Efferent Arteriolar Diameters Research Articles

Evaluation of glomerular hemodynamic changes by sodium-glucose-transporter 2 inhibition in type 2 diabetic rats using in vivo imaging

The mechanisms responsible for glomerular hemodynamic regulation with sodium-glucose co-transporter 2 (SGLT2) inhibitors in kidney disease due to type 2 diabetes remain unclear. Therefore, we investigated changes in glomerular hemodynamic function using an animal model of type 2 diabetes, treated with an SGLT2 inhibitor alone or in combination with a renin-angiotensin-aldosterone system inhibitor using male Zucker lean (ZL) and Zucker diabetic fatty (ZDF) rats. Afferent and efferent arteriolar diameter and single-nephron glomerular filtration rate (SNGFR) were evaluated in ZDF rats measured at 0, 30, 60, 90, or 120 minutes after the administration of a SGLT2 inhibitor (luseogliflozin). Additionally, we assessed these changes under the administration of the adenosine A1 receptor (A1aR) antagonist (8-cyclopentyl-1,3-dipropylxanthine), along with coadministration of luseogliflozin and an angiotensin II receptor blocker (ARB), telmisartan. ZDF rats had significantly increased SNGFR, and afferent and efferent arteriolar diameters compared to ZL rats, indicating glomerular hyperfiltration. Administration of luseogliflozin significantly reduced afferent vasodilatation and glomerular hyperfiltration, with no impact on efferent arteriolar diameter. Urinary adenosine levels were increased significantly in the SGLT2 inhibitor group compared to the vehicle group. A1aR antagonism blocked the effect of luseogliflozin on kidney function. Co-administration of the SGLT2 inhibitor and ARB decreased the abnormal expansion of glomerular afferent arterioles, whereas the efferent arteriolar diameter was not affected. Thus, regulation of afferent arteriolar vascular tone via the A1aR pathway is associated with glomerular hyperfiltration in type 2 diabetic kidney disease.

PS-BPB08-6: REGULATORY MECHANISM OF GLOMERULAR FILTRATION RATE BY THE KEAP1/NRF2 PATHWAY

The activation of the Kelch like ECH associated protein 1 (Keap1)/nuclear factor (erythroid derived 2) like 2 (Nrf2) pathway increases glomerular filtration rate (GFR) in patients with type 2 diabetes and chronic kidney disease. However, the mechanisms whereby the Keap1/Nrf2 pathway regulates GFR are unknown. C57BL/6 mice (WT), Nrf2 deficient mice, and Nrf2 activated Keap1 knockdown mice were compared, and RTA dh404 (CDDO dhTFEA) was used as a Nrf2 activator. Glomerular hemodynamics and the permeability of glomeruli to macromolecules were evaluated using laser Doppler techniques and in vivo imaging. Calcium influx and the rearrangement of the actin cytoskeleton in response to angiotensin II, H2O2, the Nrf2 activator dh404, and transient receptor potential cation channel, subfamily C (TRPC) channel inhibitors were evaluated in podocytes. Finally, the effects of a TRPC6 inhibitor on GFR were investigated in vivo. Pharmacological and genetic Keap1/Nrf2 activation increased renal blood flow (p < 0.05), glomerular volume (p < 0.05), and GFR (p < 0.05), but did not alter afferent or efferent arteriolar diameter or glomerular permeability. Calcium influx into podocytes through TRPC channels in response to angiotensin II or H2O2 was suppressed by Keap1/Nrf2 activation. TRPC inhibitors and dh404 reduced the activation of the small GTPase(RhoA)/Rho associated kinase 1/LIM domain kinase 1/cofilin pathway and the subsequent actin rearrangement induced by angiotensin II or H2O2. Treatment with a TRPC6 inhibitor increased single nephron GFR in WT mice. In conclusion, the Keap1/Nrf2 pathway regulates GFR through changes in ultrafiltration via intracellular calcium signaling and cellular contractility, mediated through TRPC activity, in glomerular cells.

Effects of serelaxin on renal microcirculation in rats under control and high-angiotensin environments.

