William J. Arendshorst

Activation of TRPV1 by Dietary Capsaicin Improves Endothelium-Dependent Vasorelaxation and Prevents Hypertension

Some plant-based diets lower the cardiometabolic risks and prevalence of hypertension. New evidence implies a role for the transient receptor potential vanilloid 1 (TRPV1) cation channel in the pathogenesis of cardiometabolic diseases. Little is known about impact of chronic TRPV1 activation on the regulation of vascular function and blood pressure. Here we report that chronic TRPV1 activation by dietary capsaicin increases the phosphorylation of protein kinase A (PKA) and eNOS and thus production of nitric oxide (NO) in endothelial cells, which is calcium dependent. TRPV1 activation by capsaicin enhances endothelium-dependent relaxation in wild-type mice, an effect absent in TRPV1-deficient mice. Long-term stimulation of TRPV1 can activate PKA, which contributes to increased eNOS phosphorylation, improves vasorelaxation, and lowers blood pressure in genetically hypertensive rats. We conclude that TRPV1 activation by dietary capsaicin improves endothelial function. TRPV1-mediated increase in NO production may represent a promising target for therapeutic intervention of hypertension.

Pathogenesis of acute renal failure following temporary renal ischemia in the rat.

In this study, we characterized the sequence of several intrarenal events and evaluated their relative importance in the pathogenesis of unilateral oliguric acute renal failure induced experimentally in rats by complete occlusion of a renal artery for 1 hour. Kidneys were studied prior to occlusion and 1–3 hours and 22–26 hours after release of the temporary occlusion. Renal blood flow measured by an electromagnetic flow transducer was reduced to 40-50percent of control during both postocclusion periods. Flow of tubular fluid was markedly reduced, and the damaged kidneys were oliguric. Proximal and distal convolutions were filled with fluid and dilated 1–3 hours after occlusion; their pressures were greatly heterogeneous and were elevated, on the average, to 31 and 16 mm Hg, respectively. Glomerular capillary pressure at this time was normal or slightly increased. Histological sections showed extensive tubular obstruction. We conclude that initially the oliguria is primarily due to intraluminal obstruction in the absence of predominant increases in preglomerular vascular resistance. Observations at 22–26 hours after occlusion indicated acute tubular necrosis. Moreover, the combined involvement of preglomerular vasoconstriction, persisting tubular obstruction, and passive backflow of tubular fluid appeared to be important in the maintenance of the oliguria. Glomerular capillary, proximal intratubular, and peritubular capillary hydrostatic pressures were reduced below control values. After acute volume expansion, the reduced pressures and renal blood flow were reversed, yet the experimental kidneys remained oliguric. Thus, it is clear that tubular obstruction is a significant factor responsible for both the genesis and the maintenance of oliguria in this experimental model of ischemia-induced acute renal failure.

Regulation of angiotensin II receptor AT1 subtypes in renal afferent arterioles during chronic changes in sodium diet.

Studies determined the effects of chronic changes in sodium diet on the expression, regulation, and function of different angiotensin II (ANG II) receptor subtypes in renal resistance vessels. Rats were fed low- or high-sodium diets for 3 wk before study. Receptor function was assessed in vivo by measuring transient renal blood flow responses to bolus injections of ANG II (2 ng) into the renal artery. ANG II produced less pronounced renal vasoconstriction in rats fed a low- compared with high-sodium diet (16% vs. 56% decrease in renal blood flow, P < 0.001). After acute blockade of ANG II formation by iv enalaprilat injection in sodium-restricted animals, ANG II produced a 40% decrease in renal blood flow, a level between untreated dietary groups and less than high salt diet. Intrarenal administration of angiotensin II receptor type 1 (AT1) receptor antagonists losartan or EXP-3174 simultaneously with ANG II caused dose-dependent inhibition of ANG II responses. Based on maximum vasoconstriction normalized to 100% ANG II effect in each group, AT1 receptor antagonists produced the same degree of blockade in all groups, with an apparent maximum of 80-90%. In contrast, similar doses of the angiotensin II receptor type 2 (AT2) receptor ligand CGP-42112 had only a weak inhibitory effect. In vitro equilibrium-saturation 125I-ANG II binding studies on freshly isolated afferent arterioles indicated that ANG II receptor density was lower in the low- vs. high-sodium animals (157 vs. 298 fmol/mg, P < 0.04); affinity was similar (0.65 nM). Losartan and EXP-3174 displaced up to 80-90% of the ANG II binding; fractional displacement was similar in both diet groups. In contrast, the AT2 receptor analogues PD-123319 and CGP-42112 at concentrations < 10(-6) M had no effect on ANG II binding. RT-PCR assays revealed the expression of both angiotensin II receptor type 1A (AT(1A)) and angiotensin II receptor type 1B (AT(1B)) subtypes in freshly isolated afferent arterioles, while there was very little AT2 receptor expression. Total AT1 receptor mRNA expression was suppressed by low sodium intake to 66% of control levels, whereas it was increased to 132% of control by high-sodium diet, as indicated by ribonuclease protection assay. Receptor regulation was associated with parallel changes in AT(1A) and AT(1B) expression; the AT(1A)/AT(1B) ratio was stable at 3.7. We conclude that AT1 receptors are the predominant ANG II receptor type in renal resistance vessels of 7-wk-old rats. Chronic changes in sodium intake caused parallel regulation of expression and amount of receptor protein of the two AT1 receptor genes that modulate receptor function and altered reactivity of renal vessels to ANG II.

