Effects of daily sodium intake and ANG II on cortical and medullary renal blood flow in conscious rats

Implanted optical fibers and laser-Doppler flow measurement techniques were used for the sequential measurement of regional renal blood flow in conscious rats to determine the effects of an increase of daily NaCl intake on the renal cortical blood flow and blood flow to the outer and inner medulla. Cortical blood flow was increased significantly (32%) by the second day when NaCl intake was increased from 1 to 7 meq/day and was increased further (50%) on the second day after a further elevation of NaCl intake to 13 meq/day. Blood flow to the outer and inner medulla was not changed as NaCl intake was elevated. The increase in renal cortical flow was closely associated with significant reductions in circulating concentrations of ANG II from 31 to 16 pg/ml. Rats given a continuous infusion of nonpressor does of ANG II (5.0 ng.kg(-1).min-1) to maintain constant plasma concentrations of ANG II as sodium intake was increased exhibited no increase of cortical flow. We conclude that reductions of plasma ANG II associated with incremental increases of daily sodium intake result in a rise of renal cortical flow. The elevated blood flow to the renal cortex may enhance sodium excretion and contribute to long-term sodium homeostasis.

A multiunit model of solute and water removal by inner medullary vasa recta

A recent model of volume and solute microcirculatory exchange in the renal medulla based on a single descending vasa rectum (DVR) was extended to account for the varying number of vessels along the corticomedullary axis. The assumption that concentration polarization at the walls of ascending vasa recta (AVR) during volume uptake eliminates transmural oncotic pressure gradients was examined. In this limiting case, small hydrostatic pressure gradients can drive AVR volume uptake if the pressure in the interstitium exceeds that in the AVR lumen. The calculated hydraulic pressure difference across AVR yielding agreement between predicted and measured values of AVR-to-DVR blood flow rate ratios was found to be smaller than the reported maximum pressure difference AVR can sustain. Simulations also confirmed previous conclusions suggesting that the presence of urea transporters in DVR counterbalances that of water channels that would otherwise decrease the efficiency of small solute trapping in the renal medulla.

Regulation of intrarenal blood flow in experimental heart failure: role of endothelin and nitric oxide

Congestive heart failure(CHF) is associated with a marked decrease in cortical blood flow and preservation of medullary blood flow. In the present study we tested the hypothesis that changes in the endothelin (ET) and nitric oxide (NO) systems in the kidney may contribute to the altered intrarenal hemodynamics in rats with aortocaval fistula, an experimental model of CHF. Cortical and medullary blood flow were measured simultaneously by laser-Doppler flowmetry in controls and rats with compensated and decompensated CHF. As previously reported [K. Gurbanov, I. Rubinstein, A. Hoffman, Z. Abassi, O. S. Better, and J. Winaver. Am. J Physiol. 271 (Renal Fluid Electrolyte Physiol. 40): F1166-F1172, 1996], administration of ET-1 in control rats produced a sustained cortical vasoconstriction and a transient medullary vasodilatory response. In rats with decompensated CHF, cortical vasoconstriction was severely blunted, whereas ET-1-induced medullary vasodilation was significantly prolonged. This prolonged response was mimicked by IRL-1620, a specific ETB agonist, and partially abolished by NO synthase (NOS) blockade. In line with these findings, expression of ET-1, ETA and ETB receptors, and endothelial NOS (eNOS), assessed by RT-PCR, and eNOS immunoreactivity, assessed by Western blotting, was significantly higher in the medulla than in the cortex. Moreover, expression of ET-1 mRNA in the cortex and eNOS mRNA in the cortex and the medulla increased in proportion to the severity of heart failure. These findings indicate that CHF is associated with altered regulation of intrarenal blood flow, which reflects alterations in expression and activity of the ET and NO systems. It is further suggested that exaggerated NO activity in the medulla contributes to preservation of medullary blood flow in the face of cortical vasoconstriction in CHF.

