Reactivity of gomerular afferent and efferent arterioles in renal hypertension

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Effects of angiotensin II on regional afferent and efferent arteriole dimensions and the glomerular pole

The diversity of renal arteriole diameters in different cortical regions has important consequences for control of glomerular capillary pressure. We examined whether intrarenal angiotensin II (ANG II; 0.1, 1, or 5 ng. kg(-1). min(-1)) in anesthetized rabbits acts preferentially on pre- or postglomerular vessels using vascular casting. ANG II produced dose-related reductions in afferent and efferent diameters in the outer, mid, and inner cortex, without effecting arterial pressure. Afferent diameter decreased more than efferent in the outer and mid cortex (P < 0.05) but by a similar extent in juxtamedullary nephrons (P = 0.58). Calculated efferent resistance increased more than afferent, especially in the outer cortex (127 vs. 24 units; 5 ng. kg(-1). min(-1) ANG II). ANG II produced significant dose-related increases in the distance between the arterioles at the entrance to the glomerular pole in all regions. Thus afferent diameter decreased more in response to ANG II, but efferent resistance rose more due to smaller resting luminal dimensions. The results also indicate that glomerular pole dimensions change in response to ANG II.

Tissue-localized angiotensin II enhances cardiac and renal disorders in Tsukuba hypertensive mice

OBJECTIVE

To evaluate the relation of tissue-localized angiotensin II (Ang II) concentration with cardiac hypertrophy and glomerulosclerosis in Tsukuba hypertensive mice (THM) carrying both human renin and angiotensinogen genes.

DESIGN

Thirty THM aged 12 weeks were distributed equally to a lisinopril dosage group, a hydralazine dosage group, and an untreated group. Ten age-matched C57BL/6 mice were used as normal controls. Administration was performed for 8 weeks from 12 weeks of age. All mice were euthanized at 20 week of age, and the heart-to-body weight ratio, the renal glomerulosclerosis score, tissue Ang II concentration and tissue catecholamine concentration were measured.

RESULTS

In the untreated group, a significant increase in every examination item was found as compared with that in C57BL/6 mice. In the lisinopril group, the observed value of every item was significantly lower than that in the untreated group. In the hydralazine group, tissue Ang II and catecholamine concentrations and the heart-to-body weight ratio were not different from those in the untreated group. Although the glomerulosclerosis score in the hydralazine group was significantly less than that in the untreated group, this was significantly higher than that in the lisinopril group.

CONCLUSION

Tissue Ang II concentration is more important than hypertension in causing cardiac hypertrophy, and both tissue Ang II level and hypertension are important in causing glomerulosclerosis in THM.

Inhibition of tissue angiotensin converting enzyme activity prevents malignant hypertension in TGR(mREN2)27

BACKGROUND

Activation of the renin-angiotensin system has been implicated strongly in the transition from benign to malignant hypertension. However, the concomitant rise in blood pressure might also have a direct effect on the vascular wall by initiating fibrinoid necrosis and myointimal proliferation. Ascertaining the relative importance of these two factors in this process has proved difficult. TGR(mREN2)27 heterozygotes (HanRen2/Edin- -) have previously been shown to develop malignant hypertension spontaneously and exhibit the characteristic features of human malignant hypertension.

OBJECTIVE

Tissue renin-angtiotensin systems have been implicated in the pathogenesis of malignant hypertension. We set out to determine whether inhibition of this system might protect against development of the disease in a rat model.

METHOD

Male TGR(mREN2)27 heterozygotes (n = 24) were given a non-hypotensive dose of the angiotensin converting enzyme inhibitor ramipril (5 microg/kg per day) from 28 to 120 days of age, untreated rats acting as controls (n = 40). The incidences of malignant hypertension were compared. Systolic blood pressure was measured by tail-cuff plethysmography during treatment; tissue and plasma angiotensin converting enzyme levels and renal histological changes were assessed at the end of the treatment period or upon development of malignant hypertension.

RESULTS

Sixty-three per cent of control rats and 4% of angiotensin converting enzyme inhibitor-treated rats had developed malignant hypertension by 120 days despite there having been no significant difference in systolic blood pressure throughout the course of treatment. Angiotensin converting enzyme activities in kidney, heart and resistance vessels, though not that in plasma, were significantly lower in the treated rats. The degree of medial wall thickening did not differ between the two groups whereas evidence of tissue injury (e.g. intimal fibrosis, fibrinoid necrosis and nephron injury) was significantly less common among rats in the angiotensin converting enzyme inhibitor-treated group.

CONCLUSIONS

Tissue angiotensin converting enzyme inhibition at a non-hypotensive dose almost completely prevented mortality from malignant hypertension and significantly reduced tissue injury in this model, implicating angiotensin II rather than high blood pressure as the principal 'vasculotoxic' agent in malignant hypertension.

Contribution of tubuloglomerular feedback to renal arteriolar angiotensin II responsiveness

The purpose of the present study was to test the hypothesis that a component of the afferent arteriolar vasoconstrictor response to angiotensin II (Ang II) requires an intact tubuloglomerular feedback (TGF) mechanism. Enalaprilat-treated male Sprague-Dawley rats served as tissue donors for study of renal microvascular function using the in vitro blood-perfused juxtamedullary nephron technique. Arteriolar lumen diameter responses to exogenous Ang II were determined before and after TGF blockade (papillectomy or 50 microM furosemide). Before TGF blockage, 10 nM Ang II significantly reduced diameters of both mid-afferent (53 +/- 5%) and efferent (43 +/- 9%) arterioles. TGF blockade did not alter baseline diameter of either arteriole, but significantly blunted the mid-afferent vasoconstriction evoked by 10 nM Ang II (44 +/- 7% inhibition by papillectomy; 43 +/- 10% inhibition by furosemide). Similar behavior was observed at afferent arteriolar sites near the glomerulus; however, efferent arteriolar Ang II responsiveness was not altered by papillectomy. The impact of TGF blockade on afferent arteriolar Ang II responsiveness was most prominent at high peptide concentrations (10 nM), while not significantly influencing the response to 1 nM Ang II. In contrast, the afferent vasoconstrictor effect of norepinephrine was unaffected by papillectomy. These data indicate that the vasoconstrictor influence of exogenous Ang II on afferent, but not efferent, arterioles of intact juxtamedullary nephrons includes both TGF-dependent and TGF-independent components.