Nephron Stop-Flow Pressure Response to Obstruction for 24 Hours in the Rat Kidney

Complete ureteral ligation of 24-h duration significantly reduced stop-flow and estimated glomerular capillary pressures in nephrons accessible to micropuncture in obstructed kidneys. In kidneys without ureteral obstruction, a similar response occurred in single tubules blocked for 24 h without affecting nearby unblocked tubules. Thus, the response to tubular obstruction occurs on an individual nephron basis and results from constriction of individual afferent arterioles. The mechanism leading to the response is unknown, but a feedback mechanism operating through the juxtaglomerular apparatus of individual nephrons is an attractive possibility.

Angiotensin AT1B receptor mediates calcium signaling in vascular smooth muscle cells of AT1A receptor-deficient mice.

Our studies on angiotensin II receptor subtype 1A (AT1A) knockout mice define how endogenous receptors other than AT1A receptors stimulate changes in cytosolic calcium concentration ([Ca2+]i) in cultured aortic vascular smooth muscle cells (VSMCs). Wild-type cells have a 1.7 ratio of AT1A/AT1B receptor mRNA as determined by semiquantitative reverse transcriptase-polymerase chain reaction. Mutant cells express AT1B receptor mRNA but not that for the AT1A receptor. In wild-type cells with AT1A present, Ang II (10(-7) mol/L) produces a characteristic rapid peak increase in [Ca2+]i of 150 to 180 nmol/L, followed by a plateau phase characterized by a sustained 70 to 80 nmol/L increase in [Ca2+]i. An unexpected finding was that the magnitude and time-dependent pattern of [Ca2+]i changes produced by Ang II were similar in cells that lacked AT1A receptors but possessed AT1B receptors. The response in mutant cells indicates effective coupling of an Ang II receptor to one or more second messenger systems. The similarity of response patterns between cells with and without AT1A receptors suggests that non-AT1A receptors are functionally linked to similar signal transduction pathways in mutant cells. The fact that mutant and wild-type cells exhibit similar patterns of calcium mobilization and entry supports the notion that AT1A and non-AT1A receptors share common signal transduction pathways. The AT2 receptor ligands PD-123319 and CGP-42112 do not alter Ang II effects in either VSMC type, suggesting a paucity of AT2 receptors and/or an absence of their linkage to [Ca2+]i pathways. The nonpeptide AT1 receptor blocker losartan antagonizes Ang II-induced [Ca2+]i increases in both cell groups, supporting mediation by native AT1B receptors and effective coupling of this subtype to second messenger systems leading to calcium entry and mobilization. Our results demonstrate that Ang II causes calcium signaling in AT1A-deficient VSMCs that is mediated by an endogenous losartan-sensitive AT1B receptor.

