The relative contributions of reabsorptive rate and redistributed nephron filtration rate to changes in proximal tubular fractional reabsorption during acute saline infusion and aortic constriction in the rat

One physiology does not fit all: a path from data variability to "physiogenetics"?

Data variability is a costly complication of biomedical experimentation because the same experiment must be repeated a sufficient number of times so that the sample mean becomes a credible representation of the entire population. Since sampling is ideally done randomly in populations normalized for environmental and genetic backgrounds, data variability is viewed as a purely statistical issue reflecting the distribution in the population and captured as the standard deviation of the sampled data. The factors contributing to data variability are not analyzed by statistical methods; for want of a better explanation, data scatter is simply attributed to random noise and/or methodological limitations. In this commentary, evidence is discussed that documents an important role of interindividual biological diversity as a cause for data variability based on studies in which repeated sampling in the same individual permitted statistical comparisons between individuals in the same sample. Significant differences were found for proximal fluid reabsorption and plasma renin concentration between sample means of individuals of the same population. Furthermore, arterial blood pressure varied significantly between individual mice independently of strain and sex. Recognition of the extent of interindividual variability has important implications for data reproducibility, data collection, and data presentation in physiological research. Such nonrandom data variability may have different causes, but DNA modifications by genetic or epigenetic mechanisms could generate phenotype variants without being associated with disease symptoms. Exploration of the heritability of phenotypical diversity in physiology may be defined as "physiogenetics," and it would thus be the physiological corollary of pharmacogenetics and pharmacogenomics.

The effect of reducing proximal tubular fluid delivery on the rate of filtration of single nephrons

The rate of delivery of tubular fluid from the proximal tubule (PT) is thought to reset nephron filtration rate (SNGFR). In micropuncture experiments in rats we tested this hypothesis by reducing the efflux from the PT by simultaneously "double collecting" (DC) tubular fluid from the early distal tubule (DT) and from the last convolution of the PT of the same nephrons. SNGFR measured by total collection of tubular fluid was 34 +/- 3 nl/min at the DT and 34 +/- 3 nl/min at the PT (p > 0.97, n = 42). The simultaneous collection from proximal and distal sampling site was performed between these two paired measurements. It yielded an average SNGFR of 40 +/- 3 nl/min (p < 0.02). This may be due to the collection, at the distal site, of the extra amount of inulin stored between distal and proximal pipette, prior to starting the aspiration of tubular fluid. Since this error would decrease in longer collections, the difference in SNGFR between single and double collections was plotted against the duration of collections. In fact it was negatively correlated with the sampling time (p < 0.01), indicating no difference in SNGFRs for collections > 4 minutes. Reduction and complete interruption of the delivery of native proximal tubular fluid to the Macula Densa does not seem to influence the measurement of SNGFR. Filtration rate is not significantly different when measured within few minutes at the DT and PT of the same nephrons.

The role of prostaglandins in Na retention of porta-cava shunted rats

The importance of prostaglandins (PG) in Na and water retention of liver cirrhosis was studied in rats with porta-cava shunt (PCS) compared to control, non-shunted animals. Balance studies were performed in metabolic cages with diets of high, normal and low Na. An experimental phase, during which the animals received either 5 mg X kg-1 of indomethacin daily or placebo, was preceded by a control period and followed by a post-indomethacin period identical to the control phase. In each diet, indomethacin, but non placebo, caused a positive Na balance, correlated with Na intake, which in overall pooled data amounted to -1453 +/- 255 muEq in PCS rats, significantly larger than that measured in controls, of -295 +/- 320 muEq (P less than 0.01). This was attended by a reverse change in K balance of -35.6 +/- 349 muEq versus -1566 +/- 582 muEq (P less than 0.01); glomerular filtration rate (GRF) was unchanged. These data demonstrate that PGs contribute to the control of Na homeostasis in the presence of PCS.

Failure of changes in intracapillary pressures to alter proximal fluid reabsorption

To determine the role that peritubular capillary oncotic and hydraulic pressures play in regulating urinary sodium excretion in the euvolemic state, experiments were carried out in rats under conditions which altered these pressures without volume expanding the animal. In cross-circulation experiments, the donor rat was expanded with plasma or Ringer's solution while the recipient rat remained euvolemic. Micropuncture measurements in the euvolemic recipients demonstrated significant increases in efferent plasma flow rate (QEA), capillary hydraulic pressure (Pc), and decreases in mean capillary oncotic pressure (pi c). There were no changes in nephron GFR (SNGFR), absolute proximal reabsorption (APR), or UNaV. In additional studies, peritubular oncotic pressure was lowered markedly by plasmapheresis of the experimental animal. Large decreases in pi c were produced without any change occurring in SNGFR, APR, or UNaV. Measurements of interstitial hydraulic pressure (Pi) with a subcapsular pressure pipet revealed that Pi was unaltered under all of these conditions but rose markedly in rats undergoing a saline-expansion diuresis. Our findings indicate that APR and UNaV can remain constant despite large changes in pi c, Pc, and QEA in nonexpanded animals. Furthermore, the changes in pi c, Pc, and QEA induced in the euvolemic non-diuretic rats were the same as those in the saline-expanded diuretic rats. We conclude that under euvolemic experimental conditions, urinary sodium excretion and APR do not correlate with intracapillary pressures or flow rates in the renal cortex. The only difference found between the nondiuretic and diuretic rats was a rise in Pi in the latter group.

