Search

Subpressor doses of angiotensin II (SP-Ang II) cause a slow increase in blood pressure in rats as assessed by tail cuff plethysmography (TCP), reflecting either sustained hypertension or an exaggerated pressor response to diverse stimuli. We examined whether subpressor doses of Ang II enhance blood pressure responses to simple stress (handling of trained rats for TCP). We implanted telemetry in Sprague-Dawley rats. After 10 days of recovery and TCP training, we implanted osmotic minipumps with either SP-Ang II (50 ng/kg per minute) or vehicle, and then measured systolic blood pressure continuously in unrestrained rats for 13 days. We also recorded telemetry readings while obtaining TCP measurements every 2 days. SP-Ang II increased blood pressure from 134±19 to 159±22 mm Hg by TCP, which matched the simultaneous telemetry readings of 131±20 to 154±25 mm Hg. In contrast, SP-Ang II did not change the blood pressure in the unrestrained rats (measured with continuous telemetry: 124±2 versus 127±1 mm Hg). The blood pressure in the control rats did not change in the unrestrained state (125±3 versus 128±5 mm Hg on days 0 and 12, respectively), and only slightly increased during TCP (11±5 and 6±4 mm Hg by TCP and simultaneous telemetry, respectively; P =NS). In summary, SP-Ang II, although unable to provoke sustained hypertension, nonetheless magnifies the pressor response to otherwise trivial stimuli. We speculate that even modestly elevated Ang II levels may contribute to hypertensive complications because such levels promote the punctuation of an apparent normotensive state by episodic hypertension occasioned by seemingly innocuous stimuli.

Abstract — — Chronically infusing a subpressor dose of angiotensin (Ang) II increases blood pressure via poorly defined mechanisms. We found that this hypertensive response is accompanied by increased oxidant stress and is prevented by blocking endothelin (ET) receptors. Thus, we now tested whether blocking oxidant stress decreases both blood pressure and ET levels. We infused Sprague-Dawley rats (via osmotic pumps) with either vehicle (group 1) or Ang II (5 ng · kg −1 · min −1 ; groups 2 to 4) for 15 days. Groups 3 and 4 also received either tempol in the drinking water (1 mmol/L) or vitamin E (5000 IU/kg diet), respectively, for 15 days. We measured systolic blood pressure (SBP) and urinary nitrite excretion every 3 days, and on day 15 we measured systemic and renal venous plasma levels of ET, isoprostanes, and thiobarbituric acid reactive substances (TBARS). SBP in Group 1 did not change throughout the study, whereas Ang II increased SBP (from 132±5 to 151±7 mm Hg). In addition, Ang II increased the systemic and renal venous levels of isoprostanes, TBARS, and ET and caused a transient decrease in urinary nitrites (that returned to control levels by day 9). Both tempol and vitamin E prevented Ang II-induced hypertension and either prevented or tended to blunt the increase in systemic and renal isoprostanes, TBARS, and ET. Finally, both antioxidants abolished the transient decrease in urinary nitrites. These results together with our previous study suggest that subpressor-dose Ang II increases oxidant stress (and isoprostanes). This in turn increases ET levels, which participate in the hypertensive response to Ang II.

We tested the hypothesis that angiotensin II (Ang II)–induced stimulations of endothelin (ET) and isoprostanes are implicated in the slow pressor responses to Ang II. We infused either vehicle (group 1) or Ang II (groups 2 to 4) intravenously at 5 ng/kg per minute via osmotic pumps for 15 days into Sprague-Dawley rats. Groups 3 and 4 received 30 mg/kg per day of either losartan (Ang II type 1 receptor blocker) or bosentan (ET A and ET B receptor blocker) in their drinking water. We measured systolic blood pressure (SBP) every 3 days during the infusion. Plasma levels of Ang II, ET, isoprostanes, and urinary nitrites were determined at 15 days. Vehicle infusion did not change SBP (from 138±13 to 136±2 mm Hg at day 15). Circulating Ang II, ET, and isoprostane levels were 35±9, 39±3, and 111±10 pg/mL, respectively, whereas urinary nitrites were 2.3±0.4 μg/d. Ang II increased SBP (from 133±10 to 158±8 mm Hg), plasma Ang II (179±77 pg/mL), and isoprostanes (156±19 pg/mL) without altering ET levels (38±5 pg/mL) or urinary nitrites (1.8±0.5 μg/d). Losartan prevented Ang II–induced increases in SBP and isoprostanes (SBP went from 137±5 to 120±4 mm Hg; isoprostanes were 115±15 pg/mL) while increasing urinary nitrite levels (5.2±1.1 μg/d). Losartan did not alter Ang II (141±57 pg/mL) or ET (40±4 pg/mL) levels. Bosentan also blocked Ang II–induced hypertension (from 135±4 to 139±3 mm Hg) but did not decrease isoprostanes (146±14 pg/mL). Ang II (63±11 pg/mL), ET levels (46±2 pg/mL), and urinary nitrites (2.8±0.4 μg/d) were not altered. In conclusion, our results suggest that low-dose Ang II increases isoprostanes via its Ang II type 1 receptor and causes an ET-dependent hypertension, without altering circulating ET levels.

