Ang II-induced Vasoconstriction Research Articles

Synergistic vascular effects of dietary sodium supplementation and angiotensin II administration.

The hypertensinogenic action of dietary sodium supplementation was investigated in angiotensin II (ANG II)-treated rats. We hypothesized that high-sodium diet would potentiate ANG II-induced vasoconstriction and hypertension, including the development of structural vascular changes, through synergistic action with ANG II in stimulating sympathetic activity and vascular growth. Mesenteric vasoconstrictor responses to ANG II, norepinephrine (NE), arginine vasopressin (AVP), and periarterial nerve stimulation were measured in Sprague-Dawley rats after 7-10 days of the following treatments: 200 ng.kg-1.min-1 ip ANG II (n = 12), 4% NaCl diet (n = 11), ANG II + 4% NaCl diet (n = 7), and 0.7% NaCl diet (controls) (n = 15). Additional rats received 50 ng.kg-1.min-1 sc ANG II (n = 8), 2% NaCl diet (n = 9), ANG II + 2% NaCl diet (n = 6), or 0.7% NaCl diet (controls) (n = 10) for 12 wk. Systolic blood pressure (SBP) values were measured weekly for 4 wk and then every other week for 8 wk. Then, the wall-to-lumen ratio (W/L) of mesenteric resistance arteries (< 150 microns OD) was measured after in situ perfusion-fixation. After 7-10 days of treatment, there were no significant changes in SBP in any of the groups. High-sodium diet increased vasoconstrictor responses to ANG II (P < 0.01) and nerve stimulation (P < 0.02), but not to NE or AVP, and in combination with ANG II treatment further potentiated vasoconstrictor responses to ANG II (synergism). After 12 wk of treatment, ANG II increased W/L of small resistance arteries by 11% (P < 0.05) without a significant rise in SBP. ANG II and 2% NaCl diet in combination raised SBP by 36 mmHg (P < 0.01) and increased small artery W/L by 28% (P < 0.001) compared with values obtained in control rats. To test the specificity of the interaction between ANG II and high-sodium diet, all the experiments were repeated during phenylephrine (PE, 10 micrograms.kg-1.min-1 sc) treatment of rats. PE by itself or in combination with high-sodium diet had no effect on the measured parameters. Thus short-term administration of high-sodium diet appears to potentiate vasoconstrictor responses to ANG II by facilitating sympathetic neurotransmission, and long-term administration of high-sodium diet raises SBP by potentiating the trophic vascular effects of ANG II. The interaction appears to be specific to ANG II and is occurring on the vascular level.

Membrane potential measurements in renal afferent and efferent arterioles: actions of angiotensin II.

An adaptation of the in vitro perfused hydronephrotic rat kidney model allowing in situ measurement of arteriolar membrane potentials is described. At a renal perfusion pressure of 80 mmHg, resting membrane potentials of interlobular arteries (22 +/- 2 microns) and afferent (14 +/- 1 microns) and efferent arterioles (12 +/- 1 microns) were -40 +/- 2 (n = 8), -40 +/- 1 (n = 45), and -38 +/- 2 mV (n = 22), respectively (P = 0.75). Using a dual-pipette system to stabilize the impalement site, we measured afferent and efferent arteriolar membrane potentials during angiotensin II (ANG II)-induced vasoconstriction. ANG II (0.1 nM) reduced afferent arteriolar diameters from 13 +/- 1 to 8 +/- 1 microns (n = 8, P = 0.005) and membrane potentials from -40 +/- 2 to -29 +/- mV (P = 0.012). ANG II elicited a similar vasoconstriction in efferent arterioles, decreasing diameters from 13 +/- 1 to 8 +/- 1 microns (n = 8, P = 0.004), but failed to elicit a significant depolarization (-39 +/- 2 for control; -36 +/- 3 mV for ANG II; P = 0.27). Our findings thus indicate that resting membrane potentials of pre- and postglomerular arterioles are similar and lie near the threshold activation potential for L-type Ca channels. ANG II-induced vasoconstriction appears to be closely coupled to membrane depolarization in the afferent arteriole, whereas mechanical and electrical responses appear to be dissociated in the efferent arteriole.

Renal oxygenases: differential contribution to vasoconstriction induced by ET-1 and ANG II.

