Biomedical subjects

W J Arendshorst

Publications and source records attributed to W J Arendshorst.

At least 37 records · Page 2Linked to original sources

PubMed1999

Actions of angiotensin II on the renal microvasculature.

Angiotensin II (AngII) exerts powerful effects on the renal microcirculation to influence a variety of functions. This review summarizes some of the major findings over the past 10 years as they elucidate the multiple roles that AngII plays in the regulation of whole kidney blood flow, perfusion of cortical and medullary regions, and renal autoregulation. Topics of discussion include localization of AngII receptor types and subtypes in the renal vasculature, action of AngII on vascular smooth muscle cells of the afferent and efferent arterioles, and intracellular signaling pathways. Within the microvasculature, AngII causes potent constriction in both the afferent and efferent arterioles, with responses modulated by paracrine and autocrine factors of endothelial and macula densa origins. With regard to renal autoregulatory mechanisms consisting of the myogenic response and the tubuloglomerular feedback mechanism, the myogenic response appears to operate independent of the renin-angiotensin system. On the other hand, tubuloglomerular feedback activity is often directly proportional to concentrations of AngII, especially in high renin states. Of the two types defined to date, the AT1 is the predominant receptor in the adult rat kidney mediating the vascular effects of AngII. AT2 receptor is highly expressed in the fetal kidney and is important for renal development, but is very weakly expressed in adult animals. Nevertheless, AT2 receptors may mediate vasodilation under certain conditions. The signaling transduction pathways for AT1 receptors include Gq/11-protein and protein kinase C activation. AngII causes constriction of the afferent arteriole primarily by stimulation of calcium entry via voltage-sensitive, L-type channels, whereas AngII effects on the efferent arteriole are due to calcium release from intracellular stores and calcium entry through voltage-independent calcium entry channels. Future experiments should contribute to a more in-depth understanding of the modulation of AngII effects by other vasoactive agents and interactions between different second-messenger signaling pathways in health and disease.

Angiotensin II↗

PubMed1999

Effects of candesartan on angiotensin II-induced renal vasoconstriction in rats and mice.

This study determined the inhibitory effect of the angiotensin II (AngII) type I (AT1) receptor blocker candesartan on renal vascular reactivity in vivo. Reactivity to AngII before and during candesartan administration was assessed by measuring (by electromagnetic or ultrasonic flowmetry) renal blood flow responses to AngII in rats and mice. AngII produced greater renal vasoconstriction in 7-wk-old, spontaneously hypertensive rats than in Wistar-Kyoto rats. After indomethacin treatment, AngII (2 ng) produced 40% reductions in renal blood flow in both rat strains, without affecting systemic arterial pressure. Coadministration of candesartan blocked AngII effects in a dose-dependent manner, with similar levels of inhibition in spontaneously hypertensive rats and Wistar-Kyoto rats; maximal inhibition was 80%. In rats that had been pretreated (for 30 min) with intravenous candesartan, AngII-induced renal vasoconstriction was inhibited dose dependently up to 98%. To evaluate receptor subtype mediation, responses were compared in mice with or without the AT1A receptor (deleted by gene targeting). Intrarenal AngII (1 ng) caused a 32% reduction of renal blood flow in wild-type mice and an 8% reduction of renal blood flow in AT1A receptor-knockout mice. Ten nanograms of AngII were required to elicit 20% renal vasoconstriction in these mutant mice. Concurrent injection of candesartan caused dose-dependent inhibition of AngII up to 80%. The candesartan IC50 values for percentage changes in renal blood flow did not differ in the two groups of mice. These studies establish that candesartan is an effective, highly selective, AT1 receptor blocker, inhibiting renal vasoconstriction in rodents in a concentration- and time-dependent manner. Candesartan effectively blocks AT1A and AT1B receptors in renal resistance vessels of rodents, with similar efficacies in rats and mice.

Angiotensin II↗

PubMed1999

Candesartan normalizes exaggerated tubuloglomerular feedback activity in young spontaneously hypertensive rats.

