Deleterious effects of calcium channel blockade on pressure transmission and glomerular injury in rat remnant kidneys

Potential risks of calcium channel blockers in chronic kidney disease

Antihypertensive therapy remains the most effective strategy for slowing the progression of chronic kidney disease (CKD). However, in proteinuric nephropathies, calcium channel blockers (CCBs) are less effective than other antihypertensives unless normotension is achieved. This is because the glomerular capillaries, rather than larger vessels, are the primary site of hypertensive injury in proteinuric nephropathies. CCBs impair renal autoregulation, which protects glomerular capillaries against the transmission of systemic pressures. CCBs' renoprotective inferiority in the comparator group likely accounts for the greater renoprotection observed with renin-angiotensin system blockade rather than blood pressure (BP)-independent renoprotective superiority. Nevertheless, CKD patients are at greater absolute risk for cardiovascular events rather than end-stage renal disease. Therefore, if the needed BP reductions cannot be achieved with other agents, it may be appropriate to use CCBs because of their antihypertensive effectiveness, provided care is taken to ensure normotension and to closely monitor proteinuria and renal disease progression.

Elevations in serum creatinine with RAAS blockade: why isn't it a sign of kidney injury?

PURPOSE OF REVIEW

The aim of this article is to review the pertinent physiology and pathophysiology of the renin-angiotensin-aldosterone system (RAAS), summarize the proven beneficial cardiovascular and renal effects of RAAS blockade, examine clinical situations in which RAAS blockade may induce reductions in glomerular filtration rate, and explore why increases in serum creatinine in the setting of angiotensin-converting enzyme inhibitor (ACEi) or angiotensin receptor blocker (ARB) therapy do not necessarily signify the presence of clinically relevant kidney failure.

RECENT FINDINGS

RAAS inhibition appears to reduce the likelihood of atrial fibrillation. RAAS inhibition leads to improved insulin sensitivity and glycemic control, but does not appear to prevent diabetes. The beneficial effects of ACEi/ARB therapy extend to those with significant renal disease. Combination ACEi/ARB is safe, and reduces proteinuria more than either agent alone in patients with macroalbuminuric nephropathy. Acute deteriorations in renal function that result from RAAS inhibition are usually reversible.

SUMMARY

RAAS blockade exerts potent hemodynamic, antihypertensive, and antiinflammatory effects, and slows progression of kidney disease beyond that due to lowering of blood pressure. The benefit extends to those with advanced disease. In spite of established benefit, ACEi and ARB therapy remains underutilized, in part due to concerns about acute deteriorations in renal function that result from interruption of the RAAS.

Renoprotective effect of calcium channel blockers

The advancing chronic renal failure is at most the consequence of secondary haemodynamic and metabolic factors as intraglomerular hypertension and glomerular hypertrophy. Although tight blood pressure control is the major preventive mechanism for progressive renal failure, ACE inhibitors and angiotensin receptor blockers have some other renoprotective mechanisms beyond the blood pressure control. That is why these two groups of antihypertensive drugs traditionally have advantages in treating renal patients especially those with proteinuria over 400-1000 mg/day. Even if earlier experimental studies have shown renoprotective effect of calcium channel blockers, later clinical studies did not prove that calcium channel blockers have any advantages in renal protection over ACE inhibitors given as monotherapy or in combination with ACE inhibitors. It was explained by action of calcium channel blockers on afferent but not on efferent glomerular arterioles; a well known mechanism that leads to intraglomerular hypertension. New generations of dihydropiridine calcium channel blockers can dilate even efferent arterioles not causing unfavorable haemodynamic disturbances. This finding was confirmed in clinical studies which showed that renoprotection established by calcium channel blockers was not inferior to that of ACE inhibitors and that calcium channel blockers and ACE inhibitors have additive effect on renoprotection. Newer generation of dihydropiridine calcium channel blockers seem to offer more therapeutic possibilities in renoprotection by their dual action on afferent and efferent glomerular arterioles and, possibly by other effects beyond the blood pressure control.

Calcium antagonists: Do they equally protect against kidney injury?

Calcium antagonists are a heterogeneous group of drugs that affect not only different channels but different parts of the same channel. These differences translate into different renal hemodynamic effects. This Commentary discusses the implications of these differences for surrogate markers of outcome.

Adverse renal consequences of obesity

Emerging evidence indicates that obesity, even in the absence of diabetes, contributes significantly to the development and progression of chronic kidney disease (CKD). Glomerular hyperfiltration/hypertrophy in response to the increased metabolic needs of obesity are postulated to lead to the development of glomerulosclerosis (GS) in a manner analogous to that in reduced renal mass states. Nevertheless, the individual risk for developing GS with obesity is very low. It is proposed that glomerular hyperfiltration/hypertrophy are per se not pathogenic in the absence of an enhanced glomerular blood pressure (BP) transmission, and the modest preglomerular vasodilation that is likely present in the large majority of obese individuals is not sufficient to result in such increased BP transmission. However, in the small subset of obese individuals who are also born with a substantially reduced nephron number, there is a greater risk of enhanced glomerular BP transmission due to the substantially greater preglomerular vasodilation. Of perhaps greater clinical importance, similar additive deleterious effects of obesity on BP transmission would be expected in individuals with reduced renal mass, either congenital or acquired, or with concurrent renal disease, leading to accelerated progression. Of note, a low birth weight may be a risk factor for not only reduced nephron numbers at birth, but also for obesity and hypertension, resulting in a clustering of risk factors for progressive GS. Therefore, even though the individual risk for developing obesity GS is low, the cumulative impact of obesity on the public health burden of CKD is likely to be large because of its huge prevalence.

