Role of chloride in constriction of descending vasa recta by angiotensin II

GLP-1 Promotes Cortical and Medullary Perfusion in the Human Kidney and Maintains Renal Oxygenation During NaCl Loading

Background GLP-1 (glucagon-like peptide-1) receptor agonists exert beneficial long-term effects on cardiovascular and renal outcomes. In humans, the natriuretic effect of GLP-1 depends on GLP-1 receptor interaction, is accompanied by suppression of angiotensin II, and is independent of changes in renal plasma flow. In rodents, angiotensin II constricts vasa recta and lowers medullary perfusion. The current randomized, controlled, crossover study was designed to test the hypothesis that GLP-1 increases renal medullary perfusion in healthy humans. Methods and Results Healthy male participants (n=10, aged 27±4 years) ingested a fixed sodium intake for 4 days and were examined twice during a 1-hour infusion of either GLP-1 (1.5 pmol/kg per minute) or placebo together with infusion of 0.9% NaCl (750 mL/h). Interleaved measurements of renal arterial blood flow, oxygenation (R₂*), and perfusion were acquired in the renal cortex and medulla during infusions, using magnetic resonance imaging. GLP-1 infusion increased medullary perfusion (32±7%, P<0.001) and cortical perfusion (13±4%, P<0.001) compared with placebo. Here, NaCl infusion decreased medullary perfusion (-5±2%, P=0.007), whereas cortical perfusion remained unchanged. R₂* values increased by 3±2% (P=0.025) in the medulla and 4±1% (P=0.008) in the cortex during placebo, indicative of decreased oxygenation, but remained unchanged during GLP-1. Blood flow in the renal artery was not altered significantly by either intervention. Conclusions GLP-1 increases predominantly medullary but also cortical perfusion in the healthy human kidney and maintains renal oxygenation during NaCl loading. In perspective, suppression of angiotensin II by GLP-1 may account for the increase in regional perfusion. Registration URL: https://www.clinicaltrials.gov; Unique identifier: NCT04337268.

Spontaneous activity in the microvasculature of visceral organs: role of pericytes and voltage-dependent Ca(2+) channels

The microvasculature plays a primary role in the interchange of substances between tissues and the circulation. In visceral organs that undergo considerable distension upon filling, the microvasculature appears to display intrinsic contractile properties to maintain their flow. Submucosal venules in the bladder or gastrointestinal tract generate rhythmic spontaneous phasic constrictions and associated Ca(2+) transients. These events are initiated within either venular pericytes or smooth muscle cells (SMCs) arising from spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) and the opening of Ca(2+) -activated chloride channels (CaCCs) that trigger Ca(2+) influx through L-type voltage-dependent Ca(2+) channels (VDCCs). L-type VDCCs also play a critical role in maintaining synchrony within the contractile mural cells. In the stomach myenteric layer, spontaneous Ca(2+) transients originating in capillary pericytes appear to spread to their neighbouring arteriolar SMCs. Capillary Ca(2+) transients primarily rely on SR Ca(2+) release, but also require Ca(2+) influx through T-type VDCCs for their synchrony. The opening of T-type VDCCs also contribute to the propagation of Ca(2+) transients into SMCs. In visceral microvasculature, pericytes act as either spontaneously active contractile machinery of the venules or as pacemaker cells generating synchronous Ca(2+) transients that drive spontaneous contractions in upstream arterioles. Thus pericytes play different roles in different vascular beds in a manner that may well depend on the selective expression of T-type and L-type Ca(2+) channels.

Disruption of vascular Ca2+-activated chloride currents lowers blood pressure

High blood pressure is the leading risk factor for death worldwide. One of the hallmarks is a rise of peripheral vascular resistance, which largely depends on arteriole tone. Ca2+-activated chloride currents (CaCCs) in vascular smooth muscle cells (VSMCs) are candidates for increasing vascular contractility. We analyzed the vascular tree and identified substantial CaCCs in VSMCs of the aorta and carotid arteries. CaCCs were small or absent in VSMCs of medium-sized vessels such as mesenteric arteries and larger retinal arterioles. In small vessels of the retina, brain, and skeletal muscle, where contractile intermediate cells or pericytes gradually replace VSMCs, CaCCs were particularly large. Targeted disruption of the calcium-activated chloride channel TMEM16A, also known as ANO1, in VSMCs, intermediate cells, and pericytes eliminated CaCCs in all vessels studied. Mice lacking vascular TMEM16A had lower systemic blood pressure and a decreased hypertensive response following vasoconstrictor treatment. There was no difference in contractility of medium-sized mesenteric arteries; however, responsiveness of the aorta and small retinal arterioles to the vasoconstriction-inducing drug U46619 was reduced. TMEM16A also was required for peripheral blood vessel contractility, as the response to U46619 was attenuated in isolated perfused hind limbs from mutant mice. Out data suggest that TMEM16A plays a general role in arteriolar and capillary blood flow and is a promising target for the treatment of hypertension.