Abstract —Chronic intravenous infusion of subpressor doses of angiotensin II causes blood pressure to increase progressively over the course of several days. The mechanisms underlying this response, however, are poorly understood. Because high-dose angiotensin II increases oxidative stress, and some compounds that result from the increased oxidative stress (eg, isoprostanes) produce vasoconstriction and antinatriuresis, we tested the hypothesis that a subpressor dose of angiotensin II also increases oxidative stress, as measured by 8-epi-prostaglandin F 2α (isoprostanes), which may contribute to the slow pressor response to angiotensin II. To test this hypothesis, we infused angiotensin II (10 ng/kg per minute for 28 days via an osmotic pump) into 6 conscious normotensive female pigs (30 to 35 kg). We recorded mean arterial pressure continuously with a telemetry system and measured plasma isoprostanes before starting the angiotensin II infusion (baseline) and again after 28 days with an enzyme immunoassay. Angiotensin II infusion significantly increased mean arterial pressure from 121±4 to 153±7 mm Hg ( P P =NS). However, the plasma concentrations of free isoprostanes increased significantly, from 38.3±5.8 to 54.7±10.4 pg/mL ( P

Abstract —In this review, we examine the possibility that small increments in angiotensin II are responsible for an increase in blood pressure and maintenance of hypertension through the stimulation of oxidative stress. A low dose of angiotensin II (2 to 10 ng · kg −1 · min −1 , which does not elicit an immediate pressor response), when given for 7 to 30 days by continuous intravenous infusion, can increase mean arterial pressure by 30 to 40 mm Hg. This slow pressor response to angiotensin is accompanied by the stimulation of oxidative stress, as measured by a significant increase in levels of 8-iso-prostaglandin F 2α (F 2 -isoprostane). Superoxide radicals and nitric oxide can combine chemically to form peroxynitrite, which can then oxidize arachidonic acid to form F 2 -isoprostanes. F 2 -isoprostanes exert potent vasoconstrictor and antinatriuretic effects. Furthermore, angiotensin II can stimulate endothelin production, which also has been shown to stimulate oxidative stress. In this way, a reduction in the concentration of nitric oxide (which is quenched by superoxide) along with the formation of F 2 -isoprostanes and endothelin could potentiate the vasoconstrictor effects of angiotensin II. We hypothesize that these mechanisms, which underlie the development of the slow pressor response to angiotensin II, also participate in the production of hypertension when circulating angiotensin II levels appear normal, as occurs in many cases of essential and renovascular hypertension.

Nitric oxide mediates the vasodilator and hypotensive responses of acetylcholine infusion. It has been reported that nitric oxide could be protected from free radical destruction by forming an S-nitrosothiol compound. Furthermore, sulfhydryl donors such as N-acetylcysteine or thiosalicylic acid enhance nitric oxide production from nitroglycerin. Consequently, the hypotensive effect of intravenous acetylcholine infusion might be potentiated during the simultaneous administration of sulfhydryl donors. The objective of the present study was to test in Okamoto spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats (1) whether the hypotensive effect of acetylcholine (10 micrograms/kg per minute) was affected by the simultaneous administration of N-acetylcysteine (10 micrograms/kg per minute) or thiosalicylic acid (10 micrograms/kg per minute), and (2) whether NG-nitro-L-arginine-methyl ester (100 micrograms/kg per minute) administration was able to reverse the changes induced by acetylcholine plus N-acetylcysteine or acetylcholine plus thiosalicylic acid. The administration of acetylcholine reduced (P < .05) mean arterial pressure in WKY rats (13 +/- 2%) and SHR (14 +/- 2%) without affecting urine flow rate, urinary sodium excretion, and glomerular filtration rate. In the presence of N-acetylcysteine, the acetylcholine-induced reduction in mean arterial pressure was potentiated (P < .05) in WKY rats (24 +/- 4%) and SHR (20 +/- 2%). These changes in mean arterial pressure were accompanied by significant reductions in urine flow rate and urinary sodium excretion in WKY rats, as well as in glomerular filtration rate in SHR.2