Cortical and medullary hemodynamics in deoxycorticosterone acetate-salt hypertensive mice

The effect of acutely increasing renal perfusion pressure or extracellular fluid volume on renal medullary and cortical blood flow was examined in the low-renin deoxycorticosterone acetate (DOCA)-salt hypertension model in mice. A 50-mg DOCA tablet was implanted, and 1% saline was given as drinking water for 3 wk. Medullary and cortical blood flow were determined with laser-Doppler flowmetry, and whole-kidney blood flow was measured with a transit-time ultrasound flowprobe around the renal artery. In control mice, total renal blood flow ranged from 6.3 and 7.6 ml/min per g kidney weight and in DOCA-salt mice from 4.3 and 4.7 ml/min per g kidney weight, respectively, and was minimally affected as renal perfusion pressure was increased. Renal vascular resistance increased correspondingly. During stepwise increases in renal artery pressure from 90 to 140 mmHg, medullary blood flow progressively increased in control mice to 125% of baseline values, whereas cortical blood flow did not change. In DOCA-salt mice, increasing BP from 100 to 154 mmHg had no effect on either cortical or medullary blood flow. Urine flow and sodium excretion were lower in DOCA-salt mice than in controls and increased nearly to the same extent in both groups after volume expansion with isotonic saline. Total renal blood flow increased after saline loading, more in controls than in DOCA-salt mice. Increases in medullary blood flow after saline loading were up to 122% of baseline values in controls and demonstrated a significantly steeper slope than the 110% of baseline increases in DOCA-salt mice. Cortical blood flow, however, was not different between the groups. Thus, medullary blood flow is not as tightly autoregulated as cortical blood flow in normal mice. Natriuresis with acute volume loading is facilitated by increased medullary blood flow. In DOCA-salt mice, the medullary blood flow reaction to renal perfusion pressure increases is abolished, whereas flow increases with extracellular volume expansion are diminished. These results suggest that diminished pressure-natriuresis responses in DOCA-salt mice are related to perturbed medullary blood flow.

Role of AT1 receptors in the renal papillary effects of acute and chronic nitric oxide inhibition

Nitric oxide (NO) is a vasodilator substance controlling renal papillary blood flow (PBF) in the rat. In this study we have evaluated the role of AT1 angiotensin II receptors as modulators of the whole kidney and papillary vasoconstrictor effects induced by the acute or chronic inhibition of NO synthesis. Experiments have been performed in anesthetized, euvolemic Munich-Wistar rats prepared for the study of renal blood flow (RBF) and PBF. In normal rats, acute administration of the NO synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) increased mean arterial pressure (MAP) and decreased RBF and PBF. Either acute or chronic treatment with the AT1 receptor blocker losartan did not modify the decreases in RBF or PBF secondary to L-NAME. In animals made hypertensive by chronic inhibition of NO, basal MAP was higher, whereas RBF and PBF were lower than in the controls. In these animals, acute or chronic administration of losartan decreased MAP and increased both RBF and PBF significantly. These results indicate that, under normal conditions, the decreases in RBF or PBF induced by the acute inhibition of NO synthesis are not modulated by AT1-receptor stimulation. However, the arterial hypertension, renal vasoconstriction, and reduced PBF present in chronic NO-deficient hypertensive rats is partially due to the effects of angiotensin II, via stimulation of AT1-receptors.

Response of isolated rat descending vasa recta to bradykinin

Outer medullary descending vasa recta (OMDVR) were dissected from the outer medullary vascular bundles of young rats, perfused in vitro, and loaded with fura 2 for measurement of intracellular calcium concentration ([Ca2+]i) by fluorescent ratio imaging. Fluorescent video images revealed that fura 2 selectively loads into endothelial cells but not pericytes. Bradykinin (BK), at concentrations > 10(-11) M, elicited an increase in [Ca2+]i from baseline values in the range from 50 to 100 nM to peak values of 600-800 nM followed by a sustained plateau of 150-250 nM. The vasopressin V1-receptor agonist [Phe2,Ile3,Orn8]vasopressin constricted OMDVR but yielded no observable [Ca2+]i response, a finding that is consistent with an endothelial cell origin for the fura 2 fluorescent signal. The BK [Ca2+]i response was blocked by the selective BK B2-receptor antagonists D-Arg-[Hyp3,Thi5.8,D-Phe7]BK and D-Arg-[Hyp3,D-Phe7,Leu8]BK but not the B1 antagonist des-Arg9-[Leu8]BK. BK vasodilated microperfused OMDVR that had been preconstricted with 10(-8) M angiotensin II. We conclude that the [Ca2+]i response of OMDVR endothelia can be selectively studied with fura 2, that BK increases endothelial [Ca2+]i via the B2 receptor, and that BK can vasodilate descending vasa recta.