Descending vasa recta endothelial cells and pericytes form mural syncytia

Using patch clamp, we induced depolarization of descending vasa recta (DVR) pericytes or endothelia and tested whether it was conducted to distant cells. Membrane potential was measured with the fluorescent voltage dye di-8-ANEPPS or with a second patch-clamp electrode. Depolarization of an endothelial cell induced responses in other endothelia within a millisecond and was slowed by gap junction blockade with heptanol. Endothelial response to pericyte depolarization was poor, implying high-resistance myo-endothelial coupling. In contrast, dual patch clamp of neighboring pericytes revealed syncytial coupling. At high sampling rate, the spread of depolarization between pericytes and endothelia occurred in 9 ± 2 or 12 ± 2 μs, respectively. Heptanol (2 mM) increased the overall input resistance of the pericyte layer to current flow and prevented transmission of depolarization between neighboring cells. The fluorescent tracer Lucifer yellow (LY), when introduced through ruptured patches, spread between neighboring endothelia in 1 to 7 s, depending on location of the flanking cell. LY diffused to endothelial cells on the ipsilateral but not contralateral side of the DVR wall and minimally between pericytes. We conclude that both DVR pericytes and endothelia are part of individual syncytia. The rate of conduction of membrane potential exceeds that for diffusion of hydrophilic molecules by orders of magnitude. Gap junction coupling of adjacent endothelial cells may be spatially oriented to favor longitudinal transmission along the DVR axis.

Mural propagation of descending vasa recta responses to mechanical stimulation

To investigate the responses of descending vasa recta (DVR) to deformation of the abluminal surface, we devised an automated method that controls duration and frequency of stimulation by utilizing a stream of buffer from a micropipette. During stimulation at one end of the vessel, fluorescent responses from fluo4 or bis[1,3-dibutylbarbituric acid-(5)] trimethineoxonol [DiBAC₄(3)], indicating cytoplasmic calcium ([Ca²⁺]CYT) or membrane potential, respectively, were recorded from distant cells. Alternately, membrane potential was recorded from DVR pericytes by nystatin whole cell patch-clamp. Mechanical stimulation elicited reversible [Ca²⁺)]CYT responses that increased with frequency. Individual pericyte responses along the vessel were initiated within a fraction of a second of one another. Those responses were inhibited by gap junction blockade with 18 β-glycyrrhetinic acid (100 μM) or phosphoinositide 3 kinase inhibition with 2-morpholin-4-yl-8-phenylchromen-4-one (50 μM). [Ca²⁺]CYT responses were blocked by removal of extracellular Ca²⁺ or L-type voltage-gated channel blockade with nifedipine (10 μM). At concentrations selective for the T-type channel blockade, mibefradil (100 nM) was ineffective. During mechanostimulation, pericytes rapidly depolarized, as documented with either DiBAC4(3) fluorescence or patch-clamp recording. Single stimuli yielded depolarizations of 22.5 ± 2.2 mV while repetitive stimuli at 0.1 Hz depolarized pericytes by 44.2 ± 4.0 mV. We conclude that DVR are mechanosensitive and that rapid transmission of signals along the vessel axis requires participation of gap junctions, L-type Ca²⁺ channels, and pericyte depolarization.

Murine vasa recta pericyte chloride conductance is controlled by calcium, depolarization, and kinase activity

We used the whole cell patch-clamp technique to investigate the regulation of descending vasa recta (DVR) pericyte Ca(2+)-dependent Cl(-) currents (CaCC) by cytoplasmic Ca(2+) concentration ([Ca](CYT)), voltage, and kinase activity. Murine CaCC increased with voltage and electrode Ca(2+) concentration. The current saturated at [Ca](CYT) of ∼1,000 nM and exhibited an EC(50) for Ca(2+) of ∼500 nM, independent of depolarization potential. Activation time constants were between 100 and 200 ms, independent of electrode Ca(2+). Repolarization-related tail currents elicited by stepping from +100 mV to varying test potentials exhibited deactivation time constants of 50-200 ms that increased with voltage when electrode [Ca](CYT) was 1,000 nM. The calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7, 30 μM) blocked CaCC. The myosin light chain kinase blockers 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-7, 1-50 μM) and 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-9, 10 μM) were similarly effective. Resting pericytes were hyperpolarized by ML-7. Pericytes exposed to ANG II (10 nM) depolarized from a baseline of -50 ± 6 to -29 ± 3 mV and were repolarized to -63 ± 7 mV by exposure to 50 μM ML-7. The Ca(2+)/calmodulin-dependent kinase inhibitor KN-93 reduced pericyte CaCC only when it was present in the electrode and extracellular buffer from the time of membrane break-in. We conclude that murine DVR pericytes are modulated by [Ca](CYT), membrane potential, and phosphorylation events, suggesting that Ca(2+)-dependent Cl(-) conductance may be a target for regulation of vasoactivity and medullary blood flow in vivo.

