Selective effect of tempol on renal medullary hemodynamics in spontaneously hypertensive rats

The present study assessed the short- and long-term effect of tempol, a membrane-permeable mimetic of superoxide dismutase, on renal medullary hemodynamics in spontaneously hypertensive rats (SHR). Tempol was given in the drinking water (1 mM) for 4 days or 7 wk (4-11 wk of age), and medullary blood flow (MBF) was measured over a wide range of renal arterial pressure by means of laser-Doppler flowmetry in anesthetized rats. In addition, the response of the medullary circulation to angiotensin II (5-50 ng x kg(-1) x min(-1) iv) was determined in SHR treated for 4 days with tempol. Compared with control SHR, short- and long-term treatment with tempol decreased arterial pressure by approximately 20 mmHg and increased MBF by 35-50% without altering total renal blood flow (RBF) or autoregulation of RBF. Angiotensin II decreased RBF and MBF dose dependently (approximately 30% at the highest dose) in control SHR. In SHR treated with tempol, angiotensin II decreased RBF (approximately 30% at the highest dose) but did not alter MBF significantly. These data indicate that the antihypertensive effect of short- and long-term administration of tempol in SHR is associated with a selective increase in MBF. Tempol also reduced the sensitivity of MBF to angiotensin II. Taken together, these data support the idea that tempol enhances vasodilator mechanisms of the medullary circulation, possibly by interacting with the nitric oxide system. Increased MBF and reduced sensitivity of MBF to angiotensin II may contribute to the antihypertensive action of tempol in SHR.     

Differential control of intrarenal blood flow during reflex increases in sympathetic nerve activity

The role of renal sympathetic nerve activity (RSNA) in the physiological regulation of medullary blood flow (MBF) remains ill defined, yet regulation of MBF may be crucial to long-term arterial pressure regulation. To investigate the effects of reflex increases in RSNA on intrarenal blood flow distribution, we exposed pentobarbital sodium-anesthetized, artificially ventilated rabbits (n = 7) to progressive hypoxia while recording RSNA, cortical blood flow (CBF), and MBF using laser-Doppler flowmetry. Another group of animals with denervated kidneys (n = 6) underwent the same protocol. Progressive hypoxia (from room air to 16, 14, 12, and 10% inspired O(2)) significantly reduced arterial oxygen partial pressure (from 99 +/- 3 to 65 +/- 2, 51 +/- 2, 41 +/- 1, and 39 +/- 2 mmHg, respectively) and significantly increased RSNA (by 8 +/- 3, 44 +/- 25, 62 +/- 21, and 76 +/- 37%, respectively, compared with room air) without affecting mean arterial pressure. There were significant reductions in CBF (by 2 +/- 1, 5 +/- 2, 11 +/- 3, and 14 +/- 2%, respectively) in intact but not denervated rabbits. MBF was unaffected by hypoxia in either group. Thus moderate reflex increases in RSNA cause renal cortical vasoconstriction, but not at vascular sites regulating MBF.     

Modulation of V1-receptor-mediated renal vasoconstriction by epoxyeicosatrienoic acids

This study examined the effects of renal arterial infusion of a selective cytochrome P-450 epoxygenase inhibitor, N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH; 2 mg/kg plus 1.5 mg.kg(-1).h(-1)), on renal hemodynamic responses to infusions of [Phe(2),Ile(3),Orn(8)]vasopressin and ANG II into the renal artery of anesthetized rabbits. MS-PPOH did not affect basal renal blood flow (RBF) or cortical or medullary blood flow measured by laser-Doppler flowmetry (CLDF/MLDF). In vehicle-treated rabbits, [Phe(2),Ile(3),Orn(8)]vasopressin (30 ng.kg(-1).min(-1)) reduced MLDF by 62 +/- 7% but CLDF and RBF were unaltered. In MS-PPOH-treated rabbits, RBF and CLDF fell by 51 +/- 8 and 59 +/- 13%, respectively, when [Phe(2),Ile(3),Orn(8)]vasopressin was infused. MS-PPOH had no significant effects on the MLDF response to [Phe(2),Ile(3),Orn(8)]vasopressin (43 +/- 9% reduction). ANG II (20 ng.kg(-1).min(-1)) reduced RBF by 45 +/- 10% and CLDF by 41 +/- 14%, but MLDF was not significantly altered. MS-PPOH did not affect blood flow responses to ANG II. Formation of epoxyeicosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids (DiHETEs) was 49% lower in homogenates prepared from the renal cortex of MS-PPOH-treated rabbits than from vehicle-treated rabbits. MS-PPOH had no effect on the renal formation of 20-hydroxyeicosatetraenoic acid (20-HETE). Incubation of renal cortical homogenates from untreated rabbits with [Phe(2),Ile(3),Orn(8)]vasopressin (0.2-20 ng/ml) did not affect formation of EETs, DiHETEs, or 20-HETE. These results do not support a role for de novo EET synthesis in modulating renal hemodynamic responses to ANG II. However, EETs appear to selectively oppose V(1)-receptor-mediated vasoconstriction in the renal cortex but not in the medullary circulation and contribute to the relative insensitivity of cortical blood flow to V(1)-receptor activation [corrected].     

