Perfusion pressure dependency of in vivo renal tubular dynamics

Martin Rodriguez-Porcel1, Lilach O. Lerman1, Patrick F. Sheedy II2, and J. Carlos Romero1

1 Department of Physiology and Biophysics and 2 Department of Diagnostic Radiology, Mayo Clinic and Foundation, Rochester, Minnesota 55905

    ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References

To examine whether changes in renal perfusion pressure (RPP) within the range of autoregulation induce detectable changes in tubular dynamics in an entire nephron population of the intact kidney, we measured, using electron beam computed tomography (EBCT), transit times (TT, s) and intratubular concentration (%) of filterable contrast media in various nephron segments simultaneously with renal regional perfusion. In seven dogs (group A) this was performed at the upper and lower limits of autoregulation (RPP = 130 and 95 mmHg, respectively) while group B (n = 5) served as control. In group A alone, a decrease in RPP led to an increase in TT by 40%, 68%, and 32% in the proximal tubules, ascending limb of Henle's loop, and distal tubules, respectively, in association with an increase in intratubular concentration (+50%, 80%, and 42%, respectively). Papillary perfusion decreased, whereas perfusion of the adjacent, outlying inner medulla increased. The decrease in papillary perfusion correlated positively with the concurrent change in sodium excretion (R = 0.81). This study demonstrates that changes in RPP within the autoregulatory range elicit changes of tubular sodium reabsorption mainly in proximal, distal, and ascending tubules, in which most of the nephrons participate. These tubular changes are associated with an alteration of perfusion circumscribed to two areas of the inner renal medulla.

electron beam computed tomography; renal function; renal circulation; tubular function; contrast media

    INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References

IT HAS BEEN LONG KNOWN THAT the kidney alters flow and sodium excretion in response to acute changes in renal perfusion pressure (RPP) (24). Since the glomerular filtration rate (GFR) is well autoregulated, pressure-related natriuresis is the result of decreased reabsorption by the renal tubules rather than an increased filtered load. The mechanism that, under these conditions, is responsible for coupling the changes in renal hemodynamics to the corresponding alterations in tubular sodium transport has remained elusive.

It has been suggested that changes in blood flow to the renal medulla may be responsible for altering renal interstitial pressure, which may ultimately modify sodium reabsorption (5, 19, 24) per se (8) or by the release of natriuretic autacoids, such as nitric oxide or prostaglandins (23). However, measurements of medullary circulation with laser-Doppler flowmetry and videomicroscopy have yielded controversial results (4, 15, 21). This may be because these techniques are based on assessing the velocity and number of erythrocytes (22) circulating in medullary capillaries, and changes in circulatory velocity do not necessarily correlate directly with changes in blood flow (2). Moreover, in this invasive technique the region of interest examined is confined to only few capillaries (22); consequently, samples from the same region of interest are difficult to obtain consistently, and measurements performed on these regions of interest cannot be extrapolated to a particular anatomical region in its entirety.

Micropuncture has been the most common research tool with which to study changes in tubular sodium reabsorption evoked by alterations of RPP. Although this technique has yielded accurate results, it involves examination of a specific nephron site at a given moment, so that the results cannot necessarily be extrapolated to the entire nephron population. In fact, under certain physiological conditions such as pressure natriuresis (18), there are significant differences between sodium reabsorption in superficial and deep nephrons (6). These limitations makes it difficult to examine the relationship between a given circulatory change in a large region of interest and the corresponding tubular change.

These considerations prompted us to study the potential application of fast computed tomography (CT) to determine the manner in which changes in renal hemodynamics were associated with specific changes in tubular sodium excretion. Electron beam CT (EBCT) is capable of providing simultaneous assessments of renal perfusion and tubular dynamics, thereby describing the coupling between perfusion and tubular dynamics (14), that may account for the pressure-dependent natriuresis. We have previously validated the capabilities of this technique in detecting changes in renal perfusion and volume reabsorption, secondary to the infusion of a loop diuretic (furosemide) (14). In the current study, we investigated the capability of EBCT to detect these changes following a change in RPP within the autoregulatory range of renal blood flow (RBF).

    METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References

This study was performed according to institutional animal care and use guidelines. The two groups of dogs studied were an experimental (group A, n = 7, body wt 17-21 kg) and a control (group B, n = 5, body wt 17-20 kg) group.