Oligoclonal CD8+ T Cells Play a Critical Role in the Development of Hypertension

Recent studies have emphasized a role of adaptive immunity, and particularly T cells, in the genesis of hypertension. We sought to determine the T-cell subtypes that contribute to hypertension and renal inflammation in angiotensin II–induced hypertension. Using T-cell receptor spectratyping to examine T-cell receptor usage, we demonstrated that CD8+ cells, but not CD4+ cells, in the kidney exhibited altered T-cell receptor transcript lengths in V&bgr;3, 8.1, and 17 families in response to angiotensin II–induced hypertension. Clonality was not observed in other organs. The hypertension caused by angiotensin II in CD4−/− and MHCII−/− mice was similar to that observed in wild-type mice, whereas CD8−/− mice and OT1xRAG-1−/− mice, which have only 1 T-cell receptor, exhibited a blunted hypertensive response to angiotensin II. Adoptive transfer of pan T cells and CD8+ T cells but not CD4+/CD25− cells conferred hypertension to RAG-1−/− mice. In contrast, transfer of CD4+/CD25+ cells to wild-type mice receiving angiotensin II decreased blood pressure. Mice treated with angiotensin II exhibited increased numbers of kidney CD4+ and CD8+ T cells. In response to a sodium/volume challenge, wild-type and CD4−/− mice infused with angiotensin II retained water and sodium, whereas CD8−/− mice did not. CD8−/− mice were also protected against angiotensin-induced endothelial dysfunction and vascular remodeling in the kidney. These data suggest that in the development of hypertension, an oligoclonal population of CD8+ cells accumulates in the kidney and likely contributes to hypertension by contributing to sodium and volume retention and vascular rarefaction.

Nitric oxide blunts myogenic autoregulation in rat renal but not skeletal muscle circulation via tubuloglomerular feedback.

This rat renal blood flow (RBF) study quantified the impact of nitric oxide synthase (NOS) inhibition on the myogenic response and the balance of autoregulatory mechanisms in the time domain following a 20 mmHg‐step increase or decrease in renal arterial pressure (RAP). When RAP was increased, the myogenic component of renal vascular resistance (RVR) rapidly rose within the initial 7–10 s, exhibiting an ∼5 s time constant and providing ∼36% of perfect autoregulation. A secondary rise between 10 and 40 s brought RVR to 95% total autoregulatory efficiency, reflecting tubuloglomerular feedback (TGF) and possibly one or two additional mechanisms. The kinetics were similar after the RAP decrease. Inhibition of NOS (by l‐NAME) increased RAP, enhanced the strength (79% autoregulation) and doubled the speed of the myogenic response, and promoted the emergence of RVR oscillations (∼0.2 Hz); the strength (52%) was lower at control RAP. An equi‐pressor dose of angiotensin II had no effect on myogenic or total autoregulation. Inhibition of TGF (by furosemide) abolished the l‐NAME effect on the myogenic response. RVR responses during furosemide treatment, assuming complete inhibition of TGF, suggest a third mechanism that contributes 10–20% and is independent of TGF, slower than the myogenic response, and abolished by NOS inhibition. The hindlimb circulation displayed a solitary myogenic response similar to the kidney (35% autoregulation) that was not enhanced by l‐NAME. We conclude that NO normally restrains the strength and speed of the myogenic response in RBF but not hindlimb autoregulation, an action dependent on TGF, thereby allowing more and slow RAP fluctuations to reach glomerular capillaries.

Nitric Oxide/cAMP Interactions in the Control of Rat Renal Vascular Resistance

This study aimed to characterize the interaction between nitric oxide (NO)- and cAMP-related pathways in the control of renal blood flow. Using the isolated perfused rat kidney model, we determined the effects of inhibition of NO formation by Nomega-nitro-L-arginine methyl ester (L-NAME; 1 mmol/L) and of NO administration by sodium nitroprusside (SNP, 10 micromol/L) on renal vascular resistance under conditions of elevated vascular cAMP levels. cAMP levels were increased either by adenylate cyclase activation via isoproterenol or by inhibition of cAMP phosphodiesterases (PDEs) 1, 3, and 4. We found that L-NAME markedly increased vascular resistance and that this effect was completely reversed by SNP. Both isoproterenol and inhibitors of the cAMP PDEs lowered basal vascular resistance. In the presence of isoproterenol (3 nmol/L) and inhibitors of PDE-1 [8-methoxymethyl-l-methyl-3-(2-methylpropyl)-xanthine; 8-MM-IBMX, 20 micromol/L] and PDE-4 (rolipram, 20 micromol/L), L-NAME again substantially increased vascular resistance, and this effect of L-NAME was completely reversed by SNP. In the presence of the PDE-3 inhibitors milrinone (20 micromol/L) and trequinsin (200 nmol/L), however, both L-NAME and SNP failed to exert any additional effects. Because PDE-3 is a cGMP-inhibited cAMP PDE and because the vasodilatory effect of SNP was abrogated by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) (20 micromol/L), our findings are compatible with the idea that an action of NO on PDE-3 could account for the vasodilatory properties of NO on the renal vasculature. Moreover, our findings suggest that PDE-3 activity is an important determinant of renal vascular resistance.