Effects of angiotensin II on isolated rabbit afferent arterioles

We examined the effects of angiotensin II (Ang II) on isolated rabbit afferent arterioles to assess the direct effect of Ang II at the resistance vessel level in the kidney. We microdissected the superficial afferent arteriole from the kidney of New Zealand White rabbits. The afferent arteriole was cannulated with a micropipette system, and the intraluminal pressure was set at 80 mmHg. Ang II did not change the lumen diameter of the afferent arterioles. After the afferent arterioles were pretreated with aspirin DL-lysine or indomethacin, Ang II (10(-7) M) caused transient vasoconstriction in the afferent arterioles. Ang II (10(-7) M) caused persistent constriction in the afferent arterioles pretreated with NG-nitro-L-arginine (10(-4) M). Physiological doses of Ang II decreased the lumen diameter of the isolated afferent arterioles pretreated with NG-nitro-L-arginine and aspirin DL-lysine. Dup753 (10(-6) M), an AT1-receptor antagonist, abolished the vasoconstrictor effects of Ang II. These findings suggest that the isolated rabbit afferent arteriole has AT1 receptors, and the vasoconstrictor response of Ang II is counteracted by vasodilatory prostaglandins and nitric oxide.

Angiotensin II and renal prostaglandin release in the dog. Interactions in controlling renal blood flow and glomerular filtration rate

The relationship between angiotensin II and renal prostaglandins, and their interactions in controlling renal blood flow (RBF) and glomerular filtration rate (GFR) were investigated in 18 anaesthetized dogs with acutely denervated kidneys. Intrarenal angiotensin II infusion increased renal PGE2 release (veno-arterial concentration difference times renal plasma flow) from 1.7 +/- 0.9 to 9.1 +/- 0.4 and 6-keto-PGF1 alpha release from 0.1 +/- 0.1 to 5.3 +/- 2.1 pmol min-1. An angiotensin II induced reduction in RBF of 20% did not measurably change GFR whereas a 30% reduction reduced GFR by 18 +/- 8%. Blockade of prostaglandin synthesis approximately doubled the vasoconstrictory action of angiotensin II, and all reductions in RBF were accompanied by parallel reductions in GFR. When prostaglandin release was stimulated by infusion of arachidonic acid (46.8 +/- 13.3 and 15.9 +/- 5.4 pmol min-1 for PGE2, and 6-keto-PGF1 alpha, respectively), angiotensin II did not change prostaglandin release, but had similar effects on the relationship between RBF and GFR as during control. In an ureteral occlusion model with stopped glomerular filtration measurements of ureteral pressure and intrarenal venous pressure permitted calculations of afferent and efferent vascular resistances. Until RBF was reduced by 25-30% angiotensin II increased both afferent and efferent resistances almost equally, keeping the ureteral pressure constant. At greater reductions in RBF, afferent resistance increased more than the efferent leading to reductions in ureteral pressure. This pattern was not changed by blockade of prostaglandin synthesis indicating no influence of prostaglandins on the distribution of afferent and efferent vascular resistances during angiotensin II infusion. In this ureteral occlusion model glomerular effects of angiotensin II will not be detected, and it might well be that the shift from an effect predominantly on RBF to a combined effect on both RBF and GFR induced by inhibition of prostaglandin synthesis is located to the glomerulus. We therefore postulate that renal prostaglandins attenuate the effects of angiotensin II on glomerular surface area and the filtration barrier, and not on the afferent arterioles as previously suggested.

Pressure-induced contraction of the juxtamedullary afferent arterioles in spontaneously hypertensive rats

1. In the SHR juxtamedullary nephron preparation, the increase of the perfusion pressure from 80 to 160 mmHg increased the diameters of arcuate arteries but produced a pressure-dependent contraction of the afferent arterioles, a response that can account for renal autoregulation. 2. The pressure-induced contractions of the afferent arterioles were abolished by 1 microM nifedipine and by 10 microM furosemide, suggesting that the autoregulatory responses are mainly mediated by tubuloglomerular mechanisms and can be abolished by calcium antagonists.

Juxtamedullary afferent and efferent arterioles constrict to renal nerve stimulation

Sympathetic neural control of afferent and efferent arterioles of inner cortical (juxtamedullary) glomeruli has not been established, in part, because of difficulty accessing these vessels, normally located deep below the kidney surface. In this study we utilized the rat hydronephrotic kidney model to visualize the renal microcirculation and to quantitate the responses of juxtamedullary arterioles to brief (30 sec) renal nerve stimulated (RNS). Juxtamedullary afferent and efferent arterioles constricted in a frequency-dependent fashion to RNS, achieving a maximal constriction of 35% to 8 Hz stimulation. In these same kidneys, outer cortical afferent arterioles also constricted to RNS but outer cortical efferent arterioles did not. Microinjection of norepinephrine (NE) around single outer cortical efferent arterioles (to avoid the constriction of preglomerular vessels) constricted the efferent arterioles. However, the afferent arterioles of the same glomeruli were considerably more responsive to microinjected NE. Thus, the lack of constriction of outer cortical efferent arterioles to RNS may relate, in part, to their low sensitivity to NE, the primary neurotransmitter. These direct observations indicate that the juxtamedullary efferent arterioles are responsive to renal nerve stimulation whereas the outer cortical efferent vessels are not. These results, which should be cautiously extrapolated to normal filtering kidneys, indicate that glomerular hemodynamic changes evoked by the sympathetic nervous system are different for outer cortical and inner cortical glomeruli.