Animals in group A were anesthetized (pentobarbital sodium, 30 mg/kg iv), intubated, and ventilated (using a Harvard respirator) with room air. One femoral vein was cannulated, and a polyethylene PE-200 catheter was placed for a constant infusion of inulin throughout the experiment (a priming bolus of 20 ml followed by a constant infusion of 1 ml/min for the remainder of the experiment). To maintain anesthesia, pentobarbital sodium (1.5 mg/ml) was included in the inulin infusion. The other femoral vein was cannulated for constant infusion of 0.9% saline (1-2 ml/min) throughout the experiment. A femoral artery was cannulated, and a PE-240 catheter was advanced to the level of the renal artery, for continuous monitoring of RPP with a pressure transducer (Statham model P23ID; Gould, Hato Rey, PR). For injection of contrast media, a Rodriguez catheter (no. 8 French) was advanced under fluoroscopy via the common carotid artery to the midthoracic descending aorta. Each dog was then placed in dorsal suspension following exposure of the left kidney and its renal artery by a ventrocostal flank incision and placement of a Transonic flow probe around the renal artery for continuous recording of RBF. Both the superior mesenteric and the celiac arteries were ligated to increase blood flow and perfusion pressure to the renal territories. Two plastic vascular clamps were placed on the aorta, one above and the other below the level of the left renal artery, for manipulation of RPP. The corresponding ureter was catheterized with a PE-200 catheter for collection of urine.

Animals in group B were prepared similarly, except for cannulation of the urinary bladder instead of the ureter, and no flow probe was placed around the renal artery.

Following surgical preparation, each animal was transferred and positioned in the EBCT (model C-100; Imatron, South San Francisco, CA) scanning gantry. After a 1-h recovery period, EBCT scans were performed. These were performed in five of the dogs of group A at the upper limit, in two dogs of group A at the lower limit of RPP autoregulation (to avoid bias due to differences in directional changes of RPP), and in group B at the basal RPP level.

Iopamidol (Isovue-370; Squibb Diagnostics, Princeton, NJ) was used as the contrast medium. This is a nonionic, low-osmolar, intravascular contrast medium (mol wt 777). Like most low-osmolar urographic contrast media, it is an inert monomer derivative of triiodinated benzoic acid (16). It is cleared primarily (over 95%) by glomerular filtration, with the remainder being excreted in the feces (1). It is not significantly secreted or reabsorbed in the renal tubules at clinical doses and is essentially equivalent to the standard marker for glomerular filtration, inulin (16). Iopamidol does not appreciably bind to plasma proteins (1).

Following a recovery period of ~30 min, during which fluids administration was resumed, EBCT studies were repeated, in group A at the lower (in 5 dogs) or upper (in 2 dogs) limit of autoregulation and in group B at the same basal level of RPP. In group A, the upper limit of autoregulation was selected in the following manner. After a baseline RBF value was recorded, the lower aortic clamp was tightened until the pressure monitor showed the highest level of RPP that was associated with a value of RBF no more than 10% different from baseline levels. A similar procedure (this time adjusting the upper aortic clamp) was followed to decrease RPP.

Mean arterial pressure was recorded before and after each study. In group A, RBF was recorded throughout the experiment, and GFR was measured as a further reassurance that studies were performed in a state of autoregulation. In group B, RBF was measured from EBCT-derived data. In both groups, urine was collected for 10 min before and after each EBCT study for measurement of urinary flow rate and osmolarity. In the experimental group urinary sodium excretion (UNaV) was also calculated.

EBCT Procedure

Each study was performed during respiratory suspension at end expiration. Using abdominal localization scans, we identified all tomographic levels containing the left kidney (from cranial to caudal pole). One midhilar tomographic level was then selected in the left kidney for performance of a flow study.

For the study of perfusion and tubular dynamics, one midhilar tomographic level was scanned in the high-resolution mode. Forty consecutive scans were performed after a bolus injection (0.5 ml/kg) of the contrast medium. The first 20 scans were performed at the rate of 1 scan/0.6 s for 6 s, 1 scan/s for 4 s, and 1 scan/2.5 s for 4 s. The last 20 scans were performed at a rate of 1 scan/5 s, with a total scanning time of 124 s, as previously described (14). Each dog received assisted ventilation between scans during the last 20 scans.