ANG II type 2 receptors and neural control of intrarenal blood flow

We tested the hypothesis that activation of angiotensin type 2 (AT(2)) receptors, by both exogenous and endogenous ANG II, modulates neurally mediated vasoconstriction in the renal cortical and medullary circulations. Under control conditions in pentobarbital-anesthetized rabbits, electrical stimulation of the renal nerves (RNS; 0.5-8 Hz) reduced renal blood flow (RBF; -88 +/- 3% at 8 Hz) and cortical perfusion (CBF; -92 +/- 2% at 8 Hz) more than medullary perfusion (MBF; -67 +/- 6% at 8 Hz). Renal arterial infusion of ANG II, at a dose titrated to reduce RBF by approximately 40-50% (5-50 ng.kg(-1).min(-1)) blunted responses of MBF to RNS, without significantly affecting responses of RBF or CBF. Subsequent administration of PD123319 (1 mg/kg plus 1 mg.kg(-1).h(-1)) during continued renal arterial infusion of ANG II did not significantly affect responses of RBF or CBF to RNS but enhanced responses of MBF, so that they were similar to those observed under control conditions. In contrast, administration of PD123319 alone blunted responses of CBF and MBF to RNS. Subsequent renal arterial infusion of ANG II in PD123319-pretreated rabbits restored CBF responses to RNS back to control levels. In contrast, ANG II infusion in PD123319-pretreated rabbits did not alter MBF responses to RNS. These data indicate that exogenous ANG II can blunt neurally mediated vasoconstriction in the medullary circulation through activation of AT(2) receptors. However, AT(2)-receptor activation by endogenous ANG II appears to enhance neurally mediated vasoconstriction in both the cortical and medullary circulations.     

Renal tissue NO and intrarenal haemodynamics during experimental variations of NO content in anaesthetised rats.

Direct renal nitric oxide (NO) measurements were infrequent and no simultaneous measurements of renal cortical and medullary NO and local perfusion. Large-surface NO electrodes were placed in renal cortex and medulla of anaesthetised rats; simultaneously, renal blood flow (RBF, index of cortical perfusion) and medullary laser-Doppler flux (MBF) were determined. NO synthase inhibitors: nonselective (L-NAME) or selective for neuronal NOS (nNOS) (S-methyl-thiocitrulline, SMTC), and NO donor (SNAP), were used to manipulate tissue NO. Baseline tissue NO was significantly higher in medulla (703+/-49 NM) than in cortex (231+/-17 nM). Minimal cortical and medullary NO current measured after maximal L-NAME dose (2.4 mg kg(-1) i.v.) was taken as tissue NO zero kevel. This dose decreased RBF and MBF significantly (-43%). SMTC, 1.2 mg kg(-1) h(-1) i.v., significantly decreased tissue NO by 105+/-32 nM in cortex and 546+/-64 nM in medulla, RBF and MBF decreased 30% and 20%, respectively. Renal artery infusion of SNAP, 0.24 mg kg(-1) min(-1) significantly increased tissue NO by 139+/-18 nM in cortex and 948+/-110 nM in medulla. Since inhibition of nNOS decreased medullary NO by 80% and MBF by 20% only, this isoform has probably minor role in the maintenance of medullary perfusion.     

Selected contribution: Gender differences in the endothelin-B receptor contribution to basal cutaneous vascular tone in humans.