Voltage-gated divalent currents in descending vasa recta pericytes

Multiple voltage-gated Ca(2+) channel (Ca(V)) subtypes have been reported to participate in control of the juxtamedullary glomerular arterioles of the kidney. Using the patch-clamp technique, we examined whole cell Ca(V) currents of pericytes that contract descending vasa recta (DVR). The dihydropyridine Ca(V) agonist FPL64176 (FPL) stimulated inward Ca(2+) and Ba(2+) currents that activated with threshold depolarizations to -40 mV and maximized between -20 and -10 mV. These currents were blocked by nifedipine (1 μM) and Ni(2+) (100 and 1,000 μM), exhibited slow inactivation, and conducted Ba(2+) > Ca(2+) at a ratio of 2.3:1, consistent with "long-lasting" L-type Ca(V). In FPL, with 1 mM Ca(2+) as charge carrier, Boltzmann fits yielded half-maximal activation potential (V(1/2)) and slope factors of -57.9 mV and 11.0 for inactivation and -33.3 mV and 4.4 for activation. In the absence of FPL stimulation, higher concentrations of divalent charge carriers were needed to measure basal currents. In 10 mM Ba(2+), pericyte Ca(V) currents activated with threshold depolarizations to -30 mV, were blocked by nifedipine, exhibited voltage-dependent block by diltiazem (10 μM), and conducted Ba(2+) > Ca(2+) at a ratio of ∼2:1. In Ca(2+), Boltzmann fits to the data yielded V(1/2) and slope factors of -39.6 mV and 10.0 for inactivation and 2.8 mV and 7.7 for activation. In Ba(2+), V(1/2) and slope factors were -29.2 mV and 9.2 for inactivation and -5.6 mV and 6.1 for activation. Neither calciseptine (10 nM), mibefradil (1 μM), nor ω-agatoxin IVA (20 and 100 nM) blocked basal Ba(2+) currents. Calciseptine (10 nM) and mibefradil (1 μM) also failed to reverse ANG II-induced DVR vasoconstriction, although raising mibefradil concentration to 10 μM was partially effective. We conclude that DVR pericytes predominantly express voltage-gated divalent currents that are carried by L-type channels.

Methodological aspects of measuring absolute values of membrane potential in human cells by flow cytometry

The bis-barbituric acid oxonol, DiBAC(4)(3) is used as a standard potentiometric probe in human cells. However, its fluorescence depends not only on membrane potential but also varies with nonpotential related changes in the amount of intracellular free and bound dye. This study demonstrates the influence of different experimental conditions on this nonspecific fluorescence proportion. IGR1 melanoma cells as a model were specifically altered in cell volume and protein content by depolarizing treatments or cell cycle synchronization. Flow cytometry was performed over a wide range of extracellular DiBAC(4)(3) concentrations. Fixation and increase in protein content led to a nonspecifically enhanced fluorescence, while changes in the amount of free intracellular dye as a result of altered cell volume proved to be negligible. To establish a calibration curve using totally depolarized cells, the pore-forming action of gramicidin should be preferred to fixation. Below 100 nM DiBAC(4)(3), the logarithmic relation between cell fluorescence and dye concentration turned into a virtually linear function intersecting with zero. Consequently, calibration can then be confined to determination of the fluorescence of depolarized cells stained with the same concentration as used for the actual measurement of membrane potential. Unexpectedly, quenching of fluorescence occurred in totally depolarized cells at concentrations higher than 6,250 nM. Linearity and quenching could be confirmed by additional experiments on Chinese hamster ovary CHO-K1 and B lymphoblastoid LCL-HO cells.