In both groups, each flow study was immediately followed by a renal volume study, as previously described (12). To minimize the potential effects of contrast on renal hemodynamics, additional contrast medium was not administered for this study; hence, individual cortical and medullary volumes and blood flows were not separately quantified (11). The product of renal perfusion [ml · min-1 · (cm3 tissue)-1] and volume (ml) yields RBF (ml/min) and therefore eliminated the need for laparotomy in group B, which is otherwise necessary for the use of electromagnetic flow measurements of RBF. In group B, on-line measurement of RBF was necessary to define the limits of autoregulation.

Following completion of the studies, each dog was killed with Sleepaway (Fort Dodge Laboratories, Fort Dodge, Iowa). Urinary volume obtained in each of the 10-min control periods was measured in a graduated cylinder, and urinary osmolarity was measured using a Micro Osmometer (Precision Systems). Urinary sodium was measured using a flame photometer.

Data Analysis

The images were reconstructed, and regions of interest were selected from the cross-sectional images as follows. The cortex was defined during the vascular phase as the highly opacified zone at the circumference of the kidney and was arbitrarily further divided into outer and inner cortex. The outer medulla was defined as the anatomical area clearly outlined immediately following the vascular phase, as a ring of contrast that was moving toward the inner medulla. The inner medulla was subdivided in two regions; this was performed due to the known inherent heterogeneity of medullary blood flow, which may vary as a function of distance from the corticomedullary junction (28). Subdivision to only two large regions of interest (rather than a larger number of smaller regions) enables maintenance of a high signal-to-noise ratio and thereby accurate measurements of flow. These two inner medullary regions were anatomically selected as follows: the "deep inner medulla" was identified between adjacent calyxes (Fig. 1). The region of interest immediately above the crest, but below the outer medulla, was defined as "outlying inner medulla."


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Fig. 1.   Schematic of a cross section of the kidney, illustrating the anatomic renal components.

The computer then generated for each region of interest distinctive time-density curves, describing the change in tissue density consequent to transit of contrast in that region.

Perfusion. Perfusion rate, as milliliters of blood per cubic centimeter of tissue per minute, was calculated from the first peak of the time-density curve obtained in each region of interest, representing the passage of contrast through the vascular compartment, using the algorithm (11)
Perfusion = (<IT>h</IT><SUB>peak</SUB> /<IT>A</IT><SUB>a</SUB>) × 60
where hpeak is the peak height of the tissue curve, and Aa is the area under the aortic curve.

Renal volume. Renal volume (cm3) was calculated from each tomographic level by manually tracing the renal contour. The areas obtained were then summed and multiplied by the slice thickness (6 mm) to obtain the total renal volume (12).

RBF. RBF (ml/min) was calculated as the product of whole kidney perfusion and volume (11).

Transit times. Each peak observed after the vascular phase was analyzed separately (see Fig. 2). Transit time (s) was calculated as the difference between appearance and disappearance time of each peak (first inflection and deflection, respectively) (14).


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Fig. 2.   Demonstration of time-density curves obtained in a dog kidney with electron beam computed tomography (EBCT), describing change of tissue density [in Hounsfield units (HU), (reflecting contrast concentration)] in the various nephron segments contained in the renal cortex (bullet ) and inner medulla (black-square). Labels correspond to transit of contrast through a particular tubular segment. Area under each curve and the time from its appearance to disappearance are calculated separately to compute relative contrast concentration and transit time, respectively, in each nephron segment (see text).

Relative contrast concentration. The area enclosed under each tubular and aortic curve (between first inflection and deflection, respectively) was calculated (Fig. 2) using a data analysis/graphics computer program. The ratio of the area under each tubular peak to that under the aortic curve (29) was then utilized to assess the process of concentration (or dilution) of contrast in each nephron segment relative to pure blood (%) (14).

This calculation is based on the assumption that vascular and tubular volume fraction (i.e., proportion of renal parenchyma) remain unchanged or at least change very little relative to intravascular or intratubular density changes. Nevertheless, it is not inconceivable that a small degree of an increase in density in a region of interest would be subsequent to vascular or tubular expansion.

