Medullary Thick Ascending Limb Buffer Vasoconstriction of Renal Outer-Medullary Vasa Recta in Salt-Resistant But Not Salt-Sensitive RatsNovelty and Significance
We have demonstrated previously that paracrine signaling occurs between medullary thick ascending limb (mTAL) and the contractile pericytes of outer-medullary vasa recta (VR), termed “tubular-vascular cross-talk.” The aim of the current study was to determine whether tubular-vascular cross-talk has a functional effect on vasoconstrictor responses to angiotensin II and to determine whether this is altered in the Dahl salt-sensitive (SS) rat. Studies were performed on salt-resistant consomic SS.13 Brown Norway (BN) and SS rats using a novel outer medullary tissue strip preparation in which freshly isolated VRs within VR bundles were perfused either alone or in combination with nearby mTAL. In VRs from SS.13 bn rats, angiotensin II (1 µmol/L) increased VR bundle intracellular Ca2+ concentration 19 ± 9 nmol/L (n=8) and reduced focal diameter in perfused VRs by −20 ± 7% (n=5). In the presence of nearby mTAL, however, VR bundle intracellular Ca2+ concentration (−9 ± 8 nmol/L; n=8) and VR diameter (−1 ± 4%, n=7) in SS.13bn rats were unchanged by angiotensin II. In contrast, in Dahl SS rats, angiotensin II resulted in rapid and sustained increase in VR bundle intracellular Ca2+ concentration (89 ± 48 nmol/L, n=7; 50 ± 24%, n=8) and a reduction in VR diameter of (−17 ± 7%, n=7; −11 ± 4%, n=5) in both isolated VRs and VRs with nearby mTAL, respectively. In VRs with mTAL from SS13bn rats, inhibiton of purinergic receptors resulted in an increase in VR bundle intracellular Ca2+ concentration, indicating that purinergic signaling buffers vasoconstriction. Importantly, our in vitro data were able to predict medullary blood flow responses to angiotensin II in SS and SS.13bn rats in vivo. We conclude that paracrine signaling from mTAL buffers angiotensin II vasoconstriction in Dahl salt-resistant SS.13bn rats but not SS rats.
Renal medullary ischemia promotes sodium and water retention and the development of hypertension.1 The renal outer medulla is perfused by descending vasa recta (VR) capillaries that branch from the efferent arterioles of juxtamedullary glomeruli.2–4 Unlike most capillary beds, VRs are surrounded by numerous pericyte cell bodies that impart contractility, allowing vascular resistance to be altered at the capillary level.3–5 Dysfunction of paracrine and autocrine signaling within the local outer medullary milieu may result in hypersensitivity of the medullary circulation to vasoconstrictor agents, medullary ischemia, and the development of hypertension.1
Angiotensin II has a direct vasoconstrictor action on descending VR. Angiotensin II–induced constriction of VR is mediated by activation of pericyte angiotensin II type 1 receptors and subsequent depolarization of the plasma membrane and Ca2+ entry.6,7 Despite the direct constrictor effects of angiotensin II on VRs, in normotensive rats in vivo, medullary perfusion is relatively insensitive to the vasoconstrictor actions of angiotensin II.8 However, in Dahl salt-sensitive (SS) or NG-nitro-l-arginine methyl ester (l-NAME)–treated Sprague Dawley rats, medullary perfusion is reduced in response to angiotensin II, and chronic angiotensin II infusion results in hypertension.9–11 The enhanced susceptibility of the medullary circulation to small amounts of circulating angiotensin II in these models likely predisposes these animals to the development of hypertension.1,11
In a recent study we demonstrated that medullary thick ascending limb (mTAL) tubular elements of salt-resistant consomic SS.13 Brown Norway (BN) rats produce NO in response to angiotensin II stimulation and that this NO can then diffuse to nearby VR pericytes.12 Importantly, NO production was blunted in mTAL from Dahl SS rats and NO did not diffuse to neighboring pericyte cell bodies unless tissue superoxide levels were reduced with TEMPOL.12 As NO reduces vascular tone in isolated VR,5,13,14 we hypothesized that diffusion of NO from mTAL to nearby VR pericytes may buffer VR constriction to angiotensin II.
