The relationship between glomerular filtration rate and sodium reabsorption by the proximal tubule of the rat nephron

Demonstration of independent roles of proximal tubular reabsorption and intratubular load in the phenomenon of glomerulotubular balance during aortic constriction in the rat

The mechanism of glomerulotubular balance was investigated by microperfusion of the rat proximal tubule at two different rates before and after contriction of the aorta sufficient to produce a 50% reduction in whole kidney filtration rate and plasma flow. At a perfusion rate of 28 nl/min the absolute rate of proximal tubular reabsorption averaged 4.80+/-0.28 nl/mm.min in the absence of aortic constriction. Reducing the perfusion rate by one-half resulted in only a 22% decrease in the absolute rate of reabsorption, and imbalance between load and reabsorption resulted as fractional reabsorption of the perfused volume increased from 0.56 to 0.83 at 3 mm length of perfused tubule. These observations support other studies indicating that changing the load presented to the individual proximal tubule does not change reabsorptive rate sufficiently to result in glomerulotubular balance. Aortic constriction decreased the absolute rate of proximal tubular reabsorption approximately 50%, resulting in imbalance between load and reabsorption at the higher perfusion rate (fractional reabsorption of the perfused volume fell to 0.23 at 3 mm). Thus, the decrease in proximal tubular reabsorption necessary for glomerulotubular balance will occur independent of a change in the load presented for reabsorption. Balance between load and reabsorption was produced artificially by combining aortic constriction and a reduction in perfusion rate proportional to the reduction in whole kidney filtration rate. Mathematical analysis of the data suggests that the absolute rate of reabsorption along the accessible length of the proximal tubule is constant and is not proportional to the volume of fluid reaching a given site. Thus, there appears to be no contribution to glomerulotubular balance of any intra- or extratubular mechanism directly coupling load and the rate of proximal tubular reabsorption. It is concluded that glomerulotubular balance during aortic constriction is a consequence of hemodynamic effects of the maneuver to decrease filtration rate and the rate of proximal tubular reabsorption independently but in an approximately proportional manner.

Increased ureteral back pressure enhances renal tubular sodium reabsorption

Moderate increases of ureteral back pressure usually cause decreases of glomerular filtration rate and even greater decreases of sodium excretion. It has been assumed previously that increased ureteral back pressure does not enhance renal tubular sodium reabsorption directly and that the decreases of sodium excretion are caused by the decreases of glomerular filtration rate. In the experiments reported here, the effect of increased ureteral back pressure on urinary sodium excretion was studied in dogs in which changes of filtration rate were minimized by infusing saline while ureteral back-pressure was increased. When ureteral back pressure was increased on one side by 10-23 cm of water, the inulin clearance of the experimental kidney decreased by only 3-12% in 21 experiments, did not change significantly (+/-2%) in eight experiments, and increased by 3-8% in seven experiments. The sodium excretion of the experimental kidney decreased in all experiments regardless of whether its inulin clearance increased, decreased, or was unchanged from control values. When the inulin clearance of the experimental kidney increased or remained unchanged during increased ureteral back pressure, its reabsorption of sodium increased more than could be accounted for by the increase of filtered sodium. When the inulin clearance of the experimental kidney decreased during increased ureteral back pressure, its reabsorption of sodium decreased less than could be accounted for by the decrease of filtered sodium.Therefore, the effect of increased ureteral back pressure to decrease urinary sodium excretion is caused in part by increased tubular reabsorption of sodium.