Protective effect of angiotensin II-induced increase in nitric oxide in the renal medullary circulation

This study examined the effect of intravenous infusion of subpressor doses of angiotensin (Ang II) on renal medullary blood flow (MBF), medullary partial oxygen pressure (PO2), and nitric oxide (NO) concentration under normal conditions and during reduction of the medullary nitric oxide synthase (NOS) activity in anesthetized rats. With laser Doppler flowmetry and polarographic measurement of PO2 with microelectrodes, Ang II (5 ng/kg per minute) did not alter renal cortical and medullary blood flows or medullary PO2. N(omega)-nitro-L-arginine methyl ester (L-NAME) was infused into the renal medullary interstitial space at a dose of 1.4 microg/kg per minute, a dose that did not significantly alter basal levels of MBF or PO2. Intravenous infusion of Ang II at the same dose in the presence of L-NAME decreased MBF by 23% and medullary PO2 by 28%, but it had no effect on cortical blood flow or arterial blood pressure. An in vivo microdialysis-oxyhemoglobin NO trapping technique was used in other rats to determine tissue NO concentrations using the same protocol. Ang II infusion increased tissue NO concentrations by 85% in the renal cortex and 150% in the renal medulla. Renal medullary interstitial infusion of L-NAME (1.4 microg/kg per minute) reduced medullary NO concentrations and substantially blocked Ang II-induced increases in NO concentrations in the renal medulla, but not in the renal cortex. Tissue slices of the renal cortex and medulla were studied to determine the effects of Ang II and L-NAME on the nitrite/nitrate production. Ang II stimulated the nitrite/nitrate production predominately in the renal medulla, which was significantly attenuated by L-NAME. We conclude that small elevations of circulating Ang II levels increase medullary NO production and concentrations, which plays an important role in buffering the vasoconstrictor effects of this peptide and in maintaining a constancy of MBF.

Effects of insulin-like growth factor I on the renal juxtamedullary microvasculature

To characterize the effects on the rat renal preglomerular microvasculature of insulin-like growth factor I (IGF-I), experiments were performed using the in vitro blood-perfused juxtamedullary nephron preparation. IGF-I induced a reversible vasodilation of pre- but not postglomerular microvessels in a dose-dependent manner (10(-9)-10(-7) M). The IGF-I-induced vasodilation was similar in all preglomerular vascular segments: interlobular artery, 11.5 +/- 1.2% of control (n = 16); mid-afferent arterioles, 11.6 +/- 1.7% (n = 24); and juxtaglomerular afferent segments, 16.1 +/- 2.8% (n = 19). Renal autoregulatory capacity was not reduced by IGF-I. Pretreatment with the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (10(-4) M) completely inhibited the vasodilatory response to IGF-I. IGF-I induced a rapid increase of NO concentration in intact renal microvessels, monitored by a NO-selective voltametric microelectrode. Pretreatment with the cyclooxygenase inhibitor indomethacin (10(-5) M) not only abrogated the IGF-I-induced dilation, but, moreover, IGF-I elicited a small but significant (approximately 10%) vasoconstriction in all preglomerular vessels. These results indicate that the renal vascular effects of IGF-I involve activation of two endogenous vasodilators (NO and vasodilatory prostaglandins). In addition, IGF-I may also release an undefined vasoconstrictor.

Renal medullary blood flow and renal medullary antihypertensive mechanisms

It has long been recognised that the kidneys take part in blood pressure control via both their exocrine and endocrine functions. An endocrine antihypertensive function of the renal medulla has been proposed. The renal medullary depressor substances ("medullipins"), are released in response to increased renal perfusion pressure. It has been suggested that the release of "medullipin" is controlled via changes in renal medullary blood flow. Recent observations also suggest that renal medullary blood flow is involved in the control of the pressure/natriuretic-diuretic action of the kidney. In this review we outline a unified hypothesis for blood pressure control via a combination of the plasma volume regulating pressure-natriuresis mechanism and the powerful antihypertensive actions of the "medullipins" (i.e. vasodilatation, inhibition of sympathetic drive and a diuretic action). It is hypothesised that the activity of both these systems are under control by renal medullary blood flow.