Endothelium-derived relaxing factor/nitric oxide modulates angiotensin II action in the isolated microperfused rabbit afferent but not efferent arteriole

It has been reported that sensitivity to angiotensin II (Ang II) is higher in efferent (Ef) than afferent (Af) arterioles (Arts). We tested the hypothesis that this is due to arteriolar differences in the interaction between Ang II and endothelium-derived relaxing factor/nitric oxide (EDNO). Rabbit Af-Arts with glomerulus intact were microperfused in vitro at a constant pressure. Ef-Arts were perfused from the distal end of either the Af-Art (orthograde perfusion) or the Ef-Art (retrograde perfusion) to eliminate influences of the Af-Art or glomerulus, respectively. Ang II did not alter Af-Art luminal diameter until the concentration reached 10(-9) M, which decreased the diameter by 11 +/- 2.6% (n = 11; P < 0.002). In contrast, Ef-Arts became significantly constricted at concentrations as low as 10(-11) M with either perfusion. Surprisingly, the decrease in Ef-Art diameter at 10(-10), 10(-9), and 10(-8) M was significantly greater with retrograde perfusion (44 +/- 6.9%, 70 +/- 5.6%, and 74 +/- 4.1%, respectively; n = 5) than with orthograde perfusion (16 +/- 4.2%, 25 +/- 2.9%, and 35 +/- 3.5%; n = 9). ENDO synthesis inhibition with 10(-4) M nitro-L-arginine methyl ester (L-NAME) decreased the diameter to a greater extent in Af-Arts (22 +/- 3.0%; n = 11) compared to Ef-Arts with either orthograde (9.5 +/- 2.3%; n = 8) or retrograde perfusion (1.2 +/- 2.1%; n = 6). With L-NAME pretreatment, Af-Art constriction induced by 10(-10) M (14 +/- 4.0%, n = 9) and 10(-9) M Ang II (38 +/- 3.9%) was significantly greater compared to nontreated Af-Arts. In contrast, L-NAME pretreatment had no effect on Ang II-induced constriction in Ef-Arts with either perfusion. In conclusion, this study demonstrates higher sensitivity of Ef-Arts to Ang II, particularly with retrograde perfusion. Our results suggest that EDNO significantly modulates the vasoconstrictor action of Ang II in Af-Arts II but not Ef-Arts, contributing to the differential sensitivity to Ang II.

Renal functional alterations induced by angiotensin-converting enzyme inhibitors in heart failure

OBJECTIVE

To review the effects of angiotensin-converting enzyme (ACE) inhibitors on renal pathophysiology and the compensatory mechanisms involved in heart failure. A clinical application of the use of these agents in the setting of concomitant heart failure and renal insufficiency also is presented.

DATA SOURCES

A MEDLINE search was conducted using the terms heart failure, congestive; renal insufficiency; and angiotensin-converting enzyme inhibitors.

STUDY SELECTION

All applicable animal and human trials were reviewed.

DATA SYNTHESIS

Advances in the management of heart failure have led to new insights into the complex pathophysiology of this condition, particularly the favorable clinical effects noted with the ACE inhibitors. The net effect of ACE inhibitors on the renin-angiotensin system in patients with heart failure is to augment renal blood flow to a greater extent than cardiac output. Glomerular filtration rate is either unchanged or decreased by ACE inhibition. Sodium excretion is augmented primarily by alterations in glomerular hemodynamic parameters. ACE inhibitors may alter renal tubular function, both directly and indirectly, leading to increased diuresis and natriuresis.

CONCLUSIONS

Although ACE inhibitors exert beneficial pharmacodynamic effects on the renal bed in patients with heart failure, clinical consideration of volume and neurohormonal status prior to institution of therapy is prudent.

Morphological study of bovine lung grafted into the hamster cheek pouch

The hamster cheek pouch has been used extensively to study the modulation of microvascular responsiveness of native and transplanted tissues, because it is immunologically privileged. The purpose of this study was to determine the structural changes that occur over time in bovine lung tissue (donor) that was grafted into the hamster cheek pouch (recipient). Lungs from adult cows were cut into 1-mm-thick slices and grafted into the cheek pouch of adult Syrian golden hamsters (n = 60). After induction of anesthesia, bovine lung tissue was placed under the avascular tissue covering the cheek pouch, and the overlying skin was sutured. Intravital microscopy (IM) and transmission electron microscopy (TEM) of the cheek pouch and grafted tissue were performed 1, 7, 14, and 28 days after grafting. By using IM, we found blood flow throughout the grafted bovine lung tissue between days 7 and 14 post-transplantation. Both IM and TEM showed that the grafted tissue contained patent microvessels anastomosing with cheek pouch microvessels, alveolar structures, and interstitial tissue. Mast cell infiltration around microvessels of the grafted tissue was evident in all animals between days 14 and 28 post-grafting. No other inflammatory cells were identified throughout the observation period. By day 28 post-grafting, the entire lung tissue became fibrotic. We conclude that bovine lung tissue can be successfully transplanted into the hamster cheek pouch, that blood flow is established throughout the graft, and that prominent mast cell infiltration is associated with fibrosis of the graft 28 days after transplantation. We suggest that this model can be useful in studying pulmonary microvascular responses in situ.

ACE inhibitors in renal disease

Recent experimental studies suggest that the resistance state of the preglomerular and postglomerular capillary arterioles may determine if a particular class of antihypertensive agents will protect the kidney from hemodynamically mediated glomerular injury. This review discusses (1) the effects of angiotensin II on the renal microcirculation, (2) the pathophysiology of essential hypertensive renal disease, (3) the renal pharmacology of angiotensin-converting enzyme (ACE) inhibitors, and (4) the hypothesis that renal protection is dependent on control of systemic and glomerular hypertension.

