Transplantation studies of the role of the kidney in long-term blood pressure reduction following brief ACE inhibitor treatment in young spontaneously hypertensive rats

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Effects of ACE inhibition on proximal tubule sodium transport

Angiotensin-converting enzyme (ACE) inhibitors such as captopril, which block ANG II formation, are commonly used for treatment of hypertension. There is substantial evidence that the proximal tubule (PT) is a primary target site for captopril but the molecular mechanisms for its action in PT are not well defined. The aim of this study was to determine the physiological and molecular changes in PT provoked by acute captopril treatment in the absence of changes in blood pressure or glomerular filtration rate (GFR). Captopril (infused at 12 μg/min for 20 min) did not change blood pressure or GFR but induced an immediate (<10 min) increase in PT flow measured with a nonobstructive optical method (to 117 ± 14% of baseline) along with a rapid diuresis from 2.1 ± 0.6 mg/min (baseline) to 3.7 ± 0.9 mg/min (captopril). Captopril also provoked a significant retraction of PT Na+/H+exchanger isoform 3 (NHE3), NHE regulatory factor (NHERF)-1, myosin-VI, and Na+-Picotransporter type 2 (NaPi2), but not ACE, out of apical microvillus-enriched membranes. Proteomic analysis with MALDI-TOF MS revealed an additional eight abundant membrane-associated proteins that redistributed out of the microvillus-enriched membrane during captopril treatment: megalin, myosin II-A, clathrin, aminopeptidase N, DPPIV, ezrin, moesin, and vacuolar H+-ATPase subunit β2. In summary, captopril can rapidly depress PT reabsorption in the absence of a change in GFR or BP and provokes the redistribution of a set of transporters and transporter-associated proteins that likely participate in the decrease in PT reabsorption and may also contribute to the blood pressure-lowering effect of ACE inhibitors.

Inflammation, not hyperhomocysteinemia, is related to oxidative stress and hemostatic and endothelial dysfunction in uremia

BACKGROUND

Several cardiovascular risk factors are present in patients with chronic renal failure (CRF), among which are systemic inflammation and hyperhomocysteinemia. Increased oxidative stress, endothelial activation/dysfunction, and coagulation activation are considered integral components of the inflammatory response, but have also been proposed as mediators of plasma homocysteine (tHcy)-induced cell damage. Using correlation analysis, we assessed the relative contributions of inflammation and hyperhomocysteinemia in the abnormal oxidative stress, endothelial activation/dysfunction, and hemostasis activation in patients with CRF.

METHODS

The relationships of inflammatory proteins and tHcy with plasma markers of these processes were studied in 64 patients with CRF (serum creatinine 526 +/- 319 micromol/L) on conservative treatment, comparing the results with healthy controls (N = 15 to 40, depending on the measured variable) of similar sex and age.

RESULTS

Patients had significant increases in inflammatory cytokines (TNF-alpha and IL-8) and acute-phase proteins (C-reactive protein, fibrinogen and alpha1-antitrypsin). tHcy was increased in 87.5% of patients (mean = 27.1 micromol/L, range 6.5 to 118). Patients had significant increases in (1) indices of oxidative stress: TBARS (thiobarbituric acid-reactive species), a marker of lipid peroxidation and AOPP (advanced oxidation protein products), a marker of protein oxidation; (2) endothelial cell markers such as von Willebrand factor (vWF:Ag), soluble ICAM-1 and soluble thrombomodulin (sTM); (3) markers of intravascular thrombin generation: thrombin-antithrombin complexes (TAT) and prothrombin fragment F(1+2) (PF(1+2)); and (4) indices of activation of fibrinolysis: plasmin-antiplasmin complexes (PAP), fibrin degradation products (FnDP) and fibrinogen degradation products (FgDP). tHcy was significantly correlated with plasma creatinine (r = 0.29, P < 0.018) and with serum folate (r = -0.38, P < 0.002). However, no significant correlations were observed between tHcy and TBARS, AOPP, vWF:Ag, sICAM-1, sTM, TAT, F(1+2), sTF, PAP, FnDP, and FgDP. Conversely, acute-phase proteins showed significant, positive correlations with most markers of oxidative stress, endothelial dysfunction and hemostatic activation.

CONCLUSIONS

Systemic inflammation, which is closely associated with augmented oxidative stress, endothelial cell dysfunction and hemostatic activation, emerges as a major cardiovascular risk factor in CRF. tHcy is unrelated to these events. Thus, alternative mechanisms through which hyperhomocysteinemia could predispose to vascular lesion and thrombotic events in CRF needs to be investigated.