Renal tubular flow dynamics during angiotensin diuresis in the rat

1. Tubular size and lissamine green transit times were measured in rat kidneys undergoing a diuretic response to angiotensin II (0.5 mug/kg per min), and compared with the changes observed during diuresis induced by osmotic diuretics, noradrenaline and chlorothiazide.2. Angiotensin always caused a marked prolongation in proximal and distal tubular transit times; individual distal convolutions were coloured for prolonged periods, and lissamine green appeared in high concentration in distal tubules.3. Marked changes were observed in superficial tubular calibre during a stable diuretic response to angiotensin. Where distal tubular diameter was normal for the rate of urine flow, proximal tubular volume was generally reduced. In a number of experiments, however, distal tubules were markedly dilated, and in these cases proximal tubular volume was also often increased. Angiotensin may therefore be capable of causing a degree of internal hydronephrosis in the rat kidney.4. Prolongation of dye transit times, and the appearance of a concentrated lissamine green bolus in distal tubules, was suggestive of a decreased superficial nephron flow rate, indicating that the diuretic effect of angiotensin may take place only through deeper nephrons.

Reversible stimulation of sodium transport in the toad bladder by stretch

Short-circuit current and transepithelial potential difference were measured in toad hemibladders mounted as sacs on glass cannulae. When sac volume was changed by adding or removing fluid, short circuit current responded by increasing or decreasing during the ensuing half-hour. The time course of the response and its magnitude indicated that it was not artefactual. Furthermore, net sodium flux responded similarly. Sac volume, and thus bladder surface area, could be varied from 0.03 to 0.4 cm(2)/mg wet weight. The mean response to either decreases or increases was 10 muA/cm(2). Everted hemibladders, however, responded less. Neither hydrostatic pressure, nor increased chloride conductance, nor increased access of oxygen or glucose to the mucosa was responsible for the response. Tissue conductance did vary markedly with volume, and may have played a role, but sodium conductance did not vary with volume in a consistent manner. The results indicate the existence of an intrinsic mechanism in this tissue which alters sodium transport in response to stretch.

The relationship between peritubular capillary protein concentration and fluid reabsorption by the renal proximal tubule

The relationship between peritubular capillary protein concentration and rate of sodium reabsorption by the rat proximal tubule was examined using free-flow recollection micropuncture techniques. Tubule fluid-to-plasma inulin ratios were measured before, during, and at successive intervals after brief (15-25 sec) intra-aortic injections (at the level of the renal artery) of colloid-free, isoncotic, and hyperoncotic solutions. Arterial hematocrit and protein concentrations were measured simultaneously in these rats. In other rats, total protein concentration of peritubular capillary blood plasma was determined before, during, and after these same infusions with a newly described submicroliter fiber-optic colorimeter. In the 15-25 sec interval necessary to infuse 2 ml of these test solutions, fractional and absolute sodium reabsorption varied directly with peritubular capillary colloid osmotic pressure, declining during infusion of colloid-free solutions, increasing during hyperoncotic infusions, and remaining unchanged during isoncotic infusions. In the subsequent 20-min interval after intra-aortic injection of these test solutions, capillary protein concentration remained at (isoncotic infusions) or returned to (colloid-free and hyperoncotic fluids) control values. Whereas reabsorption after colloid-free solutions returned to base line levels in parallel with the return in capillary protein concentration, after colloid infusions (which resulted in continued expansion of extracellular fluid volume), a progressive decline in reabsorption was observed. These results afford strong evidence that peritubular capillary colloid osmotic pressure is one important determinant of proximal sodium reabsorption. Nevertheless it is apparent that mechanisms other than or in addition to this must be invoked to explain the delayed inhibition of reabsorption that accompanies expansion of extracellular fluid volume by colloid solutions.

Failure to demonstrate a hormonal inhibitor of proximal sodium reabsorption

Recently, it has been reported that a humoral inhibitor of proximal sodium reabsorption could be detected in plasma, and dialysates of plasma, of rats and dogs undergoing saline diuresis. We have repeated these studies using similar techniques and protocols. Fractional sodium reabsorption by the proximal tubule (as estimated in free-flow micropuncture studies from tubule fluid-to-plasma inulin ratios) was found not to be lower during infusion of "natriuretic" plasma than during subsequent infusion of "hydropenic" plasma. Similarly, infusion of natriuretic plasma failed to prolong reabsorptive half-time of the shrinking drop beyond that seen during hydropenic plasma infusion. No increase in urine volume or rate of sodium excretion was observed during the period of natriuretic plasma infusion, nor did natriuretic plasma result in an increase in these measures in rats undergoing water diuresis. It also has been reported that dialysates of natriuretic plasma, but not of hydropenic plasma, when placed directly into the tubule lumen, inhibit proximal sodium reabsorption. In double blind studies carried out independently in Bethesda, London, and Cologne, we failed to detect the presence of a dialyzable inhibitor in natriuretic plasma. Finally, in contrast to other recent reports, we were unable to detect inhibitory activity in plasma obtained from dogs during the "escape" phase of chronic deoxycorticosterone acetate administration.