To test whether the contribution of endothelin-B (ET-B) receptors to resting vascular tone differs between genders, we administered the ET-B receptor antagonist BQ-788 into the forearm skin of 11 male and 11 female subjects by intradermal microdialysis. Skin blood flow was measured using laser-Doppler flowmetry at the microdialysis site. The probe was perfused with Ringer solution alone, followed by BQ-788 (150 nM) and finally sodium nitroprusside (28 mM) to effect maximal cutaneous vasodilation. Cutaneous vascular conductance (CVC) was calculated (laser-Doppler flowmetry/mean arterial pressure) and normalized to maximal levels (%max). In male subjects, baseline CVC was (mean +/- SE) 19 +/- 3%max and increased to 26 +/- 5%max with BQ-788 (P < 0.05 vs. baseline). In female subjects, baseline CVC was 13 +/- 1%max and decreased to 10 +/- 1%max in response to BQ-788. CVC responses to BQ-788 differed with gender (P < 0.05); thus the contribution of ET-B receptors to resting cutaneous vascular tone differs between men and women. In men, ET-B receptors mediate tonic vasoconstriction, whereas, in women, ET-B receptors mediate tonic vasodilation.     

alpha(2)-adrenergic receptor-mediated increase in NO production buffers renal medullary vasoconstriction

The present study was designed to investigate the role of nitric oxide (NO) in modulating the adrenergic vasoconstrictor response of the renal medullary circulation. In anesthetized rats, intravenous infusion of norepinephrine (NE) at a subpressor dose of 0.1 microgram. kg(-1). min(-1) did not alter renal cortical (CBF) and medullary (MBF) blood flows measured by laser-Doppler flowmetry nor medullary tissue PO(2) (P(m)O(2)) as measured by a polarographic microelectrode. In the presence of the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME) in the renal medulla, intravenous infusion of NE significantly reduced MBF by 30% and P(m)O(2) by 37%. With the use of an in vivo microdialysis-oxyhemoglobin NO-trapping technique, we found that intravenous infusion of NE increased interstitial NO concentrations by 43% in the renal medulla. NE-stimulated elevations of tissue NO were completely blocked either by renal medullary interstitial infusion of L-NAME or the alpha(2)-antagonist rauwolscine (30 microgram. kg(-1). min(-1)). Concurrently, intavenous infusion of NE resulted in a significant reduction of MBF in the presence of rauwolscine. The alpha(1)-antagonist prazosin (10 microgram. kg(-1). min(-1) renal medullary interstitial infusion) did not reduce the NE-induced increase in NO production, and NE increased MBF in the presence of prazosin. Microdissection and RT-PCR analyses demonstrated that the vasa recta expressed the mRNA of alpha(2B)-adrenergic receptors and that medullary thick ascending limb and collecting duct expressed the mRNA of both alpha(2A)- and alpha(2B)-adrenergic receptors. These subtypes of alpha(2)-adrenergic receptors may mediate NE-induced NO production in the renal medulla. We conclude that the increase in medullary NO production associated with the activation of alpha(2)-adrenergic receptors counteracts the vasoconstrictor effects of NE in the renal medulla and may play an important role in maintaining a constancy of MBF and medullary oxygenation.     

Angiotensin II and nitric oxide in neural control of intrarenal blood flow

We investigated the roles of the renin-angiotensin system and the significance of interactions between angiotensin II and nitric oxide, in responses of regional kidney perfusion to electrical renal nerve stimulation (RNS) in pentobarbital sodium-anesthetized rabbits. Under control conditions, RNS (0.5-8 Hz) reduced total renal blood flow (RBF; -89 +/- 3% at 8 Hz) and cortical perfusion (CBF; -90 +/- 2% at 8 Hz) more than medullary perfusion (MBF; -55 +/- 5% at 8 Hz). Angiotensin II type 1 (AT(1))-receptor antagonism (candesartan) blunted RNS-induced reductions in RBF (P = 0.03), CBF (P = 0.007), and MBF (P = 0.04), particularly at 4 and 8 Hz. Nitric oxide synthase inhibition with N(G)-nitro-L-arginine (L-NNA) enhanced RBF (P = 0.003), CBF (P = 0.001), and MBF (P = 0.03) responses to RNS, particularly at frequencies of 2 Hz and less. After candesartan pretreatment, L-NNA significantly enhanced RNS-induced reductions in RBF (P = 0.04) and CBF (P = 0.007) but not MBF (P = 0.66). Renal arterial infusion of angiotensin II (5 ng.kg(-1).min(-1)) selectively enhanced responses of MBF to RNS in L-NNA-pretreated but not in vehicle-pretreated rabbits. In contrast, greater doses of angiotensin II (5-15 ng.kg(-1).min(-1)) blunted responses of MBF to RNS in rabbits with intact nitric oxide synthase. These results suggest that endogenous angiotensin II enhances, whereas nitric oxide blunts, neurally mediated vasoconstriction in the renal cortical and medullary circulations. In the renal medulla, but not the cortex, angiotensin II also appears to be able to blunt neurally mediated vasoconstriction.     