In the rat isolated perfused kidney, 5,8,11,14-eicosatetraynoic acid, an inhibitor of all pathways of arachidonic acid (AA) metabolism, diminished endothelin-1 (ET-1)- and angiotensin II (ANG II)-induced renal vasoconstriction by approximately 60-70%. We then examined the individual contribution of each oxygenase, cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P-450 (CYP) to the vasoconstrictor effects of ET-1 and ANG II. Inhibition of COX with indomethacin reduced by 30-40% the vasoconstrictor responses to ET-1 and ANG II. Inhibition of 12-LOX with baicalein and 5- and 12-LOX with 5,8,11-eicosatriynoic acid attenuated ANG II-induced renal vasoconstriction by approximately 40-60% but did not affect responses to ET-1. In contrast, 12,12-dibromododec-11-enoic acid (DBDD), an inhibitor of the CYP omega/omega 1-hydroxylase pathway, diminished ET-1-induced renal vasoconstriction by 30-40%, an effect reproduced by depletion of CYP enzymes with CoCl2. Neither DBDD nor CoCl2 affected renal vasoconstriction elicited by ANG II. ET-1 increased efflux of 19- and 20-hydroxyeicosatetraenoic acid, an effect reduced by DBDD. Thus products of the COX and CYP pathways contribute to the renal vasoconstrictor response to ET-1, whereas COX- and LOX-derived eicosanoids contribute to the response to ANG II, accounting for > or = 80% of the vasoactivity of the peptides.

Adenosine modulates vasomotor tone in outer medullary descending vasa recta of the rat.

Adenosine is generated within the renal medulla under hypoxic conditions and is known to induce net vasoconstriction within the renal cortex while increasing medullary blood flow and oxygenation. To test the hypothesis that vasoconstriction of outer medullary descending vasa recta (OMDVR) is modulated by adenosine, we examined the effects of adenosine and adenosine Al and A2 receptor subtype agonists on in vitro perfused control and preconstricted rat OMDVR. Constriction with angiotensin II (ANG II, 10(-9) M) was attenuated by adenosine in a concentration-dependent manner (EC50 = 2.0 x 10(-7)M, P < 0.05). Similarly, an adenosine A2 agonist (CGS-21680, 10(-7) M), but not an adenosine Al agonist (cyclohexyladenosine, 10(-6) M), attenuated ANG II-induced vasoconstriction. Under control conditions, ablumenal application of adenosine (10(-12) to 10(-5) M) elicited a biphasic response. Additionally, cyclohexyladenosine (10(-6) M) caused vasoconstriction and CGS-21680 (10(-6) M) had no effect on untreated vessels. Finally, an influence of ANG II receptor stimulation on adenosine Al receptor-mediated vasoconstriction could not be shown. These data suggest that OMDVR possess both Al and A2 adenosine receptors and that they mediate constriction and dilatation, respectively. We conclude that adenosine is a potent modulator of OMDVR vasomotor tone and that its net effect is dependent upon local concentrations.

Tissue specific expression of vascular smooth muscle angiotensin II receptor subtypes during ovine pregnancy.

Uteroplacentral responses to infused angiotensin II (ANG II) are less than those elicited by systemic vasculature. This does not reflect ANG II receptor (AT) downregulation but may reflect differences in AT-receptor subtypes expressed. We examined AT-receptor subtypes in smooth muscle (SM) from uterine (UA), mesenteric, renal, and mammary arteries and aorta from nulliparous (n = 12), pregnant (n = 18; 105-140 days, term = 145 days), postpartum (n = 5; 6-9 days after delivery), and nonpregnant parous (n = 14) ewes by assessing displacement of 125I-labeled ANG II binding by [Sar1, Ile8]ANG II (AT1 and AT2), losartan (AT1) PD-123319 (AT2), and CGP-42112A (AT2). AT2 receptors accounted for 75-90% of total binding in UA. Except for mammary arteries, other arteries expressed only AT1 receptors. Receptor subtype expression was not altered by reproductive state in any artery studied. With the use of autoradiography, AT2 receptors appear to predominate in media of small intramyometrial arteries, whereas AT1 receptors predominate in the luminal portion. We therefore determined which subtype mediates endothelium-derived ANG II-induced increases in UA PGI2 synthesis during pregnancy. ANG II (0.05 microM) increased PGI2 synthesis 62%, from 214 +/- 13 to 346 +/- 23 pg.mg-1.h-1 (P < 0.05). Losartan (1.0 microM) inhibited the rise in PGI2 (257 +/- 24 vs. 238 +/- 25 pg.mg-1.h-1), whereas 1.0 microM PD-123319 had no effect (231 +/- 23 vs. 337 +/- 31 pg.mg-1.h-1; P < 0.05). AT2 receptors do not mediate ANG II-induced vasoconstriction, thus differences in uteroplacental and systemic sensitivity to ANG II may reflect predominance of AT2 receptors in UASM and ANG II-induced increases in UA prostacyclin synthesis by endothelial AT1 receptors.

