high distal sodium loads were achieved by progressive 1/2 isotonic saline infusion. C(H2O) at high distal sodium loads was depressed by expansion of the ECF volume with hypotonic saline. The difference in free water formation between dogs which did and did not receive hypotonic saline was accounted for by the difference in sodium excretion. In one dog hypotonic saline expansion failed to depress free water formation; likewise the level of natriuresis in this dog was severely attenuated. The results of these experiments provide strong evidence that the natriuresis that occurs following ECF volume expansion with saline is a consequence of alteration in function of the distal nephron.

Sodium chloride and water transport in the medullary thick ascending limb of Henle. Evidence for active chloride transport

Transport of NaCl and water was examined in the rabbit medullary thick ascending limb of Henle (ALH) by perfusing isolated segments of these nephrons in vitro. Osmotic water permeability was evaluated by perfusing tubules against imposed osmotic gradients. In these experiments the net transport of fluid remained at zero when segments of thick ALH were perfused with isotonic ultrafiltrate in a bath of rabbit serum in which the serum osmolality was increased by the addition of either 239+/-8 mosmol/liter of raffinose or 232+/-17 mosmol of NaCl indicating that the thick ascending limb of Henle is impermeant to osmotic flow of water. When these tubules were perfused at slow rates with isosmolal ultrafiltrate of same rabbit serum as used for the bath, the effluent osmolality was consistently lowered to concentrations less than the perfusate and the bath. That this decrease in collected fluid osmolality represented salt transport was demonstrated in a separate set of experiments in which it was shown that the sodium and chloride concentrations decreased to 0.79+/-0.02 and 0.77+/-0.02 respectively when compared with the perfusion fluid concentrations. In each instance the simultaneously determined transtubular potential difference (PD) revealed the lumen to be positive with the magnitude dependent on the perfusion rate. At flow rates above 2 nl.min(-1), the mean transtubular PD was stable and equal to 6.70+/-0.34 mv. At stop-flow conditions this PD became more positive. Ouabain and cooling reversibly decreased the magnitude of this PD. The transtubular PD remained positive, 3.3+/-0.2 mV, when complete substitution of Na by choline was carried out in both the perfusion fluid and the bathing media. These results are interpreted to indicate that the active transport process is primarily an electrogenic chloride mechanism. The isotopic permeability coefficient for Na was 6.27+/-0.38 x 10(-5) cm.s(-1) indicating that the thick ALH is approximately as permeable to Na as the proximal convoluted tubule. The chloride permeability coefficient for the thick ALH was 1.06+/-0.12 x 10(-5) cm.s(-1) which is significantly less than the chloride permeability of the proximal tubule. These data demonstrate that the medullary thick ascending limb of Henle is water impermeable while having the capacity for active outward solute transport as a consequence of an electrogenic chloride pump. The combination of these characteristics allows this segment to generate a dilute tubular fluid and participate as the principal energy source for the overall operation of the countercurrent multiplication system.

Micropuncture studies of sodium transport in the remnant kidney of the dog. The effect of graded volume expansion

Proximal and distal tubule micropuncture studies were performed to examine the response to graded extracellular volume (ECV) expansion in 10 normal dogs (stage I), 11 dogs with a unilateral remnant kidney (stage II), and 7 dogs with a remnant kidney after removal of the contralateral kidney (stage III). Before ECV expansion in stage III, there was a suggestive reduction in proximal tubule as well as loop fractional reabsorption of sodium. After ECV expansion to 3% body weight proximal tubule reabsorption was depressed in all groups of animals, while little further inhibition was observed in this segment with additional expansion to 10% body weight. In contrast, the fraction of filtered sodium remaining in the distal tubule rose progressively in all three groups after graded ECV expansion, suggesting that the graded natriuretic response found in the final urine was largely due to a similar response in the loop of Henle rather than that in the proximal tubule. The distal tubule response of the remnant kidney in both stages II and III was greater than that in stage I. These data indicate that although enhanced sodium excretion per nephron in chronic renal failure may be related to uremia, its exaggerated response to ECV expansion is due, at least in part, to certain as yet unidentified intrarenal factors consequent to reduction in functioning renal mass.