This study examines the effect of systemic blockade of angiotensin II AT1 receptors by candesartan on the exaggerated tubuloglomerular feedback (TGF) activity in 7-wk-old, euvolemic spontaneously hypertensive rats (SHR) and in Wistar-Kyoto rats (WKY). TGF activity was assessed by stop-flow pressure (SFP) and early proximal flow rate (EPFR) measurements during perfusion of Henle's loop. During the control period, SHR exhibited a greater maximal SFP response (19 versus 0.11 mmHg), and a lower tubular flow rate elicited half-maximal response (turning point) (12.7 versus 14.1 nl/min). In addition, EPFR at a high perfusion rate (40 nl/min) was lower in SHR, indicating exaggerated TGF activity. Blockade of AT1 receptors was achieved by intravenous injection of 0.05 mg/kg candesartan, which did not affect mean arterial pressure. Renal blood flow and mean arterial pressure responses to injections of angiotensin II were blocked by >95%. Maximum SFP response in SHR decreased to 11 mmHg, and turning point increased to 16.5 nl/min. The slope of the TGF response curve at the half-maximal SFP response (reactivity) decreased from -5.5 to -2.0 mmHg/nl per min. In contrast, maximum SFP response and TGF reactivity were unchanged by AT1 receptor blockade in euvolemic WKY. A small effect was noted as an increase in turning point to 18.0 nl/min after candesartan treatment. Thus, the exaggerated TGF activity in young SHR is markedly attenuated by systemic administration of candesartan, whereas TGF was basically unchanged in euvolemic WKY. These results demonstrate that angiotensin II plays an important role in the enhanced TGF activity observed in young SHR. Significant TGF activity, essentially at normal levels for euvolemic animals, persists during AT1 receptor blockade in both groups of rats.

Angiotensin Receptor Antagonists↗

PubMed1998

ANG II and vasopressin stimulate calcium entry in dispersed smooth muscle cells of preglomerular arterioles.

Calcium signaling mechanisms were examined in vessel segments and dispersed single smooth muscle cells (SMC) of interlobular arteries and afferent arterioles (< 50 microns diameter) from the rat kidney. These resistance vessels were isolated from rat kidneys, using an iron oxide-sieving technique with subsequent collagenase digestion. Individual cells were identified by their characteristic oval appearance and positive staining for smooth muscle-specific alpha-actin and heavy chain myosin SM-1 and SM-2. Cytosolic calcium concentration ([Ca2+]i) was measured using fura 2 ratiometric fluorescence at 340 and 380 nm wavelength with a microscope-based photometer. Angiotensin II (ANG II) and arginine vasopressin (AVP), at concentrations of 10(-10)-10(-6) M, produced dose-dependent increases in [Ca2+]i; maximum increases were 221 +/- 49 nM for ANG II and 237 +/- 49 nM for AVP. The temporal response patterns for both agonists were characterized by a square-shaped, immediate step increase in [Ca2+]i to a near maximum level that was maintained through the recording period of 150-200 s. Responses of individual dispersed SMC and short vessel segments were similar. Losartan antagonized the action of ANG II, indicating mediation by AT1 receptors on preglomerular arteriolar SMC. The V1-selective antagonist [d(CH2)5Tyr(Me)2Tyr(NH2)9]AVP completely inhibited AVP-induced [Ca2+]i changes. The importance of calcium entry in hormone-induced changes in [Ca2+]i was demonstrated by the finding that neither ANG II nor AVP elicited a [Ca2+]i response in media rendered nominally calcium free by addition of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Calcium entry occurred primarily through L-type, voltage-gated calcium channels as the dihydropyridine, nifedipine, completely prevented or reversed [Ca2+]i changes normally elicited by either hormone. Our results provide new information about the similarity of calcium signaling in single SMC and short segments freshly isolated from renal interlobular arteries and afferent arterioles. The observations indicate that AT1 and V1 receptors are coupled to signal transduction pathways leading to rapid changes in [Ca2+]i. Calcium mobilization appears to play a minor to nonexistent role under the experimental conditions. The predominant mechanism involves calcium entry through dihydropyridine-sensitive, voltage-gated calcium channels in single SMC from these resistance vessels.

Angiotensin II↗

PubMed1998

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.

Angiotensin II↗

PubMed1997

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.

Actins↗

PubMed1997

Angiotensin II responses in AT1A receptor-deficient mice: a role for AT1B receptors in blood pressure regulation.