Serelaxin is a novel recombinant human relaxin-2 that has been investigated for the treatment of acute heart failure. However, its effects on renal function, especially on the renal microcirculation, remain incompletely characterized. Our immunoexpression studies localized RXFP1 receptors on vascular smooth muscle cells and endothelial cells of afferent arterioles and on principal cells of collecting ducts. Clearance experiments were performed in male and female normotensive rats and Ang II-infused male rats. Serelaxin increased mean arterial pressure slightly and significantly increased renal blood flow, urine flow, and sodium excretion rate. Group analysis of all serelaxin infusion experiments showed significant increases in GFR. During infusion with subthreshold levels of Ang II, serelaxin did not alter mean arterial pressure, renal blood flow, GFR, urine flow, or sodium excretion rate. Heart rates were elevated during serelaxin infusion alone (37 ± 5%) and in Ang II-infused rats (14 ± 2%). In studies using the in vitro isolated juxtamedullary nephron preparation, superfusion with serelaxin alone (40 ng/ml) significantly dilated afferent arterioles (10.8 ± 1.2 vs. 13.5 ± 1.1 µm) and efferent arterioles (9.9 ± 0.9 vs. 11.9 ± 1.0 µm). During Ang II superfusion, serelaxin did not alter afferent or efferent arteriolar diameters. During NO synthase inhibition (l-NNA), afferent arterioles also did not show any vasodilation during serelaxin infusion. In conclusion, serelaxin increased overall renal blood flow, urine flow, GFR, and sodium excretion and dilated the afferent and efferent arterioles in control conditions, but these effects were attenuated or prevented in the presence of exogenous Ang II and NO synthase inhibitors.

High-salt diet induces outward remodelling of efferent arterioles in mice with reduced renal mass.

The glomerular filtration rate (GFR) falls progressively in chronic kidney disease (CKD) which is caused by a reduction in the number of functional nephrons. The dysfunctional nephron exhibits a lower glomerular capillary pressure that is induced by an unbalance between afferent and efferent arteriole. Therefore, we tested the hypothesis that oxidative stress induced by CKD differentially impairs the structure or function of efferent vs. afferent arterioles. C57BL/6 mice received sham operations (sham) or 5/6 nephrectomy (RRM) and three months of normal- or high-salt diet or tempol. GFR was assessed from the plasma inulin clearance, arteriolar remodelling from media/lumen area ratio, myogenic responses from changes in luminal diameter with increases in perfusion pressure and passive wall compliance from the wall stress/strain relationships. Mice with RRM fed a high salt (vs. sham) had a lower GFR (553±25 vs. 758±36μLmin-1 g-1 kidney, P<0.01) and a larger efferent arteriolar diameter (9.6±0.8 vs. 7.4±0.7μm, P<0.05) resulting in a lower media/lumen area ratio (1.4±0.1 vs. 2.4±0.2, P<0.01). These alterations were corrected by tempol. The myogenic responses of efferent arterioles were about one-half that of afferent arterioles and were unaffected by RRM or salt. Passive wall compliance was reduced by high salt in both afferent and efferent arterioles. A reduction in renal mass with a high-salt diet induces oxidative stress that leads to an outward eutrophic remodelling in efferent arterioles and reduced wall compliance in both afferent and efferent arterioles. This may contribute to the lower GFR in this model of CKD.

Nebivolol-induced vasodilation of renal afferent arterioles involves β3-adrenergic receptor and nitric oxide synthase activation

Nebivolol is a β(1)-adrenergic blocker that also elicits renal vasodilation and increases the glomerular filtration rate (GFR). However, its direct actions on the renal microvasculature and vasodilator mechanism have not been established. We used the in vitro blood-perfused juxtamedullary nephron technique to determine the vasodilator effects of nebivolol and to test the hypothesis that nebivolol induces vasodilation of renal afferent arterioles via an nitric oxide synthase (NOS)/nitric oxide (NO)/soluble guanylate cyclase (sGC)/cGMP pathway and the afferent arteriolar vasodilation effect may be mediated through the release of NO by activation of NOS via a β(3)-adrenoceptor-dependent mechanism. Juxtamedullary nephrons were superfused with nebivolol either alone or combined with the sGC inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or the NOS inhibitor N(ω)-nitro-l-arginine (l-NNA) or the β-blockers metoprolol (β(1)), butoxamine (β(2)), and SR59230A (β(3)). Nebivolol (100 μmol/l) markedly increased afferent and efferent arteriolar diameters by 18.9 ± 3.0 and 15.8 ± 1.8%. Pretreatment with l-NNA (1,000 μmol/l) or ODQ (10 μmol/l) decreased afferent vasodilator diameters and prevented the vasodilator effects of nebivolol (2.0 ± 0.2 and 2.4 ± 0.6%). Metoprolol did not elicit significant changes in afferent vasodilator diameters and did not prevent the effects of nebivolol to vasodilate afferent arterioles. However, treatment with SR59230A, but not butoxamine, markedly attenuated the vasodilation responses to nebivolol. Using a monoclonal antibody to β(3)-receptors revealed predominant immunostaining on vascular and glomerular endothelial cells. These data indicate that nebivolol vasodilates both afferent and efferent arterioles and that the afferent vasodilator effect is via a mechanism that is independent of β(1)-receptors but is predominantly mediated via a NOS/NO/sGC/cGMP-dependent mechanisms initiated by activation of endothelial β(3)-receptors.