Effect of frusemide on the reactivity of rat portal vein

Short lengths of rat portal vein were suspended in Krebs bicarbonate solution. Contractile responses to noradrenaline and angiotensin II and the parameters of spontaneous contractions were determined before and after addition of frusemide at 10–6 and 10–4 g/ml. Frusemide at 10–6 g/ml slightly suppressed the amplitude of spontaneous contractions and slightly decreased the responses to noradrenaline and angiotensin II. At 10–4 g/ml the drug had a much greater inhibitory effect on endogenous and exogenous stimulation. The concentrations used appear to be similar to those achieved during effective diuretic and hypotensive treatment. It is postulated that frusemide may have a direct inhibitory action on the reactivity of vascular smooth muscle mrticularlv in the capacitance vessels.

Urine concentration during solute diuresis in potassium-depleted rabbits. Evidence for a defect in tubular sodium transport

1. The relationship between osmolal clearance (C(osm)) and the reabsorption of solute-free water by the kidney (T(H2O) (c)) was examined during 10% mannitol and 2.3% saline diuresis in normal and potassium-depleted rabbits.2. In normal rabbits at osmolal clearances close to 3.0 ml./min, T(H2O) (c) during mannitol diuresis was 0.87 +/- 0.06 ml./min and during saline diuresis 1.19 +/- 0.07 ml./min. The mean difference in T(H2O) (c) of 0.32 +/- 0.05 ml./min was highly significant (P < 0.001).3. In one group of potassium-depleted rabbits with a reduction in maximal urinary concentration, T(H2O) (c) during both mannitol and saline diureses was reduced significantly below normal and the increment in T(H2O) (c) normally seen during saline diuresis was abolished.4. In a second group of potassium-depleted rabbits maximal urinary concentration (1253 +/- 88 m-osmole/kg H(2)O) was not significantly different from that in normal rabbits (1272 +/- 116 m-osmole/kg H(2)O). In these animals, T(H2O) (c) at osmolal clearances close to 3.0 ml./min was not significantly different during mannitol diuresis (0.83 +/- 0.07 ml./min) from that in normal animals, whereas it was reduced significantly during saline diuresis (0.89 +/- 0.07 ml./min, P < 0.001) and the difference in T(H2O) (c) normally seen between mannitol and saline diuresis was abolished.5. The inability to increase T(H2O) (c) during saline diuresis above that achieved during mannitol appears to be the earliest manifestation of the concentrating defect associated with potassium depletion. It probably results from an impairment of sodium transport by the ascending limb of the loop of Henle. This is supported by the fact that potassium-depleted rabbits excreted a greater percentage of the filtered load of sodium than did normal controls.

Characteristics of NaCl and water transport in the renal proximal tubule

Renal proximal tubular transport of salt and water has been examined using isolated perfused rabbit tubules. In this method direct measurements can be made under controlled conditions not readily achieved in vivo. The results are in general agreement with previous micropuncture studies in other species, supporting the validity of both sets of measurements. In the present studies, absorption of sodium salts and water occurred without change in the concentration of Na in the lumen except when a poorly reabsorbed solute (raffinose) was present, in which case, mean concentration of Na in the lumen reached a steady-state value 33-35 mEq liter(-1) less than in the bath. The tubule is very permeable to sodium salts (sodium permeability = 9.3 x 10(-5) cm sec(-1), sigma(NaCl) = 0.68-(0.71) and to water (hydraulic conductivity [L(p)] = 2.9 to 6.3 x 10(-5) cm sec(-1) atm(-1)). Net reabsorptive flux of Na was only 20% of the unidirectional Na flux. The steady-state concentration difference for Na in the presence of raffinose and sigma(NaCl) in the present studies was the same as previously found by micropuncture in the rat. On the other hand Na permeability, net Na transport rate, and L(p) were all from one-half to one-third as great in the isolated rabbit tubule as in the rat in vivo. Apparently, although the transport mechanisms appear to be basically the same in the two species, there are fewer transport units and passive permeability paths per unit length in the rabbit tubule than in the rat.