Most of the classic functions of the renin-angiotensin system are mediated by type 1 (AT1) angiotensin receptors, of which two subtypes, AT1A and AT1B, have been identified. However, distinct functions for these two AT1 receptors have been difficult to separate. We examined the pressor effects of angiotensin II in Agtr1A -/- mice, which lack AT1A receptors. In enalapril-pretreated Agtr1A -/- mice, angiotensin II caused significant and dose-proportional increases in mean arterial pressure. This pressor response was not blocked by pretreatment with sympatholytic agents but was completely inhibited by the AT1-receptor antagonists, losartan and candesartan, suggesting that it is directly mediated by AT1B receptors. Chronic treatment of Agtr1A -/- mice with losartan reduced systolic blood pressure from 80 +/- 5 to 72 +/- 4 mmHg (P < 0.04), suggesting a role for AT1B receptors in chronic blood pressure regulation. These studies provide the first demonstration of in vivo pressor effects mediated by AT1B receptors and demonstrate that, when AT1A receptors are absent, the AT1B receptor contributes to the regulation of resting blood pressure.

Angiotensin II↗

PubMed1997

Calcium signaling mechanisms in renal vascular responses to vasopressin in genetic hypertension.

Previous blood flow studies demonstrated that arginine vasopressin (AVP) produces exaggerated renal vasoconstriction in young spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto control rats (WKY). The purpose of the present study was to determine the role of postreceptor calcium signaling pathways in AVP-induced renal vasoconstriction in vivo. Renal blood flow (RBF) was measured by electromagnetic flowmetry in anesthetized, water-loaded, 8-week-old WKY and SHR pretreated with indomethacin to avoid interactions with prostaglandins. AVP was injected into the renal artery to produce a transient 25% to 30% decrease in RBF without affecting arterial pressure. To achieve similar control levels of vasoconstriction, SHR received a lower dose (2 versus 5 ng). Coadministration of nifedipine with AVP produced dose-dependent inhibition of the AVP-induced renal vasoconstriction. Nifedipine exerted maximum inhibition by blocking 30% to 35% of the peak AVP response, indicating the involvement of dihydropyridine-sensitive voltage-dependent calcium channels. To evaluate intracellular calcium mobilization, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) or heparin was coadministered with AVP. Each agent produced a dose-dependent inhibition of up to 65% of the maximum blood flow change produced by AVP. The degrees of inhibition produced by maximum effective doses of nifedipine and TMB-8 were additive; the combination blocked up to 85% of the response to AVP. These observations indicate that about one third of the AVP-induced constriction of renal resistance vessels is mediated by voltage-dependent L-type calcium channels responsive to the dihydropyridine nifedipine. Approximately two thirds of the change in vascular tone is due to inositol 1,4,5-trisphosphate-mediated calcium mobilization from intracellular sources sensitive to TMB-8 and heparin. The results suggest that the exaggerated renal vascular reactivity to AVP challenge in SHR is probably not due to a strain difference in postreceptor calcium signal transduction. After AVP receptor stimulation, calcium mobilization and calcium entry signaling pathways participate to similar degrees in WKY and SHR.

Animals↗

PubMed1996

Oxygen modulates alpha 1B-adrenergic receptor gene expression by arterial but not venous vascular smooth muscle.

Blood and tissue O2 levels are major determinants of short-term autoregulatory adjustments in vascular smooth muscle cell (SMC) tension and may effect long-term alterations in SMC catecholamine responsiveness. We examined the hypothesis that prolonged hypoxia altered gene expression of alpha 1-adrenoceptors. After exposure of cultured aortic (in vitro) SMC to 3% O2 for 8 h, alpha 1B mRNA increased to 523% (P = 0.02) of control cells (21% O2) and to 205% (P = 0.04) in in situ organ-cultured aortic SMC. In vivo hypoxic hypoxia (10% inspired O2) similarly increased aortic SMC alpha 1B mRNA 180% (P = 0.02). In contrast, alpha 1D, alpha-actin and beta-actin mRNA levels were not changed in aortic SMC by low O2 in the in vitro, in situ, or in vivo models. Unlike aortic SMC, vena caval SMC alpha 1B mRNA expression did not change with low-O2 exposure in vitro or in vivo, nor did alpha 1D, alpha-actin or beta-actin mRNA. Aortic SMC alpha 1B transcription rate increased 360% (P = 0.02), whereas alpha 1D, alpha-actin, and beta-actin transcription was unchanged. Neither alpha 1B nor alpha 1D mRNA stability was altered by low-O2 exposure. Total alpha 1-adrenoceptor density ([3H]prazosin binding) increased 12% (P = 0.04) after 24 h of 3% O2. This was associated with a 200% increase (P < 0.01) in the chloroethylclonidine (CEC)-sensitive alpha 1-adrenoceptor population and no change in CEC-insensitive alpha 1-adrenoceptor density. Exposure of aortic SMC to 24 h of 3% O2 increased the maximum response of norepinephrine-evoked elevations in intracellular Ca2+ as measured using fura 2. Low O2 did not change responses to another G protein-coupled receptor, angiotensin II. These data suggest that reduced O2, during prolonged hypoxemia or tissue ischemia, may selectively increase expression of functionally coupled alpha 1B-adrenoceptors in arterial blood vessels.