To test this hypothesis, we first performed studies in acutely anesthetized Dahl SS and consomic SS.13bn rats in vivo in which outer medullary blood flow responses were determined using laser Doppler flowmetry in response to angiotensin II infusion. To determine whether these responses may be mediated by tubule-vascular cross-talk between mTAL segments and VRs, we developed a unique in vitro model in which freshly isolated, perfused VRs could be stimulated with vasoactive agents either alone or in the presence of mTAL while maintaining their natural anatomic relationships.
Studies used 9- to 13-week–old male SS and SS.13bn rats (MCW inbred strains15) weighing 250 to 350 g maintained ad libitium on water and a purified AIN-76 rodent diet containing 0.4% NaCl (Dyets, Inc, Bethlehem, PA) since weaning in the animal resource center of the Medical College of Wisconsin. All of the protocols were approved by the Institutional Animal Care Committee.
Hanks’ balanced salt solution (HBSS) was purchased from Invitrogen (Invitrogen, Grand Island, NY), HEPES (20 mmol/L per L) and l-arginine (100 µmol/L) added, and pH adjusted to 7.40. Angiotensin II, l-NAME, l-arginine, and BSA were purchased from Sigma (Sigma Pharmaceutical Co, St Louis, MO).
Renal Hemodynamic Responses to Angiotensin II in SS and SS.13bn Rats In Vivo
Medullary blood flow responses to angiotensin II infusion were determined in acutely anesthetized SS (n=6) and SS.13bn (n=6) rats using laser Doppler flowmetry. Rats were anesthetized with thiobutabarbital (Inactin; Sigma, 60 mg/kg IP) and prepared as described previously.16 Mean arterial pressure, heart rate, total renal blood flow, and outer medullary blood flow (OMBF) were recorded throughout the experimental protocol. The experimental protocol consisted of five 20-minute periods. In the first two 20-minute periods, rats received only maintenance infusions (2 mL/h of 2% BSA; Sigma). After the initial 2 baseline periods, angiotensin II was added to the vehicle infusion so that rats received 25 nmol/min per kilogram of angiotensin II IV. The angiotensin II infusion was maintained over an additional two 20-minute infusion periods. At the conclusion of these periods, the angiotensin II infusion was stopped and a final 20-minute recovery period begun in which animals again received only maintenance fluids. At the cessation of the experimental protocol, animals were euthanized with an overdose of sodium pentobarbital. Background OMBF signal was taken as the signal obtained after the cessation of blood flow.
Isolation of Outer Medullary Tubular and Vascular Segments
Rats were anesthetized with sodium pentobarbital (60 mg/kg IP), the left kidney flushed with chilled HBSS via retrograde aortic perfusion, and the kidney excised and prepared for microdissection.17 Vascular bundles containing VR were identified and either cleared of all visible tubular tissue (isolated VR) or tubular tissue was left remaining intact with the natural anatomic relationships preserved (VR with mTAL). Care was taken to ensure that similar populations of VR were used in both isolated VRs and VRs with mTAL preparations. Specifically, where possible, a portion of a tissue strip would be used for VR+mTAL measurements, whereas from a separate portion of the same strip, the VRs were isolated. This ensured that the same VR elements were studied in both groups. VR bundles were visually inspected to ensure that all mTALs were removed at ×40. Tissue strips containing either isolated VRs or VRs with mTAL were adhered to coverslips coated with the tissue adhesive Cell-Tak (BD Biosciences, Bedford, MA). Tissue was then allowed to rest in chilled HBSS solution for ≈30 minutes before being transferred to a heated imaging chamber maintained at 37°C (Warner Instruments, Hamden, CT) and mounted on the stage of an inverted microscope.