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.

The regulation of glomerular and tubular functions with eicosanoids

The aim of the present study was to evaluate the effects of inhibition of cytochrome P-450 (CYP) activity by 1-aminobenzotriazole (ABT) and by CoCl2, first, on the development of hypertension when treatment was started in young male heterozygous Ren-2 transgenic rats (TGR) and, second, on blood pressure (BP) when treatment was started in adult TGR with established hypertension. Normotensive Hannover Sprague–Dawley (HanSD) rats served as controls. In addition, the renal cortical activities of ω-hydroxylase, the enzyme catalyzing the formation of 20-hydroxyeicosatetraenoic acid (20-HETE), and of epoxygenase, the enzyme responsible for epoxyeicosatrienoic acids (EETs) production, and urinary excretion of 20-HETE and EETs in TGR and HanSD rats were assessed. TGR have higher renal tissue ω-hydroxylase activity and urinary excretion of 20-HETE but have significantly lower renal epoxygenase activity and urinary excretion of EETs than HanSD rats. Treatment of young TGR with ABT and CoCl2 attenuated the development of hypertension and cardiac hypertrophy and prevented glomerulosclerosis. Administration of ABT and CoCl2 in adult TGR decreased BP, cardiac hypertrophy, but did not reduce glomerulosclerosis. Our data suggest that altered production and/or action of CYP-derived metabolites play a permissive role in the development and maintenance of hypertension in TGR by enhancing ANG II-induced vasoconstriction.

Angiotensin II 1 receptor-mediated contraction of pulmonary artery and its modulation by prolylcarboxypeptidase.

To determine the subtype of angiotensin II (ANG II) receptor involved in the contraction of pulmonary artery and to elucidate its possible modulation by endogenous peptidases, we studied canine isolated pulmonary arterial rings under isometric conditions in vitro. Addition of ANG II caused a concentration-dependent contraction, an effect that was not altered by the ANG II 2 receptor antagonist EXP655 but was depressed by the ANG II 1 receptor antagonist DuP 753 so that the ANG II response curves were displaced to higher concentration by 1.5-2.0 log U (P < 0.001). Pretreatment of tissues with the prolylcarboxypeptidase (PCP) inhibitor p-methylphenyl sulfonyl-fluoride potentiated the ANG II-induced contraction, with the concentration required to produce a half-maximal effect of ANG II being decreased from 4.1 +/- 0.9 x 10(-9) to 3.8 +/- 0.5 x 10(-10) M (P < 0.001), whereas other peptidase inhibitors such as p-chloromercuriphenyl sulfonic acid, amastatin, and phosphoramidon had no effect. The p-methylphenyl sulfonylfluoride-induced potentiation was abolished by the removal of endothelium, but it was still observed in the presence of NG-nitro-L-arginine methyl ester in the endothelium-intact tissues. The PCP activity in the tissues was reduced by the removal of endothelium from 645 +/- 88 to 91 +/- 29 nmol.mg protein-1.h-1 (P < 0.001), and cultured endothelium had the activity of 404 +/- 39 nmol.mg protein-1.h-1. These results suggest that ANG II contracts pulmonary artery via ANG II 1 receptor and that PCP localized to the endothelium may have a modulatory role in the ANG II-induced pulmonary vasoconstriction.

Segmental differences in angiotensin receptor subtypes in interlobular artery of hydronephrotic rat kidneys.

Recent studies demonstrate that angiotensin II (ANG II)-induced vascular action is mediated preferentially by AT1 receptors. Although autoradiographic studies indicate the presence of AT2 receptors in large preglomerular vessels, functional roles for AT2 receptors in ANG II-induced renal vasoconstriction remain undetermined. We examined the effects of DuP-753 and PD-123319 on ANG II-induced vasoconstriction of interlobular arteries (ILA) in isolated perfused hydronephrotic rat kidneys to directly assess the AT1- and AT2-mediated action of ANG II on renal microvessels. Both DuP-753 (0.1-10 microM) and PD-123319 (0.1-10 microM) elicited dose-dependent vasodilation of ANG II-induced ILA constriction, with 86 +/- 4% and 36 +/- 4% inhibition by 10 microM DuP-753 and PD-123319, respectively. The reversal by DuP-753 of ANG II-induced ILA vasoconstriction was greater in small-caliber segments than in large-caliber segments. In contrast, the ability of PD-123319 (10 microM) to inhibit the vasoconstriction was augmented as the vessel diameter increased (slope = +0.46, correlation coefficient = +0.68; P < 0.01). Thus, although AT1 predominantly mediates the ANG II-induced ILA vasoconstriction, PD-123319-sensitive ANG II receptors (e.g., AT2 or AT1B) may also participate partly in the ILA vasoconstriction, particularly at large-caliber segments. In conclusion, distribution of ANG II receptor subtypes may differ depending on the size of the renal microvasculature.