Three-dimensional microcomputed tomography of renal vasculature in rats

Current microscopic methods to view renal microvasculature reveal only a very limited portion of the total renal volume. Identification of connectivity for postglomerular vessels in the cortex and the medulla during functional states related to changes in sodium excretion will help better to understand the coupling of renal vasculature to tubular function. The purpose of this study was to investigate the possibility of visualizing the continuity of pre- and postglomerular vasculature using three-dimensional micro-computed tomography (micro-CT). Kidneys from normal rats were perfusion fixed in situ at physiological pressure, filled with latex microfil containing lead chromate, and embedded in plastic. The micro-CT scans of the intact kidneys were carried out on a rotating stage illuminated either by a synchrotron x-ray source or a conventional x-ray spectroscopy tube. Images were reconstructed by a filtered backprojection algorithm and volume-rendering techniques were utilized to display the vasculature. The reconstructed images clearly showed the large distribution vessels and the venous drainage of the kidneys, while pre- and postglomerular vessels and their vascular connections throughout the kidney were displayed in great detail. Efferent arterioles showed the characteristics of their peritubular capillary beds in the cortical and medullary regions. The vascular volume of the cortex was 27%, the outer stripe of the outer medulla 18%, the inner stripe of the outer medulla 30%, and the inner medulla 18%. In conclusion, micro-CT is a promising method to evaluate renal vascular architecture relative to physiological and pathological alterations.

Interactions between nitric oxide and angiotensin II on renal cortical and papillary blood flow

This study examined the role of angiotensin II (Ang II) on the effects of nitric oxide (NO) synthesis blockade on renal cortical and papillary blood flow in innervated and denervated kidneys of volume-expanded Munich-Wistar rats with hormonal influences on the kidney that were held constant by intravenous infusion. Cortical (CBF) and papillary (PBF) blood flow were measured by laser-Doppler flowmetry. A low dose of N omega-nitro-L-arginine methyl ester (L-NAME, 3.7 nmol x kg[-1] x min[-1]) reduced CBF only in innervated kidneys, and this effect was abolished by subsequent administration of valsartan (an AT1 antagonist). L-NAME 3.7 nmol x kg(-1) x min(-1) improved PBF autoregulation by lowering PBF to the range of 100 to 140 mm Hg of perfusion pressure, and this effect was attenuated or abolished by valsartan in innervated and denervated kidneys, respectively. These results indicate that the cortical and medullary vasoconstriction induced by a low dose of L-NAME are caused by potentiation of the vasoconstrictor influence of renal sympathetic nerves and Ang II. A higher dose of L-NAME (37 nmol x kg[-1] x min[-1]) lowered CBF and PBF in both innervated and denervated kidneys. This effect of L-NAME on the cortical circulation was abolished by valsartan, but this AT1 antagonist had no effect on the medullary vasoconstriction produced by NO synthesis blockade. Therefore, a higher dose of L-NAME induces a renal cortical vasoconstriction through potentiation of the renin-angiotensin system, whereas the fall of PBF seen after L-NAME 37 nmol x kg(-1) x min(-1) seems to be caused primarily by NO suppression. This Ang II potentiation produced by L-NAME in the renal cortex seems to be mediated by AT1 receptors, because it was unaffected by PD123319 (an AT2 antagonist). The results of the present study indicate that NO is an important modulator of the vasoconstrictor influence of Ang II in the renal cortical circulation of the rat. However, although there are some interactions between NO and renal nerves and Ang II on the medullary circulation, the renal medullary vasoconstriction produced by L-NAME appears to be caused primarily by NO suppression, with little influence of the renal vasoconstrictor systems.