Mechanisms underlying angiotensin II-induced calcium oscillations

To gain insight into the mechanisms that underlie angiotensin II (ANG II)-induced cytoplasmic Ca2+ concentration ([Ca]cyt) oscillations in medullary pericytes, we expanded a prior model of ion fluxes. ANG II stimulation was simulated by doubling maximal inositol trisphosphate (IP3) production and imposing a 90% blockade of K+ channels. We investigated two configurations, one in which ryanodine receptors (RyR) and IP3 receptors (IP3R) occupy a common store and a second in which they reside on separate stores. Our results suggest that Ca2+ release from stores and import from the extracellular space are key determinants of oscillations because both raise [Ca] in subplasmalemmal spaces near RyR. When the Ca2+-induced Ca2+ release (CICR) threshold of RyR is exceeded, the ensuing Ca2+ release is limited by Ca2+ reuptake into stores and export across the plasmalemma. If sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pumps do not remain saturated and sarcoplasmic reticulum Ca2+ stores are replenished, that phase is followed by a resumption of leak from internal stores that leads either to [Ca]cyt elevation below the CICR threshold (no oscillations) or to elevation above it (oscillations). Our model predicts that oscillations are more prone to occur when IP3R and RyR stores are separate because, in that case, Ca2+ released by RyR during CICR can enhance filling of adjacent IP3 stores to favor a high subsequent leak that generates further CICR events. Moreover, the existence or absence of oscillations depends on the set points of several parameters, so that biological variation might well explain the presence or absence of oscillations in individual pericytes.

Membrane current oscillations in descending vasa recta pericytes

We investigated the origin of spontaneous transient inward current (STIC) oscillations in descending vasa recta (DVR) pericytes. In cells clamped at -80 mV, angiotensin II (ANG II; 10 nmol/l) induced oscillations with mean amplitude and frequency of -65.5 pA and 1.2 Hz. Simultaneous recording of cytoplasmic calcium ([Ca(2+)](CYT)) and membrane current oscillations verified their synchrony and the correlation of their amplitudes. Confocal recording in fluo-4-loaded DVR showed that ANG II can induce either stable pericyte [Ca(2+)](CYT) elevation or oscillations, while decreasing adjacent endothelial [Ca(2+)](CYT). Oscillating currents reversed sign at -30.2 mV and were blocked by niflumic acid, implicating charge transfer via Cl(-) ion. Removal of extracellular Ca(2+), blockade of Ca(2+) influx with SKF96365 (30 micromol/l), ryanodine (30 micromol/l), or caffeine (10 mmol/l) inhibited oscillations. In contrast, they were insensitive to removal of extracellular Na(+) and exposure to either nifedipine (1 micromol/l) or 2-aminoethoxydiphenyl borate (10 micromol/l). Ouabain (100 nmol/l) increased basal pericyte [Ca(2+)](CYT) and the frequency of resting STICs but did not affect the larger oscillations that followed ANG II stimulation. We conclude that [Ca(2+)](CYT) oscillations stimulate Cl(-) currents. The former are most likely maintained by repetitive cycles of ryanodine-sensitive SR Ca(2+) release and SKF96365-sensitive store refilling.

Descending vasa recta endothelia express inward rectifier potassium channels

Descending vasa recta (DVR) are capillary-sized microvessels that supply blood flow to the renal medulla. They are composed of contractile pericytes and endothelial cells. In this study, we used the whole cell patch-clamp method to determine whether inward rectifier potassium channels (K(IR)) exist in the endothelia, affect membrane potential, and modulate intracellular Ca(2+) concentration ([Ca(2+)](cyt)). The endothelium was accessed for electrophysiology by removing abluminal pericytes from collagenase-digested vessels. K(IR) currents were recorded using symmetrical 140 mM K(+) solutions that served to maximize currents and eliminate cell-to-cell coupling by closing gap junctions. Large, inwardly rectifying currents were observed at membrane potentials below the equilibrium potential for K(+). Ba(2+) potently inhibited those currents in a voltage-dependent manner, with affinity k = 0.18, 0.33, 0.60, and 1.20 microM at -160, -120, -80, and -40 mV, respectively. Cs(+) also blocked those currents with k = 20, 48, 253, and 1,856 microM at -160, -120, -80, and -40 mV, respectively. In the presence of 1 mM ouabain, increasing extracellular K(+) concentration from 5 to 10 mM hyperpolarized endothelial membrane potential by 15 mV and raised endothelial [Ca(2+)](cyt). Both the K(+)-induced membrane hyperpolarization and the [Ca(2+)](cyt) elevation were reversed by Ba(2+). Immunochemical staining verified that both pericytes and endothelial cells of DVR express K(IR)2.1, K(IR)2.2, and K(IR)2.3 subunits. We conclude that strong, inwardly rectifying K(IR)2.x isoforms are expressed in DVR and mediate K(+)-induced hyperpolarization of the endothelium.

