Autoregulation of renal medullary blood flow in rabbits

We examined the extent of renal medullary blood flow (MBF) autoregulation in pentobarbital-anesthetized rabbits. Two methods for altering renal arterial pressure (RAP) were compared: the conventional method of graded suprarenal aortic occlusion and an extracorporeal circuit that allows RAP to be increased above systemic arterial pressure. Changes in MBF were estimated by laser-Doppler flowmetry, which appears to predominantly reflect erythrocyte velocity, rather than flow, in the kidney. We compared responses using a dual-fiber needle probe held in place by a micromanipulator, with responses from a single-fiber probe anchored to the renal capsule, to test whether RAP-induced changes in kidney volume confound medullary laser-Doppler flux (MLDF) measurements. MLDF responses were similar for both probe types and both methods for altering RAP. MLDF changed little as RAP was altered from 50 to >or=170 mmHg (24 +/- 22% change). Within the same RAP range, RBF increased by 296 +/- 48%. Urine flow and sodium excretion also increased with increasing RAP. Thus pressure diuresis/natriuresis proceeds in the absence of measurable increases in medullary erythrocyte velocity estimated by laser-Doppler flowmetry. These data do not, however, exclude the possibility that MBF is increased with increasing RAP in this model, because vasa recta recruitment may occur.     

Biphasic actions of angiotensin IV on renal blood flow in the rat

Our aim was to investigate the changes in renal blood flow during brief exposure of the renal vasculature to angiotensin IV (Ang IV). Total renal blood flow was measured by electromagnetic flowmetry in pentobarbital-anesthetized male Sprague Dawley rats. Intrarenal injection of Ang I, Ang II and Ang III produced a dose-dependent vasoconstriction. In contrast, Ang IV and Ang-(3-10) produced a dose-dependent rapid vasoconstriction (lasting seconds) followed by a transient vasodilatation (lasting minutes). The biphasic response to Ang IV was unchanged in rats pre-treated with captopril, whereas the Ang-(3-10) response was abolished implying that its vasoactive activity was due to the generation of Ang IV. The vasodilatory component of Ang IV was unaffected by indomethacin. The biphasic vasoactive response of Ang IV was unaffected by divalinal-Ang IV (AT(4) receptor antagonist) or PD 123319 (AT(2) receptor antagonist), but greatly reduced by losartan or L-158,809 (AT(1) receptor antagonists). These results suggest that Ang IV is distinct from other angiotensins in that it possesses non-prostaglandin mediated renal vasodilatory activity that is apparently linked to the renal vascular AT(1) receptor.     

HO-1 induction lowers blood pressure and superoxide production in the renal medulla of angiotensin II hypertensive mice

Heme oxygenase-1 (HO-1) induction can attenuate the development of angiotensin II (ANG II)-dependent hypertension. However, the mechanism by which HO-1 lowers blood pressure in this model is not clear. The goal of this study was to test the hypothesis that induction of HO-1 in the kidney can attenuate the increase in reactive oxygen species (ROS) generation in the kidney that occurs during ANG II-dependent hypertension. Mice were divided into four groups, control (Con), cobalt protoporphyrin (CoPP), ANG II, and ANG II + CoPP. CoPP treatment (50 mg/kg) was administered in a single subcutaneous injection 2 days prior to implantation of an osmotic minipump that infused ANG II at a rate of 1 microg x kg(-1) x min(-1). At the end of this period, mean arterial blood pressure (MAP) averaged 93 +/- 5, 90 +/- 5, 146 +/- 8, and 105 +/- 6 mmHg in Con, CoPP-, ANG II-, and ANG II + CoPP-treated mice. To determine whether HO-1 induction resulted in a decrease in ANG II-stimulated ROS generation in the renal medulla, superoxide production was measured. Medullary superoxide production was increased by ANG II infusion and normalized in mice pretreated with CoPP. The reduction in ANG II-mediated superoxide production in the medulla with CoPP was associated with a decrease in extracellular superoxide dismutase protein but an increase in catalase protein and activity. These results suggest that reduction in superoxide and possibly hydrogen peroxide production in the renal medulla may be a potential mechanism by which induction of HO-1 with CoPP lowers blood pressure in ANG-II dependent hypertension.     