Perfusion of Descending VR Capillaries In Vitro
In studies in which VR diameter was determined, VRs were perfused at 5 nL/min with HBSS using pulled glass micropipettes. Perfusion of VR was carried out to prevent the tendency of nonperfused VR capillaries to otherwise collapse, making it difficult to accurately quantify reductions of vascular diameter in response to stimuli (Figure 1). In addition to allowing us to more easily identify vascular dimensions, perfusion of VR produced luminal pressure that would facilitate observations of vessel dilation.
Determination of Buffering Capacity of Tubules From SS and SS.13bn Rats
To determine whether tubular segments buffered angiotensin II–induced vasoconstriction in VRs from Dahl SS and SS.13bn rats, vasoconstrictor responses to 1 µmol/L of angiotensin II were determined in the following: (1) isolated VRs from SS.13bn rats; (2) VRs from SS.13bn rats with mTAL; (3) isolated VRs from SS rats; and (4) VRs from SS rats with mTAL. Before the experimental protocol, a region of VR in which multiple pericytes could be identified was selected. The upper and lower focal planes in which VRs could be identified were then established and recorded using imaging software (MetaMorph, Universal Imaging, Downingtown, PA). In each group, perfused VRs were initially superfused with warmed 37°C HBSS for 10 minutes and images of individual perfused VR recorded at 300 seconds and 60 seconds before the administration of angiotensin II. The average of these measurements was taken as baseline resting-state diameter. The superfusion solution was then rapidly exchanged with HBSS containing 1 µmol/L of angiotensin II and images recorded at 30, 60, and 300 seconds after exchange of the buffer solution.
Images were captured using a Nikon TE2000 inverted microscope with an ×100 oil immersion (numeric aperture 1.45) total internal reflection fluorescence objective lens. The signal was detected using a high-resolution digital camera (Photometrics Cascade 512B, Roper Scientific, Tucson, AZ). At each time point, 40 images were taken at ≈1.5- to 2.0-µmol/L distance (distance between upper and lower focal plane divided by 40 optical sections) along the Z plane so that the optimal level of focus could be obtained at any plane using a motorized Z stage controller (Prior Scientific Inc, Rockland, MA) and MetaMorph imaging software. High-resolution stacked images were then saved for analysis.
Quantification of Vessel Diameter
At the time of analysis, 1 to 3 points were identified in each image corresponding with points in which pericyte cell bodies were present (Figure 2). The plane of best focus was then identified for each point by scanning through the stacked image and then luminal ID at each point determined using a MetaMorph image calibrated software package. Measurements at each point were repeated in each of the 5 stacked images corresponding with each of the time points in which images were taken (300 and 60 seconds preangiotensin II and 30, 60, and 300 seconds postangiotensin II) in a single preparation.
Determination of VR [Ca2+]
VR and VR with mTAL were isolated as for diameter measurements and loaded with the Ca2+-sensitive dye Fura-2, as described previously (see the online-only Data Supplement for details). VRs were visualized at ×40× and Ca2+ determined using Fura-2AM and Metafluor imaging software (Universal Imaging; see the online-only Data Supplement for details). Identifiable cell bodies were selected for measurement within each VR bundle to quantify changes in fluorescent intensity.
Data and Statistical Analysis
In Vivo Measurements of Renal Perfusion
The average total renal blood flow, OMBF, and mean arterial pressure measurements over each period were calculated and the responses compared between SS and SS.13bn rats with 2-way repeated-measures ANOVA and Bonferroni post hoc test. Data are expressed as mean±SE.
Mean vessel diameter at 300 and 60 seconds before administration of angiotensin II was taken to equal resting vessel diameter. The response of VR to angiotensin II was then calculated at each of the 3 time points after administration of angiotensin II (30, 60, and 300 seconds) by dividing the average vessel diameter at each of these time points with resting vessel diameter and the data expressed as percentage of change from rest. All of the data are expressed as mean±SE. Responses in mTAL of SS and SS.13bn rats were compared with 2-way repeated-measures ANOVA using a Bonferroni post hoc test.