Descending vasa recta endothelium is an electrical syncytium

We examined gap junction coupling of descending vasa recta (DVR). DVR endothelial cells or pericytes were depolarized to record the associated capacitance transients. Virtually all endothelia and some pericytes exhibited prolonged transients lasting 10-30 ms. Carbenoxolone (100 microM) and 18beta-glycyrrhetinic acid (18betaGRA; 100 microM) markedly shortened the endothelial transients. Carbenoxolone and heptanol (2 mM) reduced the pericyte capacitance transients when they were prolonged. Lucifer yellow (LY; 2 mM) was dialyzed into the cytoplasm of endothelial cells and pericytes. LY spread diffusely along the endothelial monolayer, whereas in most pericytes, it was confined to a single cell. In some pericytes, complex patterns of LY spreading were observed. DVR cells were depolarized by voltage clamp as fluorescence of bis(1,3-dibarbituric acid)-trimethine oxanol [DiBAC(4)(3)] was monitored approximately 200 microm away. A 40-mV endothelial depolarization was accompanied by a 26.1 +/- 5.5-mV change in DiBAC(4)(3) fluorescence. DiBAC(4)(3) fluorescence did not change after 18betaGRA or when pericytes were depolarized. Similarly, propagated cytoplasmic Ca(2+) responses arising from mechanical perturbation of the DVR wall were attenuated by 18betaGRA or heptanol. Connexin (Cx) immunostaining showed predominant linear Cx40 and Cx43 in endothelia, whereas Cx37 stained smooth muscle actin-positive pericytes. We conclude that the DVR endothelium is an electrical syncytium and that gap junction coupling in DVR pericytes exists but is less pronounced.

Vasa recta pericytes express a strong inward rectifier K+ conductance

Strong inward rectifier potassium channels are expressed by some vascular smooth muscle cells and facilitate K+-induced hyperpolarization. Using whole cell patch clamp of isolated descending vasa recta (DVR), we tested whether strong inward rectifier K+ currents are present in smooth muscle and pericytes. Increasing extracellular K+ from 5 to 50 and 140 mmol/l induced inward rectifying currents. Those currents were Ba2+ sensitive and reversed at the K+ equilibrium potential imposed by the electrode and extracellular buffers. Ba2+ binding constants in symmetrical K+ varied between 0.24 and 24 micromol/l at -150 and -20 mV, respectively. Ba2+ blockade was time and voltage dependent. Extracellular Cs+ also blocked the inward currents with binding constants between 268 and 4,938 micromol/l at -150 and -50 mV, respectively. Ba2+ (30 micromol/l) and ouabain (1 mmol/l) depolarized pericytes by an average of 11 and 24 mV, respectively. Elevation of extracellular K+ from 5 to 10 mmol/l hyperpolarized pericytes by 6 mV. That hyperpolarization was reversed by Ba2+ (30 micromol/l). We conclude that strong inward rectifier K+ channels and Na+-K+-ATPase contribute to resting potential and that KIR channels can mediate K+-induced hyperpolarization of DVR pericytes.

KATP channel conductance of descending vasa recta pericytes

Using nystatin-perforated patch-clamp and whole cell recording, we tested the hypothesis that K(ATP) channels contribute to resting conductance of rat descending vasa recta (DVR) pericytes and are modulated by vasoconstrictors. The K(ATP) blocker glybenclamide (Glb; 10 microM) depolarized pericytes and inhibited outward currents of cells held at -40 mV. K(ATP) openers pinacidil (Pnc; 10 microM) and P-1075 (1 microM) hyperpolarized pericytes and transiently augmented outward currents. All effects of Pnc and P-1075 were fully reversed by Glb. Inward currents of pericytes held at -60 mV in symmetrical 140 mM K(+) were markedly augmented by Pnc and fully reversed by Glb. Ramp depolarizations in symmetrical K(+), performed in Pnc and Pnc + Glb, yielded a Pnc-induced, Glb-sensitive K(ATP) difference current that lacked rectification and reversed at 0 mV. Immunostaining identified both K(IR)6.1, K(IR)6.2 inward rectifier subunits and sulfonurea receptor subtype 2B. ANG II (1 and 10 nM) and endothelin-1 (10 nM) but not vasopressin (100 nM) significantly lowered holding current at -40 mV and abolished Pnc-stimulated outward currents. We conclude that DVR pericytes express K(ATP) channels that make a significant contribution to basal K(+) conductance and are inhibited by ANG II and endothelin-1.