Localization of endothelin-converting enzyme-1 in human kidney

The distribution of endothelin-converting enzyme-1 (ECE-1) mRNA and protein was investigated in human kidney excised because of renal tumors. ECE-1 immunoreactivity was detected by immunohistochemistry throughout the different areas of the kidney in the vascular and tubular structures. In the cortex, ECE-1 immunostaining was present in the endothelial surface of arcuate and interlobular arteries and in arterioles. Weak specific immunoreactivity was present over some proximal and distal tubules. Few endothelial glomerular cells contained ECE-1 protein. In the medulla, ECE-1 immunoreactivity was observed in the vasa recta bundles and capillaries. ECE-1 immunostaining was also detected in the outer and inner medullary collecting ducts and thin limbs of Henle's loops. Immunohistochemical detection of the von Willebrand factor on adjacent sections confirmed the endothelial nature of the vascular cells that exhibited ECE-1 immunostaining. The distribution patterns of ECE-1 mRNA, investigated by in situ hybridization, appeared similar to that obtained by immunohistochemistry in the cortical and medullary vasculature and in different portions of the nephron. Northern blot and densitometric analyses demonstrated that ECE-1 mRNA levels were quantitatively similar in both the renal cortex and medulla. These results demonstrate that vascular endothelial and tubular epithelial cells in the cortex and medulla of the human kidney synthesize ECE-1, which, in turn, may play an important role in regulating endothelin production in physiological and pathological conditions.

Evidence for the presence of smooth muscle alpha-actin within pericytes of the renal medulla

This study was designed to determine whether smooth muscle alpha-actin mRNA and smooth muscle alpha-actin contractile protein elements were present within the renal medullary pericytes. Extraction of total RNA from microdissected outer medullary descending vasa recta allowed for the detection of smooth muscle alpha-actin mRNA expression using reverse transcription-polymerase chain reaction (RT-PCR). Expression of smooth muscle alpha-actin was specific to the descending vasa recta and not a result of tubular contamination because RT-PCR amplification of the vasopressin V2 receptor, which is a specific tubular marker, did not occur. To determine the exact cell type(s) that translate the mRNA into protein, we performed immunohistochemistry on the renal outer and inner medulla using a monoclonal smooth muscle alpha-actin antibody, whose specificity was determined by immunoblot analysis. Smooth muscle alpha-actin protein was found selectively within the pericytes surrounding the descending vasa recta from the outer and inner medullary tissue sections. This study demonstrates that the pericytes alone that surround the descending vasa recta within the outer and inner medulla contain smooth muscle alpha-actin mRNA and protein and are therefore the site of the contractile elements that could play a vasomodulatory role in the control of renal medullary blood flow and its distribution within the renal medulla.

Simultaneous recording of tissue ion content and blood flow in rat renal medulla: evidence on interdependence

The relationship of renal medullary tissue ion concentration and medullary blood flow (MBF) has never been closely evaluated because of limitations of available measuring methods. In an attempt to overcome this difficulty, an integrated probe was developed for simultaneous recording in rat renal medulla of tissue electrical admittance (Y), an index of interstitial ion concentration, and tissue perfusion with blood (laser-Doppler method). During spontaneous-selective MBF variations tissue Y showed inverse changes (r = -0.77, P < 0.001). The inverse correlation of the two variables was also seen after MBF has been reduced (-43%) by indomethacin, 5 mg/kg body wt iv (r = -0.77, P < 0.01). A modest selective MBF reduction (15%) induced by glibenclamide, an inhibitor of ATP-dependent K channels, did not alter medullary tissue admittance. The data support experimentally the concept that the rate of medullary tissue perfusion with blood is one determinant of interstitial solute concentration; however, changes in the latter were demonstrable only with major alterations of the MBF.

Lovastatin prevents development of hypertension in spontaneously hypertensive rats

The present study evaluated the effects of lovastatin on renal function and the development of hypertension in spontaneously hypertensive rats (SHR). Four-week-old SHR were given lovastatin (10 mg/kg) or vehicle twice daily by gavage. After 4 weeks of treatment, mean arterial pressure was significantly lower in lovastatin-treated SHR (131 +/- 4 mm Hg, n=5) than in control animals (160 +/- 4 mm Hg, n=12) (P<.05). The fall in arterial pressure in lovastatin-treated rats was accompanied by changes in renal function. The slope of the relationship between arterial pressure and sodium excretion was threefold greater in lovastatin-treated SHR (n=6) than in control rats (n=6), and this was associated with significant elevations in renal medullary blood flow and renal interstitial hydrostatic pressure. Glomerular filtration rate was 17% higher in lovastatin-treated SHR (n=6) than in control rats (n=6) (0.94 +/- 0.05 versus 0.81 +/- 0.07 mL/min per g of kidney weight, P<.05). The wall-to-lumen area ratio of renal arterioles was significantly reduced in lovastatin-treated SHR compared with vehicle-treated rats (0.86 +/- 0.05 versus 1.08 +/- 0.04 for vessels with inner diameters <50 microm and 0.62 +/- 0.02 versus 0.75 +/- 0.04 for vessels with inner diameters of 50 to 100 microm, P<.05). These results indicate that chronic treatment with lovastatin shifts the relations between renal medullary blood flow, renal interstitial pressure, sodium excretion, and renal perfusion pressure to lower levels of arterial pressure and attenuates the development of hypertension and renal vascular hypertrophy in SHR.