Perfusion and tubular function analysis is based on the indicator-dilution theory (25). The curves analyzed are derived from renal anatomical regions (e.g., cortex, medulla), and the nephron segments named are not individually visualized but inferred. This is done by observing the spatial and temporal location of the bolus of contrast as it moves among the different regions and by inferring a nephron-site location accordingly, on the basis of the known microanatomy of the kidney. However, with our method, determinations of alterations in segmental tubular dynamics (sodium reabsorption and transit time) during a particular period of time do not consider the effect on fluid reabsorption of changes in fluid delivery from more proximal tubular segments.

Statistical Analysis

All data are reported as means ± SE. A paired t-test was used to compare values obtained in the same animal. Regressions were calculated by the least-squares fit. Statistical significance was assumed as P <=  0.05.

    RESULTS
Top
Abstract
Introduction
Methods
Results
Discussion
References

As shown in Table 1, a change of -26.7 ± 3.1% (-34.6 mmHg) in RPP was followed by an efficient hemodynamic autoregulatory response, with no significant changes in either GFR or RBF. As predictable for pressure-induced antinatriuresis, this decrement in RPP was associated with a statistically significant reduction in both urinary flow rate (-44%, P = 0.043) and UNaV (-53.1%, P = 0.025) (Table 1). This change was not observed in group B, in which RPP remained unaltered (118.0 ± 4.6 vs. 119.4 ± 6.4 mmHg, respectively, P = 0.67) as did urinary flow rate (P = 0.1) and RBF (P = 0.68).

                              
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Table 1.   Renal hemodynamics and urinary excretion measured in group A (n = 7) at the higher vs. lower limits of autoregulation

In group A, whole kidney perfusion, as well as perfusion of the cortex and outer medulla, did not show significant changes during the modification of RPP (Fig. 3). Furthermore, no redistribution of blood flow was observed between the outer and inner cortex at high [4.98 ± 0.45 and 4.24 ± 0.37 ml · min-1 · (cm3 tissue)-1, respectively] vs. low [4.83 ± 0.44 and 4.19 ± 0.39 ml · min-1 · (cm3 tissue)-1, respectively] RPP (P > 0.05). Although average perfusion of the total inner medulla did not show a significant change (P = 0.12), the two components of the inner medulla did show different responses to changes in RPP. The outlying inner medullary perfusion exhibited a significant increase (+19.1%, P = 0.017), whereas perfusion in the deep inner medulla decreased significantly (-36.5%, P = 0.027) (Fig. 3). This decrease in papillary perfusion was significantly correlated with the decrease in concurrent UNaV (Fig. 4). In the control group, comparison between the two scans revealed no significant changes, either in global or regional perfusion (Table 2).


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Fig. 3.   Regional renal perfusion [ml · min-1 · (cm3 tissue)-1] as measured with EBCT in group A (n = 7) 15 min after changing renal perfusion pressure (RPP). Measurements were obtained at the higher (solid bars) vs. lower (open bars) limits of autoregulation, achieved by changing RPP from 130 to 95 mmHg, respectively. * P <=  0.05 compared with high RPP.

                              
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Table 2.   Regional renal perfusion as measured with EBCT in group B (n = 5)

In group A, EBCT-derived measurements of transit times revealed definite and different patterns between the two levels of RPP. A decrease in RPP was associated with a prolongation in transit times through the proximal tubule (an increase of 39.3 ± 11.9%, P = 0.009), ascending limb of the loop of Henle (+68.0 ± 26.2%, P = 0.024), and the distal tubule (+32.1 ± 10.8%, P = 0.013), whereas no significant change was observed at the level of the bend of the loop (Fig. 4). However, this reduction in RPP induced a decrease in transit time in the descending limb of Henle's loop (-28.0 ± 6.2%, P = 0.007) (Fig. 5).


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Fig. 4.   Correlation between the change in deep inner medullary perfusion, detected with EBCT in group A (n = 7) after a decrease in RPP within the range of autoregulation, and the concurrent change in urinary sodium excretion.


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Fig. 5.   Renal tubular transit times (s) as measured with EBCT in group A (n = 7), at the higher (solid bars) vs. lower (open bars) limits of autoregulation. Measurements were obtained 15 min after changing RPP. * P <=  0.05 compared with high RPP.