Descending vasa recta pericytes express voltage operated Na+ conductance in the rat

We studied the properties of a voltage-operated Na+ conductance in descending vasa recta (DVR) pericytes isolated from the renal outer medulla. Whole-cell patch-clamp recordings revealed a depolarization-induced, rapidly activating and rapidly inactivating inward current that was abolished by removal of Na+ but not Ca+ from the extracellular buffer. The Na+ current (I(Na)) is highly sensitive to tetrodotoxin (TTX, Kd = 2.2 nM). At high concentrations, mibefradil (10 microM) and Ni+ (1 mM) blocked I(Na). I(Na) was insensitive to nifedipine (10 microM). The L-type Ca+ channel activator FPL-64176 induced a slowly activating/inactivating inward current that was abolished by nifedipine. Depolarization to membrane potentials between 0 and 30 mV induced inactivation with a time constant of approximately 1 ms. Repolarization to membrane potentials between -90 and -120 mV induced recovery from inactivation with a time constant of approximately 11 ms. Half-maximal activation and inactivation occurred at -23.9 and -66.1 mV, respectively, with slope factors of 4.8 and 9.5 mV, respectively. The Na+ channel activator, veratridine (100 microM), reduced peak inward I(Na) and prevented inactivation. We conclude that a TTX-sensitive voltage-operated Na+ conductance, with properties similar to that in other smooth muscle cells, is expressed by DVR pericytes.

Depolarization induces Rho-Rho kinase-mediated myosin light chain phosphorylation in kidney tubular cells

Myosin-based contractility plays important roles in the regulation of epithelial functions, particularly paracellular permeability. However, the triggering factors and the signaling pathways that control epithelial myosin light chain (MLC) phosphorylation have not been elucidated. Herein we show that plasma membrane depolarization provoked by distinct means, including high extracellular K(+), the lipophilic cation tetraphenylphosphonium, or the ionophore nystatin, induced strong diphosphorylation of MLC in kidney epithelial cells. In sharp contrast to smooth muscle, depolarization of epithelial cells did not provoke a Ca(2+) signal, and removal of external Ca(2+) promoted rather than inhibited MLC phosphorylation. Moreover, elevation of intracellular Ca(2+) did not induce significant MLC phosphorylation, and the myosin light chain kinase (MLCK) inhibitor ML-7 did not prevent the depolarization-induced MLC response, suggesting that MLCK is not a regulated element in this process. Instead, the Rho-Rho kinase (ROK) pathway is the key mediator because 1) depolarization stimulated Rho and induced its peripheral translocation, 2) inhibition of Rho by Clostridium difficile toxin B or C3 transferase abolished MLC phosphorylation, and 3) the ROK inhibitor Y-27632 suppressed the effect. Importantly, physiological depolarizing stimuli were able to activate the same pathway: L-alanine, the substrate of the electrogenic Na(+)-alanine cotransporter, stimulated Rho and induced Y-27632-sensitive MLC phosphorylation in a Na(+)-dependent manner. Together, our results define a novel mode of the regulation of MLC phosphorylation in epithelial cells, which is depolarization triggered and Rho-ROK-mediated but Ca(2+) signal independent. This pathway may be a central mechanism whereby electrogenic transmembrane transport processes control myosin phosphorylation and thereby regulate paracellular transport.

Calcium handling in afferent arterioles

The cytosolic intracellular calcium concentration ([Ca(2+)](i)) is a major determining factor in the vascular smooth muscle tone. In the afferent arteriole it has been shown that agonists utilizing G-protein coupled receptors recruit Ca(2+) via release from intracellular stores and entry via pathways in the plasma membrane. The relative importances of entry vs. mobilization seem to differ between different agonists, species and preparations. The entry pathway might include different types of voltage sensitive Ca(2+) channels located in the plasmalemma such as dihydropyridine sensitive L-type channels, T-type channels and P/Q channels. A role for non-voltage sensitive entry pathways has also been suggested. The importance of voltage sensitive Ca(2+) channels in the control of the tone of the afferent arteriole (and thus in the control of renal function and whole body control of extracellular fluid volume and blood pressure) sheds light on the control of the membrane potential of afferent arteriolar smooth muscle cells. Thus, K(+) and Cl(-) channels are of importance in their role as major determinants of membrane potential. Some studies suggest a role for calcium-activated chloride (Cl(Ca)) channels in the renal vasoconstriction elicited by agonists. Other investigators have found evidence for several types of K(+) channels in the regulation of the afferent arteriolar tone. The available literature in this field regarding afferent arterioles is, however, relatively sparse and not conclusive. This review is an attempt to summarize the results obtained by others and ourselves in the field of agonist induced afferent arteriolar Ca(2+) recruitment, with special emphasis on the control of voltage sensitive Ca(2+) entry. Outline of the Manuscript: This manuscript is structured as follows: it begins with an introduction where the general role for [Ca(2+)](i) as a key factor in the regulation of the tone of vascular smooth muscles (VSMC) is detailed. In this section there is an emphasis is on observations that could be attributed to afferent arteriolar function. We then investigate the literature and describe our results regarding the relative roles for Ca(2+) entry and intracellular release in afferent arterioles in response to vasoactive agents, with the focus on noradrenalin (NA) and angiotensin II (Ang II). Finally, we examine the role of ion channels (i.e. K(+) and Cl(-) channels) for the membrane potential, and thus activation of voltage sensitive Ca(2+) channels.