Mechanisms of the impaired diuretic and natriuretic responses to a sustained and moderate saline infusion in rats with experimental cirrhosis

Kidney function and tubular handling of water and sodium by superficial nephrons, packed cell volume, total plasma proteins and albumin distribution space were studied in control and cirrhotic rats before and after a moderate and sustained saline infusion (3% body weight per 30 min + reposition of urinary losses). Tubular fluid samples were obtained from late proximal, early distal and late distal convolutions of superficial nephrons using micropuncture. Protein distribution was assessed by intravenous injection of 0.5 muCi of (125I)-albumin. In basal conditions, both groups of rats showed similar glomerular filtration rate and renal plasma flow, but cirrhotic animals had lower sodium excretion (fractional excretion of sodium = 0.04 +/- 0.01% vs. 0.22 +/- 0.02%, p less than 0.05) and urinary volume (4.31 +/- 0.41 vs. 7.57 +/- 0.53 microliter per min; p less than 0.05). After saline infusion, total plasma proteins decreased more in cirrhotic than in control rats (-18.5 +/- 2.7 vs. -12.9 +/- 2.2%, p less than 0.05). The opposite was observed for albumin distribution space (34.5 +/- 6.1 vs. 22.1 +/- 3.5%, p less than 0.05). Fractional sodium excretion increased to 2.98 +/- 0.15% in control rats but only to 0.61 +/- 0.080% in cirrhotic rats. The ratio single nephron glomerular filtration rate/glomerular filtration rate increased from 19.6 +/- 0.7 to 21.2 +/- 1.0 (X10(-6), p less than 0.005) in control animals but did not change in cirrhotic rats. These animals were unable to decrease adequately fractional fluid reabsorption in the proximal tubule and the loop of Henle.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of alterations in extracellular fluid volume on urinary kallikrein in the conscious rat

The effect of alterations in extracellular fluid volume (ECV) and solute concentration on excretion of urinary kallikrein was examined in conscious Sprague-Dawley rats. Animals were given infusions of either dextrose and water, saline, or albumin according to a variety of protocols. These were designed to evaluate possible relationships between excretion of kallikrein, volume, sodium, and potassium. A reproducible pattern of kallikrein excretion was noted in all volume expanded groups. This consisted of a short lived increase during the initial hour of expansion with a subsequent fall to lower levels than baseline and a gradual recovery. To define the role of aldosterone in these studies, an adrenalectomized group and a group of appropriately prepared sham controls were expanded with saline. Adrenalectomy did not effect this pattern. We postulate a tubular "washout" phenomenon as the etiology of these observations. Results of these studies fail to demonstrate a consistent relationship between urinary volume, sodium, or potassium and the simultaneous amount of kallikrein found in the urine.

Experimental dissociation of the effects of prostaglandins on renal sodium and water reabsorption by cyclo-oxygenase inhibitors in the rat

1 The relative importance of the effect of prostaglandins on renal sodium and water reabsorption was assessed in rats. 2 Clearance experiments were performed on 24 anaesthetized rats divided into 3 groups. Each group was infused throughout either with Ringer solution at 9 ml/h (Protocol I), or at 3 ml/h (Protocol II) or with hypotonic fluid at 5 ml/h (Protocol III). Clearance periods were performed before and after intravenous injection of indomethacin (5 mg/kg) and then of aspirin (20 mg/kg). The natriuretic response to different degrees of volume expansion was not modified during the action of the inhibitors. 3 When baseline urine osmolality (Uosm) was high (Protocol II) no further increase occurred in the presence of prostaglandin inhibition. Conversely, Uosm rose from 771 +/- 134 to 1356 +/- 414 and from 575 +/- 245 to 841 +/- 407 mosm/kg (P less than 0.05) in Protocol I and Protocol III respectively, when antidiuretic hormone secretion was inhibited by the higher degree of volume expansion. 4 There was a significant correlation between the change in urine flow rate induced by cyclooxygenase inhibitors and the attendant variations in Na excretion, r = 0.42, n = 41, P less than 0.01. 5 Thus, prostaglandins affect Na loss during saline load as a side effect of their action on water permeability. They could play an important role in volume depletion by counterbalancing the large secretion rate of renal vasoconstrictors.

Volume reabsorption by the loop of Henle: a micropuncture study

The observed extension of glomerulo-tubular beyond the proximal tube is thought to be due either to flow dependent reabsorption by non-accessible proximal segments and pars recta, or to osmotic volume flow out of the descending limb of the loop of Henle.