Prehypertension: a possible target for antihypertensive medication

Vascular changes associated with elevated blood pressure may precede the clinical diagnosis of hypertension. Even after the diagnosis is made, associated coronary heart disease and renal disease continue to progress, despite adequate blood pressure control. Early treatment of blood pressure may reduce the incidence of clinical hypertension and reduce the long-term consequences of hypertension. Animal studies have shown that early blood pressure lowering, through blockade of the renin-angiotensin system, prevents long-term hypertension. Prevention is an important goal in the treatment of hypertension. Our best attempts to prevent hypertension use nonpharmacologic methods of diet and exercise. These methods are fraught with difficulties of implementation and compliance that limit their success. Finding novel approaches to prevent hypertension may have a major impact on the incidence of hypertension. We are investigating the effect of 2 years of treatment with an angiotensin receptor blocker (candesartan cilexitil) compared with placebo, followed by 2 years of follow-up, on the incidence of hypertension in patients with high-normal blood pressure. Incidence of hypertension after discontinuation of active treatment will be compared with the incidence in the placebo group. There will be 1000 patients enrolled in the study, which will be completed in 2004.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Long-term effects of angiotensin-converting enzyme inhibition on renal medullary neutral lipid in spontaneously hypertensive rats

Short-term treatment of young spontaneously hypertensive rats (SHR) with angiotensin-converting enzyme (ACE) inhibitors reduces systolic blood pressure. Renal medullary neutral lipids (RMNLs) have vasodilator properties that may explain the effects of ACE inhibition. We measured RMNL levels of SHR treated between 6 and 10 weeks of age with (1) vehicle, (2) ramipril 1 mg. kg-1. d-1, (3) the bradykinin B2 receptor antagonist icatibant 0.5 mg. kg-1. d-1, or (4) icatibant 0.5 mg. kg-1. d-1 plus ramipril 1 mg. kg-1. d-1. RMNLs were quantified by oil red O fluorescence at 10 and 20 weeks of age. Systolic blood pressure (BP) was measured by tail-cuff plethysmography. Ramipril reduced BP at 10 weeks of age and increased RMNLs compared with controls (0.99+/-0.07% versus 0.56+/-0. 06%, P<0.01). Icatibant alone had no significant effect on RMNLs (0.55+/-0.04%) but attenuated the increase in RMNLs by ramipril (0. 81+/-0.05%). In control SHR, the increase in BP between 10 and 20 weeks of age was associated with a significant increase in RMNLs (0.79+/-0.09%). SHR that had received ramipril had significantly lower BP than controls at 20 weeks of age, but RMNL was not significantly different (0.92+/-0.10%). Therefore, in young SHR, ACE inhibition increases RMNLs and reduces blood pressure, an effect that appears to depend on bradykinin. The changes in RMNLs at the age of 10 weeks paralleled long-term BP effects and may be involved in setting the BP track in SHR.

Renal medulla and bradykinin during the development of hypertension in SHR

1. The long-term reduction in blood pressure following ACE inhibitor treatment in young spontaneously hypertensive rats (SHR) appears to depend on both the kidney and bradykinin. 2. The aim of this experiment was to examine the effects of ACE inhibition and bradykinin on renal morphology and blood pressure in SHR. 3. Between 6 and 10 weeks of age male SHR received one of four treatments: water (n = 26), ramipril (1 mg/kg per day; n = 24), ramipril (1 mg/kg per day) plus Hoe 140 (0.5 mg/kg per day; n = 25) or Hoe 140 (0.5 mg/kg per day; n = 25). 4. Renal medullary and cortical volumes were determined stereologically at 10 and 20 weeks of age. 5. After 4 weeks of treatment, ramipril reduced the size of the renal medulla while Hoe 140 increased medullary volumes compared to control. Ten weeks after treatment was stopped the renal medulla of the ramipril group had returned to normal, however, there was a persistent increase in medullary volume of both Hoe 140 treated groups. 6. Our results imply that bradykinin may influence the size of the renal medulla which may have important effects on the development of hypertension in SHR.

Defective G protein activation of the cAMP pathway in rat kidney during genetic hypertension

The development of hypertension in the spontaneously hypertensive rat (SHR) is associated with renal dysfunction and vasoconstriction. The kidneys of young SHRs exhibit exaggerated reactivity to angiotensin II (Ang-II) and attenuated responses to vasodilators that normally activate the cAMP signal to buffer hormone-induced vasoconstriction. The present study investigates the mechanism(s) responsible for this abnormality in activation of the cAMP second-messenger pathway in hypertensive animals. Renal vascular reactivity was assessed in 7-week-old anesthetized SHRs and normotensive Wistar-Kyoto rats. The animals were pretreated with indomethacin to block prostanoid production throughout an experiment. Ang-II was injected into the renal artery either alone or mixed with the vasodilator fenoldopam, a dopamine-receptor agonist. These two opposing vasoactive agents were administered before and during intrarenal infusion of NaF or cholera toxin, two activators of G proteins that stimulate cAMP production. The results show that Ang-II reduced renal blood flow by 45% in both strains. In Wistar-Kyoto rats, fenoldopam reduced the Ang-II-induced decrease in renal blood flow from -45% to -30%. This protective effect of fenoldopam was increased further during infusion of NaF or cholera toxin (-18% or -19% decrease in renal blood flow). In SHRs, fenoldopam failed to attenuate Ang II-mediated vasoconstriction (-45% vs. -44%). In contrast, fenoldopam effectively blunted the Ang-II-induced vasoconstriction when it was given concurrently with NaF or cholera toxin (-27 or -31% decrease in renal blood flow). These findings provide evidence for defective interaction between receptor coupling and activation of guanine nucleotide stimulatory factor proteins in the renal microcirculation of 7-week-old SHRs. Such a deficiency could play an important role in renal dysfunction associated with the development of genetic hypertension.