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)

Pulmonary and systemic vascular responsiveness to TNF-alpha in conscious rats.

Endotoxin decreases pulmonary vascular reactivity. Because tumor necrosis factor-alpha (TNF-alpha) is a primary mediator of endotoxemia, we tested whether TNF-alpha altered pulmonary vascular reactivity in conscious adult female rats. Osmotic pumps were implanted intraperitoneally, and low-dose TNF-alpha (62 micrograms, TNF62; n = 7), high-dose TNF-alpha (> or = 250 micrograms, TNF250; n = 5), or saline (n = 5) was administered for 2 wk. Pulmonary pressor responses to 14% O2 and angiotensin II (ANG II, 0.0206 micrograms/min for 10 min) were measured without (day 13) or after (day 14) administration of nitro-L-arginine (4.4 mg/kg iv), an inhibitor of endothelium-derived relaxing factor (EDRF). TNF-alpha administration slightly decreased (P < or = 0.08) baseline pulmonary arterial pressure in TNF250 rats and decreased (P < or = 0.05) hypoxia- and ANG II-induced constrictions in TNF62 and TNF250 rats. Whereas nitro-L-arginine potentiated (P < or = 0.05) pressure responses in control rats, it had no effect on hypoxic responses in TNF-alpha-treated rats. Nitro-L-arginine increased (P < or = 0.05) ANG II-induced vasoconstriction in TNF-alpha-treated rats, but the pulmonary arterial pressure response was still lower (P < or = 0.05) in TNF250 than in control and TNF62 rats. These results suggest that chronic TNF-alpha decreases 1) pulmonary vascular reactivity in the intact rat, 2) hypoxic pulmonary vasoconstriction by a mechanism that is independent of EDRF, and 3) ANG II-induced constriction by a mechanism that is partly EDRF dependent.

Impaired ability of prostaglandins to buffer renal vasoconstriction in genetically hypertensive rats.

The purpose of this study was to gain insight into the mechanism(s) responsible for the exaggerated angiotensin II (ANG II)-induced renal vasoconstriction during the development of hypertension. In previous studies we observed that ANG II produces a twofold larger decrease in renal blood flow (RBF) in spontaneously hypertensive (SHR) compared with Wistar-Kyoto (WKY) rats before but not after cyclooxygenase inhibition. We suggested that this strain difference could be attributed to differences in renal prostaglandin (PG) levels and/or action. To evaluate these possibilities, measurements of RBF were made in 6-wk-old, anesthetized SHR and WKY pretreated with indomethacin. ANG II was injected intrarenally before and during continuous intrarenal infusion of a low dose of PGE2, viprostol (PGE2 analogue), PGI2, iloprost (PGI2 analogue), and bradykinin. In the control period ANG II reduced RBF by 50% in both strains. Infusion of PGs reduced the vasoconstrictor effect of ANG II in WKY, but had no effect in SHR. In contrast, infusion of bradykinin blunted the ANG II-induced vasoconstriction to a similar degree in both WKY and SHR. To investigate whether this lack of protection in SHR is due to strain differences in the number and/or affinity of renal receptors of PGs, radiolabeled ligand binding studies for PGE2 and PGI2 receptors were undertaken in glomeruli isolated from young WKY and SHR. Scatchard analysis revealed a single, high-affinity receptor site for PGE2 that was similar in both strains of rats. Both strains also exhibited a single, high-affinity PGI2 receptor site. No differences were observed in the PGE2 or PGI2 receptor number between WKY and SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Uterine prostaglandin production in ovine pregnancy: effects of angiotensin II and indomethacin.