Adaptation of laser-Doppler flowmetry to measure cerebral blood flow in the fetal sheep.

The purpose of this study was to devise a means to use laser-Doppler flowmetry to measure cerebral perfusion before birth. The method has not been used previously, largely because of intrauterine movement artifacts. To minimize movement artifacts, a probe holder was molded from epoxy putty to the contour of the fetal skull. A curved 18-gauge needle was embedded in the holder. At surgery, the holder, probe, and skull were fixed together with tissue glue. Residual signals were recorded after fetal death and after maternal death 1 h later. These averaged <5% of baseline flow signals, indicating minimal movement artifact. To test the usefulness of the method, cerebral flow responses were measured during moderate fetal hypoxia induced by giving the ewes approximately 10% oxygen in nitrogen to breathe. As fetal arterial PO(2) decreased from 21.1 +/- 0.5 to 10.7 +/- 0.4 Torr during a 30-min period, cerebral perfusion increased progressively to 56 +/- 8% above baseline. Perfusion then returned to baseline levels during a 30-min recovery period. These responses are quantitatively similar to those spot observations that have been recorded earlier using labeled microspheres. We conclude that cerebral perfusion can be successfully measured by using laser-Doppler flowmetry with the unanesthetized, chronically prepared fetal sheep as an experimental model. With this method, relative changes of perfusion from a small volume of the ovine fetal brain can be measured on a continuous basis, and movement artifacts can be reduced to 5% of measured flow values.     

Increased H(2)O(2) counteracts the vasodilator and natriuretic effects of superoxide dismutation by tempol in renal medulla

A membrane-permeable SOD mimetic, 4-hydroxytetramethyl-piperidine-1-oxyl (tempol), has been used as an antioxidant to prevent hypertension. We recently found that this SOD mimetic could not prevent development of hypertension induced by inhibition of renal medullary SOD with diethyldithiocarbamic acid. The present study tested a hypothesis that increased H2O2 counteracts the effects of tempol on renal medullary blood flow (MBF) and Na+ excretion (UNaV), thereby restraining the antihypertensive effect of this SOD mimetic. By in vivo microdialysis and Amplex red H2O2 microassay, it was found that interstitial H2O2 levels in the renal cortex and medulla in anesthetized rats averaged 55.91 +/- 3.66 and 102.18 +/- 5.16 nM, respectively. Renal medullary interstitial infusion of tempol (30 micromol x min-1x kg-1) significantly increased medullary H2O2 levels by 46%, and coinfusion of catalase (10 mg x min-1x kg-1) completely abolished this increase. Functionally, removal of H2O2 by catalase enhanced the tempol-induced increase in MBF, urine flow, and UNaV by 28, 41, and 30%, respectively. Direct delivery of H2O2 by renal medullary interstitial infusion (7.5-30 nmol x min-1x kg-1) significantly decreased renal MBF, urine flow, and UNaV, and catalase reversed the effects of H2O2. We conclude that tempol produces a renal medullary vasodilator effect and results in diuresis and natriuresis. However, this SOD mimetic increases the formation of H2O2, which constricts medullary vessels and, thereby, counteracts its vasodilator actions. This counteracting effect of H2O2 may limit the use of tempol as an antihypertensive agent under exaggerated oxidative stress in the kidney.     