Ang Ⅱ拮抗剂对慢性肾衰大鼠肾血流量和肾内氧耗的影响

目的观察血管紧张素II(AngⅡ)拮抗剂对5/6(ablation/infarction,A/I)肾切除诱导慢性肾衰竭(CRF)大鼠肾功能、肾血流量及肾内氧耗的影响。方法制备5/6(A/I)肾切除诱导慢性肾衰大鼠模型,设正常组(A组,n=14只),模型组(B组,n=14只),AngⅡ拮抗剂治疗组(氯沙坦钾联合福辛普利钠)(C组,n=14只)。给予相应干预,疗程60 d。分别测量尾动脉收缩压(SBP)、舒张压(DBP),检测大鼠尾静脉血清肌酐(Scr)、尿素氮(BUN)、血红蛋白(Hb),计算内生肌酐清除率(Ccr)。干预60 d后,检测肾血流量(RBF)、腹主动脉和肾静脉血气(AABG and RVBG),左肾静脉压(RVpO2),计算残余肾内氧耗(QO2/TNa)及观察残肾组织病理变化。结果 (1)造模后与A组比较,B、C两组的Scr、BUN和尾动脉SBP、DBP显著增加(P0.01),Ccr、Hb显著降低(P0.01),提示造模成功。(2)干预后与B组比较,C组的Scr、尾动脉SBP、DBP、QO2/TNa明显下降(P0.01),BUN降低(P0.05),Hb、Ccr、RVpO2显著升高(P0.01),RBF升高(P0.05)。(3)残肾组织病理形态学变化显示,C组的肾组织病理变化明显减轻,优于B组。结论 AngⅡ拮抗剂可以增加慢性肾衰大鼠肾血流量,降低肾内氧耗,改善肾功能及减轻肾组织病理变化,其肾脏保护作用机制可能与其调节细胞能量代谢,改善肾内氧耗有关。     

Central angiotensin II increases biosynthesis of tyrosine hydroxylase in the rat adrenal medulla

Angiotensin II acting centrally contributes to the regulation of blood pressure and water intake and stimulates the release of catecholamines from the adrenal medulla. We hypothesized that the central angiotensin II is one mediator of biosynthesis of catecholamines in the adrenal medulla. Rats were administered i.c.v. angiotensin II or saline, and TH mRNA and protein levels in adrenal medulla were measured 1 or 3 h later. Angiotensin II did not change TH mRNA or protein 1 h later. However, by 3 h, angiotensin II increased TH mRNA and protein levels. Centrally administered angiotensin II elevates TH mRNA expression and protein levels in the adrenal medulla. In conclusion, one component of central angiotensin II elevation of blood pressure may be the result of increased catecholamine synthesis in the adrenal gland and elevated TH synthesis represents one underlying mechanism.     

The role of angiotensin II in renal growth.

Angiotensin II (AII) has many of the features of the archetypical growth factors and appears to be a growth regulator in the kidney. AII binds to specific cell surface receptors present on a number of different renal cell types including mesangial, vascular smooth muscle, tubular and interstitial cells, and activates many of the intracellular signalling pathways associated with cell growth. In vitro AII can potentiate the mitogenic effect of other growth factors such as EGF. AII induces hypertrophy of vascular smooth muscle cells but the role of AII in the growth of other renal cell types has not been systematically studied.     

Autoregulation of diaphragmatic blood flow in dogs

In eight anesthetized spontaneously breathing dogs, we determined whether diaphragmatic blood flow is dependent on arterial blood pressure (Pa) or whether it is autoregulated. We also determined whether diaphragmatic muscular activity affects the degree of autoregulation. We measured blood flow through the left phrenic artery (Qphr) with an electromagnetic flow probe and decreased Pa in steps by controlled hemorrhage. Phrenic venous blood was sampled to allow the calculation of diaphragmatic O2 consumption (VO2phr). Diaphragmatic energy demands were varied by using three inspiratory resistances (R1, R2, and R3), which increased peak transdiaphragmatic pressure two-, three-, and fourfold, respectively. During quiet breathing, Qphr was independent of Pa between Pa of 90 and 120 mmHg (i.e., plateau of pressure-flow relation), but at lower Pa, Qphr was directly related to Pa. During inspiratory loading, the Qphr plateau ended at a higher Pa than with quiet breathing, but within the normal ranges of Pa there still was a plateau. VO2phr at a given work load was constant between Pa of 70 and 120 mmHg, but at Pa of 50-55 mmHg, VO2phr declined with all work loads. We conclude that in spontaneously breathing dogs 1) Qphr is autoregulated over the normal range of blood pressures and 2) VO2phr is maintained over wider ranges of Pa than Qphr.     

