Isotonic saline loading and intrarenal distribution of glomerular filtration in dogs

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

Mechanism of sodium modulation of glomerular angiotensin receptors in the rat

Specific binding of 125I-angiotensin to high affinity glomerular receptors varies directly with the level of dietary sodium. To investigate the mechanism of sodium regulation of glomerular angiotensin receptors, groups of Sprague-Dawley rats were maintained on one of three levels of sodium intake for at least 5 d: high sodium (7.39 meq/24 h), moderate sodium (0.88 meq/24 h), and low sodium diets (0.01 meq/24 h). An additional group was given low sodium diet with daily injections of furosemide (1 mg/kg i.p.). To dissociate the effects of dietary sodium from those of circulating angiotensin II levels on glomerular receptor regulation, a fifth group was placed on high sodium diet and given a continuous infusion of angiotensin via an implanted minipump (100 ng/min) for 21 d. There was a strong negative correlation (r = -0.98, P less than 0.01) between plasma angiotensin II and glomerular angiotensin receptor density. Dietary sodium, potassium, or water consumption did not correlate with angiotensin II receptor concentration. The affinity constant did not vary in any of the groups (2.33 +/- 0.30 X 10(8) M-1). The time course of sodium regulation of glomerular angiotensin II receptors was studied in rats switched from a moderate sodium to either a high sodium diet or a low sodium diet plus furosemide. Receptor density was unchanged at 24 h, varied directly with sodium intake for 1-5 d when induction was maximal, and remained constant for at least 21 d. The time course of receptor regulation closely paralleled changes in plasma angiotensin II. Additional studies were undertaken to demonstrate that glomerular angiotensin II receptors are down-regulated by circulating hormone. Rats maintained on moderate sodium intake were killed 2 min after the induction of anesthesia with pentobarbital (50 mg/kg i.p.) or by rapid decapitation. Despite a 50-fold elevation of plasma angiotensin II in anesthetized rats (424 +/- 154 vs. 8.6 +/- 1.0 pg/ml, P less than 0.001) angiotensin receptor density was unchanged (anesthetized, 1,016 +/- 126 vs. unanesthetized, 1,290 +/- 84 fmol/mg). The infusion of angiotensin II (100 mg/min) for 15 min or 2 h into anesthetized rats maintained on moderate sodium intake resulted in a 50% reduction in specific angiotensin binding that could not be reversed by the dissociation of endogenous angiotensin. These data are compatible with modulation of receptor density by circulating hormone and can not be accounted for by prior receptor occupancy.

Renal blood flow distribution at varying perfusion pressure in the alloperfused dog kidney

Tissue blood flow (TBF), its percent distribution and glomerular blood flow (GBF) were measured using labelled microspheres 15 micrometer in diameter (M) and chicken red blood cells (CRBC) at perfusion pressures (PP) of 17.3, 12.8 and 8.0 kPa (130, 95 and 60 mm Hg) in isolated alloperfused dog kidneys. Renal blood flow (RBF) was never interrupted during the isolation. Experiments with M showed a marked inequality of the tissue blood flow in different parts of the renal cortex at a constant PP of 17.3 kPa. TBF was highest in the outermost quarter and lowest in the juxtamedullary one. Using CRBC, a homogeneous TBF was observed in the outer 3/4 of the renal cortex with a lower flow in the innermost quarter. With M, a typical percent "redistribution" of TBF and GBF into the inner cortical regions was indicated during PP reduction. With CRBC, this phenomenon was observed only at PP below the range of RBF autoregulation (8.0 kPa) and was much less conspicuous than with M. The smaller size and higher elasticity of CRBC as compared with M, may result in a more realistic reflection of cortical blood flow distribution. The GBF of outermost superficial glomeruli decreases, even with CRBC, with each PP reduction, the difference exhibiting only a 5% significance level. The lower limit of BF autoregulation in these glomeruli seems to be somewhat higher than that of total RBF autoregulation.

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.