Control of right atrial pressure at constant cardiac output suppresses volume natriuresis in anesthetized rats

The blood volume of anesthetized rats was expanded acutely by 33% with donor blood while a caval snare was gradually tightened so that right atrial pressure (RAP) was prevented from rising (n = 6). In control experiments (n = 5) an aortic snare was used to hold mean arterial blood pressure near the values found in the experimental series. However, RAP was allowed to change freely and increased by 1.6 +/- 0.4 mmHg (1 mmHg = 133.322 Pa) during volume expansion. When the two groups were compared, there were no significant differences between their mean arterial blood pressures (near 110 mmHg) or in their cardiac outputs (near 0.25 mL X min-1 X g body weight-1). There were, however, significant differences between their renal responses to the volume load. When RAP was free to change, the rate of volume excretion (V) increased to 30 +/- 15 (SEM) microL X min-1 X g kidney weight-1 (KW) from its control value of 3.49 +/- 0.31 and the rate of sodium excretion (UNaV) increased to 3.59 +/- 0.20 muequiv X min-1 X g KW-1 from its preinfusion value of 0.42 +/- 0.10. When RAP was not allowed to increase during volume loading, V and UNaV did not change from their respective preinfusion values (2.99 +/- 0.46 microL X min-1 X g KW-1 and 0.35 +/- 0.10 muequiv X min-1 X g KW-1). The results imply that during acute blood volume expansion increased central vascular pressure is a prerequisite for the homeostasis of body water and salt.     

Effect of histamine and cimetidine on retinal and choroidal blood flow in humans

Intravenous administration of histamine causes an increase in choroidal blood flow and retinal vessel diameter in healthy subjects. The mechanism underlying this effect remains to be elucidated. In the present study, we hypothesized that H2 receptor blockade alters hemodynamic effects of histamine in the choroid and retina. Eighteen healthy male nonsmoking volunteers were included in this randomized, double-masked, placebo-controlled two-way crossover study. Histamine (0.32 microg.kg(-1).min(-1) over 30 min) was infused intravenously in the absence (NaCl as placebo) or presence of the H2 blocker cimetidine (2.3 mg/min over 50 min). Ocular hemodynamic parameters, blood pressure, and intraocular pressure were measured before drug administration, after infusion of cimetidine or placebo, and after coinfusion of histamine. Subfoveal choroidal blood flow and fundus pulsation amplitude were measured with laser-Doppler flowmetry and laser interferometry, respectively. Retinal arterial and venous diameters were measured with a retinal vessel analyzer. Retinal blood velocity was assessed with bidirectional laser-Doppler velocimetry. Histamine increased subfoveal choroidal blood flow (+14 +/- 15%, P < 0.001), fundus pulsation amplitude (+11 +/- 5%, P < 0.001), retinal venous diameter (+3.0 +/- 3.6%, P = 0.002), and retinal arterial diameter (+2.8 +/- 4.2%, P < 0.01) but did not change retinal blood velocity. The H2 antagonist cimetidine had no significant effect on ocular hemodynamic parameters. In addition, cimetidine did not modify effects of histamine on choroidal blood flow, fundus pulsation amplitude, retinal venous diameter, and retinal arterial diameter compared with placebo. The present data confirm that histamine increases choroidal blood flow and retinal vessel diameters in healthy subjects. This ocular vasodilator effect of histamine is, however, not altered by administration of an H2 blocker. Whether the increase in blood flow is mediated via H1 receptors or other hitherto unidentified mechanisms remains to be elucidated.     

Evaluation of laser-Doppler flowmetry as a measure of tissue blood flow

In this study the technique of laser-Doppler flowmetry was evaluated for the measurement of tissue blood flow by comparing laser-Doppler flow (LDF) signal in the renal cortex, gracilis muscle, and cremaster muscle of anesthetized rats to whole-organ blood flow measured with an electromagnetic flowmeter or radioactive microspheres. In vitro, LDF signal was closely correlated (r = 0.99) to changes in erythrocyte velocity generated with a rotating wheel. Although individual LDF readings varied in situ, mean LDF signal calculated from multiple readings on the tissue surface were significantly correlated (r = 0.74-0.95) with tissue blood flows measured at various perfusion pressures. However, significant differences in the slope of the LDF signal vs. blood flow relationship were observed in different tissues and with different methods of measurement in the same tissue. This study indicates that mean laser-Doppler flow signal provides a good estimate of tissue blood flow, provided a sufficient number of points is scanned. However, there appears to be no universal calibration factor for the method.     