Renal effects of angiotensin-converting enzyme inhibition in congestive heart failure

Some studies report that inhibition of angiotensin-converting enzyme (ACE) improves renal function in patients with congestive heart failure, whereas others report that renal deterioration is a frequent complication of treatment with ACE inhibitors. This article explores the mechanisms by which antagonism of the renin-angiotensin system improves kidney function in some patients while causing harm in others. ACE inhibition may alter renal blood flow, glomerular perfusion pressure, basement membrane activity and renal tubular function both directly and indirectly. In most patients, renal function is maintained as other neurohormonal mechanisms compensate for the negative effects and permit the positive effects (such as improved renal flow) to predominate. However, when physiologic characteristics or iatrogenic interventions (such as volume reduction or prostaglandin inhibition) limit the effectiveness of neurohormonal compensation to maintain renal autoregulation, clinically important deterioration in renal function may occur. An understanding of the renal effects of ACE inhibitors permits their safe and effective use in most patients with congestive heart failure.

A narrow segment of the efferent arteriole controls efferent resistance in the hydronephrotic rat kidney

We microscopically examined the narrow segment of the efferent arteriole (NSEA) in the hydronephrotic kidney of rats in vivo. This segment is routinely found at the proximal efferent arteriole adjacent to the glomerulus and is characterized by a narrowing due to a cellular structure which bulges into the lumen. The existence of NSEA has been described in a previous study from our laboratory, but it was not examined in detail; its functional behavior in particular is still unknown. In the present study, we measured changes in luminal diameter of NSEA during reduction of renal perfusion pressure and in response to angiotensin II (Ang II), saralasin, and nitroprusside. The mean luminal diameter of NSEA at its narrowest point was 5.1 microns and its effective length was 8.4 microns. Reduction of renal perfusion pressure from 115 to 90, 70, and 40 mm Hg decreased NSEA diameter by 9, 15, and 23%, respectively. Topical application of saralasin (10(-5) M) or nitroprusside (10(-4) M) to the kidney dilated NSEA by 10 or 20%, respectively, and attenuated the pressure-dependent constriction of NSEA. Intravenous infusion of Ang II at rates of 50, 100, 200, and 400 ng/min/kg constricted NSEA lumen by 7, 15, 20, and 21%, respectively. The vascular bed of the hydronephrotic kidney was scanned morphologically, and a mathematical model was developed from these data. With this model, we could calculate the effect of segmental diameter changes on total renal resistance as well as flow and pressure at different vascular levels. NSEA contributed 4% to the total renal resistance and 20% to the efferent resistance under control conditions. Lowering of renal perfusion pressure increased the efferent arteriolar resistance up to 50% above the control value mainly as a result of constriction of NSEA. Our data indicate that efferent resistance increases at reduced renal perfusion pressure and leads to a stabilization of glomerular pressure. The rise of efferent resistance is mediated primarily by NSEA constriction, and the renin-angiotensin system is involved in this regulation.

Role of the kidney in congestive heart failure. Relationship of cardiac index to kidney function

Because of inappropriate signals from the volume-regulation system and a reduction in renal blood flow, the kidney is not able to prevent sodium and water retention in chronic congestive heart failure (CHF). A brief summary of normal renal function and renal involvement in CHF is given and a study of renal function in patients with moderate or severe chronic CHF is presented. To evaluate the impact of cardiac output reduction on the regulation of the glomerular filtration rate (GFR) in heart failure, GFR (inulin clearance), renal plasma flow [p-aminohippurate (PAH) clearance], invasive haemodynamics, blood volume, plasma renin and plasma catecholamines were measured in 34 patients with chronic CHF. The patients were divided into 3 groups according to their cardiac index (CI): CI greater than 2.0 L/min/m2 (group A), CI 1.5 to 2.0 L/min/m2 (group B), and CI less than 1.5 L/min/m2 (group C). Differences in the relationship between GFR, renal plasma flow and filtration fraction for the 3 groups emerged. Despite an intergroup reduction in the renal fraction of cardiac output and renal blood flow, GFR was similar in groups A and B (62 and 67 ml/min/1.73 m2, respectively), and was accompanied by a compensatory increase in filtration fraction, from 24% in group A to 35% in group B. Group C had a much lower GFR (38 ml/min/1.73 m2), however, the filtration fraction (28%) was intermediate in value between those of groups A and B. The differences in GFR were reflected by blood urea nitrogen levels but not by serum creatinine levels. The occurrence of a non-compensatory filtration fraction response in the patients with the greatest impairment of CI and renal blood flow suggests that GFR becomes dependent on afferent arteriolar flow in the most severe heart failure, despite stimulation of haemodynamic and hormonal pathways, which would normally increase efferent arteriolar tone. It is thus suggested that GFR becomes flow dependent in patients in the most severe stage of chronic CHF.

Pressure-induced vasoconstriction of renal microvessels in normotensive and hypertensive rats. Studies in the isolated perfused hydronephrotic kidney

The capacity of small arteries to respond to increased intravascular pressure may be altered in hypertension. In the kidney, hypertension is associated with a compensatory shift in the autoregulatory response to pressure. To directly determine the effects of established hypertension on the renal microvascular response to changes of perfusion pressure, we evaluated pressure-induced vasoconstriction in hydronephrotic kidneys isolated from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Vessel diameters of interlobular arteries (ILAs) and afferent and efferent arterioles were determined by computer-assisted videomicroscopy during alterations in renal arterial pressure (RAP) from 80 to 180 mm Hg. Increased RAP induced a pressure-dependent vasoconstriction in preglomerular vessels (afferent arterioles and ILAs), but not in postglomerular vessels (efferent arterioles). The calcium antagonist nifedipine prevented pressure-induced afferent arteriolar vasoconstriction with a similar half-maximal inhibitory concentration (IC50) (WKY, 63 +/- 27 vs. SHR, 60 +/- 32 nM). The pressure-activation curves for ILAs in SHR and WKY were similar. In contrast, the pressure-activation curve for afferent arterioles in SHR kidneys exhibited a rightward shift, which was observed at every segment of the afferent arteriole (i.e., near ILA, at midportion, and near glomerulus). These findings demonstrate that the ILA and the afferent arteriole both possess the ability to constrict in response to increased pressure, whereas this property is lacking in the efferent arteriole. Hypertension was associated with a compensatory shift in the pressure response of the afferent arteriole, such that higher RAPs were required to elicit vasoconstriction in this vessel.