Determination of changes in intratubular concentration of contrast media (relative to input function) in group A revealed a statistically significant concentration of contrast in the proximal tubule (an increase of 49.2 ± 22.8%, P = 0.05), ascending limb (+79.5 ± 30.2%, P = 0.006), and distal tubule (+41.8 ± 18.8%, P = 0.047) (Fig. 5); whereas contrast medium was significantly less concentrated in the descending limb of Henle's loop (-20.7 ± 6.9%, P = 0.038). Once again, no significant change was observed when this parameter was measured at the level of the bend of the loop (Fig. 6).


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Fig. 6.   Renal intratubular contrast concentration relative to blood (%) as measured with EBCT in group A (n = 7), 15 min after reaching the higher (solid bars) or lower (open bars) limits of autoregulation. * P <=  0.05 compared with high RPP.

Therefore, in the nephron segments where transit times were prolonged (proximal, ascending, and distal tubules), the contrast medium was proportionally concentrated, whereas in the segment where transit time was shorter at low compared with high RPP (descending limb), contrast medium was found to be less concentrated.

In the control group (group B), no statistically significant changes were observed in either transit times or intratubular contrast concentration (Table 3).

                              
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Table 3.   Renal tubular dynamics (transit times and relative contrast concentration) as measured with EBCT in group B (n = 5)

    DISCUSSION
Top
Abstract
Introduction
Methods
Results
Discussion
References

This study demonstrates that a change in RPP leads to changes in volume reabsorption in the proximal tubule, ascending limb, and distal tubule in the majority of the nephrons of all the kidneys sampled. These alterations occurred in the presence of hemodynamic changes circumscribed to the inner medulla.

The animals of the experimental group experienced a very efficient hemodynamic autoregulation. A change in aortic pressure from 139 to 95 mmHg was attended by no significant change in either RBF or GFR, whereas urine sodium excretion fell by 44%. In the control group, on the other hand, no significant changes were observed in any of the parameters studied. These findings allow us to describe, for the first time, the concurrent intrarenal hemodynamic and tubular changes that are detectable with fast CT within the range of autoregulation.

We succeeded at our goal of developing a method capable of estimating parameters of tubular function (14). We previously demonstrated the capability of the fast CT technique to study the relationship between blood flow distribution and tubular solute handling (14). This technique is based on the fact that radiographic contrast medium (like inulin) is freely filtered by the glomeruli, and its transit time through the tubules can be externally recorded and followed as it appeared and then disappeared from the region of interest. Furthermore, since contrast media are neither reabsorbed nor secreted in the tubules, a change in X-ray density (relative contrast concentration) results from changes in tubular fluid reabsorption within a nephron segment, and vice versa.

We measured tubular transit times (s) and relative contrast (solute) concentration (proportional to aortic blood), analogous to fluid/plasma ratio. Since our density measurements represented an average across the region of interest, they represent whole populations of different nephron segments contained in these regions. Indeed, our measurements of proximal tubular transit times in the control group (21.8 ± 2.5 s) were highly comparable to those previously measured in normal dogs using Lissamine green, i.e., 26 ± 5.7 s (17) and 21.6 ± 1.3 s (26). Our Henle's loop basal passage time (33.3 ± 4.1 s) was also very similar to previously reported values (26) obtained with Lissamine green (32.3 ± 1.1 s), although our distal tubular transit time (71.5 ± 1.6 s) was a little longer than the 56 ± 12 s previously reported in the dog (17).

Our results revealed that a reduction in RPP was accompanied by prolongation of transit times in the proximal tubule, ascending limb of Henle's loop, and distal tubule. Since these changes were associated with an increase in relative contrast concentration in the same tubules, it is reasonable to conclude that tubular fluid was being reabsorbed in these segments compared with the condition prevailing during high RPP.

It should be mentioned here that, in the dog, Navar et al. (17) have failed to observe any significant change of sodium reabsorption in proximal tubules of superficial nephrons when RPP was decreased within the range of autoregulation. However, no information was obtained in that study regarding changes of proximal reabsorption in deep nephrons. In the rat, on the other hand, Haas et al. (6) found that changes in proximal tubule sodium reabsorption during pressure natriuresis occurred only in deep nephrons, whereas sodium delivery to distal tubules in superficial nephrons remained unchanged. It is quite possible that a pattern similar to that described for the rat exists in the dog as well. Our results would therefore reflect the integrative nature of our CT methodology, in which measurements of tubular dynamics are derived from a large population of nephrons, rather than being based on those observed in single nephron, like the micropuncture technique.