VR Ca2+ Concentration
[Ca2+]VR was calculated at 3-second intervals for 300 seconds after the addition of angiotensin II (1 µmol/L) to the bath using the formula described (see Equation in the online-only Data Supplement). Responses between either SS and SS.13bn rats or SS.13bn vehicle-treated versus SS.13bn l-NAME (100 µmol/L)–treated preparations were compared with 2-way repeated-measures ANOVA. Data are expressed as mean±SE. All of the other data were compared by unpaired t test.
Renal Medullary Perfusion In Vivo
Before angiotensin II infusion, anesthetized mean arterial pressure was greater in Dahl SS rats (127 ± 6 mm Hg) compared with SS.13bn rats (103 ± 4 mm Hg; P<0.05). Baseline total renal blood flow did not differ between Dahl SS (5.8 ± 0.8 mL/min) and SS.13bn (5.4 ± 0.4 mL/min) rats. Similarly, baseline OMBF did not differ between Dahl SS (60 ± 10 arbitrary units) and SS.13bn rats (68 ± 11 arbitrary units). Whereas mean arterial pressure tended to increase during the initial 30 minutes of infusion, there was no significant effect of IV infusion of angiotensin II (25 ng/ kg per minute IV) on average mean arterial pressure in either Dahl SS or Dahl SS.13bn rats over the two 30-minute infusion periods. Angiotensin II infusion reduced total renal blood flow similarly in both Dahl SS and Dahl SS.13bn rats (Figure 2B). Importantly, angiotensin II reduced OMBF in Dahl SS rats but not in Dahl SS.13bn rats (Figure 2C; P<0.05), indicating greater sensitivity of the medullary circulation of Dahl SS rats to the vasoconstrictor actions of angiotensin II compared with salt-resistant SS.13bn rats.
Diameter changes in VR were visualized using a ×100 oil immersion (numeric aperture 1.45) total internal reflection fluorescence objective lens and high-resolution images saved for analysis. Figure 3 shows a typical response of VRs from SS.13bn rats with nearby mTAL. Changes in VR diameter were determined between 1 and 3 pericyte cell bodies on each image, because these are the sites of contraction in VRs (Figure 3). VR diameter was determined at 30, 60, and 300 seconds after administration of angiotensin II to the bath (Figure 3). Exchange of the vehicle superfusate with bath medium containing angiotensin II (1 µmol/L) resulted in a rapid vasoconstriction of isolated VRs from Dahl SS.13bn rats (20 ± 7%; P<0.05) that was maintained over the 300-second protocol (Figure 4). In VRs from SS.13bn rats with mTAL, angiotensin II resulted in an initial vasoconstriction; however, this was not maintained, and VR diameter was not different from nonstimulated levels at 300 seconds postangiotensin II (Figure 4). In contrast, in Dahl SS rats, angiotensin II resulted in a rapid and sustained reduction in VR diameter in both isolated VRs and VRs with mTAL (Figure 4), indicating no functional buffering of VR vasoconstriction by mTAL in these animals.
VR Ca2+ Levels
VR pericytes contain contractile elements capable of altering VR diameter in response to increased intracellular Ca2+.7 Baseline [Ca2+]VR (in nanometers) in each experimental protocol is shown in the Table and represents the average intracellular Ca2+ levels in all of the cell bodies identified. In VR alone from SS.13bn rats, [Ca2+]VR increased in response to angiotensin II, and this increase was maintained over the entire 300-second protocol (Figure 5). Administration of angiotensin II to VR from SS.13bn rats in the presence of nearby mTAL did not significantly increase [Ca2+]VR. (Figure 5). The Ca2+ response of VR cell bodies in the presence of nearby mTAL to angiotensin II was significantly different from that of VR alone (P<0.05). In VRs from Dahl SS rats in the absence of mTAL, [Ca2+]VR increased in response to angiotensin II (Figure 5). In contrast to the response in SS.13bn rats, in VRs with mTAL from SS rats, [Ca2+]VR levels remained elevated after administration of angiotensin (Figure 5), and this response was not significantly different from that of VR alone.