The nature of transtubular Na and K transport in isolated rabbit renal collecting tubules

In order to investigate the mechanism of Na and K transport, rabbit cortical collecting tubules were perfused in vitro and the concentrations of Na and K in lumen and bathing fluid and the transtubular electrical potential difference (PD) were measured. When the perfusate and external bath contained 150 Na-5 K (mEq liter(-1)), the sodium concentration decreased and the potassium concentration increased by an approximately equal amount in collected tubular fluid. The transtubular electrical potential was equal at both ends of the tubule in the steady state and ranged between 21 and 67 mv, lumen negative. In all tubules perfused at rates less than 0.5 nl min(-1), the K concentration of the collected fluid was higher and the Na concentration lower than that predicted for electrochemical equilibrium between lumen fluid and external bath, evidence for active transtubular transport of both cations. These results differ from those observed in rat distal tubule in which potassium secretion is passive. Active Na and K transport and the transtubular PD were decreased by (a) ouabain, (b) removal of sodium from the perfusate, or (c) removal of potassium from the external bath, evidence of interdependence of Na and K transport. The dependence of active K secretion on intraluminal Na concentration accounts for the phenomenon of "distal" Na-K exchange noted previously in clearance and stop-flow studies. The mechanism of Na transport may in part be electrogenic since the rate of decline of the transtubular PD in low K media was faster than could be accounted for on the basis of a reduction in cell potassium concentration.

Micropuncture study of hypertonic mannitol diuresis in the proximal and distal tubule of the dog kidney

Fractional reabsorption of water, sodium, and potassium at proximal and distal tubular sites within the nephron was studied by recollection-micropuncture experiments on dogs undergoing hypertonic mannitol diuresis. After an initial control hydropenic phase, 16% mannitol in modified Ringer's solution was administered intravenously, resulting in marked increases in fractional excretion of water (28.7%), sodium (12.6%), and potassium (63.9%). Inulin clearance decreased significantly from 35.1 to 25.2 ml/min. Analysis of paired micropuncture data revealed a significant decrease in tubule fluid to plasma (TF:P) inulin ratios in both the proximal tubule (1.63-1.45) and distal tubule (5.38-1.94). There was also a significant decrease in proximal TF:P sodium ratios (0.99-0.93) and potassium ratios (1.05-0.98). Distal TF:P sodium ratios, in contrast, rose significantly (0.38-0.59), while TF:P potassium ratios tended towards unity whether initially greater or less than one. Fractional reabsorption of sodium and water decreased by 5% and 10% respectively in the proximal tubule, but to a lesser extent than the resulting increases in fractional urinary excretion. The nonreabsorbed fraction, however, had increased sharply at the point of distal puncture for water (32%), sodium (26%), and potassium (26%), indicating a large inhibitory effect within the loop of Henle in addition to the smaller proximal effects.

An inhibitory effect of furosemide on sodium reabsorption by the proximal tubule of the rat nephron

The evidence from previous micropuncture studies for an inhibitory effect of furosemide on proximal sodium reabsorption in the rat has been conflicting. Intrinsic reabsorptive capacity, estimated in free flow and shrinking drop experiments, has been reported to be depressed, whereas fractional reabsorption usually remains unchanged. We have recently reported that, during conditions of elevated intraluminal hydrostatic pressure, unless care is taken to prevent retrograde flow of tubule fluid from more distal sites, the concentration of inulin in late proximal fluid is often factitiously elevated. Since furosemide raises intraluminal pressures, often markedly, the failure to detect a depression of fractional reabsorption might be the consequence of retrograde contamination during fluid collection. Experiments were designed to compare the effect of furosemide on fractional sodium reabsorption by the proximal tubule when collections were obtained with distal oil blocks of conventional length as well as with unusually long blocks of oil of low and high viscosities. When reflux is prevented, fractional sodium reabsorption is usually depressed by furosemide, whereas when conventional distal blocks are used, the calculated values for fractional reabsorption either remain unchanged or increase. Simultaneous measurements of nephron glomerular filtration rate indicate that the latter is the consequence of retrograde contamination.