Tempol improves renal hemodynamics and pressure natriuresis in hyperthyroid rats

Hyperthyroidism in rats is associated with increased oxidative stress. These animals also show abnormal renal hemodynamics and an attenuated pressure-diuresis-natriuresis (PDN) response. We analyzed the role of oxidative stress as a mediator of these alterations by examining acute effects of tempol, a superoxide dismutase mimetic. The effects of increasing bolus doses of tempol (25-150 micromol/kg) on mean arterial pressure (MAP), renal vascular resistance (RVR), and cortical (CBF) and medullary (MBF) blood flow were studied in control and thyroxine (T4)-treated rats. In another experiment, tempol was infused at 150 micromol.kg(-1).h(-1) to analyze its effects on the glomerular filtration rate (GFR) and on PDN response in these animals. Tempol dose dependently decreased MAP and RVR and increased CBF and MBF in control and T4-treated rats, but the T4 group showed a greater responsiveness to tempol in all of these variables. The highest dose of tempol decreased RVR by 13.5 +/- 2.1 and 5.5 +/- 1.2 mmHg.ml(-1).min(-1) in hyperthyroid (P < 0.01) and control rats, respectively. GFR was not changed by tempol in controls but was significantly increased in the hyperthyroid group. Tempol did not change the absolute or fractional PDN responses of controls but significantly improved those of hyperthyroid rats, although without attaining normal values. Tempol increased the slopes of the relationship between renal perfusion pressure and natriuresis (T4+tempol: 0.17 +/- 0.05; T4: 0.09 +/- 0.03 microeq.min(-1).g(-1).mmHg(-1); P < 0.05) and reduced 8-isoprostane excretion in hyperthyroid rats. These results show that antioxidant treatment with tempol improves renal hemodynamic variables and PDN response in hyperthyroid rats, indicating the participation of an increased oxidative stress in these mechanisms.     

Neonatal sympathectomy reduces NADPH oxidase activity and vascular resistance in spontaneously hypertensive rat kidneys

Neonatal sympathectomy reduces arterial pressure in spontaneously hypertensive rats (SHR). In SHR transplanted with a kidney from sympathectomized SHR, arterial pressure was lower and less Na+ sensitive than in SHR transplanted with a kidney from hydralazine-treated SHR. This study was performed to identify underlying renal mechanisms. Tests for differential renal mRNA expression of nine a priori selected genes revealed robust differences for renal medullary expression of the NADPH oxidase subunit p47phox. Therefore, we investigated the effects of neonatal sympathectomy on renal mRNA expression of NADPH oxidase subunits, NADPH oxidase activity, and renal function. In 10-wk-old sympathectomized SHR fed a 0.6% NaCl diet, medullary p47phox and gp91phox expression was 40% less than in hydralazine-treated SHR. Also, after a 1.8% NaCl diet, medullary p47phox mRNA expression was lower in sympathectomized than in hydralazine-treated SHR. We found lower cortical (-30%, P<0.01) and medullary (-30%, P<0.05) NADPH oxidase activities in sympathectomized than in hydralazine-treated or untreated SHR. Glomerular filtration rate, renal blood flow, medullary blood flow, and fractional Na+ excretion in kidney grafts from sympathectomized and hydralazine-treated donors (n=8 per group) were similar at baseline and in response to a 20-mmHg rise in renal perfusion pressure. Renal vascular resistance was lower in kidneys from sympathectomized than hydralazine-treated donors (25+/-2 vs. 32+/-4 mmHg.min.ml-1, P<0.05). The results indicate that the sympathetic nervous system contributes to the level of renal NADPH oxidase activity and to perinatal programming of alterations in renal vascular function that lead to elevated renal vascular resistance in SHR.     