Role of Paracrine Signaling Agents
To test the role of paracrine signaling agents NO and purines in the functional buffering response of mTAL in VR from SS.13bn rats in response to angiotensin II, we pretreated tissue strips with either the NO synthase inhibitor l-NAME or purinergic receptor antagonist suramin (300 µmol/L). In the presence of l-NAME, angiotensin II elicited a sustained elevation of [Ca2+]VR levels in VR in the absence of mTAL (Figure 6), which was greater than that of vehicle-treated VR. Although L-NAME did significantly increase [Ca2+]VR at 300 seconds postangiotensin II in VRs with mTAL from SS.13bn rats when compared with vehicle-treated VRs with mTAL (Figure 6B), significant buffering of VR constriction remained (Figure 6A). In contrast, suramin completely abolished the buffering response of mTAL on [Ca2+]VR in tissue from SS.13bn rats (Figure 7).
The major finding of this study is that renal outer mTAL tubular elements buffer angiotensin II–induced vasoconstriction in the salt-resistant consomic SS.13bn rats but not Dahl SS rats. Medullary blood flow is an important modulator of the renal pressure-natriuresis response, and altered responsiveness of the medullary circulation to vasoconstrictor agents, such as angiotensin II or vasopressin, results in medullary ischemia, Na+, and water retention and the development of hypertension.1,11 Our current data provide the first functional evidence that paracrine factors released from tubular elements buffer angiotensin II–mediated vasoconstriction in the outer medulla. Importantly, because these buffering responses were observed only in SS.13bn rats but not in Dahl SS rats, these data indicate that tubular dysfunction in the Dahl SS rat contributes to enhanced medullary vascular sensitivity and may predispose these animals to the development of hypertension.
The observation that tubular dysfunction in SS rats results in reduced buffering capacity of the renal medullary capillary circulation is an important step forward in our understanding of the development of SS hypertension. Although the SS rat is a low renin model of hypertension, a number of studies have demonstrated that intrarenal levels of angiotensin II are abnormally high in the high-salt–fed SS rats.18–20 Increased local production of angiotensin II within the kidney of SS rats fed a high-salt diet, then, may lead to enhanced constriction of medullary VRs and the development of hypertension in this strain because of reduced tubular buffering of the vasoconstrictor actions of this peptide. Consistent with this hypothesis, we have demonstrated previously that intrarenal infusion of angiotensin II leads to a reduction in MBF and chronic hypertension in SS rats but not in salt-resistant rats.9 In addition, although the current study focused on the response to angiotensin II, other vasoconstrictor agents may also be important in the development of VR constriction and hypertension in SS rats fed a high-salt diet in vivo. In addition to angiotensin II, we have demonstrated previously that the renal medullary circulation of Dahl SS rats or rats with reduced medullary NO, such as the Dahl rat,11 are hypersensitive to a number of vasoconstrictor agents, including norepinephrine21,22 and vasopressin.23 Because the mechanisms that buffer angiotensin II–induced vasoconstriction in VR are likely shared with other vasoactive agents, it is probable that the mechanism identified in the current study, tubular dysfunction, underlies the hypersensitivity of the medullary circulation of the SS rat to many of these vasoactive agents. Such a phenomenon may also explain the relatively high doses of angiotensin II required to mediate significant vasoconstriction in our studies, because in vivo smaller elevations of angiotensin II than those used in the current study may act synergistically with other vasoconstrictor stimuli to promote vasoconstriction and the development of chronic hypertension in high-salt–fed SS rats.
