Effect of saline infusions on intrarenal distribution of glomerular filtrate and proximal reabsorption in the dog

Intrarenal venous and cortical catheter pressures in the dog kidney

To examine the validity of intrarenal venous pressure (IRVP) as a measure of peritubular capillary pressure when obtained with a method applicable in man, IRVP was measured with a 0.9 mm o.d. catheter introduced retrograde into interlobar veins of anesthetized dogs and was compared with a modified needle pressure (cortical catheter pressure = RCCP) measured simultaneously in the same kidneys. In twelve dogs with a mean experimental kidney control sodium excretion of 91 +/- 15 (SEM) micronmol/min IRVP averaged 16.0 +/- 1.1 mmHg and was significantly lower than the average RCCP of 22.6 +/- 1.1 mmHg (P less than 0.001). These pressures compare well with the reported micropuncture pressures in the peritubular capillaries and proximal tubules, respectively, at comparable levels of sodium excretion. IRVP fell significantly during reduction of renal perfusion pressure within the range of autoregulation of renal blood flow and increased during elevation of renal pelvic pressure (PP). At at PP of 60 mmHg, when urine flow had stopped, the PP-IRVP gradient was 22.7 +/- 3.1 and increased to 36.7 +/- 3.8 (P less than 0.001) at a PP of 80. Acute renal vein constriction always increased IRVP before renal vein pressure reached the preceeding control level of IRVP. Increased urine flow during saline volume expansion and furosemid infusion was associated with increased IRVP. The results when compared with micropuncture data indicate that IRVP is a satisfactory expression of peritubular capillary pressure in the experimental conditions included in the study.

Effects of prostaglandin inhibition on intrarenal hemodynamics in acutely saline-loaded rats

We studied the effect of inhibition of the prostaglandin (PG)-synthesizing enzyme system in female Sprague-Dawley rats following acute expansion of the extracellular fluid volume (ECV). In 57 conscious rats expansion of the ECV with isotonic saline corresponding to an increase in body weight of 10% was induced. Prior to ECV expansion 31 rats received indomethacin (10 mg/kg of body wt) by stomach tube. In six non-ECV-expanded rats indomethacin had no effect on glomerular filtration rate (GFR) and renal plasma flow (RPF). In ECV-expanded rats pretreated with indomethacin, GFR was unaltered but 125I-hippuran clearance decreased, and filtration fraction significantly increased. Intrarenal 86Rb distribution was similar in control and ECV-expanded rats. Indomethacin caused a slight increase in relative cortical 86 RB activity in non-ECV-expanded rats, but had no effect on intrarenal 86Rb distribution in ECV-expanded rats. No difference in intracortical glomerular perfusion was noted between control and ECV-expanded rats. In indomethacin-treated ECV-expanded rats an increase in relative inner cortical perfusion was observed. Absolute perfusion remained unaltered. Thus the decrease in total RPF was entirely due to decreased perfusion of outer cortical nephrons. Renal prostaglandins therefore may play a permissive role for physical factors to promote renal sodium excretion in acute ECV expansion via changes in intrarenal hemodynamics.

Renal intracortical blood flow distribution, function and sodium excretion in response to saline loading of anesthetized and unanesthetized dogs

In order to study the effect of anesthesia on the canine response to saline loading, experiments were performed on 10 dogs, first while awake and then during pentobarbital anesthesia. Individual kidney function and intrarenal blood flow response to saline loading (7.5% body weight) were measured in each condition and all data are reported as the average of a single kidney. CIN is considerably reduced under anesthesia (24.7 +/- 3.2 vs. 43.2 +/- 3.9 ml/min, P less than 0.01). A directionally similar reduction of PAH clearance was noted (89 +/- 17 vs. 122 +/- 13 ml/min). The natriuretic response to saline loading of the dogs reached 290 +/- 61 muEq/min while awake, but only 70 +/- 27 muEq/min while anesthetized. No measurable increase of CIN or CPAH occurred in response to saline loading either in the anesthetized or unanesthetized state. The natriuresis was entirely due to a rise of CNA/GFR in both circumstances. The change of CNA/GFR in response to saline load was also appreciably larger while awake (1.2 leads to 4.7% vs. 0.7 leads to 1.8%). Although the fraction of blood flow to the outermost quarter of the kidney was initially the same (31 +/- 3 vs. 29 +/- 3%) awake or anesthetized, the changes with saline loading were in the opposite direction and the values reached were significantly different (37 +/- 3, awake, vs. 27 +/- 3%, P less than 0.05). We conclude that while increased outer cortical blood flow is not necessary for natriuresis, it may occur during sodium loading and may facilitate sodium excretion.