Our results also showed that, when RPP was decreased, there was an acceleration in the transit of contrast media in the descending limb of Henle's loop, associated with a lower intratubular contrast concentration. This implies that between its transit in the proximal and descending tubules, the contrast was undergoing less concentration than it did during high RPP. The physiological significance of this phenomenon remains unclear.

Micropuncture studies have demonstrated that changes in RPP induce changes in sodium excretion in the proximal tubule of superficial and deep nephrons, as well as in the thin ascending limb of Henle's loop (6, 20). Sodium reabsorption in the outer medullary thick ascending limb of the loop of Henle is highly load dependent and can prevent changes in sodium excretion when proximal reabsorption is inhibited (7, 9). However, many investigators have observed that chloride reabsorption in the loop was not changed when proximal delivery was increased by elevations of RPP (3, 10, 17, 20). This means that the major contribution of the thick ascending limb to pressure natriuresis is not confined to altering the increase sodium load that is being delivered from the proximal tubules (19). Consistent with this demonstration, we found that following the decrease in RPP, there was a 70-80% increase of fluid reabsorption in the region of interest corresponding to the thick ascending limb. These observations allow, for the first time, to extend the observations made by micropuncture techniques to the whole nephron segment population.

Our EBCT technique also enabled studying concurrent renal hemodynamics. The hemodynamic changes observed in this study resemble those reported by our group (13) in studies performed under similar experimental conditions but using another fast CT scanner, the dynamic spatial reconstructor. In that study (13), we found that when RPP was decreased, cortical and outer medullary blood flow exerted an efficient hemodynamic autoregulation, whereas papillary (deep inner medulla) blood flow decreased significantly. In the present study we have separately sampled an additional region of interest, which was the more peripheral (outlying) zone of the inner medulla (Fig. 1). We found that, as reported before, changes in RPP were associated with similar directional changes in deep inner medullary blood flow. Opposite directional changes of blood flow, however, were systematically recorded in the outlying inner medullary perfusion.

The reasons for these reciprocal changes of flow in the inner medulla are not known. Nonetheless, they were reproducible from animal to animal and always occurred in a reciprocal fashion whether RPP was initially increased (5 dogs) or decreased (2 dogs). In brief, this observation emphasizes the fact that significant changes in tubular sodium reabsorption are associated with constant and reproducible changes and localized to a relatively small area of the inner renal medulla.

Several techniques, mainly radioactive microspheres and more recently laser-Doppler flowmetry, have attempted to study the hemodynamic renal response to alterations in RPP. However, radioactive microspheres have a preferential regional distribution within the renal cortex, and do not reach the medulla, therefore providing unreliable and incomplete blood flow measurements of these regions (28). Moreover, it is conceivable that the papillary region could be perfused by vessels regulated independently from those that provide circulation to the more outlying inner medulla or that the increase in outlying inner medullary perfusion observed with a decrease in RPP is at expenses of blood flow to the deep inner medulla and vice versa. These alterations may be difficult to detect using laser-Doppler techniques, since it requires one to position the laser detector in a very specific area of the renal medulla.

On the other hand, it is quite possible that placement of the laser-Doppler probe in different anatomical regions of the inner medulla may explain the contradictory results obtained by different investigators. Using laser-Doppler flowmetry, some investigators (21) hypothesized that changes in medullary circulation play a central role in regulating sodium excretion by producing parallel alterations in renal interstitial pressure. In a dog model, Majid et al. (15) could not detect changes in medullary circulation during changes in RPP within the range of autoregulation, whereas Strick et al. (27) found good medullary autoregulation only on the upper part of the autoregulatory curve. From our study it is difficult to determine whether the observed changes in papillary flow were sufficiently significant to alter renal interstitial pressure and/or release renal autacoids. However, this is conceivable since there was a significant positive correlation between UNaV and percent change in papillary (deep inner medullary) flow. Furthermore, the slope of this correlation (Fig. 4) indicates an amplification of this relationship, so for any increase in papillary perfusion there was more than a twofold increase in sodium excretion. This may suggest a causal relationship between the two. However, our experimental protocol and resolution do not allow us to determine whether this is indeed the case or whether the observed concomitant change in medullary flow is just an epiphenomenon or the consequence of changes in the tubular pressure.