Abstract 1495: Existence of Hyperfiltration in an Early Diabetic Rat was Visualised by Multiphoton Microscopy

To visualise glomerular filtration under physiological conditions in an early diabetic rat and to evaluate filtration molecularly and quantitatively. We used 4 week STZ (50 mg/kg) model as early diabetic rats. We measured diameters of afferent (aff) and efferent (eff) arterioles, glomerular red blood cell velocities and left renal blood flow with our intravital videomicro-scope. Glomerular filtration was visualised by multiphoton microscopy. Various sizes of dextran (3k, 10k, 40k and 70k dalton (Dal)) conjugated with Texas Red in 0.5 ml were administered intravenously by bolus shot and for the next 60 seconds glomerular filtration was observed. FITC conjugated 500k Dal dextran was administered to dye intravascular area. The time course of Texas Red intensity in Bowman’s space was measured from the recorded images. Intensity peak values were normalized by the peak value of 3k Dal as sieving coefficients since the intensity reached its peak within 10 seconds after administration of Texas Red conjugated dextrans. Afferent arteriolar diameters were larger than efferent arteriolar diameters in both control (n=6) and diabetic (n=7) rats. Increases of both afferent and efferent diameters were remarkable in diabetic rats (p&lt;0.05) (aff/eff diameter, DM rats: 14.0 ± 1.9, 9.4 ± 0.7, Control rats: 11.9 ± 0.8, 8.8 ± 0.7 μm). Glomerular red blood cell velocity in diabetic rats was larger than that in control rats (p&lt;0.05) (DM/Control rats: 0.236 ± 0.030, 0.196 ± 0.076 mm/s). Left renal blood flow in diabetic rats was larger than that in control rats (p&lt;0.05) (DM/Control rats: 1.84 ± 0.16, 1.60 ± 0.17 ml/100 gBW). All of these findings suggest the existence of hyperfiltration in early diabetic rats. In fact, filtration of the larger dextrans (40k and 70k Dal) were clearly visualized by multiphoton microscopy in early diabetic rats indicating greater leakage even from the early stage of diabetes with a significant difference by about 20% for the 40k and 70k Dal sieving coefficients comparing with the control group (p&lt;0.05). Both afferent and efferent arteriolar diameters were increased in early diabetic rats suggesting the existence of hyperfiltration and the hyperfiltration was clearly visualised by multiphoton microscopy.

The olfactory isoform of adenylyl cyclase (AC3) in the renal macula densa serves as a key regulator of glomerular filtration rate

Recent work shows that olfactory receptors (ORs) are functionally expressed in non‐olfactory tissues. We find that key components of the OR signaling pathway, olfactory G protein (Golf) and the olfactory adenylyl cyclase (AC3), are detectable in the kidney by RT‐PCR and western blot. AC3 and Golf colocalize with one another and with markers of the distal convoluted tubule. Most notably, AC3 is expressed in the macula densa (MD). The MD regulates glomerular filtration rate (GFR) through both the tubuloglomerular feedback (TGF) and renin secretion pathways. Renal function studies using AC3−/− mice and wild‐type littermates demonstrated that GFR in AC3−/− mice is significantly decreased (AC3−/−:0.42 ± 0.08 ml/min/100g BW, n=8; AC3+/+: 0.87 ± 0.06, n=6, p&lt;0.01). This is not due to alterations in TGF, as TGF assayed by micropuncture is normal in AC3−/−; however, AC3−/− have a ~50% reduction in plasma renin concentration (p&lt;0.001). By RT‐PCR, we have identified six ORs in mouse kidney, suggesting that the chemosensors that participate in this pathway are also present in renal epithelia. These studies indicate that the olfaction machinery present in the kidney is necessary for proper control of GFR, presumably through modulation of renin secretion to regulate efferent arteriolar diameter. This OR machinery is found in the MD, an ideal localization for ORs to sense the chemical composition of the forming urine.