Pressure-dependent contraction of rat juxtamedullary afferent arterioles

Pressure-diameter relations were studied in rat afferent arterioles using an isolated, juxtamedullary nephron preparation perfused with a saline solution containing 5% albumin. Angiotensin I (10 microM), angiotensin II (0.1 microM), and norepinephrine (10 microM) increased perfusion pressure, and norepinephrine, but not angiotensin I or II, contracted afferent arterioles, indicating that the vessels are reactive. The control diameter of the afferent arterioles that exhibited pressure-dependent contraction (n = 58) averaged 30.8 +/- 1.1 micron at perfusion pressure of 80 mm Hg. When pressure was increased from 80 to 120 and then to 180 mm Hg, the diameter of these arterioles decreased by 16.4 +/- 2.1%. Glomerular capillary pressure was well autoregulated and averaged 45.2 +/- 2.2, 50.2 +/- 2.4, and 53.0 +/- 3.0 mm Hg, respectively, at perfusion pressures of 80, 120, and 180 mm Hg. Administration of vasodilators or a Ca2+-free solution eliminated the contractile response to pressure elevations; rather, the diameter of these vessels increased significantly by 17.5 +/- 5.1% and 32.0 +/- 9.4%, respectively, when pressure was increased from 80 to 180 mm Hg. Blocking tubuloglomerular feedback mechanism, with furosemide or by removal of the renal papilla (which interrupts the delivery of fluid to the macula densa), eliminated the pressure-dependent contraction of the afferent arterioles. Instead the diameter of these vessels increased by 27.0 +/- 7.8% and 36.0 +/- 5.6%, respectively, when the pressure was increased from 80 to 120 and then to 180 mm Hg. These results demonstrate that juxtamedullary nephrons perfused in vitro autoregulate glomerular capillary pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo microscopy of the cerebral microcirculation using neonatal allografts in hamsters

Studies were performed to characterize the morphology and vascular reactivity of the allografted cerebral microcirculation. Cerebral cortical tissue was allografted into the cheek pouch of the hamster so that cerebral parenchymal vessels could be studied. The vascular morphology was characterized by a large number of looping vessels. The ultrastructural examination indicated viable cerebral tissue containing typical vessels, that is, "tight" junctions, not like those of the cheek pouch. Also, the microvasculature was impermeable to 150, 70, and 20 kDa fluorescein isothiocyanate dextrans. Angiotensin II and norepinephrine caused constriction of the cerebral vessels whereas adenosine caused dilation. Isoproterenol did not affect cerebral arterioles; however, it dilated cheek pouch arterioles. Thus, this preparation provides a satisfactory model for studying the living cerebral microcirculation.

Angiotensin reactivity in the cheek pouch of the renovascular hypertensive hamster

Increased reactivity to vasoconstrictor agents and decreased arteriolar luminal diameter have been implicated in the maintenance of hypertension. The same hamster cheek pouch microvessels were tested for angiotensin I (Ang I) and angiotensin II (Ang II) reactivity before and 10 to 14 days after Grollman (two-kidney, one figure-8) or sham operation. Microvascular geometric parameters were measured before and after a maximal vasodilator dose of adenosine. Then maximal vasoconstrictions to Ang I or Ang II were measured: Ang I and Ang II were applied adjacent to arterioles (10(-2)-10(0) pmol) and venules (10(-1) pmol) in 10-microliter aliquots for 1 minute. Blood pressure (178 +/- 11/133 +/- 8 mm Hg) of renovascular hypertensive hamsters was elevated significantly over blood pressure of sham-operated hamsters (120 +/- 11/97 +/- 10 mm Hg). No change was observed in venular geometry or reactivity in renovascular hypertensive hamsters. Arteriolar luminal diameter, wall thickness, wall/lumen ratio, and wall area were not altered in hypertensive hamsters in the normal or vasodilated state; vasodilator capacity was the same in all groups. Conversion of Ang I to Ang II (response to Ang I divided by response to Ang II) for first-order and Third-order arterioles and third-order venules was 74 +/- 5, 79 +/- 3, and 72 +/- 6%, respectively, and was unaltered in renovascular hypertensive hamsters. Although vessel geometry was not altered, there was a significant shift to the left of the Ang I and Ang II dose-response curves of first-order and third-order arterioles, indicating increased sensitivity to these vasoconstrictors.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of glomerular filtration rate in chronic congestive heart failure patients

The purpose of this study was to identify the factor(s) that characterize impaired glomerular filtration rate (GFR) in congestive heart failure (CHF) patients. We studied 34 patients, measuring systemic hemodynamics, vasoactive hormones, and sodium and volume status. Renal plasma flow (para-aminohippurate) and GFR (inulin) were assessed by steady-state clearance techniques. Both linear and multiple regression analyses were performed. Impaired GFR was characterized by reduction of renal blood flow and renal fraction of cardiac output, and by an increase of renal vascular resistance. The correlation between GFR and filtration fraction (r = 0.492, P less than 0.003) indicated that individuals with greatest impairment of GFR had the lowest filtration fraction and increased overall renal vascular resistance, indicating dependence of GFR on afferent, rather than efferent vasoconstriction, under conditions of low renal perfusion. Mean vasoactive hormones and blood volume were increased, but without clear cut correlation with GFR. The greatest impairment of GFR was observed in elderly CHF patients, as renal blood flow and function demonstrated an age-dependent decline, in addition to the adverse effects of CHF. In a multiple regression model, renal blood flow and filtration fraction accounted for 69% and 25%, respectively, of the variability of GFR, with a co-linear influence of age. Thus, we have identified age-related, abnormal renal hemodynamic patterns in CHF, and the fragile nature of GFR in the elderly CHF population.