Inhibition of K+ conductance in descending vasa recta pericytes by ANG II

We tested whether K(+) channel inhibition accompanies ANG II-induced depolarization of descending vasa recta (DVR) pericytes. An increase in extracellular K(+) concentration ([K(+)](o)) from 5 to 100 mM depolarized resting pericytes but had no effect after prolonged (10 nM, 20 min) ANG II exposure. In contrast, reduction of extracellular Cl(-) concentration ([Cl(-)](o)) from 154 to 34 mM had a minor effect on resting membrane potential but strongly depolarized pericytes treated with ANG II. The K(+) channel blockers BaCl(2) (0.1, 1 mM) and tetraethylammonium (TEA; 30 mM) depolarized resting pericytes but did not affect membrane potential of ANG II-treated pericytes. Pericyte whole cell currents were reduced by ANG II and nearly eliminated by combined ANG II exposure and the Cl(-) channel blocker niflumic acid (100 muM). Augmentation of inward current induced by raising [K(+)](O) from 5 to 50 mM was eliminated by preexposure to ANG II. TEA- and BaCl(2)-sensitive outward currents, generated by depolarizing pericytes from -80 to -40 mV, were eliminated by ANG II. We conclude that ANG II depolarizes DVR pericytes by a combination of Cl(-) channel activation and K(+) channel inhibition.

Effects of chloride channel blockers on rat renal vascular responses to angiotensin II and norepinephrine

The aim of the present study was to investigate the role of Ca2+-activated Cl-channels in the renal vasoconstriction elicited by angiotensin II (ANG II) and norepinephrine (NE). Renal blood flow (RBF) was measured in vivo using electromagnetic flowmetry. Ratiometric photometry of fura 2 fluorescence was used to estimate intracellular free Ca2+concentration ([Ca2+]i) in isolated preglomerular vessels from rat kidneys. Renal arterial injection of ANG II (2-4 ng) and NE (20-40 ng) produced a transient decrease in RBF. Administration of ANG II (10-7M) and NE (5 × 10-6M) to the isolated preglomerular vessels caused a prompt increase in [Ca2+]i. Renal preinfusion of DIDS (0.6 and 1.25 μmol/min) attenuated the ANG II-induced vasoconstriction to ∼35% of the control response, whereas the effects of NE were unaltered. Niflumic acid (0.14 and 0.28 μmol/min) and 2-[(2-cyclopentenyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxo-1 H-inden-5-yl)oxy]acetic acid (IAA-94; 0.045 and 0.09 μmol/min) did not affect the vasoconstrictive responses of these compounds. Pretreatment with niflumic acid (50 μM) or IAA-94 (30 μM) for 2 min decreased baseline [Ca2+]ibut did not change the magnitude of the [Ca2+]iresponse to ANG II and NE in the isolated vessels. The present results do not support the hypothesis that Ca2+-activated Cl-channels play a crucial role in the hemodynamic effects of ANG II and NE in rat renal vasculature.

Physiology of the renal medullary microcirculation

Perfusion of the renal medulla plays an important role in salt and water balance. Pericytes are smooth muscle-like cells that impart contractile function to descending vasa recta (DVR), the arteriolar segments that supply the medulla with blood flow. DVR contraction by ANG II is mediated by depolarization resulting from an increase in plasma membrane Cl(-) conductance that secondarily gates voltage-activated Ca(2+) entry. In this respect, DVR may differ from other parts of the efferent microcirculation of the kidney. Elevation of extracellular K(+) constricts DVR to a lesser degree than ANG II or endothelin-1, implying that other events, in addition to membrane depolarization, are needed to maximize vasoconstriction. DVR endothelial cytoplasmic Ca(2+) is increased by bradykinin, a response that is inhibited by ANG II. ANG II inhibition of endothelial Ca(2+) signaling might serve to regulate the site of origin of vasodilatory paracrine agents generated in the vicinity of outer medullary vascular bundles. In the hydropenic kidney, DVR plasma equilibrates with the interstitium both by diffusion and through water efflux across aquaporin-1. That process is predicted to optimize urinary concentration by lowering blood flow to the inner medulla. To optimize urea trapping, DVR endothelia express the UT-B facilitated urea transporter. These and other features show that vasa recta have physiological mechanisms specific to their role in the renal medulla.