Autoregulation of renal blood flow in spontaneously hypertensive rats.

The relationship between renal blood flow (RBF) and arterial pressure was determined in nine anesthetized, spontaneously hypertensive rats (SHR) of the Okamoto-Aoki strain and seven Wistar-Kyoto rats (WKY). The rats had similar body weights and the average age was 14-15 weeks. Measured in vivo with a noncannulating electromagnetic flow transducer, RBF was quite stable in both groups in the pressure range of 165 to 105 mm Hg; renal vascular resistance (RVR) was directly related to pressure. Within this pressure range, RBF was approximately 6.7 ml/min in SHR and 7.3 ml/ min in WKY. When perfusion pressure was reduced from 105 to 65 mm Hg, RBF decreased progressively, roughly in proportion to pressure. RBF and RVR did not differ significantly in SHR and WKY at each level of pressure. An even more striking resemblance in the overall pressure-flow relationships was evident after RBF and RVR in each experiment were normalized to their respective values at the pressure of 115 mm Hg. The observations demonstrate that SHR autoregulate RBF in vivo as efficiently as WKY, and that the sensitivity and reactivity of the renal vasculature to acute alterations in perfusion pressure are not modified appreciably in these SHR, The increased RVR in 15-week-old SHR can be explained as an appropriate autoregulatory response of the vasculature to the elevated arterial pressure. Circ Res 44: 3-44-349, 1979

Thromboxane Receptor Mediates Renal Vasoconstriction and Contributes to Acute Renal Failure in Endotoxemic Mice

Sepsis is a major cause of acute renal failure (ARF) and death. Thromboxane A2 (TxA(2)) may mediate decreases of renal blood flow (RBF) and/or GFR associated with LPS-induced sepsis. This study tested whether TxA(2) receptor blockade, with the use of TxA(2) receptor knockout (TP-KO) mice or a selective TP receptor antagonist (SQ29,548), would alleviate LPS-induced renal vasoconstriction and ARF. Under basal conditions, anesthetized TP-KO mice displayed a lower mean arterial pressure than wild-type (WT) mice (102 versus 94 mmHg; P < 0.05). RBF, renal vascular resistance (RVR), GFR, and urine flow did not differ among groups under basal conditions, suggesting little tonic influence of TxA(2) on renal TP receptors in health. In endotoxemic WT mice, 14 h after LPS (Escherichia coli LPS 8.5 mg/kg intraperitoneally), mean arterial pressure was reduced to 85 mmHg (P < 0.001), as were RBF (5.0 versus 9.3 ml/min per g kidney wt; P < 0.001) and GFR (0.38 versus 1.03 ml/min per g kidney wt; P < 0.001). Heart rate and RVR (71 versus 47 mmHg/ml per min; P < 0.05) increased. The decreases in RBF and GFR after LPS were attenuated in TP-KO mice versus WT mice (both P < 0.05). In both TP-KO and TP antagonist-treated mice, RVR remained stable in response to LPS versus WT mice that did not receive LPS. Delayed TP-antagonist treatment (12 h after LPS injection) ameliorated RBF and RVR but did not restore GFR. In other WT animals, TP-antagonist treatment for 2 h before intravenous LPS abolished the early renal vasoconstriction and alleviated the decrease in GFR. These results demonstrate that renal vasoconstriction during endotoxemic shock induced by LPS is mediated by TP receptors as indicated by pharmacologic blockade and genetic disruption of TP receptors.