The selective response to adenosine of renal microvessels from hamster explants

The present study was undertaken to investigate the effect of adenosine on the microvasculature of the hamster kidney and the possibility of angiotensin II mediation. Renal tissue from neonatal hamsters was grafted into the cheek pouch of 33 adult hamsters. Seven to twelve days later the renal microcirculation was studied. Adenosine was tested on the pre- and postglomerular arterioles as well as on cheek pouch arterioles before and after applying an AII antagonist, saralasin. Adenosine dilated the cheek pouch arterioles and constricted the preglomerular arterioles in a dose-dependent manner. Adenosine had no effect on postglomerular arterioles. The renal vasoconstriction persisted as long as adenosine was present. Theophylline reduced the adenosine-mediated vasoconstriction of the afferent arteriole in a dose-dependent manner. These changes were not altered in the presence of saralasin at various doses, one of which was 20-fold greater than that required to abolish the vasoconstrictor response of a test dose of angiotensin II. This study indicates that the adenosine-mediated vasoconstriction of the preglomerular microvessels is not mediated via the renin-angiotensin system but may be a direct effect of adenosine.

Multiple effects of calcium entry blockers on renal function in hypertension

Characterization of the renal effects of calcium entry blockers has not been easy because the inhibition of Ca2+ cellular influx alters several regulatory functions. The ability of calcium blockers to dilate renal vasculature and to increase glomerular filtration rate is largely determined by the preexisting vascular tone. However, the increments in sodium excretion could occur without alterations in renal hemodynamics. Calcium blockers could increase sodium excretion by inducing a redistribution of renal blood flow toward juxtamedullary nephrons, by inhibiting tubuloglomerular feedback responses, or by a direct action on the tubular transport of sodium. These effects are poorly understood at present. In vitro studies show that the blockade of calcium entry enhances renin secretion and decreases prostaglandin synthesis. This dissociation has not been found during long-term administration, which has been proved to be effective for the treatment of essential hypertension with normal maintenance of renal function. In this respect, there are reports indicating that calcium blockers are particularly effective in a subgroup of patients with essential hypertension who exhibit subtle but detectable alterations in calcium metabolism. Further studies are needed to determine whether this significant response to calcium blockers is due to correction of an early defect of calcium cellular kinetics that initiated the increase in blood pressure.

Interlobular arterial resistance: influence of renal arterial pressure and angiotensin II

Pressure in the distal third of the interlobular arteries (Pila) was measured in anesthetized rats by micropuncture through corticotomy. Control Pila was 83.9 (SD 11.9) mm Hg at a renal arterial pressure (RAP) of 113.1 (SD 12.8) mm Hg. Reduction of RAP by 20 mm Hg caused no consistent change of total renal blood flow (RBF) or Pila. Relative interlobular arterial resistance, Rila = (RAP - Pila)/RBF, fell by 40 to 50%, and then remained practically unchanged at further reduction of RAP. Blood flow measured by radiolabeled microspheres (10.7 micron) showed similar values in intact cortex and in the tissue beneath the corticotomy, both varying in proportion to RBF. Intravenous infusion of angiotensin II (AII) 40 to 90 ng/min reduced RBF by 29% and increased RAP by 19 mm Hg. Pila rose by only 8 mm Hg and Rila increased to 209% of control. Reduction of RAP to control level during continued AII infusion did not change RBF, while Rila fell to 131% of control. We conclude that: dilatation and constriction of the interlobular arteries contribute importantly to autoregulation of outer cortical blood flow, probably through a myogenic mechanism (Bayliss); the constriction of interlobular arteries elicited by i.v. AII reflects mainly an autoregulatory response to increased arterial pressure, and to a smaller extent, a direct constrictor effect of AII.

The effects of angiotensin II and norepinephrine on afferent arterioles in the rat

Perfusion-fixation and vascular casting were used to determine if angiotensin II and norepinephrine cause constriction of afferent arterioles, and if so, whether or not the constriction is principally attributable to a secondary (autoregulatory) response to elevated pressure in the renal circulation. Rats were divided into six groups, and were given five minute infusions of angiotensin II, norepinephrine, or normal saline before perfusion-fixation. Angiotensin II infusions were given to animals in Groups 1 and 2; in Group 1 the renal circulation was exposed to elevated blood pressure, but in Group 2 intrarenal pressure was maintained at normal levels by partial ligation of the aorta above the renal arteries. Groups 3 and 4 were comparable to Groups 1 and 2 respectively, except that norepinephrine was infused instead of angiotensin II. Groups 5 and 6 received infusions of normal saline instead of vasopressor substances. In Group 5, renal vessels were perfusion-fixed at elevated pressures, while in Group 6 renal vessels were perfusion-fixed at normal pressures. Scanning electron microscopy was used to examine and measure vascular casts of 40 to 50 afferent arterioles from each rat. In Groups 1 through 4, casts of afferent arterioles showed severe, focal, and irregularly distributed constriction. In Groups 5 and 6 there was no qualitative evidence of afferent arteriolar constriction or irregularities in vessel caliber. Vessel diameter measurements were significantly smaller in Groups 1 to 4 when compared to Groups 5 and 6, and arteriolar constriction in the juxtamedullary and cortical regions was quantitatively comparable in Groups 1 to 4. Arteriolar diameters in Group 5 were somewhat, but not significantly smaller than those in Group 6, suggesting that perfusion-fixation at elevated pressure alone may have made a minor contribution to afferent vasoconstriction. However, this constriction was clearly not of the same pattern or magnitude as that produced by angiotensin II or norepinephrine infusions. The results indicate that both angiotensin II and norepinephrine cause severe focal constriction of afferent arterioles, and that this constriction cannot be attributed primarily to a pressure-induced autoregulatory response in the renal vasculature.