Exaggerated natriuresis in transgenic (mRen2)27 rats

BACKGROUND

Hypertension features an exaggerated natriuresis after acute volume expansion. In humans, the degree of exaggerated natriuresis appears to be correlated inversely to the level of angiotensin (Ang) II.

OBJECTIVE

To test the hypothesis that the degree of exaggerated natruresis is correlated to the level of Ang II by studying two rat models, transgenic rats (TGR) with and extra renin gene (TGR mRen2)27 and desoxycorticosterone acetate (DOCA)-salt rats, in comparison with Sprague-Dawley Hannover (SDH) rat controls.

METHODS

All of the rats were uninephrectomized for 1 month. DOCA-salt rats were implanted with a DOCA pallet and drank 1% saline. Rats were anesthetized and their left kidneys were instrumented with renal sympathetic nerve activity (RSNA) electrodes and laser-Doppler cortical and medullary flow probes. The glomerular filtration rate, diuresis, and natriuresis were measured for 120 min after sodium loading (5% body weight 0.9% saline administered during 3 min). Kidneys were examined histologically.

RESULTS

The blood pressure in TGR and DOCA-salt rats was 40-50 mmHg higher than that in SDH rats, and decreased briefly after volume expansion for all groups. The diuresis and natriuresis of TGR and DOCA-salt rats were greater than those of SDH rats. The medullary blood flow increased and the cortical blood flow in SDH decreased, whereas the cortical blood flow in TGR and DOCA-salt rats remained high. The RSNA in rats of all groups decreased; however, this decrease was greater in SDH than it was in TGR and DOCA-salt rats. The histology was affected most severely for the DOCA-salt rats.

CONCLUSIONS

Exaggerated natriuresis occurred in hypertensive rats regardless of their Ang II status. Both strains were characterized by a smaller decrease in RSNA and a preserved cortical blood flow in the face of volume expansion. These data do not support the notion that exaggerated natriuresis is a function of renin-level suppression for rats.

Effects of anastomoses on solute transcapillary exchange in countercurrent systems

OBJECTIVE

To investigate effects of anastomoses between the descending vasa recta (DVR) and the ascending vasa recta (AVR) on the distribution of small solutes in the interstitial fluid of the renal medulla.

METHODS

Countercurrent capillary loops, surrounded by a secretory epithelium, were used to model microvessels in the renal medulla. Anastomoses between the DVR and AVR were modeled as a decrease in cross-sectional area of the vessels and a decrease in flow velocity from the base to the tip of the capillary loop. When experimental data were used to evaluate parameters of the model, it was seen that diffusive transport of solute in the axial direction of the capillary was negligible, and the equations could be greatly simplified.

RESULTS

General formulae of the solute concentration distribution were derived for different degrees of shunting between the two limbs of the capillary loop. Analytical solutions for the steady-state solute distribution were obtained when the sizes of capillaries and flows in them were assumed to decrease linearly with the distance from the base to the tip of the capillary loop. When the effects of reduction in the size of the limbs were compared with the effects of reduction of flow velocities on solute distribution, it was found that, in the presence of anastomoses, change in flow velocity increases the axial gradient of the solute concentration more than change in the cross-sectional area. The combined effects of a decrease in flow velocity and cross-sectional area can easily double the axial gradient of the solute concentration for a modest degree of anastomotic shunting.

CONCLUSIONS

In this study, we separated effects of anastomotic flow between the DVR and AVR from other factors affecting the complicated countercurrent solute exchanges in the renal medulla. Results from the model show that anastomoses increase the solute concentration in the medullary interstitium and also the axial gradient of the solute concentration there.