Resetting blood pressure in spontaneously hypertensive rats. The role of bradykinin

Brief angiotensin-converting enzyme (ACE) inhibition in young spontaneously hypertensive rats (SHR) causes a persistent reduction in blood pressure. Bradykinin accumulation may contribute to these long-term effects, and to test this hypothesis we studied the consequences of bradykinin B2 receptor antagonism during ACE inhibitor treatment in young SHR. Male SHR were treated from 6 to 10 weeks of age with water, ramipril (1 mg/kg per day), Hoe 140 (0.5 mg/kg per day), or both ramipril and Hoe 140. Systolic blood pressure and body weight were measured each week from 6 to 20 weeks of age. During treatment, Hoe 140 treatment resulted in lower blood pressures than in controls. Rampiril caused a larger fall in blood pressure over the same period. The ramipril plus Hoe 140 group had the lowest blood pressures of any group during treatment. After treatment, the blood pressure of Hoe 140-treated SHR was similar to that of untreated SHR. After ramipril, blood pressure rose but plateaued significantly below values in controls. In contrast, withdrawal of combined ramipril and Hoe 140 treatment caused a rapid rise of systolic blood pressure to levels significantly higher than in ramipril-treated SHR but less than in controls. The antihypertensive effects of Hoe 140 during the development of genetic hypertension may represent a direct effect of the drug or some alteration in the normal relation between bradykinin and blood pressure. The antagonism by Hoe 140 of the long-term blood pressure reduction after ramipril withdrawal indicates that the persistent effects of ACE inhibitors may in part be due to the accumulation of bradykinin during a critical stage of development.

Blood pressure after captopril withdrawal from spontaneously hypertensive rats

The purpose of the present study was to compare the effect of chronic captopril treatment on blood pressure in young and adult spontaneously hypertensive rats (SHR) and to assess the time course for development of hypertension after captopril withdrawal. SHR received drinking water or captopril solution from 4 weeks of age and were instrumented with radiotelemetry devices at 18 weeks of age to allow continuous monitoring of blood pressure. At 23 weeks of age, mean blood pressure in the captopril group was 100 +/- 1 mm Hg compared with 157 +/- 3 mm Hg in the water group. Pulse pressure also was significantly reduced in the captopril-treated rats. Infusion of angiotensin II into a subset of captopril-treated rats increased pulse pressure and restored blood pressure to levels of water-treated rats. Captopril treatment for 6 weeks in adult, 24-week-old SHR did not reduce blood pressure to the level of rats treated from 4 weeks of age. Ten weeks after cessation of captopril, blood pressure was 125 +/- 4 and 144 +/- 4 mm Hg in SHR treated with captopril from 4 to 30 and from 24 to 30 weeks of age, respectively, compared with control hypertensive rats with mean blood pressure of 160 +/- 6 mm Hg. Results from this radiotelemetry study confirm previous findings that captopril treatment prevents the development of hypertension and produces a persistent reduction of blood pressure after treatment in young SHR. Captopril treatment produced a persistent reduction of blood pressure after discontinuation in adult rats; however, the effect was less than that observed with captopril initiated in young rats.

Blood pressure and lifespan following brief ACE inhibitor treatment in young spontaneously hypertensive rats

1. Brief treatment with angiotensin-converting enzyme (ACE) inhibitors in young spontaneously hypertensive rats (SHR) causes a reduction in blood pressure that persists into maturity. The lifetime effects of such treatment have not been studied. 2. Nineteen male SHR were treated with either water (n = 9) or perindopril (3 mg/kg per day) (n = 10) by daily gavage between 6 and 10 weeks of age and systolic blood pressure and bodyweight were measured each month until all animals died in old age. 3. Following treatment the systolic blood pressure of SHR treated with perindopril remained consistently lower than control SHR until about 82 weeks of age. After this age the blood pressure of control SHR fell spontaneously so that smaller differences were observed between the two groups in the last 4 months of the study. 4. Rats that received perindopril lived on average 1 month longer than control rats, but this difference was not statistically significant. 5. Thus, brief ACE inhibition in early life in SHR ameliorated the hypertension throughout life.