Dynamic blood pressure load and nephropathy in the ZSF1 (fa/facp) model of type 2 diabetes

Diabetes and increased blood pressure (BP) are believed to interact synergistically in the pathogenesis and progression of diabetic nephropathy. The present studies were performed to examine if there were differences in BP load and/or protective renal autoregulatory capacity between the obese diabetic Zucker fatty /spontaneously hypertensive heart failure F1 hybrid (ZSF1) ( fa/ facp) rats and their lean controls. By ∼26 wk of age, ZSF1 ( n = 13) but not their lean controls ( n = 16) had developed substantial proteinuria (180 ± 19 vs. 16 ± 1.4 mg/24 h) and glomerulosclerosis (19 ± 2.4 vs. 0.6 ± 0.2%; P < 0.001). However, average ambient systolic BP by radiotelemetry (12–26 wk of age) was modestly lower in ZSF1 than in lean controls (130 ± 1.4 vs. 137 ± 1.7 mmHg, P < 0.002), although the 24-h BP power spectra showed a mild increase at frequencies <0.1 Hz in the ZSF1. Autoregulatory capacity under anesthesia in response to step changes in perfusion pressure between 100 and 140 mmHg was similarly well preserved in both ZSF1 and lean controls at 16–18 wk of age [autoregulatory indexes (AI) <0.1]. Similarly, differences were not observed for dynamic autoregulation in conscious rats [transfer functions between BP (input) and renal blood flow (output) using chronic Transonic flow probes]. Collectively, these data indicate that the pathogenesis of nephropathy in the ZSF1 model of type 2 diabetic nephropathy is largely independent of differences in systemic BP and/or its potential renal transmission. However, these data do not exclude the possibility that the diabetic milieu may alter the glomerular capillaries in the ZSF1, such that there is an enhanced local susceptibility to injury with even normal glomerular pressures.

Manidipine-delapril combination in the management of hypertension

High blood pressure (BP) is the major cardiovascular risk factor and the main cause of death around the world. Control of blood pressure reduces the high mortality associated with hypertension and the most recent guidelines recommend reducing arterial BP values below 140/90 mmHg for all hypertensive patients (130/80 in diabetics) as a necessary step to reduce global cardiovascular risk, which is the fundamental objective of the treatment. To achieve these target BP goals frequently requires combination therapy with two or more antihypertensive agents. Although the combination of a diuretic and an angiotensin converting enzyme inhibitor (ACEI) is the most commonly used in the clinical practice, the combination of an ACEI and a calcium channel blocker may have an additive antihypertensive effect, a favorable effect on the metabolic profile, and an increased target organ damage protection. The new oral fixed combination manidipine 10 mg/delapril 30 mg has a greater antihypertensive effect than both components of the combination separately, and in non-responders to monotherapy with manidipine or delapril the average reduction of systolic and diastolic BP is 16/10 mmHg. The combination is well tolerated and the observed adverse effects are of the same nature as those observed in patients treated with the components as monotherapy. However, combination therapy reduces the incidence of ankle edema in patients treated with manidipine.

Ca2+ channel subtypes and pharmacology in the kidney

A large body of evidence has accrued indicating that voltage-gated Ca(2+) channel subtypes, including L-, T-, N-, and P/Q-type, are present within renal vascular and tubular tissues, and the blockade of these Ca(2+) channels produces diverse actions on renal microcirculation. Because nifedipine acts exclusively on L-type Ca(2+) channels, the observation that nifedipine predominantly dilates afferent arterioles implicates intrarenal heterogeneity in the distribution of L-type Ca(2+) channels and suggests that it potentially causes glomerular hypertension. In contrast, recently developed Ca(2+) channel blockers (CCBs), including mibefradil and efonidipine, exert blocking action on L-type and T-type Ca(2+) channels and elicit vasodilation of afferent and efferent arterioles, which suggests the presence of T-type Ca(2+) channels in both arterioles and the distinct impact on intraglomerular pressure. Recently, aldosterone has been established as an aggravating factor in kidney disease, and T-type Ca(2+) channels mediate aldosterone release as well as its effect on renal efferent arteriolar tone. Furthermore, T-type CCBs are reported to exert inhibitory action on inflammatory process and renin secretion. Similarly, N-type Ca(2+) channels are present in nerve terminals, and the inhibition of neurotransmitter release by N-type CCBs (eg, cilnidipine) elicits dilation of afferent and efferent arterioles and reduces glomerular pressure. Collectively, the kidney is endowed with a variety of Ca(2+) channel subtypes, and the inhibition of these channels by their specific CCBs leads to variable impact on renal microcirculation. Furthermore, multifaceted activity of CCBs on T- and N-type Ca(2+) channels may offer additive benefits through nonhemodynamic mechanisms in the progression of chronic kidney disease.

Assessment of renal autoregulation

The kidney displays highly efficient autoregulation so that under steady-state conditions renal blood flow (RBF) is independent of blood pressure over a wide range of pressure. Autoregulation occurs in the preglomerular microcirculation and is mediated by two, perhaps three, mechanisms. The faster myogenic mechanism and the slower tubuloglomerular feedback contribute both directly and interactively to autoregulation of RBF and of glomerular capillary pressure. Multiple experiments have been used to study autoregulation and can be considered as variants of two basic designs. The first measures RBF after multiple stepwise changes in renal perfusion pressure to assess how a biological condition or experimental maneuver affects the overall pressure-flow relationship. The second uses time-series analysis to better understand the operation of multiple controllers operating in parallel on the same vascular smooth muscle. There are conceptual and experimental limitations to all current experimental designs so that no one design adequately describes autoregulation. In particular, it is clear that the efficiency of autoregulation varies with time and that most current techniques do not adequately address this issue. Also, the time-varying and nonadditive interaction between the myogenic mechanism and tubuloglomerular feedback underscores the difficulty of dissecting their contributions to autoregulation. We consider the modulation of autoregulation by nitric oxide and use it to illustrate the necessity for multiple experimental designs, often applied iteratively.