Ca(2+) signaling and membrane potential in descending vasa recta pericytes and endothelia

We devised a method for removal of pericytes from isolated descending vasa recta (DVR). After enzymatic digestion, aspiration of a descending vas rectum into a micropipette strips the pericytes from the abluminal surface. Pericytes and denuded endothelia can be recovered for separate study. Using fura 2-loaded preparations, we demonstrated that 10 nM angiotensin II (ANG II) elevates pericyte intracellular Ca(2+) concentration ([Ca(2+)](i)) and suppresses endothelial [Ca(2+)](i). The anion transport blocker probenecid helps retain fura 2 in the pericyte cytoplasm. DVR endothelia were accessed for membrane potential measurement by perforated-patch whole cell recording by using the pericyte-stripping technique and by turning nondigested vessels inside out with concentric micropipettes. By either method of access, 10 nM ANG II depolarized (n = 20) and 100 nM bradykinin hyperpolarized (n = 25) the endothelia. We conclude that isolated endothelia and pericytes remain functional for study of [Ca(2+)](i) responses and that ANG II and bradykinin receptors exist separately on each cell type.

Membrane potential controls calcium entry into descending vasa recta pericytes

We tested the hypothesis that constriction of descending vasa recta (DVR) is mediated by voltage-gated calcium entry. K(+) channel blockade with BaCl(2) (1 mM) or TEACl (30 mM) depolarized DVR smooth muscle/pericytes and constricted in vitro-perfused vessels. Pericyte depolarization by 100 mM extracellular KCl constricted DVR and increased pericyte intracellular Ca(2+) ([Ca(2+)](i)). The K(ATP) channel opener pinacidil (10(-7)-10(-4) M) hyperpolarized resting pericytes, repolarized pericytes previously depolarized by ANG II (10(-8) M), and vasodilated DVR. The DVR vasodilator bradykinin (10(-7) M) also reversed ANG II depolarization. The L-type Ca(2+) channel blocker diltiazem vasodilated ANG II (10(-8) M)- or KCl (100 mM)-preconstricted DVR, and the L-type agonist BayK 8644 constricted DVR. The plateau phase of the pericyte [Ca(2+)](i) response to ANG II was inhibited by diltiazem. These data support the conclusion that DVR vasoreactivity is controlled through variation of membrane potential and voltage-gated Ca(2+) entry into the pericyte cytoplasm.

Control of descending vasa recta pericyte membrane potential by angiotensin II

Using nystatin perforated-patch whole cell recording, we investigated the role of Cl(-) conductance in the modulation of outer medullary descending vasa recta (OMDVR) pericyte membrane potential (Psi m) by ANG II. ANG II (10(-11) to 10(-7) M) consistently depolarized OMDVR and induced Psi m oscillations at lower concentrations. The Cl(-) channel blockers anthracene-9-decarboxylate (1 mM) and niflumic acid (10 microM) hyperpolarized resting pericytes and repolarized ANG II-treated pericytes. In voltage-clamp experiments, ANG II-treated pericytes exhibited slowly activating currents that were nearly eliminated by treatment with niflumic acid (10 microM) or removal of extracellular Ca(2+). Those currents reversed at -31 and -10 mV when extracellular Cl(-) concentration was 152 and 34 mM, respectively. In pericytes held at -70 mV, oscillating inward currents were sometimes observed; the reversal potential also shifted with extracellular Cl(-) concentration. We conclude that ANG II activates a Ca(2+)-dependent Cl(-) conductance in OMDVR pericytes to induce membrane depolarization and Psi m oscillations.

Renal blood flow autoregulation in blood pressure control

Although the kidney strives to maintain its perfusion within tight boundaries, considerable blood flow fluctuations do occur. The reasons for this are the rather slow acting compensatory mechanisms of renal blood flow autoregulation, the effects of renal nerves, hormonal influences, etc. It seems that variations in renal perfusion can exert a major influence on renal excretory functions, on renin release and on blood pressure. The clinical importance of renal blood flow variability is not fully understood. In many situations, the absence of normal cardiovascular oscillations seems to be a risk factor. Large fluctuations in perfusion pressure to the kidney, however, in the long run, may induce target organ damage.