The objective of the present study was to determine the role of endothelium-derived nitric oxide in mediating the renal response to extracellular volume expansion with isotonic saline (5% body weight). In anesthetized dogs (n = 7) and before volume expansion, nitric oxide synthesis was inhibited in the right kidney by continuous intrarenal infusion of NG-nitro-L-arginine-methyl ester (1 microgram/kg/min). Arterial pressure and renal hemodynamics of both kidneys did not change significantly either during nitric oxide synthesis inhibition or during 5% volume expansion. However, in response to extracellular volume expansion, increases in natriuresis, diuresis, and fractional excretion of lithium (an index of proximal sodium reabsorption) were inhibited in the right kidney by 27%, 28%, and 41%, respectively, when compared with the contralateral kidney. Increases of renal interstitial hydrostatic pressure during 5% volume expansion were not statistically different between both kidneys. In another group of dogs (n = 4), the administration of L-arginine (0.5 mg/kg/min) into the right renal artery prevented the renal effects induced by the nitric oxide synthesis inhibitor during volume expansion. The findings in this study suggest that nitric oxide production plays an important role in regulating the renal response to extracellular volume expansion. The proximal tubule seems to be involved in the reduced renal excretory response to volume expansion during nitric oxide synthesis inhibition.

Increases in renal venous pressure have been shown to consistently increase renal interstitial pressure; however, not until renal interstitial pressure is increased threefold is a natriuresis noted in normal animals. Since the intrarenal angiotensin II (Ang II) concentration has been postulated to increase with increasing renal venous pressure, the antinatriuretic action of Ang II could override the natriuretic effect of increased renal interstitial pressure. Therefore, the role of Ang II in the natriuretic response to increased renal venous pressure was examined in 10 pentobarbital-anesthetized dogs. Mean arterial pressure, renal blood flow, renal interstitial pressure, glomerular filtration rate, urinary sodium excretion, plasma renin activity, and prostaglandin E2 excretion were measured at renal venous pressures of 3, 15, and 30 mm Hg. The measurements were repeated after the administration of captopril (1 mg/kg i.v. bolus, n = 5) or [Sar1,Ile8]Ang II (50 micrograms/kg i.v. bolus + 50 micrograms/kg/hr infusion, n = 5). Under control conditions, mean arterial pressure, renal blood flow, plasma renin activity, and prostaglandin E2 excretion remained unchanged when renal venous pressure was increased. The elevations in renal venous pressure increased renal interstitial pressure from 7 +/- 2 to 12 +/- 2 and 22 +/- 4 mm Hg, while sodium excretion remained unchanged until renal venous pressure was 30 mm Hg. In the captopril-treated group, increasing renal venous pressure increased renal interstitial pressure as under control conditions; however, sodium excretion (23 +/- 4, 19 +/- 4, and 27 +/- 6 mueq/min) was not significantly increased even at the highest renal venous pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