Spontaneously reduced blood pressure load in the rat streptozotocin-induced diabetes model: potential pathogenetic relevance

The rat streptozotocin (STZ)-induced diabetes model is widely used to investigate the pathogenesis of diabetic nephropathy. However, overt nephropathy is inexplicably slow to develop in this model compared with renal mass reduction (RMR) models. To examine whether blood pressure (BP) differences correlated with the time course of glomerulosclerosis (GS), BP was measured continuously throughout the course by radiotelemetry in control (n = 17), partially insulin-treated STZ-diabetes (average blood glucose 364 +/- 15 mg/dl; n = 15), and two normotensive RMR models (systolic BP <140 mmHg)--uninephrectomy (UNX; n = 16) and 3/4 RMR by surgical excision [right nephrectomy + excision of both poles of left kidney (RK-NX); n = 12] in Sprague-Dawley rats. Proteinuria and GS were assessed at approximately 16-20 wk (all groups) and at 36-40 wk (all groups except RK-NX). At 16 wk, significantly greater proteinuria and GS had developed in the RK-NX group compared with the other three groups (not different from each other). By 36-40 wk, substantial proteinuria and GS had also developed in the UNX group, but both the control and the STZ-diabetic rats exhibited comparable modest proteinuria and minimal GS. Systolic BP (mmHg) was significantly reduced in the STZ-diabetic rats (116 +/- 1.1) compared with both control (124 +/- 1.0) and RMR (128 +/- 1.2 and 130 +/- 3.0) groups (P < 0.01). Similarly, "BP load" as estimated by BP power spectral analysis was also lower in the STZ-diabetic rats. Given the known protective effects of BP reductions on the progression of diabetic nephropathy, it is likely that this spontaneous reduction in ambient BP contributes to the slow development of GS in the STZ-diabetes model compared with the normotensive RMR models.

Progression of renal disease: renoprotective specificity of renin-angiotensin system blockade

Recent guidelines for management of patients with chronic kidney disease recommend both lower optimal BP targets and agents that block the renin-angiotensin system (RAS) for specific additional BP-independent renoprotection. Although there are other compelling rationales to use RAS blockade in patients with chronic kidney disease, including its antihypertensive effectiveness and ability to counteract the adverse effects of diuretics, a critical review of the available scientific evidence suggests that the specificity of renoprotection that is provided by RAS blockade has been greatly overemphasized. Little evidence of truly BP-independent renoprotection is observed in experimental animal models when ambient BP is assessed adequately by chronic continuous BP radiotelemetry. Although the clinical trial evidence is somewhat stronger, nevertheless, even when interpreted favorably, the absolute magnitude of the BP-independent component of the renoprotection that is observed with RAS blockade is much smaller than what is due to its antihypertensive effects.

Renal autoregulation: new perspectives regarding the protective and regulatory roles of the underlying mechanisms

When the kidney is subjected to acute increases in blood pressure (BP), renal blood flow (RBF) and glomerular filtration rate (GFR) are observed to remain relatively constant. Two mechanisms, tubuloglomerular feedback (TGF) and the myogenic response, are thought to act in concert to achieve a precise moment-by-moment regulation of GFR and distal salt delivery. The current view is that this mechanism insulates renal excretory function from fluctuations in BP. Indeed, the concept that renal autoregulation is necessary for normal renal function and volume homeostasis has long been a cornerstone of renal physiology. This article presents a very different view, at least regarding the myogenic component of this response. We suggest that its primary purpose is to protect the kidney against the damaging effects of hypertension. The arguments advanced take into consideration the unique properties of the afferent arteriolar myogenic response that allow it to protect against the oscillating systolic pressure and the accruing evidence that when this response is impaired, the primary consequence is not a disturbed volume homeostasis but rather an increased susceptibility to hypertensive injury. It is suggested that redundant and compensatory mechanisms achieve volume regulation, despite considerable fluctuations in distal delivery, and the assumed moment-by-moment regulation of renal hemodynamics is questioned. Evidence is presented suggesting that additional mechanisms exist to maintain ambient levels of RBF and GFR within normal range, despite chronic alterations in BP and severely impaired acute responses to pressure. Finally, the implications of this new perspective on the divergent roles of the myogenic response to pressure vs. the TGF response to changes in distal delivery are considered, and it is proposed that in addition to TGF-induced vasoconstriction, vasodepressor responses to reduced distal delivery may play a critical role in modulating afferent arteriolar reactivity to integrate the regulatory and protective functions of the renal microvasculature.