Visualisation of the Effects of Dilazep on Rat Afferent and Efferent Arterioles In Vivo

Although the effects of dilazep hydrochloride (dilazep), a nucleoside transport inhibitor, have been examined, there have been no visualisation studies on the physiological effects of dilazep on the glomerular arterioles. The purpose of this study was to visualise and evaluate the effects of dilazep and consequently the effects of adenosine, which dilazep augments by measuring glomelurar diameters, renal blood flow and resistance in rats in vivo. We time-sequentially examined afferent and efferent arteriolar diameter changes using an intravital videomicroscope and renal blood flow. We administered dilazep at a dose of 300 microg/kg intravenously. To further investigate the effects of dilazep, rats were pre-treated with 8-p-sulfophenyl theophylline (a nonselective adenosine receptor antagonist), 8-cyclopentyl-1,3-dipropylxanthine (an A1 receptor antagonist), or 3,7-dimethyl-1-propargylxanthine (an A2 receptor antagonist). Dilazep constricted the afferent and efferent arterioles at the early phase and dilated them at the later phase, with the same degree of vasoconstrictive and vasodilatory effect on both arterioles. A1 blockade abolished vasoconstriction and augmented vasodilatation at the later phase and A2 blockade abolished vasodilatation and augmented vasoconstriction at the early phase. Non-selective blockade abolished both early vasoconstriction and later vasodilatation. In conclusion, adenosine augmented by dilazep constricted the afferent and efferent arterioles of the cortical nephrons at the early phase and dilated both arterioles at the later phase via A1 and A2 adenosine receptor activation, respectively. That the ratio of afferent to efferent arteriolar diameter was fairly constant suggests that intraglomerular pressure is maintained in the acute phase by adenosine despite the biphasic flow change.

Vasodilatory Effect of Cilnidipine, an L-type and N-type Calcium Channel Blocker, on Rat Kidney Glomerular Arterioles

Cilnidipine is a dihydropyridine calcium channel blocker that acts on both L-type and N-type calcium channels. The effects of cilnidipine given intravenously at doses of 2.5, 5.0, and 10 microg/kg were studied using an ex vivo hydronephrosis model in spontaneously hypertensive rats. The effects of nifedipine at a dose of 10 microg/kg were also studied using the same model as a reference. Cilnidipine caused dose-dependent blood pressure reduction and dilatation of the glomerular afferent arterioles; the arteriolar diameter after cilnidipine infusion at 2.5, 5.0, and 10 microg/kg was 101% +/- 3%, 112% +/- 4%, and 123% +/- 6% relative to baseline, respectively. With cilnidipine, dilatation of the efferent arterioles was also observed; it was maximal after 5 to 10 minutes. Five minutes after administration of 2.5, 5.0, and 10 microg/kg of cilnidipine, the efferent arteriolar diameter was 103% +/- 2%, 109% +/- 4%, and 119% +/- 4% of baseline, respectively. This efferent arteriolar dilating action of cilnidipine was abolished after pretreatment with omega-conotoxin, a selective N-type calcium channel blocker. A dose-dependent increase of glomerular blood flow volume was also observed after cilnidipine infusion. Nifedipine, an L-type calcium channel blocker, at a dose of 10 microg/kg reduced systolic blood pressure to a similar extent as cilnidipine at a dose of 10 microg/kg, but only dilated the afferent arterioles and had no significant effect on efferent arterioles. Cilnidipine dilated both the afferent and efferent glomerular arterioles. The efferent arteriolar dilating effect of cilnidipine may be attributed to its inhibition of the N-type calcium channel.

Renal microvascular vasoconstrictor and vasodilator responses are reduced in angiotensin type 1 receptor double null mice (DKO)