Angiotensin II control of the renal microcirculation: effect of blockade by saralasin

The hydronephrotic rat kidney with intact circulation and innervation was split and spread out as a thin sheet in a tissue bath. The microvasculature was observed in vivo via television microscopy. We quantitated the effects of increasing concentrations (10(-9) to 10(-5) M) of saralasin (angiotensin II antagonist) applied locally in the tissue bath on microvascular diameters and on relative glomerular blood flow (measured using fluorescent labeled RBCs). Saralasin produced an increase in preglomerular diameters which was largest (37 +/- 11%) in the interlobular artery (there was no dilation in the afferent arteriole near the glomerulus), an increase in postglomerular diameters which was largest (17 +/- 4%) in the efferent arteriole near the glomerulus, and an increase in blood flow (19 +/- 4%). If these types of findings would hold for the normal kidney, it would suggest a role for angiotensin II in the control of total renal blood flow, in the regional distribution of flow, and in the control of filtration fraction. We also made control micropressure measurements using the servo-nulling approach. Pressures measured were: afferent arteriole, 65 +/- 5 mm Hg; intraglomerulus, 50 +/- 5 mm Hg; and efferent arteriole, 19 +/- 3 mm Hg. These data indicate that there is major vascular resistance near the glomerulus, especially in the efferent arteriole.

Hypertensive emergencies and urgencies: pathophysiology and clinical aspects

A hypertensive urgency should be distinguished from a hypertensive emergency. Although the distinction may not always be obvious, certain guidelines may help the clinician determine which therapeutic approaches are most appropriate for each patient. Hypertensive emergencies include those conditions in which new or progressive severe end-organ damage is present and a delay in appropriate therapy might result in permanent damage, progression of complications, and a poor prognosis. Hypertensive urgencies include those conditions with minimal to no obvious end-organ damage in which blood pressure should be lowered expeditiously. The risk of immediate complications or organ damage is less likely to occur, and thus the immediate prognosis is better, although the ultimate prognosis, if untreated, is poor. There is a marked individual, racial, sexual, and age difference in the ability to tolerate high intraarterial pressure, as evidenced by patients' symptoms and signs of end-organ damage. Patients may have no symptoms of elevated blood pressure until significant intraarterial levels are reached. If symptoms are present, they may include headache, dizziness, blurred vision, shortness of breath (especially with exertion), chest pain, rapid pulse, palpitations, malaise and fatigue, nocturia, or pedal edema. Signs of hypertensive disease vary and depend not only on the level of blood pressure but also include funduscopic changes with arteriolar narrowing, atrioventricular nicking, hemorrhages, exudates or papilledema, central nervous system changes and neurologic abnormalities, cardiac changes with gallop rhythm, cardiomegaly, tachycardia, ectopic ventricular beats, left ventricular hypertrophy or signs of congestive heart failure, pulmonary edema, and signs of renal insufficiency.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathophysiological events leading to the end-organ effects of acute hypertension

The term "hypertensive crisis" refers to a group of acute hypertensive disorders that have, in common, evidence of end-organ dysfunction. From a pathophysiological viewpoint, these disorders may be classified into two major categories. In primary hypertensive disorders, the predominant pathophysiological events and subsequent end-organ failure are directly attributable to the uncontrolled hypertension. Secondary hypertensive crises have many similar features; however, the ultimate progression from onset of hypertension to end-organ failure tends to be modified by concurrent target-organ disease. The chain of events leading to the progression from benign to malignant hypertension centers around two general theories--the pressure hypothesis and the humoral hypothesis--both of which suggest that when a critical imbalance of pressure and/or humoral factors occurs, depending variously on etiological factors, rapidity, and degree and duration of blood pressure elevation, a series of pathological events ensue leading to myointimal proliferation and fibrinoid necrosis. The primary target-organ effects of severe hypertension generally affect the central nervous, renal, and cardiovascular systems and occur when the normal compensatory mechanisms are exceeded either by a breakthrough in autoregulation, as in the central nervous system, or by an imbalance in myocardial supply and demand. To a certain extent the pathophysiological events and target-organ susceptibility will determine the manner in which the hypertensive event presents. Although effective therapy is optimally tailored to the specific underlying disease process, in the acute setting, where important clinical data may be lacking, an appreciation of these underlying mechanisms will aid in the selection of a regimen that is both safe and effective.

Microvascular reactivity of in vitro blood perfused juxtamedullary nephrons from rats

The effects of angiotensin II (AII), epinephrine, and changes in perfusion pressure on glomerular capillary and afferent arteriolar pressures were assessed using the in vitro blood perfused juxtamedullary nephron (JMN) preparation. At a perfusion pressure of 102 +/- 1 mm Hg, glomerular capillary pressure (GCP) averaged 55 +/- 1 mm Hg. Afferent arteriolar pressure (AAP), measured at early-to-mid afferent locations, was 88 +/- 2 mm Hg and decreased to 71 +/- 7 mm Hg at the most terminal segments, 50 to 80 micron from the glomerulus. In some nephrons, readjustments of GCP occurred in response to step changes in perfusion pressure within the range of 90 to 165 mm Hg. In 37 nephrons, bolus injections of AII into the blood caused dose-dependent and reversible decreases in GCP, ranging from -4 +/- 1 mm Hg (12 to 25 pg) to -26 +/- 4 mm Hg (20 ng). Similar decreases in GCP ranging from -9 +/- 3 to -22 +/- 3 mm Hg were observed in response to epinephrine (1.25 to 20 ng). Epinephrine also consistently reduced AAP by 37 +/- 10% (N = 8). In contrast, AII typically increased pressure in the early and mid segments of the afferent arteriole, but caused variable responses in the late afferent arteriole. The responses to vasoconstrictor agents were not mimicked by increases in perfusion pressure per se. These results indicate that the preglomerular vasculature of in vitro JMN can exhibit autoregulatory behavior and is responsive to humoral vasoconstrictors. The response to epinephrine was generalized, occurring along the entire preglomerular vasculature, while the predominant effects of AII were localized to terminal afferent structures, which may include intraglomerular constrictor elements.