Morphometric evidence for impairment of renal autoregulation in advanced essential hypertension

A morphometric study was performed on 22 renal biopsies from hypertensive patients with proteinuria and/or azotemia, with no evidence of other renal disease. These results were compared with our earlier study of normotensive aging kidneys. Afferent arterioles in hypertensive kidneys showed a significant increase in lumen diameter (15.7+/-4.9 vs 13.4+/-4.7 microm, P=0.0007) and wall area (1234+/-769 vs 998+/-445 microm(2), P=0.037), due primarily to shift in the distribution of arteriolar types, from predominantly normal toward predominantly hyaline arterioles in hypertension. Glomeruli were divided into four basic types: normal, hypertrophic, focal segmental glomerulosclerosis (FSGS) type, and sclerosing. Overall, glomeruli in hypertensive kidneys were much larger than in normotensive aging kidneys, for example, total capillary area (16 247+/-10 681 vs 11 624+/-5702 microm(2), P<0.00001). This increase was due primarily to an increase in size of each type, for example, for hypertrophic glomeruli: total capillary area (22 205+/-10 426 vs 15 349+/-4577 microm(2), P=0.0038). There was an excellent correlation between arteriolar lumen diameter and mean glomerular capillary area for hypertrophic/FSGS-type glomeruli (r=0.4778, P=0.0013), such that as arteriolar diameter increases the mean glomerular capillary area increases, consistent with loss of autoregulation. The morphologic correlates of loss of autoregulation, with afferent arteriolar dilatation and increase in glomerular capillary size, glomerular hypertrophy, and subsequent FSGS, are present on a focal basis in aging kidneys and, much more extensively, although still focally, in hypertensive kidneys.

Systolic blood pressure as the trigger for the renal myogenic response: protective or autoregulatory?

PURPOSE OF REVIEW

The ability of the kidney to autoregulate renal blood flow and glomerular filtration rate has long been viewed as existing to prevent fluctuations in blood pressure from causing parallel fluctuations in renal function and distal delivery of filtrate. This review, however, points out that the primary consequence of the loss of this autoregulatory capacity is not a disturbance in volume regulation, but rather an increased susceptibility to hypertensive injury. Moreover, the kinetic requirements for renal protection indicate that current views of dynamic autoregulation cannot explain how the kidney is normally protected against acute elevations in systolic blood pressure.

RECENT FINDINGS

Recent findings suggest that the kinetics of the myogenic mechanism of the afferent arteriole are uniquely suited to protect against acute elevations in the systolic blood pressure, in that this vessel not only senses this rapidly oscillating blood pressure component, but that its response is exclusively dependent on this signal.

SUMMARY

These new findings are consistent with recent data indicating that it is the systolic blood pressure elevations that most closely correlate with target organ damage. The fact that the myogenic mechanism is also a necessary component of renal autoregulation may explain the strong linkage between autoregulatory impairment and increased susceptibility to hypertensive injury.

Hypertension and atherosclerosis: clinical implications from the ALLHAT Trial

By failing to recognize the heterogeneity of hypertension, the authors of the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT) study used a faulty premise to conduct a poorly designed clinical trial. By failing to control blood pressures equally across study drug groups, ALLHAT cannot be considered to be a definitive comparative trial. Being neither a monotherapy trial nor a trial that initiated therapy for blood pressure control, ALLHAT provided no data to recommend first-line therapy for hypertension, making the conclusions invalid. Thiazide-type diuretics increase angiotensin II and consequently promote atherosclerosis and arteriolarsclerosis. Angiotensin-converting enzyme inhibitors and angiotensin receptor blockers retard atherosclerosis and are nephroprotective. Multiple randomized controlled trials show beneficial clinical outcomes, including cardioprotection and nephroprotection, with the use of angiotensin-converting enzyme inhibitors and angiotensin receptor blockers. These agents, and not thiazide-type diuretics, should be used as first-line agents to retard the process of atherosclerosis and its clinical outcomes in the setting of arterial hypertension.

Differential effects of salt on renal hemodynamics and potential pressure transmission in stroke-prone and stroke-resistant spontaneously hypertensive rats

Salt-supplemented stroke-prone spontaneously hypertensive rats (SHRsp) develop more severe hypertension-induced renal damage (HIRD) compared with their progenitor SHR. The present studies were performed to examine whether in addition to increasing the severity of hypertension salt also enhanced the transmission of such hypertension to the renal vascular bed in the SHRsp. "Step" and "dynamic" renal blood flow (RBF) autoregulation (AR) were examined in approximately 12-wk-old SHR and SHRsp after 3-5 days of an 8% NaCl diet. During step AR under anesthesia (n = 8-11), RBF was significantly higher in the SHRsp at all perfusion pressures (P < 0.01), but AR capacity was not different. Similarly, in separate conscious chronically instrumented rats (n = 8 each), both blood pressure (BP) and RBF were modestly but significantly higher at baseline before salt in the SHRsp (P < 0.05). However, transfer function analysis did not show significant differences in the admittance gain parameters. However, after 3-5 days of salt, although average BP was not significantly altered in either strain, RBF increased further in the SHRsp and there was a significantly greater transfer of BP into RBF power in the SHRsp. This was reflected in the significantly higher admittance gain parameters at most frequencies including the heartbeat frequency (P < 0.05 maximum). These differential hemodynamic effects of salt have the potential to enhance BP transmission to the renal vascular bed and also contribute to the more severe HIRD observed in the salt-supplemented SHRsp.

Recommendations for Blood Pressure Measurement in Humans and Experimental Animals

In experimental animals, as in humans, techniques for measuring blood pressure (BP) have improved considerably over the past decade. In this document, we present recommendations for measuring BP in experimental animals with the goal of helping investigators select optimal methods for BP monitoring in the research laboratory. The advantages and disadvantages of various BP measurement methods are discussed and specific recommendations are provided for selecting the optimal technique depending on the study objective. Although indirect techniques that permit only sporadic measurements of BP may be suitable for some purposes, methods for directly measuring BP are generally preferred because of their ability to monitor the highly dynamic nature of BP in a comprehensive fashion. Selection of the methods to be used should ultimately be guided by the study objectives to insure that the techniques chosen are appropriate for the experimental questions being explored.