In conclusion, we found that a decrease in RPP was associated with hemodynamic changes in inner medullary perfusion, as well as with tubular functional changes (increase in volume and probably sodium reabsorption). Most of the nephron population sampled played a role in the regulation of sodium excretion, but the specific mechanisms underlying this regulation remain unclear. EBCT provides a unique opportunity to study hemodynamics and tubular function in an integrated and noninvasive fashion.

    ACKNOWLEDGEMENTS

We gratefully acknowledge the technical assistance of the staff of the EBCT.

    FOOTNOTES

This study was supported from National Heart, Lung, and Blood Institute Grant HL-16496. M. Rodriguez-Porcel is supported with a Mayo Foundation fellowship and a University of Health Sciences fellowship (Buenos Aires, Argentina).

Address for reprint requests: J. C. Romero, Dept. of Physiology, Mayo Clinic and Foundation, 200 First St. SW, Rochester, Minnesota 55905.

Received 6 January 1997; accepted in final form 5 June 1997.

    REFERENCES
Top
Abstract
Introduction
Methods
Results
Discussion
References

1.   Almén, T., and K. Golman. Contrast media pharmacokinetics in the assessment of renal failure. In: Contrast Media: Biological Effects and Clinical Applications, edited by Z. Parvez, R. Moncada, and M. Sovak. Boca Raton, FL: CRC Press, 1987, p. 77-88.

2.   Berne, R. M., and M. N. Levy. Hemodynamics. In: Principles of Physiology, edited by S. Manning. St. Louis, MO: C. V. Mosby, 1990, p. 245-254.

3.   Chou, C.-L., and D. J. Marsh. Role of proximal convoluted tubule in pressure diuresis in the rat. Am. J. Physiol. 251 (Renal Fluid Electrolyte Physiol. 20): F283-F289, 1986[Abstract/Free Full Text].

4.   Cupples, W. A., T. Sakai, and D. J. Marsh. Angiotensin II and prostaglandins in control of vasa recta blood flow. Am. J. Physiol. 254 (Renal Fluid Electrolyte Physiol. 23): F417-F424, 1988[Abstract/Free Full Text].

5.   Garcia-Estañ, J., and R. J. Roman. Role of renal interstitial pressure in the pressure diuresis response. Am. J. Physiol. 256 (Renal Fluid Electrolyte Physiol. 25): F63-F70, 1989[Abstract/Free Full Text].

6.   Haas, J. A., J. P. Granger, and F. G. Knox. Effect of renal perfusion pressure on sodium reabsorption from proximal tubules of superficial and deep nephrons. Am. J. Physiol. 250 (Renal Fluid Electrolyte Physiol. 19): F425-F429, 1986[Abstract/Free Full Text].

7.   Howards, S. S., B. B. Davis, F. G. Knox, F. S. Wright, and R. W. Berliner. Depression of fractional sodium reabsorption by the proximal tubule of the dog. J. Clin. Invest. 47: 1561-1572, 1968[Medline].

8.   Khraibi, A. A., and F. G. Knox. Effect of decapsulation on renal interstitial hydrostatic pressure and natriuresis. Am. J. Physiol. 257 (Regulatory Integrative Comp. Physiol. 26): R44-R48, 1989[Abstract/Free Full Text].

9.   Kunau, R. T., Jr., H. L. Webb, and S. C. Borman. Characteristics of sodium reabsorption in the loop of Henle and distal tubule. Am. J. Physiol. 227: 1181-1191, 1974[Medline].

10.   Kunay, R. T., and N. H. Lameire. The effect of an acute increase of renal perfusion pressure on sodium transport in the rat kidney. Circ. Res. 39: 689-695, 1976[Abstract].

11.   Lerman, L. O., M. R. Bell, V. Lahera, J. A. Rumberger, P. F. Sheedy, A. Sanchez Fueyo, and J. C. Romero. Quantification of global and regional renal blood flow with electron beam computed tomography. Am. J. Hypertens. 7: 829-837, 1994[Medline].

12.   Lerman, L. O., M. D. Bentley, M. R. Bell, J. A. Rumberger, and J. C. Romero. Quantitation of the in vivo kidney volume with cine computed tomography. Invest. Radiol. 25: 1206-1211, 1990[Medline].