AngII induced afferent arteriolar (AA) diameter responses are reduced, while efferent arteriolar (EA) diameter responses are absent in kidneys of mice lacking the AT1A receptor compared to wild-type (WT) (AJP 284:F538-545, 2003), while responses to norepinephrine (NE) are not altered. We determined the renal microvascular reactivity of WT and DKO mice using the in vitro blood perfused juxtamedullary nephron technique. Kidneys were harvested from WT and DKO mice and bathed with NE (100–1000nM), AngII (100nM), or acetylcholine (ACh;10μM). In WT mice, AA (12.6±0.9μm; n=8) diameter was significantly reduced by 29±4 and 51±6%, and EA (11.7±0.4μm; n=8) diameter was reduced by 17±2 and 38±3% in response to 300 and 1000nM NE, respectively. Baseline AA diameters of DKO kidneys were significantly larger than WT (19.1±1.2μm; n=12), while EA diameters were not different (15.8±2.4μm; n=4). Significant constriction to 1μM NE was observed in AA and EA (18±5% AA; 23±6% EA) of kidneys from DKO mice, but were less than observed in WT mice. AngII produced a 23±7% and 28±7% reduction in AA (n=5) and EA (n=7) diameters of WT mice, respectively. Responses to AngII were absent in AA (−2±2%; n=12) and EA (−0.1±2%; n=4) of DKO mice as predicted. AA diameter increased significantly from 15.0±0.6 to 17.0±1.2μm in response to ACh in WT mice (n=6). AA diameter of DKO mice (n=6) was not significantly altered by ACh averaging 19.8±1.1 and 17.8±1.6μm (p=0.06), respectively. In summary, vasoconstrictor and vasodilator responses are reduced in renal arterioles of mice lacking both AT1A and AT1B receptor subtypes. We conclude that AngII plays a pivotal role in renal arteriolar vascular smooth muscle cell development and/or function.

An experimental study of altered nitric oxide metabolism as a mechanism of cyclosporin-induced renal vasoconstriction.

Nephrotoxicity caused by cyclosporin A (CSA) is the result of vasoconstriction of the renal microcirculation. The endothelium-derived relaxing factor nitric oxide (NO) regulates microvascular blood flow in various tissues, and mediates the microcirculatory response during hypertension and sepsis. This study investigated the role of NO in CSA-induced renal vasoconstriction. Hydronephrotic kidneys in rats were suspended in an environmentally controlled tissue bath, and interlobular, afferent and efferent arteriolar diameters and blood flow were measured by in vivo videomicroscopy. CSA was administered alone, with the nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) or with exogenous NOS substrate L-arginine. CSA significantly constricted the whole of the renal microvasculature whereas L-NAME alone preferentially constricted the preglomerular vessels. L-Arginine reversed the vasoconstriction induced by CSA whereas L-NAME had no further effect. Preglomerular basal vascular tone is dependent on continuous production of NO and alterations in the L-arginine-NO pathway contribute to CSA-induced renal vasoconstriction.

Efferent arterioles exclusively express the subtype 1A angiotensin receptor: functional insights from genetic mouse models

Angiotensin (ANG) type 1A (AT(1A)) receptor-null (AT(1A)(-/-)) mice exhibit reduced afferent arteriolar (AA) constrictor responses to ANG II compared with wild-type (WT) mice, whereas efferent arteriolar (EA) responses are absent (Harrison-Bernard LM, Cook AK, Oliverio MI, and Coffman TM. Am J Physiol Renal Physiol 284: F538-F545, 2003). In the present study, the renal arteriolar constrictor responses to norepinephrine (NE) and/or ANG II were determined in blood-perfused juxtamedullary nephrons from kidneys of AT(1A)(-/-), AT(1B) receptor-null (AT(1B)(-/-)), and WT mice. Baseline AA diameter in AT(1A)(-/-) mice was not different from that in WT mice (13.1 +/- 0.9 and 12.6 +/- 0.9 microm, n = 7 and 8, respectively); however, EA diameters were significantly larger (17.3 +/- 1.4 vs. 11.7 +/- 0.4 microm, n = 10 and 8) in AT(1A)(-/-) than in WT mice. Constriction of AA (-40 +/- 8 and -51 +/- 6% at 1 microM NE) and EA (-29 +/- 6 and -38 +/- 3% at 1 microM NE) in response to 0.1-1 microM NE was similar in AT(1A)(-/-) and WT mice. Baseline diameters of AA (13.5 +/- 0.7 and 14.2 +/- 0.9 microm, n = 9 and 10) and EA (15.4 +/- 1.0 and 15.0 +/- 0.7 microm, n = 11 and 9) and ANG II (0.1-10 nM) constrictor responses of AA (-25 +/- 4 and -31 +/- 5% at 10 nM) and EA (-32 +/- 6 and -35 +/- 7% at 10 nM) were similar in AT(1B)(-/-) and WT mice, respectively. ANG II-induced constrictions were eliminated by AT(1) receptor blockade with 4 microM candesartan. Taken together, our data demonstrate that AA and EA responses to NE are unaltered in the absence of AT(1A) receptors, and ANG II responses remain intact in the absence of AT(1B) receptors. Therefore, we conclude that AT(1A) and AT(1B) receptors are functionally expressed on the AA, whereas the EA exclusively expresses the AT(1A) receptor.