Differential effects of antihypertensive drugs on renal and glomerular hemodynamics and injury in the chronic nitric-oxide-suppressed rat

BACKGROUND/AIMS

Prolonged nitric oxide synthase (NOS) inhibition with N(omega)-nitro-L-arginine methylester in normotensive and hypertensive rats has been demonstrated to produce severe systemic and glomerular hypertension with glomerular sclerosis, and these changes have become a useful experimental model of hypertensive nephrosclerosis. This review summarizes data from our serial studies as well as work of others who are also investigating the effects of the commonly used antihypertensive drugs (including calcium antagonist, angiotensin-converting enzyme inhibitor, angiotensin II type 1 receptor blocker, aldosterone antagonist and thiazide diuretic) on renal and glomerular hemodynamics, renal function and glomerular histopathology using this model.

METHODS

A Medline search was performed to identify the relevant literature describing renal effects of antihypertensive drugs in models of hypertension and nephrosclerosis produced or exacerbated by NOS inhibition.

RESULTS

Existing data have indicated that most of these drug classes have produced dramatic renoprotective effects, structurally or functionally, on nephrosclerosis induced by prolonged NOS inhibition.

CONCLUSION

This review of experimental studies has provided strong evidence supporting the clinical benefits of antihypertensive drugs for hypertensive patients with renal impairment particularly those with endothelial dysfunction associated with NOS deficiency.

Recommendations for Blood Pressure Measurement in Humans and Experimental Animals

In experimental animals, as in humans, techniques for measuring blood pressure (BP) have improved considerably over the past decade. In this document, we present recommendations for measuring BP in experimental animals with the goal of helping investigators select optimal methods for BP monitoring in the research laboratory. The advantages and disadvantages of various BP measurement methods are discussed and specific recommendations are provided for selecting the optimal technique depending on the study objective. Although indirect techniques that permit only sporadic measurements of BP may be suitable for some purposes, methods for directly measuring BP are generally preferred because of their ability to monitor the highly dynamic nature of BP in a comprehensive fashion. Selection of the methods to be used should ultimately be guided by the study objectives to insure that the techniques chosen are appropriate for the experimental questions being explored.

Angiotensin-converting enzyme inhibitors and angiotensin receptor blockers in chronic renal disease: safety issues

Reducing the actions of the renin-angiotensin-aldosterone system with angiotensin-converting enzyme inhibitors (ACE inhibitors) and angiotensin receptor blockers (ARBs) slows nephropathy progression in patients with or without diabetes. These drug classes have proven therapeutic benefits, particularly in patients with renal insufficiency (ie, serum creatinine level 133-265 micromol/L [1.5-3.0 mg/dL]). This class of drugs could also provide renoprotective effects that are nonblood pressure-dependent when used as part of combination antihypertensive therapy in patients with more advanced renal disease. Although many studies demonstrate the use of ACE inhibitors and ARBs to delay the decline in renal function and reduce proteinuria, many physicians fail to use these drug classes in patients with renal insufficiency for fear that either serum creatinine or potassium levels will rise. Thus, because of these issues, patients are deprived of known strategies that delay progression of renal disease. A strong association exists between acute increases in serum creatinine of up to 30% to 35% after initiating ACE inhibitor therapy and long-term preservation of renal function. This association is predominantly present in people with a baseline serum creatinine of up to 3 mg/dL and usually stablizes within 2 to 3 months of therapy given blood pressure is reduced to goal. Moreover, the appropriate use of diuretics mitigates against profound increases in serum potassium. Thus, withdrawal of an ACE inhibitor in such patients should occur only when the rise in creatinine exceeds this threshold over a shorter period of time or hyperkalemia develops, ie, serum potassium level of 5.6 mmol/L or greater.

Calcium channel blockade and preservation of renal graft function in cyclosporine-treated recipients: a prospective randomized placebo-controlled 2-year study

BACKGROUND

Studies have provided conflicting results as to the protective role of calcium channel blockers (CCB) in cyclosporine-treated patients with regard to blood pressure control and preservation of renal graft function. Lacidipine is a dihydropyridine CCB that possesses antioxidative, anti-atherosclerotic, and anti-adhesion properties and was shown to prevent cyclosporine-induced nephrotoxicity in a rat model.

METHODS

We conducted a multicenter prospective, randomized, placebo-controlled study in 131 de novo recipients of a cadaveric renal allograft on cyclosporine therapy. The aim of this 2-year study was to assess the effects of lacidipine on graft function (plasma iohexol clearance), renal plasma flow, anastomotic arterial blood flow, deterioration of renal function, blood pressure, acute rejection, and hospitalization rate.

RESULTS

A total of 118 recipients were available for intention-to-treat analysis on efficacy (lacidipine: n=59; placebo: n=59). Graft function assessed by serum creatinine concentration and glomerular filtration rate measured as plasma iohexol clearance, was persistently better in lacidipine-treated patients from 1 year onwards (respectively, P<0.01 and P<0.05). Renal plasma flow and anastomotic blood flow were not significantly higher in lacidipine-treated patients. Three patients on lacidipine therapy and four on placebo experienced treatment failure defined as an increase in serum creatinine from baseline of more than 60% (log-rank test: P=0.57). Study groups did not differ in acute rejection rate, trough blood cyclosporine concentrations, blood pressure, number of antihypertensive drugs, hospitalization rate, and adverse event rate.