Autoregulation of avian renal plasma flow: contribution of the renal portal system

Summary A simplified avian kidney model was used to assess renal plasma flow (RPF) at normal (100–110 mmHg) or unilaterally reduced (40–50 mmHg) renal arterial perfusion pressure (RAPP) in domestic fowl with ambient (AMBIENT group) or restricted (RESTRICTED group) renal portal flow. Direct measurement of para-aminohippuric acid (PAH) extraction efficiencies (E_PAH) allowed avian RPF to be calculated from the clearance of PAH (C_PAH). E_PAH was unaffected by RAPP, thereby validating the use of PAH to estimate RPF during experimental hemodynamic manipulations. C_PAH and RPF were unaffected by RAPP in the AMBIENT group (perfect autoregulation), but decreased significantly compared with contralateral kidney values during reduction of RAPP in the RESTRICTED group. Urine flow and glomerular filtration rates (GFR) were reduced unilaterally along with RAPP, regardless of the portal perfusion status. The renal portal system contributes to overall RPF autoregulation in domestic fowl, helping to maintain constancy of renal blood flow even after RAPP is reduced well below the GFR autoregulatory limit.Abbreviations BW body weight - C _In Clearance of inulin - C _PAH clearance of PAH - E _PAH PAH extraction efficiency - FF filtration fraction - GFR glomerular filtration rate - LiOH lithium hydroxide - MT mammalian-type nephron - PAH para-aminohippuric acid - [PAH] _A concentration of PAH in arterial plasma - [PAH] _a chromagen corrected PAH in arterial plasma - [PAH] _E endogenous PAH-like chromagen - [PAH] _UF concentration of ultrafilterable PAH - [PAH] _v concentration of PAH in renal venous plasma - [PAH] _v chromagen corrected PAH in renal venous plasma - RAPP renal arterial perfusion pressure - RPF renal plasma flow - RT reptilian-type nephron - UFR urine flow rate - UFR per gram urine flow rate per gram kidney weight - T _M S PAH tubular secretory maximum for PAH - SEM standard error of mean     

High-affinity angiotensin receptors in rat adrenal medulla

Angiotensin II receptors have been quantitated in single rat adrenal medullas by incubation of tissue sections with 125I-[Sar1]-AII, autoradiography with exposure to 3H-sensitive Ultrofilm, computerized densitometry and comparison with 125I-labelled standards. Rat adrenal medulla contains a single class of high affinity AII receptors with a Ka of 0.84 +/- 0.02 X 10(9) M-1 and a Bmax of 3259 +/- 502 fmol/mg protein, one of the highest densities in AII receptors found in rat tissues. These observations provide evidence for a local site of action of AII in the release of adrenal medullary catecholamines.     

L-Arginine uptake affects nitric oxide production and blood flow in the renal medulla

Experiments were performed to determine whether L-arginine transport regulates nitric oxide (NO) production and hemodynamics in the renal medulla. The effects of renal medullary interstitial infusion of cationic amino acids, which compete with L-arginine for cellular uptake, on NO levels and blood flow in the medulla were examined in anesthetized rats. NO concentration in the renal inner medulla, measured with a microdialysis-oxyhemoglobin trapping technique, was significantly decreased by 26-44% and renal medullary blood flow, measured by laser Doppler flowmetry, was significantly reduced by 20-24% during the acute renal medullary interstitial infusion of L-ornithine, L-lysine, and L-homoarginine (1 micromol.kg(-1).min(-1) each; n = 6-8/group). In contrast, intramedullary infusion of L-arginine increased NO concentration and medullary blood flow. Flow cytometry experiments with 4-amino-5-methylamino-2',7'-difluorescein diacetate, a fluorophore reactive to intracellular NO, demonstrated that L-ornithine, L-lysine, and L-homoarginine decreased NO by 54-57% of control, whereas L-arginine increased NO by 21% in freshly isolated inner medullary cells (1 mmol/l each, n > 1,000 cells/experiment). The mRNA for the cationic amino acid transporter-1 was predominantly expressed in the inner medulla, and cationic amino acid transporter-1 protein was localized by immunohistochemistry to the collecting ducts and vasa recta in the inner medulla. These results suggest that L-arginine transport by cationic amino acid transport mechanisms is important in the production of NO and maintenance of blood flow in the renal medulla.     

Measurement of renal blood flow in the rat.

The radioactive microsphere technique is a simple method for measurement of RBF and intrarenal blood flow distribution in the rat that does not require surgical manipulation of the kidney or general anesthesia. The results are reproducible and compatible with other established techniques.     