Actins↗

PubMed1996

Calcium entry and mobilization signaling pathways in ANG II-induced renal vasoconstriction in vivo.

The purpose of this study was to determine the relative importance of calcium signaling pathways in angiotensin II (ANG II)-induced renal vasoconstriction in vivo. Nifedipine was used to antagonize dihydropyridine-sensitive voltage-dependent calcium channels; BAY K 8644 was employed to activate these calcium channels. Intracellular calcium mobilization was evaluated using TMB-8 or heparin to inhibit calcium release from sarcoplasmic reticulum. Renal blood flow was measured by electromagnetic flowmetry in anesthetized euvolemic Wistar-Kyoto rats. The animals were pretreated with indomethacin to avoid interactions with prostaglandins. ANG II (2 ng) or BAY K 8644 (1 microgram) was injected into the renal artery to produce a transient 30-50% decrease in renal blood flow without affecting arterial pressure. Coadministration of nifedipine with BAY K 8644 produced dose-dependent inhibition of the maximum renal vasoconstriction elicited by BAY K 8644. The calcium-channel antagonist had similar effects on ANG II-induced renal vasoconstriction. Nifedipine exerted maximum inhibition by blocking 50% of the peak ANG II response. To evaluate intracellular calcium mobilization, TMB-8 or heparin was coadministered with ANG II. Each agent produced dose-dependent inhibition of up to 50% of the maximum renal vasoconstriction produced by ANG II. The inhibitory effects of nifedipine and TMB-8 were additive; neither agent had an effect when ANG II AT1 receptors were antagonized with losartan. These observations indicate that one-half of the ANG II-induced constriction of renal resistance vessels is mediated by voltage-dependent L-type calcium channels responsive to the dihydropyridine nifedipine. The remaining 50% of the renal vasoconstriction elicited by ANG II is mediated by inositol 1,4,5-trisphosphate-mediated calcium mobilization from intracellular sources. The additive nature of the inhibitory effects indicates distinct mechanisms involving calcium mobilization and calcium entry signaling pathways that are of equal importance in ANG II activation of AT1 receptors to trigger constriction of renal resistance vessels under basal conditions.

Angiotensin II↗

PubMed1996

Exaggerated tubuloglomerular feedback activity in genetic hypertension is mediated by ANG II and AT1 receptors.

The purpose of the present study was to determine the role of endogenous angiotensin II in exaggerated tubuloglomerular feedback (TGF) in young euvolemic spontaneously hypertensive rats (SHR). TGF was characterized by measuring proximal tubular stop-flow pressure (Psf) responses to loop of Henle perfusion before and during losartan infusion in 7-wk-old SHR and Wistar-Kyoto rats (WKY). In the control period, TGF responses were exaggerated in SHR compared with WKY. This was evidenced by a larger flow-induced maximum decrease in Psf (19 vs. 13 mmHg), lower turning point (8 vs.12 nl/min), and higher reactivity (-6.4 vs. -3.0 mmHg.nl-1.min-1) in SHR. Losartan (DuP-753) was infused into the renal artery to antagonize angiotensin AT1 receptors in the experimental period. This was verified by losartan inhibiting > 90% of the decrease in whole kidney and superficial cortical blood flow produced by exogenous angiotensin II in both strains. Losartan infusion significantly attenuated TGF activity in SHR but not in WKY. In SHR losartan reduced the maximum Psf response (from 19 to 10 mmHg) and increased the turning point (from 8 to 11 nl/min). SHR values during losartan administration were similar to those obtained in WKY. WKY values were unaffected by losartan. The lack of change in maximum TGF responses after losartan treatment was not unique to WKY, inasmuch as similar results were obtained in euvolemic Munich-Wistar rats (-2.0 +/- 0.7 and -1.1 +/- 1.0 mmHg vs. -8.4 +/- 0.7 mmHg in SHR). Thus angiotensin II does not appear to play an essential role in basal TGF activity during euvolemia in normotensive animals when there is minimal stimulation of the renin-angiotensin system. In contrast, our observations indicate that the exaggerated TGF in young euvolemic SHR represents a functional resetting that is dependent on angiotensin II and losartan-sensitive AT1 receptors during the development of genetic hypertension.