The ovine and human uteroplacental vascular beds are more refractory to angiotensin II (ANG II)-induced vasoconstriction than the systemic vasculature. ANG II increases in vitro prostacyclin (PGI2) production by uterine but not omental arteries from pregnant sheep. Thus vasodilator prostaglandins may account for this difference in vascular responsiveness. We measured uterine and systemic eicosanoid production and hemodynamic responses in pregnant sheep before and during intravenous ANG II (1.15 and 11.5 micrograms/min). ANG II caused dose-related increases in arterial pressure and systemic and uterine vascular resistance (P less than 0.05). PGI2 metabolite (6-keto-PGF1 alpha) in the uterine vein rose from 166 +/- 70 (SE) to 223 +/- 114 and 631 +/- 323 pg/ml, respectively (P less than 0.05), and arterial levels increased from 67 +/- 24 to 145 +/- 78 and 312 +/- 173 pg/ml, respectively (P less than 0.05). Basal uterine venoarterial differences of 6-keto-PGF1 alpha were 99 +/- 43 pg/ml and increased during 11.5 micrograms ANG II/min to 295 +/- 181 pg/ml (P less than 0.05) but not during 1.15 micrograms/min (64 +/- 30 pg/ml). Responses were similar in gravid and nongravid uterine horns. Unilateral uterine prostaglandin inhibition with indomethacin did not alter basal uterine blood flow or systemic responses to ANG II (0.573-11.5 micrograms/min); however, ipsilateral uterine prostaglandin production fell and uterine vasoconstrictor responses increased (P less than 0.05). During ovine pregnancy ANG II increases uterine PGI2 production. PGI2 appears in part to attenuate ANG II-induced uterine vasoconstriction.

Regional circulatory contributions to increased systemic vascular conductance of pregnancy

In pregnancy, maternal systemic vascular conductance increases, a new vascular circuit grows, and the maternal systemic circulation develops a diminished pressor response to angiotensin II (ANG II). However, the quantitative contributions of the latter two circulatory changes to the increased systemic vascular conductance of pregnancy have not been explored. In this experiment, we examined regional circulatory contributions to the increased systemic vascular conductance in conscious, late-gestation guinea pigs. Systemic arterial pressure, cardiac output (dye dilution), and regional blood flows (radiolabeled microspheres) were measured during baseline conditions and progressive ANG II infusion. Systemic and regional conductances were calculated from arterial pressure and cardiac output or regional blood flows. In pregnancy, maternal systemic vascular conductance increased from 3.2 to 5.0 ml.min-1.mmHg-1 (P less than 0.001); increased nonuteroplacental conductance contributed 71% to the increase in whole body conductance. Pregnancy tended to decrease the nonuteroplacental conductance response (P = 0.072), but did not change the uteroplacental conductance response (P greater than or equal to 0.29), to ANG II. The increased uteroplacental blood flow of pregnancy was preserved during ANG II-induced vasoconstriction. We conclude that maternal systemic vascular conductance increased primarily because nonuteroplacental vascular conductance increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the Renal Microvascular Effects of Angiotensin II Antagonist, DuP 753:

The renal microvascular effects of DuP 753, an orally active imidazole angiotensin II (ANG II) receptor antagonist were assessed directly in isolated perfused hydronephrotic rat kidneys. Unilateral hydronephrosis was induced to facilitate direct visualization of renal microvessels. Hydronephrotic kidneys were perfused in vitro and microvessel diameters were measured by automated computer-assisted image processing. The administration of 0.3 nmol/L ANG II decreased afferent arteriolar (AA) and efferent arteriolar (EA) diameters by 34 +/- 3% (from 17.9 +/- 0.6 to 11.9 +/- 0.6 microns, P less than .001, n = 11) and 28 +/- 3% (from 17.1 +/- 1.3 to 12.3 +/- 1.3 microns, P less than .001, n = 11), respectively. The subsequent administration of 0.1, 1.0, and 10 mumols/L DuP 753 reversed ANG II-induced vasoconstriction of the AA by 39 +/- 10%, 81 +/- 8%, and 103 +/- 9%, and of the EA by 22 +/- 7%, 51 +/- 6%, and 87 +/- 13%, respectively. These observations indicate that DuP 753 completely blocks both the renal afferent and efferent arteriolar actions of ANG II. In light of the pathogenetic role of ANG II in mediating the deranged renal hemodynamics associated with hypertension, congestive heart failure, and some forms of renal insufficiency, our findings provide a theoretical framework for future studies assessing the potential therapeutic applicability of DuP 753 in reversing ANG II-mediated renal vasoconstriction.