The extra-cellular-matrix in hypertension: the link between renal function, autoregulation and sodium metabolism, a review and hypothesis

In this paper the nature of the extra-cellular-matrix (ECM) is outlined briefly. Changes in the ECM in hypertension have been reviewed as has their possible involvement in the pathophysiological process of hypertension. It appears that significant and widespread changes occur in the ECM in hypertensive states. Such changes may alter the characteristics of (1) the renal perfusion pressure/sodium excretion curve (2) autoregulation and (3) sodium metabolism, all of which have been implicated in the pathogenesis of hypertension. The ECM may well be the link between these three. The ECM changes will also contribute to many of the differences, e.g. altered fluid distribution, altered handling of a sodium load, altered electrolyte and water content of vascular tissue, and altered passive and active mechanical properties of vascular tissue seen in hypertensive states. The ECM changes are unlikely to be the root cause of hypertension but are a more fundamental difference than altered renal perfusion/excretion characteristics, altered fluid distribution etc. Indeed the perfusion/excretion characteristics, fluid distribution etc. appear to be dependent upon the properties of the ECM. What now requires to be done is to determine what controls the properties of the ECM and to determine what fault in the control may lead to the changes which could ultimately manifest themselves as hypertension.

Effects of renal arterial angiotensin I infusion on glomerular dynamics in sodium replete dogs

During intrarenal infusion of angiotensin I (AI), conversion to angiotensin II (AII) within the kidney has been shown to occur early enough to decrease glomerular filtration rate (GFR). To evaluate further the mechanism by which AI decreases GFR, micropuncture studies were conducted in sodium replete dogs. Feedback-mediated alterations in glomerular function were minimized by reducing renal arterial pressure to 90 mm Hg. During infusion of AI (0.82 +/- 0.01 micrograms min-1), renal blood flow (3.91 +/- 0.25 ml min-1 g-1) and GFR (0.63 +/- 0.04 ml min-1 g-1) decreased by 36.7 +/- 6.1% and 18.9 +/- 6.1%, respectively. Similarly, single nephron GFR decreased from 66.4 +/- 3.8 to 40.0 +/- 3.2 nl min-1 and estimated glomerular plasma flow (280 +/- 49 nl min-1) decreased by 55 +/- 6%. Stop-flow pressure (40.5 +/- 3.6 mm Hg) did not change significantly, while proximal tubular (21.8 +/- 1.4 mm Hg) and peritubular capillary pressures (13.2 +/- 1.8 mm Hg) decreased by 25.5 +/- 2.8% and 49.4% +/- 5.1%, respectively. Glomerular capillary and effective filtration pressures were not altered significantly. There were increases in both preglomerular (168%) and efferent (203%) arteriolar resistances, along with a decrease in the glomerular filtration coefficient (Kf) from 4.6 +/- 0.6 to 2.5 +/- 0.5 nl mm Hg-1 min-1. These data indicate that augmented intrarenal conversion of circulating AI reduces GFR as a consequence of decreases in Kf as well as in glomerular plasma flow, the latter being due to concomitant increases in preglomerular and efferent arteriolar resistances.

Transient renal dysfunction during initial inhibition of converting enzyme in congestive heart failure

Treatment with captopril in resistant normotensive congestive heart failure is associated with a pronounced reduction in blood pressure, particularly after the first dose. The effects of this reduction on renal function were assessed in 10 patients at the beginning of and during chronic treatment (at one week and three months). Renal plasma flow and glomerular filtration rates were measured by isotope clearance during water diuresis. The first dose of captopril (25 mg) led to a pronounced fall in renal plasma flow and glomerular filtration rates together with a decrease in mean arterial pressure; this fall correlated with baseline plasma renin activity. These changes were paralleled by decreases in water and sodium excretion. In contrast, by the end of the first week of treatment a similar fall in mean arterial pressure occurred together with a pronounced increase in renal plasma flow; the glomerular filtration rate was maintained and there was no decrease in water and sodium excretion. This new response pattern recurred after three months of treatment. The difference in response at different stages of treatment may reflect the balance between the different mechanisms influencing kidney dynamics in heart failure and their alteration by converting enzyme inhibition. The sustained increase in renal plasma flow during chronic treatment with captopril may account for the continued control of heart failure in these patients.

Adrenergic neurons: are they present on microvessels in cheek pouches of hamsters?

Catecholaminergic innervation was investigated in the cheek pouch, mesentery, and cremaster muscle of the hamster. These tissues were removed from normal hamsters after anesthetization with sodium pentobarbital. They were prepared immediately for visualization of catecholamines by using a glyoxylic acid fluorescent technique. Visualization was accomplished via a Zeiss fluorescent microscope with a 470-nm barrier and a 420-nm excitation filter. No fluorescence was observed in or around the microvessels of the cheek pouch; however, intense fluorescence was demonstrated around and along microvessels in the mesentery and cremaster muscle. Thus, within the limitations of this technique, this study demonstrates that local catecholamines, either as a component of adrenergic neurons or a part of the local tissue milieu, are nondetectable in the microvessels within the cheek pouch as compared to mesenteric and muscle tissue of the hamster.

Microvascular pressure distribution and responses of pulmonary allografts and cheek pouch arterioles in the hamster to oxygen

Despite extensive investigations of the pulmonary circulation using both in vitro and in vitro preparations, few direct microcirculatory studies have been made. Consequently, the mechanisms involved in the response of the pulmonary microvasculature to changes in oxygen tension remain unclear. The present study represents the first direct observation of the responses in pulmonary microvessels to alterations in oxygen tension. Neonatal lung tissue was transplanted into the hamster cheek pouch using a chamber technique. Both tissues were characterized with respect to their microvascular pressure profile and vascular response to hypoxia. The results showed the two tissues to be remarkably different. Small pulmonary and cheek pouch arterioles exhibited opposite responses to changes in oxygen environment; hypoxia elicited a constriction of pulmonary arterioles, but a dilation of cheek pouch arterioles. Pulmonary capillary pressure, although comparable to that measured in the intact lung (13 mm Hg), was substantially lower than cheek pouch capillary pressure, which was within the range of that described for several systemic vascular beds. The microcirculatory effects of oxygen on both tissues were confined to the arteriolar segments. The characteristics of this pulmonary microcirculation are such that it is a unique model for further physiological and pharmacological studies.