The effects of clonidine on the kidney function in the anesthetized dog

Studies were performed to determine the mechanism by which the antihypertensive agent clonidine increased urine flow. The response of the kidney has been examined in four combinations. The parameters of renal function have been compared during volume expansion by 1.5-2.0% body weight Ringer solution. In the control animals, volume expansion by 2% body weight, resulted in a slight increase in sodium excretion and urine flow. In 10 anesthetized dogs 1.0 microgram/kg/min of clonidine infused i.v. during 30 minutes (the total amount of clonidine infused was 30 micrograms/kg) decreased the arterial blood pressure from 136 +/- 13 mmHg to 127 +/- 12 mmHg and elevated urine flow from 2.95 +/- 1.65 ml/min to 4.34 +/- 1.77 ml/min while the urine osmolality diminished from 399 +/- 107 mosm/l to 265 +/- 90 mosm/l and the glomerular filtration remained constant. In 5 animals 0.1 microgram/kg/min of clonidine was infused into the left renal artery (this dose is corresponding to the renal fraction of the cardiac output) without any effects in the left kidney. 1.0 microgram/kg/min of clonidine infused directly into the left renal artery produced vasoconstriction in the ipsilateral kidney, decreased the glomerular filtration rate and the urine flow. By contrast in the right kidney the urine flow rose without hemodynamic changes, and the urine osmolality became hypoosmotic compared to the plasma. In ten dogs 1.0 microgram/kg/min of clonidine and 1 mU/kg/min of arginine-vasopressin were infused intravenously. The vasopressin infusion superimposed on the clonidine could not inhibit the increase of the urine excretion, and the fall of the urine osmolality. The results suggest that the clonidine increases the renal medullary blood flow possibly via a direct mechanism, decreases the sympathetic outflow to the kidney and via an indirect pathway, mediated by the renin-angiotensin system. The renal medullary flow increase produces a washout of the medullary osmotic gradient, and the water reabsorption diminishes.     

Plasma lactate concentration and muscle blood flow during dynamic exercise with negative-pressure breathing.

This study assessed the hypothesis that increasing cardiac filling pressure (CFP) would enhance contracting muscle blood flow (MBF) by stretching cardiopulmonary baroreceptors and attenuate the increase in plasma lactate concentration ([Lac(-)](p)) during dynamic exercise. Continuous negative-pressure breathing (CNPB) (-15 cmH(2)O) was used to increase the CFP by accelerating the venous return to the heart. In the first series of experiments, 10 men performed a graded exercise seated on a cycle ergometer with (N1) and without CNPB (C1). The increase in [Lac(-)](p) for N1 was attenuated at 60%, 90%, and 100% of maximal exercise intensity compared with that in C1 (P < 0.001). Also, the increases in mean arterial pressure (MAP) and plasma catecholamine concentrations were attenuated in N1 compared with those in C1 throughout the graded exercise (P < 0.05). However, heart rate and pulse pressure were not significantly influenced by CNPB. Second, we studied the impact of CNPB on forearm MBF during a rhythmic handgrip exercise in 5 of the 10 subjects. Forearm MBF was measured immediately after cessation of the exercise by venous occlusion plethysmography at rest, 30%, 50%, and 70% of maximal work load (WL(max)) with (N2) and without CNPB (C2). Forearm MBF and vascular conductance for both trials increased with the increase in intensity, but forearm skin blood flow measured by laser-Doppler flowmetry remained unchanged. MBF and vascular conductance in N2, however, increased more than in C2 at every intensity (P < 0.01) except for MBF at 70% WL(max), whereas the increase in MAP for N2 was attenuated compared with that in C2 (P < 0.05). Thus augmented active muscle vasodilation occurred in N2 with a lower increase in MAP compared with that in C2. These findings suggest that the stretch of intrathoracic baroreceptors, such as cardiopulmonary mechanoreceptors, by CNPB increased MBF by suppressing sympathetic nerve activity. The attenuation of the increase in [Lac(-)](p) might be caused, at least partially, by the increased MBF.     

Local vascular responses affecting blood flow in postural tachycardia syndrome

Postural tachycardia syndrome (POTS) is defined by orthostatic intolerance associated with abnormal upright tachycardia. Some patients have defective peripheral vasoconstriction and increased calf blood flow. Others have increased peripheral arterial resistance and decreased blood flow. In 14 POTS patients (13-19 yr) evenly subdivided among low-flow POTS (LFP) and high-flow POTS (HFP) we tested the hypothesis that myogenic, venoarteriolar, and reactive hyperemic responses are abnormal. We used venous occlusion plethysmography to measure calf venous pressure and blood flow in the supine position and when the calf was lowered by 40 cm to evoke myogenic and venoarteriolar responses and during venous hypertension by 40-mmHg occlusion to evoke the venoarteriolar response. We measured calf reactive hyperemia with plethysmography and cutaneous laser-Doppler flowmetry. Baseline blood flow in LFP was reduced compared with HFP and control subjects (0.8 +/- 0.2 vs. 4.4 +/- 0.5 and 2.7 +/- 0.4 ml.min-1.100 ml-1) but increased during leg lowering (1.2 +/- 0.5 ml.min-1. 100 ml-1) while decreasing in the others. Baseline peripheral arterial resistance was increased in LFP and decreased in HFP compared with control subjects (39 +/- 13 vs. 15 +/- 3 and 22 +/- 5 mmHg.ml-1. 100 ml. min) but decreased to 29 +/- 13 mmHg.ml-1.100 ml. min in LFP during venous hypertension. Resistance increased in the other groups. Maximum calf hyperemic flow and cutaneous flow were similar in all subjects. The duration of hyperemic blood flow was curtailed in LFP compared with either control or HFP subjects (plethysmographic time constant = 20 +/- 2 vs. 29 +/- 4 and 28 +/- 4 s; cutaneous time constant = 60 +/- 25 vs. 149 +/- 53 s in controls). Local blood flow regulation in low-flow POTS is impaired.     