Angiotensin II↗

PubMed1996

Role of protein kinase C in angiotensin II-induced renal vasoconstriction in genetically hypertensive rats.

The renal vasculature of young spontaneously hypertensive rats (SHR) responds to angiotensin II (ANG II) with exaggerated vasoconstriction, due in part to defective buffering by the adenosine 3',5'-cyclic monophosphate (cAMP) pathway. In vitro studies suggest greater activation of phospholipase C and protein kinase C (PKC) in cultured mesangial cells and vascular smooth muscle cells. The present studies evaluated the role of PKC activation in renal vascular responses to ANG II receptor activation and the relative contributions in SHR vs. Wistar-Kyoto control rats (WKY). Renal blood flow was measured in 8-wk-old anesthetized SHR and WKY pretreated with indomethacin. ANG II (2 ng) injection into the renal artery produced a transient 45-50% maximum reduction of renal blood flow in both rat strains. Intrarenal infusion of either staurosporine or chelerythrine into the renal artery effectively attenuated the vasoconstriction elicited by ANG II in a dose-dependent manner, with maximum inhibition of 60-70%. The PKC inhibitory effects were significant and independent of strain. Coadministration of the PKC inhibitors produced maximal inhibition similar to that observed with one agent, suggesting action via a common pathway. In other studies, the linkage of the PKC pathway to the AT1 receptor was evaluated using sub and maximal doses of losartan to antagonize 50-80% of ANG II-induced vasoconstriction. The same degree of inhibition was observed when a PKC inhibitor was coadministered with losartan. These findings support the views that the PKC system is a major intracellular signaling pathway coupled to the AT1 receptor in renal resistance vessels and that PKC activation is involved to similar degrees in the renal vasoconstriction elicited by ANG II in young WKY and SHR. Exaggerated vascular reactivity to vasoconstrictor agents in genetically hypertensive animals is probably due to a defect in cAMP generation in the presence of a normally operating PKC pathway.

Alkaloids↗

PubMed1996

Enhanced renal vasoconstriction induced by vasopressin in SHR is mediated by V1 receptors.

The development of hypertension in the spontaneously hypertensive rat (SHR) is associated with renal dysfunction; the observed renal vasoconstriction may reflect an imbalance of constrictor and dilator systems. The present studies evaluated renal vascular reactivity to arginine vasopressin (AVP) and mediation by V1 and/or V2 receptors. Renal blood flow (electromagnetic flowmetry) was measured in water-loaded, 8-wk-old SHR, Wistar-Kyoto rats (WKY), and Munich-Wistar rats. Injection of AVP (2 and 5 ng) into the renal artery caused dose-dependent renal vasoconstriction. The maximum blood flow response was approximately twofold larger in SHR than both normotensive strains. The strain difference was largely unaffected by indomethacin administration, although the reduction in blood flow produced by 5 ng AVP was 4-6% larger in both SHR and WKY during cyclooxygenase inhibition. The V1 receptor antagonist, [D-(CH2)5,Tyr(Me)2,Tyr(NH2)9]Arg8-vasopressin, blocked up to 90% of the renal vasoconstriction elicited by AVP. Intrarenal injection of the V1-receptor agonist [Phe2,Ile3,Org8]vasopressin produced renal hemodynamic effects similar to AVP; this agonist reduced renal blood flow, with twofold larger responses in SHR (-40 vs. -18% for 10 ng). In contrast, similar doses of the V2-receptor agonist 1-desamino-8-D-arginine vasopressin had no effect. These results indicate that AVP-induced vasoconstriction is mediated predominantly by the V1 receptor in the rat kidney. The enhanced vascular reactivity in 8-wk-old SHR may reflect an increased V1 receptor density and/or affinity or postreceptor signaling pathways, largely independent of buffering by the vascular V2 receptor or vasodilator prostaglandin activity. The strain difference in the vascular response to AVP may contribute to the renal vasoconstriction observed during the development of genetic hypertension.