Role of the renal medulla in volume and arterial pressure regulation

The original fascination with the medullary circulation of the kidney was driven by the unique structure of vasa recta capillary circulation, which Berliner and colleagues (Berliner, R. W., N. G. Levinsky, D. G. Davidson, and M. Eden. Am. J. Med. 24: 730-744, 1958) demonstrated could provide the economy of countercurrent exchange to concentrate large volumes of blood filtrate and produce small volumes of concentrated urine. We now believe we have found another equally important function of the renal medullary circulation. The data show that it is indeed the forces defined by Starling 100 years ago that are responsible for the pressure-natriuresis mechanisms through the transmission of changes of renal perfusion pressure to the vasa recta circulation. Despite receiving only 5-10% of the total renal blood flow, increases of blood flow to this region of the kidney cause a washout of the medullary urea gradient and a rise of the renal interstitial fluid pressure. These forces reduce tubular reabsorption of sodium and water, leading to a natriuresis and diuresis. Many of Starling's intrinsic chemicals, which he named "hormones," importantly modulate this pressure-natriuresis response by altering both the sensitivity and range of arterial pressure around which these responses occur. The vasculature of the renal medulla is uniquely sensitive to many of these vasoactive agents. Finally, we have found that the renal medullary circulation can play an important role in determining the level of arterial pressure required to achieve long-term fluid and electrolyte homeostasis by establishing the slope and set point of the pressure-natriuresis relationship. Measurable decreases of blood flow to the renal medulla with imperceptible changes of total renal blood flow can lead to the development of hypertension. Many questions remain, and it is now evident that this is a very complex regulatory system. It appears, however, that the medullary blood flow is a potent determinant of both sodium and water excretion and signals changes in blood volume and arterial pressure to the tubules via the physical forces that Professor Starling so clearly defined 100 years ago.

Localization of the vasopressin V1a and V2 receptors within the renal cortical and medullary circulation

Arginine vasopressin (AVP) is a potent vasoconstrictor that preferentially reduces renal medullary blood flow through the stimulation of the vasopressin V1a receptor (V1aR). Studies have also shown that the vasopressin V2 receptor (V2R) may modulate AVP-mediated vasoconstriction. At present, the distribution of the V1aR and V2R within the renal cortical and medullary microcirculation has not been determined. This study was designed to localize the transcriptional and translational sites of the V1aR and V2R in microdissected intrarenal vascular segments from both the cortex and medulla, specifically the interlobar, arcuate, and interlobular arteries; afferent and efferent arterioles; glomeruli; and single outer medullary vasa recta capillaries using reverse transcription-polymerase chain reaction and Western blot analyses. The results indicated that V1aR mRNA and proteins were present in the isolated cortical or medullary vasculature, but the V2R mRNA and proteins were not found. This study suggests that the vasoconstrictor action of AVP within the renal medulla is mediated through the V1aR and that the modulatory V2R-mediated vasodilation is probably through the release of paracrine hormones found within the renal interstitial or tubular cells.

Renal circulation and cellular metabolism during left ventricular assisted circulation: comparison study of pulsatile and nonpulsatile assists

We examined left ventricular assist during 6 h for an acute myocardial infarction model in pigs. The outflow cannula was placed in the ascending aorta and an inflow cannula in the left atrium. A pump (Pulsatile group: Zeon Medical and Nonpulsatile group: Nikkiso HPM-15) was connected to each cannula. Items measured were the regional blood flow of the cortex and the medulla in the kidney, renal arterial flow, arterial blood ketone body ratio (AKBR), lactate/pyruvic acid, BUN, creatinine and beta 2-microglobulin. After experimental study, the kidneys were removed, and a pathological study was performed. In the pulsatile assisted group, renal cortical blood flow increased but medulla blood flow decreased. On the other hand, in the nonpulsatile assisted group, both regional blood flows decreased. That means that in the pulsatile assisted group intrarenal redistribution improved, rather than in the nonpulsatile assisted group. The results of our study indicated that pulsatile assist produced superior circulation in the kidney, and the microcirculation on the cell level was superior as well in early treatment of acute left heart failure.