Reversal of renovascular hypertension: role of the renal medulla

The fall in blood pressure, which occurs when renovascular hypertension is corrected surgically, offers a means of elucidating the factors responsible for blood pressure control. When Goldblatt two-kidney, one-clip hypertension in the rat is reversed by unclipping the renal artery, or by removal of the ischaemic kidney, restoration of normal blood pressure is due to a fall in peripheral resistance. This is associated with sodium retention and cannot be modified by inhibition of the renin-angiotensin system. The fall is, however, partially inhibited by chemical removal of the renal medulla by means of 2-bromo-ethylamine hydrobromide. When normal rats are chemically medullectomized, moderate hypertension is produced, which cannot be attributed to the renin-angiotensin system or sodium retention. It is concluded that a renomedullary vasodepressor system is ablated by chemical medullectomy: further, this system plays a role in the surgical correction of Goldblatt hypertension.     

Intrarenal distribution of renal blood flow in the rat.

Intrarenal blood flow distribution was studied with the simultaneous use of the 99Tc labelled frog erythrocyte (microsphere) and the radioactive 86Rb fractionation method in the rat. The amount of blood entering the outer cortex (99Tc labelled erythrocytes method) proved to be higher than one perfusing the outer cortex (86Rb method), whereas the amount of blood entering the inner cortex (99Tc method) was less than the amount perfusing the inner cortex and medulla (86Rb method). Hence a group of the preglomerular arterioles in the outer cortex contributes to the blood supply of the inner cortex, on the other hand a group of preglomerular arteries in the inner cortex participates in the postglomerular blood supply of the medulla. Changes in the renal circulation are, however, associated with altered distribution of postglomerular vascular segments supplied by some groups of preglomerular arterioles. From this it is concluded that the postglomerular vessels of the deeper cortical layers constitute a system which is not parallelly coupled but comprises both series- and parallel-coupled sections. The contribution of these sections appears to vary depending on the actual haemodynamic conditions.     

Hypoxia and angiotensin II infusion redistribute lung blood flow in lambs

To assess the effects of alveolar hypoxia and angiotensin II infusion on distribution of blood flow to the lung we performed perfusion lung scans on anesthetized mechanically ventilated lambs. Scans were obtained by injecting 1-2 mCi of technetium-labeled albumin macroaggregates as the lambs were ventilated with air, with 10-14% O2 in N2, or with air while receiving angiotensin II intravenously. We found that both alveolar hypoxia and infusion of angiotensin II increased pulmonary vascular resistance and redistributed blood flow from the mid and lower lung regions towards the upper posterior region of the lung. We assessed the effects of angiotensin II infusion on filtration pressure in six lambs by measuring the rate of lung lymph flow and the protein concentration of samples of lung lymph. We found that angiotensin II infusion increased pulmonary arterial pressure 50%, lung lymph flow 90%, and decreased the concentration of protein in lymph relative to plasma. These results are identical to those seen when filtration pressure increases during alveolar hypoxia. We conclude that alveolar hypoxia and angiotensin II infusion both increase fluid filtration in the lung by increasing filtration pressure. The increase in filtration pressure may be the result of a redistribution of blood flow in the lung with relative overperfusion of vessels in some areas and transmission of the elevated pulmonary arterial pressure to fluid-exchanging sites in those vessels.     

Effect of furegrelate on renal plasma flow after angiotensin II infusion

We had previously shown that selective thromboxane synthetase inhibition with furegrelate increases urinary excretion of 6-ketoPGF1 alpha, the hydrolysis product of prostacyclin after stimulation of renal prostaglandin synthesis with furosemide. The present study assessed the functional significance of this "redirection" of prostaglandin formation using a more physiologic stimulus, angiotensin II. Sprague-Dawley rats (n = 27) were fitted with a transabdominal bladder cannula. Five days later they were given angiotensin II (10 mg.kg-1.min-1) by intravenous infusion. After 30 min, an infusion of furegrelate, 2 mg/kg, then 2 mg.kg-1.h-1, (n = 9); indomethacin, 2 mg/kg, then 2 mg.kg-1.h-1 (n = 9); or vehicle, 250 microL, then 0.018 mL/min (n = 9) was begun for 60 min. Clearance of [14C]para-aminohippuric acid was taken as a measure of renal plasma flow. Angiotensin II raised the mean arterial pressure in all groups. Administration of furegrelate or indomethacin did not change mean arterial pressure or heart rate. Angiotensin II reduced [14C]p-aminohippuric acid clearance by about 32% (1.42 +/- 0.18 to 0.97 +/- 0.07 mL.min-1.100 g-1, p less than 0.05). Furegrelate attenuated this renal vasoconstriction (0.97 +/- 0.07 to 1.38 +/- 0.17 mL.min-1.100 g-1, p less than 0.05), while indomethacin increased it by a further 32% (1.78 +/- 0.12 to 1.20 +/- 0.12 mL.min-1.100 g-1, p less than 0.05). Vehicle alone had no effect. Furegrelate reduced serum thromboxane B2 by 90% (6.52 +/- 0.030 to 0.7 +/- 0.21 ng/100 microL, p less than 0.05), while indomethacin reduced it by 73% (5.9 +/- 0.99 to 1.4 +/- 0.20 ng/100 microL, p less than 0.05). We conclude that furegrelate attenuates the renal vasoconstriction of angiotensin II, presumably by enhancing the formation of vasodilator prostaglandins.     