The use of the tissue strip preparation with perfusion of the VR has enabled the identification of differences in tubule-vascular cross-talk between Dahl SS and SS.13bn rats. Importantly, these in vitro data predicted intrarenal blood flow responses in these strains in response to angiotensin II in vivo. Numerous studies have attempted to identify the physiological mechanisms underlying the relative insensitivity of the renal medullary circulation to vasoconstrictor agents, such as endothelin, vasopressin, and angiotensin II.24 We have shown previously that, within the renal medullary region, Dahl SS rats are more sensitive to the vasoconstrictor actions of angiotensin II when compared with BN rats.9 In the current study we now demonstrate that medullary blood flow in Dahl SS rats is also more sensitive to the vasocontrictor actions of angiotensin II than that of salt- resistant consomic SS.13bn rats (Figure 3). Although our in vivo data are consistent with the results of our tissue strip preparation, it remains uncertain whether changes in pericyte contractility are capable of mediating changes in medullary blood flow in vivo or whether the upstream actions of angiotensin II on arterioles determines the overall response of the medullary circulation. One possibility is that changes in VR resistance as a result of pericyte constriction alter the distribution of blood flow with the renal inner medulla and papilla. Such changes in distribution could have large effects on metabolism and the medullary solute gradient, both of which could affect Na+ transport and the development of hypertension in this model. Unfortunately, present methodologies do not allow us to distinguish the role of pericyte-mediated changes in vascular resistance from other parts of the vasculature in vivo. New approaches allowing gene targeting within specific cell types, such as mTAL in rats,25 however, have become available recently and will likely allow these issues to be resolved in the future.
In the current study, we first tested the ability of NO to mediate tubular buffering of VR constriction in SS.13bn rats in response to angiotensin II. In agreement with previous findings,13,26 our data indicate that reduced NO synthase activity potentiates the contractile response of VRs from SS.13bn rats in VRs alone. Treatment with l-NAME, however, did not abolish buffering responses of mTAL in SS.13bn rats. We conclude that, whereas NO may play a role as a paracrine signaling agent between mTAL and VRs in SS.13bn rats, other paracrine signaling molecules must also be important.
mTALs are a significant source of extracellular nucleotides in the renal outer medulla,27–29 and Crawford et al30 have shown recently that VR pericytes express both P2X and P2Y receptors and that these receptors mediate nucleotide-evoked changes in VR diameter. Given these data, we tested the potential of purine metabolites as paracrine signaling agents mediating buffering of contractile responses in SS.13bn rats using the P2 receptor antagonist suramin. Our data indicate that purinergic signaling mediates buffering responses by mTAL in SS.13bn rats (Figure 7). In addition to eliminating the buffering response of mTAL, inhibition of P2 receptors reduced basal [Ca2+]VR in VRs with mTAL to levels observed in isolated VRs (Table), suggesting that purinergic signaling from mTAL to VR may result in the preconstriction of VR. Interestingly, Cabral et al31 recently published a report indicating that ATP release and subsequent activation of P2 receptors are responsible for flow-induced NO production in mTAL. When viewed in context with our current results, these data suggest that purinergic signaling may be an important outer medullary response to increased luminal flow to activate natriuretic pathways and to maintain medullary perfusion. It is intriguing to speculate that possible dysfunction in such mechanisms may be an underlying cause of ischemia, Na+ retention, and the development of hypertension in Dahl SS rats. Further studies will be required to investigate this hypothesis and to identify the specific signaling pathways involved. Also, the role of other likely signaling agents includes prostaglandin E2 or adenosine, because both of these are produced by the mTAL,27,29,32,33 have been demonstrated to cause vasodilation in isolated VRs,5,13,34,35 and need to be investigated.
The advantage of the perfused VR tissue strip preparation used in the current study over previous in vivo animal models is that, in the current study, we were able to determine the cellular source of signaling molecules important in buffering the vasoactive actions of angiotensin II. We are able to conclude that mTALs are the source of these paracrine signaling agents, because only when mTALs were present did we observed a reduced contractile response of VRs to angiotensin II. We can reject the possibility that cell types were responsible for the buffering actions that we observed in SS.13bn rats for a number of reasons. First, we used the inner stripe of the outer medulla only for dissection, so that S3 segments of the proximal tubule were not present. Second, it is easy to identify OMCDs in our preparation because of their bumpy heterogeneous cellular appearance, and we were careful to exclude these from tissue strips that were studied. Finally, whereas renal medullary interstitial cells and thin limbs were likely present in our preparation, these cell types would also have been present in VRs studied in the absence of mTAL in which we did not observe buffering of the vasoconstrictor response.