The effect on total renal and tubular function and plasma renin of a moderate isotonic saline load in rats anesthetized with amytal and inactin

The renal effects of i.v. saline loading equal to 1% body weight (b.wt.) were studied in 2 groups of rats: group I was anesthetized with Amytal (15 mg/100 g b.wt. plus supplementary doses), group II with Inactin (12.5-15.0 mg/100 g b.wt.). In group I the saline load caused an increase in urine flow (+92%), solute excretion (+67%), inulin clearance (CIN) (+24%), PAH clearance (+31%) and absolute proximal reabsorption rate (+27%). Proximal fractional reabsorption and filtration fraction (FF) remained unchanged, while plasma oncotic pressure (COP) decreased by 10%. Plasma renin fell and there was an inverse relationship between renin concentration and proximal reabsorption rate. In group II urine flow and solute excretion increased after saline (+85% and 110%, respectively); CIN and absolute proximal reabsorption rate was lower than in group I and failed to increase after saline. Proximal fractional reabsorption was also lower and decreased after saline. COP as well as renin decreased as in group I, but no relationship between renin concentration and proximal reabsorption rate could be demonstrated. The data indicate that Inactin depresses both resting proximal tubular reabsorptive capacity and the tubular response to a physiological volume expansion. The results are compatible with the hypothesis that the renin-angiotensin system is significantly involved in regulation of proximal tubular function, while they are incompatible with the idea that peritubular COP plays any major role in this adjustment.

Characteristics of salt and water transport in superficial and juxtamedullary straight segments of proximal tubules

The purpose of the present studies was to characterize the nature of salt and water transport out of the superficial (SF) and juxtamedullary (JM) straight segments of rabbit proximal tubules as examined by in vitro microperfusion techniques. When the perfusate consisted of a solution simulating ultrafiltrate of plasma, there were no differences between SF and JM straight tubules in either net reabsorption of fluid (SF=0.47 nl/mm per min; JM=0.56 nl/mm per min) or in transtubular potential difference (PD) (SF=-2.1 mV; JM=-1.8 mV). Removal of glucose and alanine from the perfusate had no effect on the magnitude of the PD in either straight segment. Ouabain decreased both the net reabsorptive rates and the PD. Isosmolal replacement of NaCL by Na-cyclamate (a presumed impermeant anion) in the perfusate and the bath caused an increase in luminal negativity in both segments wheras similar substitution of NaCL by choline-CL (nontransported cation) changed the PD TO NEAR ZERO. These studies, therefore, suggest that sodium is transported out of the proximal straight tubules by an active noncoupled process that generates a PD (electrogenic process). When the perfusate consisted of a solution with a high chloride concentration (resulting from greater HCO3 than CI reabsorption in the proximal convoluted tubule), different PDs in SF and JM tubules were generated: SF=+1.6 plus or minus 0.2 mV; JM=-1.3 plus or minus 0.3 mV. This difference in PD was attributed to relative differences in Na and CI permeabilities in these two segments. Electrophysiological and isotopic estimates of the chloride to sodium permeability revealed that the SF tubule is about twice as permeant to chloride than to sodium whereas the JM tubules are approximately twice as permeant to sodium than to chloride. It is concluded that the mechanism of active sodium transport in the straight segment of proximal tubule differs from that of the convoluted segment and that both the SF and JM straight segments differ from each other with respect os sodium and chloride permeability.

Superficial and juxtamedullary nephron function during saline loading in the dog

A modification of the microdissection technique of Hanssen was utilized in dogs to measure superficial (SNGFR) and juxtamedullary nephron filtration rate (JMGFR) in control and saline-expanded dogs. During control studies SNGFR was 60+/-4 and JMGFR was 72+/-5 nl/min. During saline loading SNGFR was 74+/-8 and JMGFR was 65+/-6 nl/min. The ratio SNGFR: JMGFR significantly increased from 0.84+/-0.03 to 1.15+/-0.08. Glomerular perfusion rate (GPR) was measured with the microsphere method during control and saline loading. Superficial GPR did not change significantly but juxtamedullary GPR increased from 225+/-42 to 323+/-39 nl/min. Calculated superficial nephron filtration fraction was unchanged after saline expansion but juxtamedullary filtration fraction decreased from 0.34+/-0.07 to 0.24+/-0.07. The data demonstrate a tendency for filtration to shift toward the superficial part and plasma flow toward the deep part of the kidney cortex. GFR in juxtamedullary nephrons appears to be less plasma flow-dependent than in superficial nephrons. The fall in filtration fraction in the deep cortex may affect sodium excretion by juxtamedullary nephrons.