Prostaglandin influences on afferent arteriolar responses to vasoconstrictor agonists

The present study was designed to evaluate, at the microvascular level, the ability of prostaglandins E2 (PGE2) and I2 (PGI2) to counteract the afferent vasoconstrictor effects of angiotensin II (ANG II) and norepinephrine (NE). The renal microvasculature of rats pretreated with captopril and indomethacin was studied directly by use of the in vitro blood-perfused juxtamedullary nephron technique combined with videomicroscopy. Afferent arterioles averaged 22.7 +/- 0.6 microns ID (n = 59) under control conditions. Topical administration of PGE2 revealed a concentration-dependent afferent vasoconstriction, whereas PGI2 (10(-7) to 10(-5) M) failed to significantly alter afferent arteriolar diameter. Afferent arterioles constricted during exposure to either 10(-9) M ANG II (-15 +/- 3%, n = 13) or 10(-7) M NE (-19 +/- 3%, n = 13). Addition of PGE2 (10(-6) M) to the bathing solution enhanced the vasoconstrictor influences of ANG II and NE by an additional 18 +/- 6 and 13 +/- 4%, respectively. In contrast, while 10(-6) M PGI2 had no effect on ANG II-induced afferent vasoconstriction, it did produce a 30% attenuation of NE-induced constriction. Furthermore, pretreatment of the tissue with 10(-6) M PGI2 prevented development of NE-induced afferent vasoconstriction. Thus, although local tissue prostanoid concentrations are unknown, it appears that low micromolar concentrations of PGE2 elicit an afferent arteriolar constriction that can accentuate the vascular actions of ANG II and NE on rat juxtamedullary afferent arterioles. In contrast, PGI2 can counteract the vasoconstrictor response to NE, but not ANG II, in this experimental setting.

Selective inhibition of angiotensin II-mediated vasoconstriction by lipoxygenase blockade

We have previously demonstrated that the lipoxygenase (LO) pathway has a specific role in the effect of angiotensin II (ANG II) on aldosterone secretion. To elucidate whether the LO pathway also participates in the vascular effects of ANG II, the nonselective LO inhibitor phenidone (PHE; 30 mg/kg) was administered to rats 1 h before graded dose ANG II infusion. PHE reduced the LO product 12-hydroxyeicosatetraenoic acid (12-HETE) in deendothelialized aortas by an average of 36% as determined by radiometric detection with high-performance liquid chromatography and radioimmunoassay methods. In parallel, the peak systolic pressor response to ANG II was lowered from 36.2 +/- 3.7 to 16.8 +/- 2.0 mmHg. The peak pressor responses to ANG II were also reduced by two other LO inhibitors, baicalein (30 mg/kg) and esculetin (60 mg/kg) (13.9 +/- 2.4 and 22.1 +/- 4.7 mmHg, respectively; P less than 0.01 compared with control rats for both), but not by the cyclooxygenase inhibitor indomethacin. The LO inhibitors baicalein (7.5 X 10(-5) M) and PHE (10(-4) M) markedly attenuated the in vitro contractile response to ANG II of femoral artery rings. In contrast, neither the in vivo nor in vitro constrictor responses to norepinephrine were affected by baicalein. Thus lipoxygenase blockade induces a direct and selective inhibition of ANG II-induced vasoconstriction. The LO pathway may have an important role in mediating the pressor effect of ANG II.

Disparate effects of Ca channel blockade on afferent and efferent arteriolar responses to ANG II

Previous reports have suggested that organic calcium antagonists only partially inhibit the renal hemodynamic actions of angiotensin II (ANG II). This study tested the hypothesis that the calcium antagonist-sensitive component of ANG II-induced vasoconstriction is localized at a preglomerular site. Videomicroscopic measurements of vascular dimension were performed on in vitro blood-perfused juxtamedullary nephrons from captopril-treated rats. Under control conditions, afferent and efferent arteriolar diameters averaged 23.0 +/- 1.6 and 21.2 +/- 2.2 microns, respectively. Topical application of 0.1 nM ANG II decreased the diameters of afferent (-17 +/- 2%) and efferent (-15 +/- 3%) arterioles. Both 50 microM verapamil and 10 microM diltiazem dilated afferent arterioles. Verapamil also elicited a modest efferent vasodilation. In the presence of either verapamil or diltiazem, the effect of ANG II to decrease efferent diameter was sustained (-15 +/- 4%); however, the effect of ANG II on afferent diameter was abolished (-1 +/- 1%). These observations document differential influences of calcium channel blockers on ANG II-mediated vasoconstriction and suggest that the pre- and postglomerular vasoconstrictor actions of ANG II may occur through different calcium entry or mobilization mechanisms.