Effect of flow rate and the extracellular fluid volume on proximal urate and water absorption

The in vivo microperfusion technique was used to examine the effect of variations in tubular flow rate and the extracellular fluid volume onf [2-14C]-urate and water absorption in the proximal tubule of the rat. In nondiuretic animals, fractional urate absorption was highest at the lowest perfusion rate examined and decreased as the rate of perfusion was increased. Increasing the initial concentration of urate in the perfusion solution had no effect on the fractional absorption of urate. Fractional water absorption was also inversely related to the rate of perfusion. Expansion of the extracellular fluid volume with isotonic saline resulted in rates of urate absorption similar to control values at any given microperfusion rate. Fractional water absorption showed the same flow rate dependency pattern observed in control animals, but at a significantly lower rate of absorption. These studies indicate that fractional urate absorption is dependent upon some parameter of tubular flow rate and that the relationship between urate absorption and perfusion rate is not related to the delivered load of urate per se and is not affected by the state of hydration of the extracellular fluid.

Reabsorption of sodium in the proximal renal tubule in cirrhosis of the liver

Proximal tubular reabsorption was examined in patients with liver cirrhosis and ascites by a variety of indirect methods. Maximal diluting ability, urine flow rate, and free water clearance were reduced. During frusemide administration V/GFR was lower than in normal control subjects, indicating that proximal fractional reabsorption is enhanced in liver cirrhosis. This, by reducing Na delivery to the loop of Henle, impairs maximal urine osmolality. Mannitol, by reducing proximal reabsorption, restores availability of Na to the loop and urine concentrating ability during osmotic diuresis, whereas volume expansion with dextran was ineffective. This could be due to a persistent increase in renal vascular resistance preventing the rise in interstitial pressure responsible for driving fluid back into the proximal tubular lumen.

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.

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.

Effect of intraluminal flow on proximal tubular reabsorption

Micropuncture techniques in the rat were used to reinvestigate the possibility that intraluminal flow rate per se may influence net volume reabsorption by the proximal tubule. An experimental design was devised which lowered intraluminal flow without affecting filtration rate of the nephron under study or without directly affecting other renal hemodynamics. In 11 rats flow of tubular fluid between early and late proximal tubular sites was reduced by partially collecting tubular fluid at the early puncture site. In 42 nephrons the rate of flow of tubular fluid was reduced an average of 45% without changing nephron filtration rate and there was an associated reduction in reabsorption between the two sites which averaged 29%. This indicated 63% balance between delivery of tubular fluid and the rate of reabsorption between two sites along proximal tubules. The results of these studies indicate that a reduction in delivery of normal filtrate along the proximal tubule is associated with a concordant reduction in the absolute rate of reabsorption. Since this relationship occurred in the absence of changes in renal hemodynamics or even a change in filtration rate of the nephron under study it is concluded that changes in intraluminal load per se play an important role in the phenomenon of glomerulotubular balance.

Renal response to chronic intravenous salt loading in the rat

The natriuresis of acute Ringer's loading is associated with a rise in the rate of delivery of fluid beyond the proximal tubule due both to a rise in glomerular filtration and a fall in absolute reabsorption, the latter being causally mediated, at least in part, by the accompanying fall in postglomerular vascular [protein]. To determine whether these factors also contribute to the renal response to chronic Ringer's loading, nine rats given continuous infusions, 30% body weight/day over 5-14 days, were studied using free-flow micro-puncture techniques. Results were compared with data from 10 chronic control rats given less than 1.5% body wt/day. Late proximal tubule fluid-to-plasma [inulin] ratios, (TF/P)(IN), single nephron glomerular filtration rate (SNGFR), absolute proximal reabsorption, and postglomerular vascular [protein] in chronic control rats and chronically loaded rats averaged 2.2+/-SE 0.1 (n = 35) and 1.5+/-0 (35), P<0.001; 37+/-2 (35) and 47+/-4 nl/min (35), P<0.05; 19+/-1 (35) and 16+/-2 nl/min (35), P>0.2; and 9.5+/-0.3 (8) and 8.6+/-0.3 g/100 ml (8), P>0.05, respectively. Thus the fall in (TF/P)(IN) and the rise in distal delivery during chronic Ringer's loading were due almost entirely to the rise in SNGFR, and not to any large fall in absolute reabsorption. Hence chronic and acute Ringer's loading increase delivery of proximal tubule fluid by different mechanisms, with chronic sodium homeostasis being governed overwhelmingly by adjustments in GFR. When, however, an acute Ringer's load was infused into chronically loaded rats, we observed significant and parallel reductions in absolute proximal reabsorption and postglomerular vascular [protein]. These findings suggest that the difference between the effects of chronic vs. acute Ringer's loading on absolute proximal reabsorption may have been due, at least in part, to the corresponding difference in the effects these two loading procedures have on postglomerular vascular [protein].