CONCLUSIONS

The use of calcium channel blockers in cyclosporine-treated renal recipients results in a significantly better allograft function at 2 years and this effect is independent of blood pressure lowering.

Hypertensive renal damage: insights from animal models and clinical relevance

Investigations using chronic blood pressure (BP) radiotelemetry in conscious animals have provided substantial insights into the pathophysiology of hypertensive renal damage. Normal renal autoregulation protects the renal microvasculature from significant injury in most patients with primary hypertension, unless BP exceeds a certain threshold, when malignant nephrosclerosis develops. However, if autoregulation is impaired, as in chronic renal disease and/or diabetes models, the threshold for renal damage is lowered and glomerulosclerosis (GS) increases linearly with increasing BP. Modest BP reductions are predicted to prevent malignant nephrosclerosis, but prevention of GS in patients with diabetes and chronic renal disease requires that BP be lowered well into the normotensive range, as recognized in the currently recommended BP goals. When BP load is accurately assessed in these experimental models, renal protection is proportional to the achieved BP reductions, and there is little evidence of BP-independent protection, even with agents that block the renin-angiotensin system (RAS). Recent clinical data also suggest that achieving lower BP targets might be vastly more important than the choice of therapeutic regimens. Nevertheless, because aggressive diuretic use is usually necessary to achieve such BP goals, RAS blockade should be included as initial therapy both for antihypertensive synergy and to minimize the potassium and magnesium depletion associated with diuretics.

Blood pressure lability and glomerulosclerosis after normotensive 5/6 renal mass reduction in the rat

BACKGROUND

Hypertension plays a major role in the progression of both experimental and clinical chronic renal disease. However, the pathogenesis of the more slowly developing glomerulosclerosis that is seen even in the absence of overt hypertension, both in renal mass reduction models and in humans with chronic renal disease, remains controversial.

METHODS

The relationship of such glomerulosclerosis to the ambient blood pressure profiles was examined in the normotensive approximately 5/6 surgical excision rat remnant kidney model. Blood pressure was radiotelemetrically monitored at 10-minute intervals for 15 to 16 weeks ( approximately 15,000 blood pressure readings) in untreated rats (N= 13), or those treated with enalapril (N= 8), amlodipine (N= 9), or a combination of hydralazine, reserpine, and hydrochlorothiazide (N= 10).

RESULTS

Even in these normotensive rats (systolic blood pressure <140 mm Hg), % glomerulosclerosis was significantly correlated with the overall average systolic blood pressure (r= 0.62, P < 0.0001; N= 40). However, much stronger correlations were observed between glomerulosclerosis and the % systolic blood pressure readings >150 mm Hg (r= 0.77, P < 0.0001) and the standard deviation of the average systolic blood pressure (r= 0.87, P < 0.0001).

CONCLUSION

These data indicate that pressure dependent injury mechanisms continue to contribute to glomerular injury even within the "normotensive" blood pressure range in rats with reduced renal mass. This most likely represents the consequence of the impairment of protective renal autoregulation and enhanced glomerular transmission of the blood pressure fluctuations into the hypertensive range characteristic of the conscious state in both experimental animals and in humans. Such pathophysiology supports the need for more aggressive and around-the-clock blood pressure control in chronic renal disease.

Hypertension but Not Sodium Intake Determines Progression of Renal Failure in Experimentally Uremic Rats

BACKGROUND/AIMS

High sodium intake is implicated in contributing to progression of chronic renal failure. We studied the effect of high sodium consumption on progression of rat experimental renal failure while sodium-induced hypertension was pharmacologically controlled.

METHODS

64 Sprague-Dawley rats underwent 5/6 nephrectomy. Subsequently, they were divided in three groups which were fed either low, normal, or high sodium diet. Only the high sodium-consuming group developed hypertension. This group was further divided in two subgroups in which hypertension was either untreated or titrated to normotension by hydralazine alone or with propranolol.

RESULTS

Sequential GFR values did not differ between the respective normotensive groups. Survival downslopes of all three normotensive groups (including the pharmacologically treated, high sodium-consuming subgroup) were also similar, extending over 10 weeks. By contrast, pharmacologically untreated animals exhibited severe hypertension and 100% mortality within 3 weeks. In all experimental groups, 24-hour urinary sodium excretion paralleled sodium intake. Proteinuria rose similarly and significantly in all animals on high sodium. A significant correlation between 24-hour sodium and proteinuria was evident throughout the experimental period.

CONCLUSIONS

(1) In 5/6 nephrectomized rats, renal function deterioration was not affected by dietary sodium, provided hypertension was pharmacologically controlled. (2) Enhanced proteinuria secondary to high sodium consumption had no adverse effect on progression of renal failure in this model.

Effects of calcium channel blockers on “dynamic” and “steady-state step” renal autoregulation

Renal autoregulation (AR) mechanisms provide the primary protection against transmission of systemic pressures and hypertensive renal damage. However, the relative merits of the “step” change vs. “dynamic” methods for the assessment of AR capacity remain controversial. The effects of 48–72 h of orally administered amlodipine (L-type) and mibefradil (T-type) calcium channel blockers (CCBs) on step and dynamic AR in Sprague-Dawley rats were compared. Both CCBs significantly impaired “steady-state step” AR (autoregulatory indexes = ∼0.5 vs. ∼0.1 in controls, P < 0.05; n = 9–10/group). By contrast, dynamic AR compensation in separate conscious rats ( n = 12) was not significantly altered by either amlodipine ( n = 10) or mibefradil ( n = 6; fractional gain in admittance ∼0.4–0.5 in all groups at frequencies in the range of 0.0025–0.025 Hz). However, both CCBs tended to attenuate the myogenic resonance peak along with shifting it to a significantly slower frequency ( P < 0.001) during dynamic AR, but no consistent effects were observed on the tubuloglomerular feedback resonance peak. While the reasons for the insensitivity of dynamic vs. steady-state step AR capacity estimates to CCBs remain to be established, the present data indicate that dynamic AR methods may have a limited utility for assessing AR capacity but may provide potentially important insights into the operational characteristics of AR control mechanisms. A strong correlation was also observed between the average conductance and the admittance gain at the heart beat frequency ( r = 0.77, P < 0.001), suggesting that such parameters may provide additional and possibly more meaningful indexes of BP transmission in conscious animals during dynamic AR.