Phospholipid metabolism in the rat renal inner medulla

In view of the importance of phospholipids as a source of precursor fatty acids for the high prostaglandin synthesis in the renal inner medulla, we studied pathways of phospholipid esterification and degradation in the rat inner medulla. De novo acylation of [14C]arachidonate occurred predominantly in position 2 of phosphatidylcholine in the microsomal fraction. This newly esterified [14C]arachidonate was accessible to deacylation by a microsomal phospholipase A2 (EC 3.1.1.4) with alkaline optimum which was Ca2+-dependent and resistant to 0.1% deoxycholate. No phospholipase A1 (EC 3.1.1.32) activity against endogenous labeled phosphatidylcholine could be demonstrated in the microsomal fraction. When exogenous phosphatidylcholine labeled at position 2 was deacylated by renomedullary homogenates, labeled free fatty acid but no labeled lysophosphatidylcholine was recovered in the reaction products. This could be attributed to further degradation of generated lysophosphatidylcholine by a cytosolic lysophospholipase (EC 3.1.1.5). Sodium deoxycholate at a concentration of 0.1% or higher inhibited the lysophospholipase and allowed the demonstration of both A2 and A1 alkaline phospholipase activities in the homogenate. The major in vitro pathway of lysophosphatidylcholine disposition is further degradation by a cytosolic lysophospholipase, while reutilization for phosphatidylcholine synthesis through the action of a predominantly microsomal acyltransferase appears to be a minor pathway. In the presence of several acyl-CoAs, reutilization of lysophosphatidylcholine is significantly increased by an acyl-CoA:lysophosphatidylcholine acyltransferase (EC 2.3.1.23) but there is no preferential transfer of arachidonyl-CoA compared to other acyl-CoAs.     

Functional role of angiotensin II type 1 and 2 receptors in regulation of uterine blood flow in nonpregnant sheep

The objective was to determine the receptor subtype of angiotensin II (ANG II) that is responsible for vasoconstriction in the nonpregnant ovine uterine and systemic vasculatures. Seven nonpregnant estrogenized ewes with indwelling uterine artery catheters and flow probes received bolus injections (0.1, 0.3 and 1 microg) of ANG II locally into the uterine artery followed by a systemic infusion of ANG II at 100 ng x kg(-1) x min(-1) for 10 min to determine uterine vasoconstrictor responses. Uterine ANG II dose-response curves were repeated following administration of the ANG II type 2 receptor (AT(2)) antagonist PD-123319 and then repeated again in the presence of an ANG II type 1 receptor (AT(1)) antagonist L-158809. In a second experiment, designed to investigate the mechanism of ANG II potentiation that occurred in the presence of AT(2) blockade, nonestrogenized sheep received a uterine artery infusion of L-158809 (3 mg/min for 5 min) prior to the infusion of 0.03 microg/min of ANG II for 10 min. ANG II produced dose-dependent decreases in uterine blood flow (P < 0.03), which were potentiated in the presence of the AT(2) antagonist (P < 0.02). Addition of the AT(1) antagonist abolished the uterine vascular responses and blocked ANG II-induced increases in systemic arterial pressure (P < 0.01). Significant uterine vasodilation (P < 0.01) was noted with AT(1) blockade in the second experiment, which was reversed by administration of the AT(2) antagonist or by the nitric oxide synthetase inhibitor N(omega)-nitro-L-arginine methyl ester. We conclude that the AT(1)-receptors mediate the systemic and uterine vasoconstrictor responses to ANG II in the nonpregnant ewe. AT(2)-receptor blockade resulted in a potentiation of the uterine vasoconstrictor response to ANG II, suggesting that the AT(2)-receptor subtype may modulate uterine vascular responses to ANG II potentially by release of nitric oxide.