Animals↗

PubMed1996

Angiotensin II-receptor stimulation of cytosolic calcium concentration in cultured renal resistance arterioles.

This study provides an initial characterization of basic morphological properties of cultures of vascular smooth muscle cells (VSMC) from rat preglomerular resistance vessels and of the functional coupling of angiotensin II (ANG II) receptors to cytosolic free calcium concentration ([Ca2+]i (fura 2 fluorescence photometry). Renal VSMC were isolated from interlobular arteries and afferent arterioles (< 50 microns) using an iron oxide sieving method and compared with rat aortic VSMC cultured under similar conditions. Quiescent monolayers maintained uniform morphology and [Ca2+]i signaling profile between passages 3 and 10. Arteriolar and aortic VSMC were spindle shaped and expressed smooth muscle-specific alpha-actin and myosin heavy chains SM-1 and SM-2. ANG II caused a rapid increase in [Ca2+]i, followed by a sustained plateau phase at 50-60% of the peak value. The initial maximum [Ca2+]i responses were dose dependent and of similar magnitude in renal arteriolar and aortic VSMC. ANG II (10(-7) M) increased [Ca2+]i from 50 to 240 nM in arteriolar and from 57 to 201 nM in aortic VSMC (P < 0.001 for both). Inhibition of ANG II effects on [Ca2+]i revealed significant signaling through distinct AT-receptor subtypes (losartan and PD-123319 sensitive) in renal arteriolar VSMC. In contrast, only losartan was effective in aortic VSMC. The AT2-receptor ligand CGP-42112 had no effect in either vessel type. Our results demonstrate that cultured arteriolar VSMC have anatomical similarities to aortic VSMC and functional differences in AT-receptor signaling in response to ANG II. This novel preparation should provide a useful approach with which to investigate cellular mechanisms concerning receptor coupling to signaling pathways involved in vascular reactivity of arteriolar VSMC in the microcirculation in general and the kidney in particular.

Animals↗

PubMed1995

Defective G protein activation of the cAMP pathway in rat kidney during genetic hypertension.

The development of hypertension in the spontaneously hypertensive rat (SHR) is associated with renal dysfunction and vasoconstriction. The kidneys of young SHRs exhibit exaggerated reactivity to angiotensin II (Ang-II) and attenuated responses to vasodilators that normally activate the cAMP signal to buffer hormone-induced vasoconstriction. The present study investigates the mechanism(s) responsible for this abnormality in activation of the cAMP second-messenger pathway in hypertensive animals. Renal vascular reactivity was assessed in 7-week-old anesthetized SHRs and normotensive Wistar-Kyoto rats. The animals were pretreated with indomethacin to block prostanoid production throughout an experiment. Ang-II was injected into the renal artery either alone or mixed with the vasodilator fenoldopam, a dopamine-receptor agonist. These two opposing vasoactive agents were administered before and during intrarenal infusion of NaF or cholera toxin, two activators of G proteins that stimulate cAMP production. The results show that Ang-II reduced renal blood flow by 45% in both strains. In Wistar-Kyoto rats, fenoldopam reduced the Ang-II-induced decrease in renal blood flow from -45% to -30%. This protective effect of fenoldopam was increased further during infusion of NaF or cholera toxin (-18% or -19% decrease in renal blood flow). In SHRs, fenoldopam failed to attenuate Ang II-mediated vasoconstriction (-45% vs. -44%). In contrast, fenoldopam effectively blunted the Ang-II-induced vasoconstriction when it was given concurrently with NaF or cholera toxin (-27 or -31% decrease in renal blood flow). These findings provide evidence for defective interaction between receptor coupling and activation of guanine nucleotide stimulatory factor proteins in the renal microcirculation of 7-week-old SHRs. Such a deficiency could play an important role in renal dysfunction associated with the development of genetic hypertension.

Angiotensin II↗

PubMed1994

Angiotensin II receptors and renin release in rat glomerular afferent arterioles.