Dynamic Modeling of Renal Blood Flow in Dahl Hypertensive and Normotensive Rats

A method is proposed in this paper which allows characterization of renal autoregulatory dynamics and efficiency using quantitative mathematical methods. Based on data from rat experiments, where arterial blood pressure and renal blood flow are measured, a quantitative model for renal blood flow dynamics is constructed. The mathematical structure for the dynamics is chosen as a "grey-box model," i.e. the model structure is inspired from physiology, but the actual parameters is found by numerical methods. Based on a number of experiments, features are extracted from the estimated parameters, which describe myogenic responses and tubuloglomerular feedback responses separately. The method is applied to data from normo- and hypertensive Dahl rats, and a discriminator that separates data from normotensive Dahl R rats and hypertensive Dahl S rats is constructed.

Slowing the Progression of Adult Chronic Kidney Disease

When kidney disease of any aetiology results in substantial loss of nephrons, a common clinical syndrome, characterised by hypertension, proteinuria and a progressive decline in renal function, ensues. This observation suggests that common mechanisms may contribute to progressive renal injury and that therapeutic interventions that inhibit these common pathways may afford renal protection. Research to date has identified several mechanisms that may contribute to progressive renal injury including glomerular haemodynamic changes, multiple effects of angiotensin II and detrimental effects of excessive filtration of plasma proteins by injured glomeruli. Clinical trials over the past decade have identified several interventions that are effective in slowing the rate of progression of chronic kidney disease (CKD). The use of ACE inhibitors, angiotensin receptor antagonists or a combination of the two should be regarded as fundamental to any therapy for slowing the rate of CKD progression. Hypertension should be treated aggressively to achieve a blood pressure target of < 130/80 mm Hg. Reduction of proteinuria to < 0.5 g/day should be regarded as an independent therapeutic goal. Although inconclusive, there is some evidence to support moderate dietary protein restriction to 0.6 g/kg/day in appropriate patients. Hyperlipidaemia may contribute to CKD progression and should be treated to reduce cardiovascular risk and potentially improve renal protection. Smoking cessation should be encouraged and, where necessary, assisted. Among diabetic patients tight glycaemic control should be achieved (glycosylated haemoglobin < 7%). These interventions are simple and relatively inexpensive. If applied to all patients with CKD they will result in substantial slowing of renal function decline in many patients and thereby reduce the number who progress to end-stage renal disease and require renal replacement therapy.

Calcium channel blockers in the prevention of end stage renal disease: a review

Hypertension and high levels of proteinuria are independent risk factors for accelerated progression of renal failure. There is increasing evidence that strict control of both blood pressure (BP) and proteinuria are beneficial in slowing the rate of progression of chronic renal disease in diabetic as well as non-diabetic nephropathy. The angiotensin-converting enzyme inhibitors (ACE-I) and angiotensin 2 receptor blockers (ARB) have clearly demonstrated their beneficial effect on both reduction of BP and proteinuria. The calcium channel blockers (CCB) have individual pharmacological and therapeutic properties that may vary, but as a group they are effective antihypertensive agents in patients with renal disease. Their effects on the kidney may extend beyond BP reduction alone. Current studies suggest that CCB do not worsen the progression of renal disease but may rather provide benefit when systemic BP has been tightly normalised. The non-dihydropyridine calcium channel blockers (NDHP), diltiazem and verapamil, slow the progression of type 2 diabetic nephropathy with overt proteinuria almost to a similar extent as observed with ACE-I. The dihydropyridine calcium channel blockers (DHP) have a variable effect on proteinuria. Pharmaceutical compounds, which inhibit the renin-angiotensin system (RAAS), remain the drugs of first choice in the treatment of hypertension and/or proteinuria in chronic nephropathy. However, a combination of two or more drugs is almost always required to attain sufficient BP reduction. CCB may have an advantage in combination with ACE-I and/or ARB.

"Step" vs. "dynamic" autoregulation: implications for susceptibility to hypertensive injury

Renal autoregulatory (AR) mechanisms provide the primary protection against transmission of systemic pressures, and their impairment is believed to be responsible for the enhanced susceptibility to hypertensive renal damage in renal mass reduction (RMR) models. Assessment of AR capacity by the "step" change methodology under anesthesia was compared with that by "dynamic" methods in separate conscious control Sprague-Dawley rats and after uninephrectomy (UNX) and (3/4) RMR (RK-NX) (n = 7-10/group). Substantially less AR capacity was seen by the dynamic vs. the step methodology in control rats. Moreover, dynamic AR capacity did not differ among controls, UNX, and RK-NX rats (fractional gain in admittance approximately 0.4-0.5 in all groups at frequencies in the range of 0.0025-0.025 Hz). By contrast, significant impairment of step AR was seen in RK-NX vs. control or UNX rats (AR indexes 0.7 +/- 0.1 vs. 0.1 +/- 0.02 and 0.2 +/- 0.04, respectively, P < 0.01). We propose that the step and dynamic methods evaluate the renal AR responses to different components of blood pressure (BP) power with the step AR assessing the ability to buffer large changes in average BP (DC power), whereas the present "dynamic" methods assess the AR ability to buffer slow BP fluctuations (<0.25 Hz) superimposed on the average BP (AC power), a substantially smaller component of total BP power. We further suggest that step but not dynamic AR methods as presently performed provide a valid index of the underlying susceptibility to hypertensive glomerular damage after RMR.