13.   Lerman, L. O., M. D. Bentley, M. J. Fiksen-Olsen, D. M. Strick, E. L. Ritman, and J. C. Romero. Pressure dependency of canine intrarenal blood flow within the range of autoregulation. Am. J. Physiol. 268 (Renal Fluid Electrolyte Physiol. 37): F404-F409, 1995[Abstract/Free Full Text].

14.   Lerman, L. O., M. Rodriguez-Porcel, P. F. I. Sheedy, and J. C. Romero. Measurement of in vivo tubular function in the intact kidney. Kidney Int. 50: 1358-1362, 1996[Medline].

15.   Majid, D. S. A., M. Godfrey, and L. G. Navar. Autoregulation of renal medullary blood flow during enhanced pressure natriuresis in anesthetized volume expanded dogs (Abstract). Hypertension 26: 572, 1995.

16.   Morris, T. W., and H. W. Fischer. The pharmacology of intravascular radiocontrast media. Annu. Rev. Pharmacol. Toxicol. 26: 143-160, 1986[Medline].

17.   Navar, L. G., P. D. Bell, and T. J. Burke. Autoregulatory response of superficial nephrons and their autoregulatory association with sodium excretion during arterial pressure alterations in the dog. Circ. Res. 41: 487-496, 1977[Medline].

18.   Ramsey, C. R., and F. G. Knox. Micropuncture and microperfusion techniques. In: Methods in Renal Toxicology, edited by R. Lash. Boca Raton, FL: CRC Press, 1996, p. 79-86.

19.   Roman, R. J. Pressure diuresis mechanism in the control of renal function and arterial pressure. Federation Proc. 45: 2878-2884, 1986[Medline].

20.   Roman, R. J. Pressure-diuresis in volume expanded rats: tubular reabsorption in superficial and deep nephrons. Hypertension 12: 177-183, 1988[Abstract].

21.   Roman, R. J., P. K. Carmines, R. Loutzenhiser, and J. D. Conger. Direct studies on the control of renal microcirculation. J. Am. Soc. Nephrol. 2: 136-149, 1991[Abstract].

22.   Roman, R. J., C. A. Smits, and J. H. Lombard. Measurement of microcirculatory blood flow by differential laser-Doppler spectroscopy in the rat gracilis muscle and kidney (Abstract). Physiologist 27: 257, 1984.

23.   Romero, J. C., M. D. Bentley, and F. G. Knox. Intrarenal mechanisms that regulate sodium excretion in relation to changes in blood pressure. Mayo Clin. Proc. 64: 1406-1424, 1989[Medline].

24.   Romero, J. C., and F. G. Knox. Mechanisms underlying pressure-related natriuresis: the role of the renin-angiotensin and prostaglandin systems. Hypertension 11: 724-738, 1988[Abstract].

25.   Rumberger, J. A., A. J. Feiring, M. J. Lipton, C. B. Higgins, S. R. Ell, and M. L. Marcus. Use of ultrafast computed tomography to quantitate regional myocardial perfusion: a preliminary report. J. Am. Coll. Cardiol. 9: 59-69, 1987[Medline].

26.   Steinhausen, M., and G. A. Tanner. Microcirculation and tubular urine flow in mammalian kidney cortex (in vivo microscopy). In: Abhandlung: Sitzungsberichte der Heidelberger Akademie der Wissenschafter, Mathematisch-naturwissenschaftliche Klasse. New York: Jahrgang, 1976, p. 6-61.

27.   Strick, D. M., M. J. Fiksen-Olsen, J. C. Lockhart, R. J. Roman, and J. C. Romero. Direct measurement of renal medullary blood flow in the dog. Am. J. Physiol. 267 (Regulatory Integrative Comp. Physiol. 36): R253-R259, 1994[Abstract/Free Full Text].

28.   Wolfgast, M. Studies on the regional renal blood flow with 32P-labeled red cells and small beta-sensitive semiconductor detector. Acta Physiol. Scand. 313: 7-109, 1968.

29.   Wu, X., D. L. Ewert, Y. H. Liu, and E. L. Ritman. In vivo relation of intramyocardial blood volume to myocardial perfusion. Circulation 85: 730-737, 1992[Abstract].


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