48 Spontaneously hypertensive rats (SHR) reportedly have inappropriately high levels of Ang II despite normal plasma renin activity (PRA). Because Ang II stimulates oxidative stress, which in turn quenches nitric oxide, it is possible that Ang II-induced increases in oxidative stress contribute to the increase in blood pressure. Indeed, administering either a converting enzyme inhibitor (to decrease Ang II) or Tempol (a potent antioxidant) reduces blood pressure in SHR. We tested whether decreasing Ang II with a converting enzyme inhibitor reduces oxidative stress as well as MAP in SHR. For this, we divided 12 weeks old SHR into two groups (n=5 each). One group was treated with captopril (100 mg/kg/day added to the drinking water) and the other served as our untreated time controls. After 16 days of treatment, the rats were anesthetised and we measured MAP and collected blood samples to determine PRA, and the plasma levels of Ang II and thiobarbituric acid-reactive substances (TBARS). The captopril-treated rats had a lower MAP than the untreated rats (92±4 vs. 160±5 mmHg, respectively) and an increased PRA (42±1 vs. 26±6 ng/ml/h; captopril-treated vs. untreated rats, respectively). The decreased MAP in the captopril-treated SHR was accompanied by reduced plasma levels of Ang II (630±47 vs. 836±205 pg/ml) and TBARS (5.4±1.0 vs. 3.0±0.2 nmol/ml). Despite the significant decrease in Ang II levels in the captopril-treated SHR, they are still 20-fold higher rhan in normotensive Sprague-Dawley rats (34.0±8.8 pg/ml). In summary, captopril-induced decreases of MAP in SHR are accompanied not only by reduced levels of Ang II, but also by reduced oxidative stress. Because antioxidants also lower MAP in SHR, it suggests that oxidative stress induced by Ang II may play a role in the pathogenesis of the increased blood pressure in SHR.

P105 The kidney is a target organ for cardiovascular risk factors, such as atherosclerosis and hypertension (HT), and is particularly susceptible to their combination. Even at an early stage, hypercholesterolemia (HC) and HT are individually associated with impaired renal perfusion responses to challenge with vasodilators. However, the effect of combined HC and HT on renal perfusion remains uncertain. To examine this, regional renal perfusion response to the renal vasodilator acetylcholine (Ach) was quantified in 4 groups of pigs, using electron beam CT, after 12 weeks of either normal (n=6), HC diet (n=6), HT (unilateral renal artery stenosis, n=5), or combined HC and HT (HC+HT, n=6). The HC and HC+HT groups had increased cholesterol levels vs. normal and HT (430±82 and 351±52 vs. 71±6 and 83±4 mg/dl, ANOVA p

44 Chronic infusion of subpressor doses of Ang II causes blood pressure to increase progressively over several days. The mechanisms underlying this response are unknown but may involve Ang II-induced generation of additional vasoconstrictor processes. In this study, we tested whether endothelin and/or oxidative stress are implicated in the slow pressor responses to Ang II. We infused either vehicle (group 1; n=6) or Ang II (group 2; n=5) intravenously at 5 ng/kg/min via osmotic pumps for 15 days into Sprague Dawley rats. In addition to the Ang II infusion, groups 3 and 4 (n=6 each) received 30 mg/kg/day of either losartan (an angiotensin AT 1 receptor blocker) or bosentan (a blocker of both endothelin receptors, ET A and ET B ) in their drinking water. We measured systolic blood pressure (SBP) during the infusion, and the levels of circulating Ang II and isoprostanes (a marker of oxidative stress) at the conclusion of the experiments. Rats infused with vehicle had no change in SBP (from 138±13 to 138±2 mmHg) and normal levels of Ang II (34.5±9 pg/ml) and isoprostanes (111±10 pg/ml). Ang II infusion increased SBP from 133±10 to 158±8 mmHg, as well as circulating levels of Ang II (144±65 pg/ml) and isoprostanes (156±19 pg/ml). Losartan treatment abolished Ang II induced increases in SBP (SBP went from 137±5 to 120±4 mmHg), and isoprostanes (115±15 pg/ml), without altering Ang II levels (101±30 pg/ml). Bosentan also blocked Ang II-induced increases in SBP (from 135±4 to 139±3) but did not alter the increased isoprostane levels (146±14 pg/ml). Surprisingly, bosentan blunted the increase in Ang II levels (51±10 pg/ml). In conclusion, low dose Ang II-induced increases in SBP and oxidant stress depend on the AT 1 receptor. Endothelin receptor blockade also reduces SBP, but it does so independently of reducing oxidative stress (as measured by isoprostanes).