The purpose of recent studies was to investigate the expression of angiotensin II (Ang II) receptor sites in afferent arterioles freshly isolated from the rat kidney, and the role of Ang II on renin release by these vessels. The method of isolation and purification of renal microvessels was based on iron oxide infusion into the kidneys and separation of the afferent arterioles from glomeruli and connective tissue with the aid of a magnetic field, successive passages through various sieves, and harvesting with collagenase. Ang II receptor characteristics were evaluated by radioligand binding studies using the non-peptide Ang II antagonists of AT1 (Dup-753 and -532) and AT2 (PD-123319 and CGP-42112) receptors. AT1 antagonists displaced up to 80% of the Ang II binding with high affinity (3 nM), whereas the remaining 20% showed low affinity for the Dup compounds and CGP-42112 (> 10 microM), and intermediate affinity for PD-123319 (12 microM). These data suggest the existence of two Ang II receptor subtypes in the renal vasculature of the rat. In separate experiments, renin release by isolated afferent arterioles in vitro was 9 ng/hr/mg under control conditions. Ang II (0.1 microM) inhibited renin secretion by 20%, whereas the adenylyl cyclase activator forskolin (10 microM) stimulated renin secretion by 50%. In arterioles isolated from rats chronically treated with a converting enzyme inhibitor (perindoprilate) to reduce endogenous formation of Ang II, renin release increased 20-fold under control conditions in vitro and was further stimulated by forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II↗

PubMed1993

Vascular interactions of prostaglandins with thromboxane in kidneys of rats developing hypertension.

We investigated the ability of the vasodilator prostaglandins E2 (PGE2) and I2 (PGI2) to counterbalance the vasoconstrictor action of thromboxane A2 (TxA2) in the rat renal vasculature during hypertension. In vivo measurements of renal blood flow (RBF) were made in 6-wk-old anesthetized spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats pretreated with indomethacin. The stable TxA2 agonist U-46619 was injected into the renal artery, and the magnitude and the kinetic parameters of the renal response were evaluated before and after continuous intrarenal infusion of a low dose of PGE2, viprostol (PGE2 analogue), PGI2, and iloprost (PGI2 analogue). The selected dose of vasodilator PGs did not affect arterial pressure and RBF. In the control period, the TxA2 agonist reduced RBF by 30% with a 90-s half time of recovery in both strains. Infusion of vasodilator PGs in young WKY significantly blunted the maximum vasoconstrictor effect of the TxA2 agonist and facilitated the recovery from vasoconstriction. In marked contrast, infusion of the vasodilator PGs in young SHR failed to affect the magnitude of the vasoconstrictor effect of the TxA2 agonist, although the recovery from vasoconstriction was facilitated as in WKY. On the other hand, infusion of bradykinin or dibutyryladenosine 3',5'-cyclic monophosphate (dibutyryl cAMP) blunted the TxA2-induced vasoconstriction to a similar degree in both strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Interactions of cAMP-mediated vasodilators with angiotensin II in rat kidney during hypertension.

In previous studies [C. Chatziantoniou and W.J. Arendshorst. Am. J. Physiol. 263 (Renal Fluid Electrolyte Physiol. 32): F573-F580, 1992], we reported that vasodilator prostaglandins (PGs) are defective in buffering the angiotensin II (ANG II)-induced vasoconstriction in the renal vasculature of spontaneously hypertensive rats (SHR). The purpose of the present study was to determine whether this defect in SHR kidneys is specific to PGs or generalized to the action of vasodilators and to gain insight into which intracellular signal(s) mediates this abnormality. Renal blood flow (RBF; electromagnetic flowmetry) was measured in 7 wk-old anesthetized, euvolemic SHR and normotensive Wistar-Kyoto (WKY) rats pretreated with indomethacin to avoid interactions with endogenous PGs. ANG II (2 ng) was injected into the renal artery before and during continuous intrarenal infusion of fenoldopam [DA1 receptor agonist and G protein-dependent stimulator of adenosine 3',5'-cyclic monophosphate (cAMP)], forskolin (G protein-independent stimulator of cAMP), dibutyryl-cAMP (soluble cAMP), and acetylcholine (cGMP stimulator). Each vasodilator was infused at a low dose that did not affect baseline arterial pressure or RBF. In the control period, ANG II reduced RBF by 50% in both strains. Infusion of fenoldopam significantly blunted the ANG II-induced vasoconstriction in WKY, but not in SHR. In contrast, forskolin, dibutyryl-cAMP, and acetylcholine effectively buffered the vasoconstriction due to ANG II in both SHR and WKY. These results suggest that renal vasodilators acting through receptor binding to stimulate the cAMP signaling pathway are ineffective in counteracting the ANG II-induced vasoconstriction in SHR kidneys. (ABSTRACT TRUNCATED AT 250 WORDS)

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