Relative contributions of Ca2+mobilization and influx in renal arteriolar contractile responses to arginine vasopressin

Experiments addressed the hypothesis that afferent and efferent arterioles differentially rely on Ca2+ influx and/or release from intracellular stores in generating contractile responses to AVP. The effect of Ca2+ store depletion or voltage-gated Ca2+ channel (VGCC) blockade on contractile responsiveness to AVP (0.01-1.0 nM) was assessed in blood-perfused juxtamedullary nephrons from rat kidney. Depletion of intracellular Ca2+ stores by 100 microM cyclopiazonic acid (CPA) or 1 microM thapsigargin treatment increased afferent arteriolar baseline diameter by 14 and 21%, respectively, but did not significantly alter efferent arteriolar diameter. CPA attenuated the contractile response to 1.0 nM AVP by 34 and 55% in afferent and efferent arterioles, respectively (P = 0.013). The impact of thapsigargin on AVP-induced afferent arteriolar contraction (52% inhibition) was also less than its effect on the efferent arteriolar response (88% inhibition; P = 0.046). In experiments probing the role of the Ca2+ influx through VGCCs, 10 microM diltiazem evoked a 34% increase in baseline afferent arteriolar diameter and attenuated the contractile response to 1.0 nM AVP by 45%, without significantly altering efferent arteriolar baseline diameter or responsiveness to AVP. Combined treatment with both diltiazem and thapsigargin prevented AVP-induced contraction of both vascular segments. We conclude that Ca2+ release from the intracellular stores contributes to the contractile response to AVP in both afferent and efferent arterioles but is more prominent in the efferent arteriole. Moreover, the VGCC contribution to AVP-induced renal arteriolar contraction resides primarily in the afferent arteriole.

Cellular mechanism for mibefradil-induced vasodilation of renal microcirculation: studies in the isolated perfused hydronephrotic kidney.

Although nifedipine and other conventional calcium antagonists elicit preferential vasodilation of renal afferent arterioles, we demonstrate that mibefradil and nickel, T-type calcium channel blockers, reverse the angiotensin II-induced constriction of both afferent and efferent arterioles. Since the angiotensin II-induced vasoconstriction involves inositol trisphosphate (IP3)-induced calcium release from the sarcoplasmic reticulum in the afferent arteriole, and both IP3- and protein kinase C (PKC)-mediated pathways in the efferent arteriole, we investigated the cellular mechanism for the mibefradil-induced dilation of angiotensin II-constricted renal arterioles, using the isolated perfused hydronephrotic rat kidney. Mibefradil caused a dose-dependent dilation of angiotensin II-constricted afferent and efferent arterioles, with 88 +/- 9% and 74 +/- 10% reversal observed at 1 micromol/L, respectively. The blockade of PKC by staurosporine did not alter the mibefradil-induced vasodilator responses of either arterioles (P > 0.5). In contrast, the pretreatment with thapsigargin, which predominantly blocked the IP3-mediated intracellular calcium release, prevented the afferent arteriolar constrictor response to angiotensin II, but caused a significant constriction of efferent arterioles. The subsequent addition of mibefradil had no effect on the efferent arteriolar diameter. Furthermore, the efferent arteriolar constriction induced by direct PKC activation by phorbol myristate acetate was refractory to mibefradil, but completely reversed by LOE908, a nonselective cation channel blocker. In summary, mibefradil markedly dilates the angiotensin II-induced renal arteriolar constriction; the action of mibefradil is most likely mediated by the inhibition of the IP3-mediated pathway, but the inhibitory action on the PKC pathway appears modest.

Renal segmental microvascular responses to ANG II in AT1A receptor null mice.