Antidiuretic hormone is known to stimulate the renal synthesis of prostaglandins. These autacoids, in turn, modulate the pressure natriuresis phenomenon. Accordingly, the present study was done to test the hypothesis that, in the absence of antidiuretic hormone and antidiuretic hormone-dependent prostaglandin synthesis, the pressure natriuresis response is blunted. Experiments were performed on Brattleboro diabetes insipidus rats (n = 7) and Long Evans control rats (n = 14). A change in perfusion pressure in the Long Evans rats from 89.3 +/- 1.0 to 108.7 +/- 1.1 mm Hg (p less than 0.05) was associated with significant increases in the fractional excretion of sodium (1.1 +/- 0.2 to 2.3 +/- 0.3%) and the urinary prostaglandin excretion (32.6 +/- 6.8 to 56.6 +/- 10.0 pg/min). In contrast, a similar change in perfusion pressure in the diabetes insipidus rat from 88.6 +/- 1.4 to 106.2 +/- 1.5 mm Hg (p less than 0.05) resulted in no significant increases in either sodium or prostaglandin excretions. Treatment of a third group of diabetes insipidus rats (n = 9) with 1-desamino-8-D-arginine vasopressin (1 microgram/day) restored the natriuretic response to increases in renal perfusion pressure. Treated diabetes insipidus and Long Evans control rats had comparable natriuretic responses to increases in renal perfusion pressure. Untreated diabetes insipidus rats, on the other hand, had blunted responses. In summary, the pressure natriuresis response in diabetes insipidus rats is blunted compared with Long Evans control rats. We conclude that antidiuretic hormone is necessary for the complete expression of the pressure natriuresis response.

Strong evidence indicates that a high protein diet accelerates end-stage renal disease by increasing glomerular capillary pressure subsequent to renal vasodilatation. The mechanisms underlying this vasodilatation remain undefined, but they have been suspected to be mediated by a pituitary factor. To test this possibility, we measured changes in renal plasma flow and glomerular filtration rate induced by an intravenous infusion of a solution of amino acids in two patients with panhypopituitarism. These patients exhibited changes in renal hemodynamics comparable to those recorded in nine healthy volunteers. The results do not support involvement of the pituitary gland in the acute renal response to amino acids.

Glomeruli were isolated from rat kidneys using a passive sieving technique to study the mechanisms of basal and beta-adrenergic stimulated renin release. Glomeruli were enclosed within glass chambers and continuously superfused with Krebs media, or modified Krebs as described below, at a rate of 0.3 ml/min. The chamber effluent was collected in 10-minute fractions and measured for renin concentration (ng angiotensin I (A-1 generated) by radioimmunoassay. Basal renin was approximately 3 ng AI/ml/hr. Beta-adrenergic stimulation with isoproterenol (ISO), 178 micron M, increased renin concentration threefold (11 +/- 2 ng AI). The beta-blocker propranolol at 12 micron M halved ISO-stimulated renin, and at 120 micron M eliminated it. Doubling Krebs sodium concentration (280 mM) had no effect upon basal or ISO-stimulated renin release. Pretreating rast with DOCA and a high salt diet significantly reduced basal and abolished ISO-stimulated renin release. Increasing Krebs calcium (10 mM) did not affect basal but abolished ISO-stimulated renin release. Calcium-free Krebs had no significant effects. Increasing Krebs potassium (50 mM) increased basal renin fourfold (14 ng AI) while the absolute increase from basal due to ISO remained the same (23 ng AI). These results suggest that basal renin and ISO-stimulated renin are released via different mechanisms.

In a group of six patients diagnosed as having unilateral renovascular hypertension due to fibromuscular dysplasia, inulin glomerular filtration rate, (GFR) and PAH renal plasma flow, (RPF) clearances, urine flow (V), urine sodium (UVNa), potassium (UVK), urinary excretion of prostaglandin E2 (UVPGE2), thromboxane B2 (UVTxB2), and 6-keto prostaglandin F1 alpha (UVPGF1 alpha) were measured in each kidney before and after the i.v. administration of furosemide (20 mg). The basal values of GFR, RPF, UVNa, UVPGE2, UVTxB2, and UV6-keto-PGF1 alpha were lower (p less than 0.01) in the stenotic kidney. Furosemide increased RPF 11% and 50%, GFR 25% and 62%, and V 142% and 280% in the contralateral and stenotic kidney respectively. The increase of UVNa was similar in the two kidneys. In the stenotic kidney, both UVPGE2 and UV6-keto-PGF1 alpha increased significantly (p less than 0.01) with furosemide while UVTxB2 remained unchanged. Furosemide did not alter the rate of excretion of the three prostaglandins measured in the contralateral kidney. We conclude that furosemide significantly improves renal circulatory and excretory function of the stenotic kidney. Since prostaglandin excretions also increased, the vasodilatation in the stenotic kidney may be prostaglandin mediated.