Role of platelet-activating factor in two-kidney, one-clip hypertension

BACKGROUND

Platelet-activating factor (PAF) is a bioactive phospholipid which is a potent hypotensive agent. To investigate the role of PAF in renovascular hypertension, we determined the PAF concentration and its production level assessed by the activity of cholinephosphotransferase (CPT) in renal tissue and examined the effect of a PAF antagonist on the mean arterial pressure (MAP) in control and two-kidney with one clipped (2K1C) hypertensive rats.

MATERIALS AND METHODS

The concentration of PAF and CPT in the renal medulla and cortex were determined by radioassay. Also, the effect of a PAF antagonist, CV-6209, on MAP was also examined in both 2K1C hypertensive and normal control rats.

RESULTS

The PAF concentration and CPT activity were significantly higher in the medulla than in the cortex in both 2K1C hypertensive and normal control rats, and both values in the medulla were also significantly higher in the clipped kidney than in the contralateral unclipped kidney or in control rat kidneys. We also observed a significant negative correlation between the PAF concentration in the medulla, and the medulla weight in the clipped kidney of 2K1C hypertensive rats. Infusion of the PAF antagonist, CV-6209, did not affect MAP in 2K1C hypertensive rats, but was significantly increased (P < 0.05) in control rats.

CONCLUSIONS

These findings suggest that PAF, whose production is induced by renal ischemia due to renal artery stenosis, plays an important role in the renomedullary vasodepressor system, but the effect of PAF as a vasodilator in the peripheral vessels is limited in 2K1C hypertension.

Molecular sieving of small solutes by outer medullary descending vasa recta

Molecular sieving of small solutes by outer medullary descending vasa recta (OMDVR). Descending vasa recta (DVR) plasma equilibrates with the medullary interstitium by volume efflux (Jv), as well as by influx of solutes. Jv is driven by transmural osmotic pressure gradients due to small hydrophilic solutes (delta pi s), NaCl and urea. DVR endothelium probably contains a "water-only" pathway most likely mediated by the aquaporin-1 (AQP1) water channel. We measured the ability of microperfused OMDVR to concentrate lumenal 22Na and [3H]raffinose when Jv was driven by transmural NaCl gradients. Collectate-to-perfusate ratios of 2 x 10(6) M(r) fluorescein isothiocyanate-labeled dextran volume marker (RDx), 22Na (RNa), and [3H]raffinose (Rraf) were measured in the absence and presence of Jv. During volume efflux (Jv > 0), RDx was 1.37 +/- 0.31. RNa increased from 0.64 +/- 0.03 when Jv = 0 to 0.82 +/- 0.05 when Jv > 0 and Rraf increased from 0.83 +/- 0.03 to 1.13 +/- 0.05: Mathematical simulations predict RNa and Rraf most accurately when the OMDVR reflection coefficient to the tracers is assigned a value near unity. This indicates that the OMDVR wall contains a pathway for osmotic volume flux that excludes small hydrophilic solutes, a behavior consistent with that of aquaporins.

Effect of ultrasound contrast medium in color Doppler and power Doppler visualization of blood flow in canine kidneys

PURPOSE

To examine the effect of an ultrasound contrast medium (UCM) in the visualization of parenchymal blood flow by means of color Doppler and power Doppler sonography.

MATERIAL AND METHODS

Nonenhanced and UCM-enhanced Doppler images of canine kidneys were obtained in a transversal plane during various states of flow reduction in the anterior branch of the renal artery. The UCM consisted of air-filled shell-stabilized microballoons (half-life approximately 2 min). The images were evaluated blindly by 4 observers who rated the amount of flow signal in the cortex and medulla, and categorized the flow state (normal, reduced or no flow) in the anterior part of the kidney.

RESULTS

The UCM increased the area of Doppler signals in the cortex and outer medulla during normal or reduced blood flow. The border between nonperfused and normally perfused parenchyma was more distinct with the UCM. The categorization of the regional flow state was more correct with the UCM. Improvement with the UCM was greatest when the nonenhanced Doppler images had suboptimal intensity, but positive effects with the UCM were also seen in recordings with adequate precontrast intensity. Color blooming artifacts sometimes occurred on the side of the kidney facing away from the transducer.

CONCLUSION

The UCM improved the color Doppler and power Doppler visualization of the parenchymal blood flow in the canine kidney, and allowed a more correct categorization to be made of the regional blood flow state.