The relative contributions of AT(1A) and AT(1B) receptors to afferent arteriolar autoregulatory capability and afferent and efferent arteriolar responses to ANG II are not known. Experiments were conducted in kidneys from wild-type (WT) and AT(1A)-/- mice utilizing the in vitro blood-perfused juxtamedullary nephron technique. Direct measurements of afferent (AAD) and efferent arteriolar diameters (EAD) were assessed at a renal arterial pressure of 100 mmHg. AAD averaged 14.8 +/- 0.8 microm for WT and 14.9 +/- 0.8 microm for AT(1A)-/- mice. AAD significantly decreased by 7 +/- 1, 16 +/- 1, and 26 +/- 2% for WT mice and by 11 +/- 1, 20 +/- 2, and 30 +/- 3% for AT(1A)-/- mice (120, 140, 160 mmHg). AAD autoregulatory capability was not affected by the absence of AT(1A) receptors. AAD responses to 10 nM ANG II were significantly blunted for AT(1A)-/- mice compared with WT (-22 +/- 2 vs. -37 +/- 5%). ANG II (0.1-10 nM) failed to elicit any change in EAD for AT(1A)-/- mice. AAD and EAD reductions in ANG II were blocked by 1 microM candesartan. We conclude that afferent arteriole vasoconstrictor responses to ANG II are mediated by AT(1A) and AT(1B) receptors, whereas efferent arteriolar vasoconstrictor responses to ANG II are mediated by only AT(1A) receptors in the mouse kidney.

Transient receptor potential channels in rat renal microcirculation: Actions of angiotensin II

This study assessed the calcium-activating mechanisms mediating glomerular arteriolar constriction by angiotensin II (Ang II). Immunohistochemical and physiological studies were carried out, using antibody against transient receptor potential (TRP)-1 and an isolated perfused kidney model. Immunohistochemical experiments demonstrated that TRP-1 proteins were transcribed on both afferent and efferent arteriolar myocytes. In the first series of physiological experiments, Ang II (0.3 nmol/L) considerably constricted afferent (20.2 +/- 0.9 to 14.9 +/- 0.7 microm) and efferent arterioles (18.4 +/- 0.7 to 14.0 +/- 0.7 microm). The addition of nifedipine (1 micromol/L) restored decrements in afferent (to 20.0 +/- 0.8 microm) but not efferent arteriolar diameters. Further administration of SKF-96365 (100 micromol/L), a TRP channel blocker, reversed efferent arteriolar constriction (to 16.2 +/- 0.8 micromol/L). In the second group, although 2-aminoethoxydiphenyl borate (100 micromol/L), an inhibitor of inositol trisphosphate-induced calcium release (IP3CR), did not alter glomerular arteriolar diameters, it prevented Ang II-induced afferent arteriolar constriction and attenuated efferent arteriolar constriction (18.8 +/- 0.8 to 16.9 +/- microm). Subsequent removal of extracellular calcium abolished residual efferent arteriolar constriction (to 19.1 +/- 0.8 microm). Our data provide evidence that Ang II elicits IP3CR, possibly inducing a cellular response that activates voltage-dependent calcium channels on afferent arterioles. The present results suggest that Ang II-induced efferent arteriolar constriction involves IP3CR and calcium influx sensitive to SKF-96365.

Altered electromechanical coupling in the renal microvasculature during the early stage of diabetes mellitus.

1. The early stage of type 1 diabetes mellitus (DM) is characterized by renal hyperfiltration, which promotes the eventual development of diabetic nephropathy. The hyperfiltration state is associated with afferent arteriolar dilation and diminished responsiveness of this vascular segment to a variety of vasoconstrictor stimuli, whereas efferent arteriolar diameter and vasoconstrictor responsiveness are typically unaltered. 2. The contractile status of preglomerular vascular smooth muscle appears to be tightly coupled to membrane potential (E(m)) and its influence on Ca(2+) influx through voltage-gated channels. Efferent arteriolar tone is largely independent of electromechanical events. Hence, defective electromechanical mechanisms in vascular smooth muscle should engender selective changes in preglomerular microvascular function, such as those evident during the early stage of DM. 3. Afferent arteriolar contractile responses to K(+)-induced depolarization and BAYK8644 are diminished 2 weeks after onset of DM in the rat. Similarly, depolarization-induced Ca(2+) influx and the resulting increase in intracellular [Ca(2+)] are abated in the preglomerular microvasculature of diabetic rats. The intracellular [Ca(2+)] response to depolarization is rapidly restored by normalization of extracellular glucose levels. These observations suggest that hyperglycaemia in DM impairs regulation of afferent arteriolar voltage-gated Ca(2+) channels. 4. Dysregulation of E(m) may also contribute to afferent arteriolar dilation in DM. Vasodilator responses to pharmacological opening of ATP-sensitive K(+) channels are exaggerated in afferent arterioles from diabetic rats. Moreover, blockade of these channels normalizes afferent arteriolar diameter in kidneys from diabetic rats. These observations suggest that increased functional availability and basal activation of ATP-sensitive K(+) channels promote afferent arteriolar dilation in DM. 5. We propose that dysregulation of E(m) (involving ATP- sensitive K(+) channels) and a diminished Ca(2+) influx response to depolarization (involving voltage-gated Ca(2+) channels) may act synergistically to promote preglomerular vasodilation during the early stage of DM.