Mechanism of natriuresis after closure of chronic arteriovenous shunts

Animals subjected to certain cardiovascular manipulations, such as arteriovenous fistulas, diminish their urinary sodium excretion. It has been shown that closure of such fistulas results in a prompt increase in the rate of sodium excretion. However, the nature of the renal mechanisms increasing the excretion of sodium when the initial cardiovascular abnormality is corrected has remained unclear. Since the elucidation of such mechanisms might provide information pertinent to other sodium-retaining states, the effect of closure of chronic Teflon-Silastic arteriovenous shunts was studied in desoxycorticosterone acetate (DOCA)-treated dogs by utilizing micropuncture techniques.Nephron filtration rates were measured first during a control period with open arteriovenous shunts and then again after closure of the shunts in 12 dogs. Nephron filtration rate rose 32% while total glomerular filtration rate (GFR) decreased 8%. After closure of the arteriovenous shunt, fractional reabsorption increased 6%, while total kidney filtration fraction increased from 0.31 to 0.35. Renal plasma flow decreased from a mean of 111 ml/min to 90 ml/min. Closure of the arteriovenous shunts increased sodium excretion from a mean of 21 mueq/min to 45 mueq/min. Concomitantly, a redistribution of filtrate to superficial nephrons occurred. Since pharmacological doses of DOCA were being administered while total GFR was not increased and fractional reabsorption of sodium in the proximal tubule was not inhibited, it was concluded that filtrate distribution to superficial nephrons may have contributed to the observed natriuresis, although alternate explanations were also deemed possible.

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 variation in dietary NaCl intake on the intrarenal distribution of plasma flow in the rat

The effect of dietary variation in sodium chloride intake on the intrarenal distribution of plasma flow was investigated in rats using the antiglomerular basement membrane antibody technique. Rats were placed on a liquid diet containing either 9.86 (n = 9) or 0 (n = 9) mEq NaCl/daily portion for 2 wk. Labeled antibody was injected and the diets were reversed. After an additional 2 wk period, antibody labeled with a different radionuclide was injected and the animals were sacrificed. Fractional plasma flow distribution was then calculated for each dietary period. No change in flow to any cortical region could be detected. In six additional awake rats on identical dietary regimen, total plasma flow was estimated by the clearance of hippuran-(131)I. No change in this parameter occurred with changes in NaCl intake. We conclude, therefore, that no change in either total renal plasma flow or intracortical distribution of plasma flow occurs with wide variations in dietary sodium chloride intake in the rat. The implications of this constancy of regional plasma flow are discussed with reference to presumed concomitant alterations in the intrarenal distribution of nephron filtration rate.

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

Extracellular fluid volume expansion with isotonic saline (7.5% of body weight) decreased maximum glucose reabsorption rate by rat kidneys at plasma glucose concentrations greater than 30 mM. Glucose reabsorption rate was 30.2 +/-1.6 (SE) mumoles/min.g kidney in nonexpanded rats; it was 18.4 +/-1.5 mumoles/min.g in volume-expanded rats. Glucose reabsorption determined by micropuncture was 92% complete at the end of accessible superficial proximal convolutions. Volume expansion resulted in a slight but statistically insignificant reduction of maximal glucose reabsorption rate in superficial nephrons from 786 +/-35 mumumoles/min.g kidney in nonexpanded rats to 720 +/-30 mumumoles/min.g in volume-expanded rats. Superficial nephron filtration rate was increased by volume expansion from 28.8 +/-1.2 nl/min.g to 36.6 +/-1.5 nl/min.g kidney. In nonexpanded rats, the ratio of glucose reabsorption to glomerular filtration (tmg/sgfr) was similar in superficial and juxtamedullary nephrons. In volume-expanded rats superficial nephron tmg/sgfr was greater than juxtamedullary nephron tmg/sgfr. Juxtamedullary nephron function was measured by puncturing loops of Henle in the exposed papillae of small rats. Volume expansion increased sgfr without much effect on tmg in superficial nephrons while it decreased tmg without much effect on sgfr in deep nephrons. Physical changes produced by volume expansion seem to exert their greatest effect on proximal tubular function in the inner cortex. The increase in heterogeneity of glomerular-tubular balance could account for increased splay of glucose titration curves previously reported to accompany volume expansion.