Evidence that renal arginine transport is impaired in spontaneously hypertensive rats

Low renal nitric oxide (NO) bioavailability contributes to the development and maintenance of chronic hypertension. We investigated whether impaired l-arginine transport contributes to low renal NO bioavailability in hypertension. Responses of renal medullary perfusion and NO concentration to renal arterial infusions of the l-arginine transport inhibitor l-lysine (10 μmol·kg(-1)·min(-1); 30 min) and subsequent superimposition of l-arginine (100 μmol·kg(-1)·min(-1); 30 min), the NO synthase inhibitor N(G)-nitro-l-arginine (2.4 mg/kg; iv bolus), and the NO donor sodium nitroprusside (0.24 μg·kg(-1)·min(-1)) were examined in Sprague-Dawley rats (SD) and spontaneously hypertensive rats (SHR). Renal medullary perfusion and NO concentration were measured by laser-Doppler flowmetry and polarographically, respectively, 5.5 mm below the kidney surface. Renal medullary NO concentration was less in SHR (53 ± 3 nM) compared with SD rats (108 ± 12 nM; P = 0.004). l-Lysine tended to reduce medullary perfusion (-15 ± 7%; P = 0.07) and reduced medullary NO concentration (-9 ± 3%; P = 0.03) while subsequent superimposition of l-arginine reversed these effects of l-lysine in SD rats. In SHR, l-lysine and subsequent superimposition of l-arginine did not significantly alter medullary perfusion or NO concentration. Collectively, these data suggest that renal l-arginine transport is impaired in SHR. Renal l-[(3)H]arginine transport was less in SHR compared with SD rats (P = 0.01). Accordingly, we conclude that impaired arginine transport contributes to low renal NO bioavailability observed in the SHR kidney.     

N-acetyl-L-cysteine improves renal medullary hypoperfusion in acute renal failure

This study evaluated the effects of N-acetyl-L-cysteine (NAC), a free radical scavenger, and N(omega)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthesis inhibitor, on the changes in renal function, intrarenal blood flow distribution (laser-Doppler flowmetry), and plasma peroxynitrite levels during the acute renal failure (ARF) produced by inferior vena cava occlusion (IVCO; 45 min) in anesthetized rats. Renal blood flow fell on reperfusion (whole kidney by -45.7%; cortex -58.7%, outer medulla -62.8%, and papilla -47.7%); glomerular filtration rate (GRF) also decreased (-68.6%), whereas fractional sodium excretion (FE(Na%)) and peroxynitrite and NO/NO plasma levels increased (189.5, 46.5, and 390%, respectively) after ischemia. Pretreatment with L-NAME (10 microg. kg(-1). min(-1)) aggravated the fall in renal blood flow seen during reperfusion (-60%). Pretreatment with NAC (150 mg/kg bolus + 715 microg. kg(-1). min(-1) iv) partially prevented those changes in renal function (GFR only fell by -29.2%, and FE(Na%) increased 119.4%) and laser-Doppler blood flow, especially in the outer medulla, where blood flow recovered to near control levels during reperfusion. These beneficial effects seen in rats given NAC seem to be dependent on the presence of NO, because they were abolished in rats pretreated with L-NAME. Also, the antioxidant effects of NAC prevented the increase in plasma peroxynitrite after ischemia. In conclusion, NAC ameliorates the renal failure and the outer medullary vasoconstriction induced by ICVO, effects that seem to be dependent on the presence of NO and the scavenging of peroxynitrite.