Effects of efonidipine hydrochloride on renal arteriolar diameters in spontaneously hypertensive rats.

Efonidipine, a calcium antagonist, has been reported to dilate not only afferent but also efferent arterioles, thereby reducing glomerular hydrostatic pressure. We investigated the effect of chronic treatment with efonidipine or lisinopril on the afferent and efferent arteriolar diameters by the vascular cast technique. Four-week-old spontaneously hypertensive rats (SHR) were divided into three groups: untreated, efonidipine (25 mg/kg/day)-treated, and lisinopril (3 mg/kg/day)-treated. At 22 weeks of age, the renal vasculatures were fixed at the maximally dilated condition. The morphometrical measurements showed that the treatments with efonidipine and lisinopril caused structural alteration of the vasculature, resulting in significantly greater efferent arteriolar diameters than in untreated SHR. In addition, lisinopril-treated rats had wider afferent lumina. The renoprotective effect of efonidipine and lisinopril might be partly due to the structurally larger efferent arteriolar lumen.

Reduced reactivity of renal microvessels to pressure and angiotensin II in fawn-hooded rats.

Fawn-Hooded rats possess an increased risk to develop glomerular damage. Both an impaired control of preglomerular resistance and an elevated postglomerular resistance have been implicated. In the present study, we directly assessed the myogenic reactivity of distal interlobular arteries and afferent arterioles from hypertensive and normotensive Fawn-Hooded rats compared with Sprague-Dawley and Wistar rats, which are known to be resistant for developing renal disease. Pressure-response curves were made in isolated perfused hydronephrotic kidneys from these rats. In addition, increasing concentrations of angiotensin II were added to the perfusate to determine the reactivity of interlobular arteries, afferent arterioles, and efferent arterioles to this peptide. Preglomerular vessels from hypertensive and normotensive Fawn-Hooded rats exhibited an impaired reactivity to both pressure and angiotensin II compared with that of Sprague-Dawley and Wistar rats. Basal efferent arteriolar diameters were similar among the 4 strains of rat. In addition, efferent arterioles from hypertensive and normotensive Fawn-Hooded rats displayed a reduced sensitivity to angiotensin II. Our observations demonstrate that in Fawn-Hooded rats, 2 components of preglomerular resistance control are impaired: the myogenic and the angiotensin II response. In addition, efferent arteriolar reactivity to angiotensin II is not elevated but lowered in these rats. Therefore, a deficit in preglomerular resistance control is the most important intrinsic factor involved in the increased susceptibility of Fawn-hooded rats to develop renal disease.

Influence of Ca(2+)-activated K(+) channels on rat renal arteriolar responses to depolarizing agonists.

Experiments were performed to evaluate the hypothesis that opening of Ca(2+)-activated K(+) channels (BK(Ca) channels) promotes juxtamedullary arteriolar dilation and curtails constrictor responses to depolarizing agonists. Under baseline conditions, afferent and efferent arteriolar lumen diameters averaged 23.4 +/- 0.9 (n = 36) and 22.8 +/- 1.1 (n = 13) microm, respectively. The synthetic BK(Ca) channel opener NS-1619 evoked concentration-dependent afferent arteriolar dilation. BK(Ca) channel blockade (1 mM tetraethylammonium; TEA) decreased afferent diameter by 15 +/- 3% and prevented the dilator response to 30 microM NS-1619. ANG II (10 nM) decreased afferent arteriolar diameter by 44 +/- 4%, a response that was reduced by 30% during NS-1619 treatment; however, TEA failed to alter afferent constrictor responses to either ANG II or arginine vasopressin. Neither NS-1619 nor TEA altered agonist-induced constriction of the efferent arteriole. Thus, although the BK(Ca) channel agonist was able to curtail afferent (but not efferent) arteriolar constrictor responses to ANG II, BK(Ca) channel blockade did not allow exaggerated agonist-induced arteriolar constriction. These observations suggest that the BK(Ca) channels evident in afferent arteriolar smooth muscle do not provide a prominent physiological brake on agonist-induced constriction under our experimental conditions.