In addition to functional measurements of VR diameter, we also measured intracellular [Ca2+] in VR bundles as an index of the contractile state of VRs. Because diameter was not measured in these studies, we did not perfuse these vessels. Nevertheless, changes in [Ca2+]VR were consistent with the functional changes that we observed in perfused VRs. Because a key goal of the current study was to maintain as close as possible the natural anatomic relationships between VR and nearby tubular elements, we chose to use VR bundles over isolated VRs to avoid disruption of these relationships. A caveat of this approach is that it is difficult to clearly distinguish abluminal VR pericytes from luminal VR endothelial cells, because in nonperfused, grouped vessels, such as those used in Ca2+ studies, the lumen is often unclear. Whereas any clearly identifiable pericytes were selected, within VR bundles we chose to measure changes in intracellular Ca2+ concentration from all of the cell bodies observed, which undoubtedly included endothelial cells, as well as pericytes. A potential caveat to this approach is a finding by Pallone et al36 that, in Fura-2–loaded VRs, endothelial cells dominated fluorescent. Importantly, we found that we were able to image Ca2+ in pericyte cell bodies even when the endothelium remained intact (Figure 1). This observation is supported by published reports in isolated pericyte-laden capillaries of the retina37 and is likely attributed to a number of factors, including an increased loading time for Fura-2 (30 minutes in the study by Pallone et al36 versus 1.5 hours the current study), the use of pluronic F-127 to distribute the dye, and the highly sensitive camera to image Ca2+ in the current studies. Our data indicating that Ca2+ is increased by angiotensin II in isolated VR bundles confirm that much of the Ca2+ signal obtained is derived from pericytes rather than endothelial cells.36
By using a novel experimental approach, we were able to demonstrate directly that tubular paracrine signaling buffers angiotensin II–induced vasoconstriction in salt-resistant SS.13bn rats but not Dahl SS rats. Our data indicate that purine signaling may be important in buffering the vasoconstrictor actions of angiotensin II in the renal medulla of salt-resistant rats.
Tubular dysfunction of paracrine signaling appears to underlie the enhanced susceptibility of the renal medullary circulation of Dahl SS rats to vasoconstrictor agents. The development of hypertension and renal disease in this strain is important because it significantly advances our understanding of the mechanisms leading to the development of SS hypertension. Our data suggest that targeting tubular dysfunction may be of benefit in preventing renal medullary ischemia and the development of hypertension in SS populations.
The unique in vitro preparation provides an excellent platform for the further identification of the specific signaling agents that mediate tubulovascular cross-talk. Combined with new gene-targeting approaches in rats in vivo, we hope that these tools will allow us to identify novel therapeutic approaches for the treatment of renal ischemia and SS hypertension.
Sources of Funding
This work was funded by National Heart, Lung, and Blood Institute grant HL-29587 and American Heart Association fellowship 0625793Z.
- Received March 12, 2012.
- Revision received April 2, 2012.
- Accepted July 26, 2012.
- © 2012 American Heart Association, Inc.
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Novelty and Significance
What Is New?
In the current study we demonstrate for the first time that mTAL buffer diameter changes in nearby VRs in response to angiotensin II in SS.13BN but not salt-sensitive Dahl SS rats.
What Is Relevant?
The Dahl SS rat model is a commonly used model of human salt-sensitive hypertension, of which the underlying causes remain unknown. Our data indicate that tubular dysfunction may be the cause of altered vascular responses and the development of hypertension in this model.
This study demonstrates that paracrine signaling from mTAL buffers the vasoconstrictor responses to angiotensin II in Dahl salt-resistant SS.13BN rats but not Dahl salt-sensitive rats and suggests that dysfunction in tubular-vascular cross-talk may contribute to the development of salt-sensitive hypertension.