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.

Effect of changes in renal perfusion pressure on the suppression of proximal tubular sodium reabsorption due to saline loading

Rapid intravenous infusion of saline is known to suppress reabsorption of sodium and water in the proximal tubule. It has previously been shown that this suppression is accompanied by two changes which in combination might account for the over-all decrease in reabsorption: a reduction in the intrinsic reabsorptive capacity of the tubular epithelium (C/pir(2)) and a reduction in the ratio between tubular volume and GFR (pir(2)d/V(o)). The present micropuncture experiments were carried out in order to study the possible role of altered peritubular physical forces (hydrostatic and colloid oncotic pressure) in mediating these two changes. Proximal tubular reabsorptive capacity, transit time, fractional reabsorption of sodium and water, pir(2)d/V(o), and intratubular hydrostatic pressure were measured in saline-loaded rats during acute changes in renal perfusion pressure induced by intermittent constriction of the abdominal aorta. We found that when renal perfusion pressure was lowered to 70-90 mm Hg, the usual effects of saline loading on C/pir(2), pir(2)d/V(o), and fractional reabsorption in the proximal tubule were greatly minimized. When the aortic clamp was released and renal perfusion pressure allowed to rise, C/pir(2), pir(2)d/V(o), and fractional reabsorption fell markedly to levels characteristically seen in saline diuresis. Reclamping of the aorta reversed all of these changes. In order to determine whether the changes in C/pir(2) accompanying changes in renal perfusion pressure were mediated by a circulating natriuretic hormone, we assayed in hydopenic rats the dialysate of plasma collected from saline-loaded rats during and after release of aortic constriction by the split oil drop method. No significant difference in reabsorptive half-time (t(1/2)) was found between the two dialysates, and t(1/2) with both dialysates was approximately the same as was found when isotonic saline was injected in the tubules of hydropenic control animals. These observations suggest that the large changes in C/pir(2) which occurred with changes in renal perfusion pressure in saline-loaded rats were not mediated by a circulating hormone. We suggest that the reduction in C/pir(2), pir(2)d/V(o), and fractional reabsorption which occurs in the proximal tubule during a saline diuresis is related to the rise in hydrostatic pressure within the kidney.

Control of fluid absorption in the renal proximal tubule

Glomerulotubular balance was investigated in isolated, perfused rabbit proximal tubules in vitro in order to evaluate some of the mechanisms proposed to account for the proportionate relationship between glomerular filtration rate and fluid absorption generally observed in vivo. The rate of fluid transport from lumen to bath in proximal convoluted tubules in vitro was approximately equal to the estimated normal rate in vivo. The absorption rate in proximal straight tubules however was approximately one-half as great. If the mechanism responsible for maintenance of glomerulotubular balance is intrinsic to the proximal tubule, as has been proposed on the basis of micropuncture studies, the rate of fluid absorption in vitro should be directly related to the perfusion rate and/or tubule volume. In the present studies absorption rate was only minimally affected when perfusion rate was increased or the tubule distended. Thus, glomerulotubular balance is not mediated by changes in velocity of flow of the tubular fluid or tubular diameter and therefore is not an intrinsic property of the proximal tubule. It has also been proposed that glomerulotubular balance results from a humoral feedback mechanism in which angiotensin directly inhibits fluid absorption by the proximal convoluted tubule. In the present experiments, angiotensin was found to have no significant effect on absorption rate.