Anatomical and functional heterogeneity of nephrons in the rabbit: microdissection studies and SNGFR measurements

The single nephron glomerular filtration rate (SNGFR) was determined in superficial (S) and juxtamedullary (JM) nephrons of 10 anesthetized rabbits by the 14C ferrocyanide infusion technique. The length of the proximal tubules and the volume of the glomeruli were also determined for the same nephrons. SNGFR was higher in JM than in S: 28.6 +/- 3.4 versus 22.6 +/- 3.0 nl/min, P less than 0.001. In JM nephrons, glomeruli were larger than in S: 1.3 +/- 0.1 versus 0.9 +/- 0.1 nl, P less than 0.001, whereas there was no difference between proximal tubule length in either category (s, 8.7 +/- 0.3 and JM, 8.9 +/- 0.5 mm). In 6 out of 8 animals, SNGFR was significantly correlated to glomerular volume. Lack of correlation was observed between SNGFR and length of proximal tubule in all animals but one. These results show that the rabbit, as well as small rodents and the dog, has a higher SNGFR in juxtamedullary than superficial glomeruli. Although this functional difference is not related to the length of the proximal tubule in each individual animal, the ratio between the mean SNGFR value and the mean length of the proximal tubule in superficial rabbit nephrons is similar to the ratio found in other species.

Effect of diuresis and nephron dimensional heterogeneity on the distribution of nephron filtration rates

Conflicting data in studies of the effect of natriuresis on intrarenal single nephron glomerular filtration rate (SNGFR) redistribution may arise from the interplay of hitherto largely overlooked factors. In the present work, the effect of diuresis induced by saline, glucose or mannitol, as well as the effect of anatomical nephron heterogeneity, were studied. A highly significant positive correlation was found between the logarithm of the urine flow per gram of kidney weight (log V) and the mean superficial (S) to mean juxtamedullary (JM) SNGFR ratio. The rise in S/JM SNGFR with diuresis was primarily a function of decreased JM SNGFR. Total proximal tubular length (TPL) was used as a measure of nephron size. The distribution of nephron sizes was evaluated as S/JM TPL. The effects of urine flow and anatomical heterogeneity on S/JM SNGFR were spearated by means of multiple regression analysis, which yielded the following equation: S/JM SNGFR = -0.049 + 0.179 log V + 0.818 S/JM TPL. Both slopes were highly significant (P less than 0.001). These findings indicate that S/JM SNGFR increases with urine flow, independently of sodium homeostasis, and that anatomical heterogeneity has a marked effect and must, therefore, be controlled. Conflicting data in the literature are harmonized with the present data when appropriate correction can be made for the dimensional factor. There is no evidence that SNGFR redistributions play a role in sodium homeostasis.

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.

Micropuncture studies of phosphate transport in the proximal tubule of the dog. The relationship to sodium reabsorption

Micropuncture studies were performed in the dog to examine the relationship between sodium and phosphate transport in the proximal tubule. In hydropenic, thyroparathyroidectomized animals, administration of parathyroid extract, saline, or acetazolamide resulted in a fall in proximal tubule fluid-to-plasma (TF/P) inulin ratio as well as a rise in tubule fluid-to-plasma ultrafilterable (TF/UF) phosphate ratio. A correlation was found between the changes in fractional reabsorption of sodium and phosphate but the phosphate changes were generally greater than those of sodium. Also, a high distal phosphate delivery in the face of low fractional excretion of phosphate in the urine in thyroparathyroidectomized dogs suggests significant phosphate reabsorption in the distal nephron. On the other hand, calcium chloride infusion to saline-loaded, normal dogs to suppress endogenous parathyroid hormone reduced proximal TF/UF phosphate without change in TF/P inulin, while both parameters remained unchanged in saline-loaded, thyroparathyroidectomized dogs after calcium infusion. An increase in proximal TF/UF phosphate associated with unchanged TF/P inulin was also demonstrated by administration of highly purified parathyroid hormone to saline-loaded, thyroparathyroidectomized dogs. It was concluded that although proximal tubule phosphate transport is generally closely related to that of sodium, the two can dissociate under certain experimental conditions, especially under the influence of parathyroid hormone. These observations also indicate that the effect of parathyroid hormone on proximal tubule phosphate transport is not solely dependent upon its effect on sodium transport.

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.