Comparison of dipyridamole and fosinopril on renal progression in nephrectomized rats

There is evidence to suggest that antiplatelet aggregation and inhibition of angiotensin converting enzyme will attenuate the progression of renal disease. In the present study, dipyridamole (DPM; 30 mg/kg per day, p.o.) or fosinopril (FOS; 20 mg/kg per day, p.o.) was given to rats for 5 weeks starting immediately after renal mass reduction (right uninephrectomy and ligation of approximately two-thirds of the blood supply to the left kidney). Renal mass reduction caused increased mean arterial blood pressure, reduced effective renal plasma flow (ERPF) and glomerular filtration rate (GFR), azotemia and proteinuria. Neither proteinuria nor hypertension was affected by DPM, although renal function improved markedly. Rats receiving FOS showed normalization of blood pressure with a significant increase in both ERPF and GFR, along with a lower degree of proteinuria. A histological examination of the remnant kidney detected the presence of vasodilation with a lower degree of podocyte swelling in both treatment groups, with a remarkable effect in the FOS group. These data indicate that both FOS and DPM attenuate the progression of glomerular disease associated with renal mass reduction in rats. However, FOS was more beneficial than DPM because it reduced proteinuria and lowered blood pressure.

Arginase inhibition slows the progression of renal failure in rats with renal ablation

Exogenous arginine slows the progression of chronic renal failure (CRF) in remnant rats through a nitric oxide (NO)-dependent mechanism. We tested whether the inhibition of arginase could induce similar results through the increased availability of endogenous arginine. Three groups of remnant rats were studied for 8 wk: 1) untreated rats (REM); 2) remnant rats treated with 1% l-arginine (ARG); and 3) remnant rats administered a Mn(2+)-free diet to inhibit arginase (MNF). Normal rats (NOR) were used as controls. Liver arginase activity was depressed in MNF rats (-35% vs. REM, P < 0.01). No difference in metabolic data was detected among the groups throughout the study; blood pressure was significantly lower in MNF vs. ARG and REM rats after 6 wk (P < 0.001). The glomerular filtration rate (GFR) was greatly depressed in REM rats (-47% vs. NOR, P < 0.03) but was higher in ARG and MNF rats (+40 and +43% vs. REM, respectively, P < 0.05), with comparable changes in renal hemodynamics. Despite the better GFR, proteinuria was decreased in both ARG and MNF rats (-42%, P < 0.05, and -57%, P < 0.01, respectively, vs. REM rats). Arginine plasma levels, significantly reduced in REM rats (-41% vs. NOR, P < 0.01), were partially restored in MNF rats (+38% vs. REM), and urinary nitrite excretion, greatly depressed in REM rats (-76% vs. NOR, P < 0.01), was significantly increased in MNF rats (+209% vs. REM, P < 0.05). At the renal level, arginase activity was only slightly depressed in MNF rats (-18% vs. REM), but intrarenal concentrations of arginine were lower in this latter group (P < 0.05 vs. other groups). Beyond the hemodynamic modifications, MNF rats showed a lower glomerular sclerosis index (P < 0.05 vs. REM and ARG). Inhibition of arginase slows the progression of CRF in remnant rats similarly to arginine-treated rats; the better histological protection in MNF rats, however, suggests that additional factors are involved in these modifications.

Morphometric study of arterioles and glomeruli in the aging kidney suggests focal loss of autoregulation

BACKGROUND

In the past it was widely assumed that hyaline afferent arteriolosclerosis was responsible for ischemic glomerulosclerosis in the aging and hypertensive kidney. However, glomerular lesions of focal segmental glomerulosclerosis are now recognized in essential hypertension. Experimentally, such lesions are associated with loss of autoregulation of blood flow and glomerular hyperperfusion, as well as initial glomerular hypertrophy. These observations challenge the notion of ischemia as a unitary explanation for glomerulosclerosis.

METHODS

A morphometric study was performed on normal portions of eight kidneys removed for tumors in aging, normotensive patients. Measurements were made of 126 pairs of afferent arterioles and their associated glomeruli. In addition, the amount of extracellular matrix (ECM) in the immediate periglomerular region was quantitated.

RESULTS

Afferent arterioles were divided into three types according to the presence or absence of hyaline deposits and whether these did or did not obstruct the lumen. Arterioles with nonobstructive hyaline deposits had lumens over twice as large as those without deposits (482 +/- 240 micro2 vs. 204 +/- 160 micro2, P=0.0000). Their associated glomeruli had significantly greater total capillary area, particularly the hilar capillaries (1276 +/- 797 micro2 vs. 667 +/- 492 micro2, P=0.002), but with larger individual capillaries elsewhere as well (P=0.03). Arterioles with obstructive deposits differed from those with nonobstructive deposits by their smaller lumens (P=0.001) and walls (P=0.004), with a higher proportion of ECM in the periglomerular region (P=0.001), all consistent with a later stage of lesion. Glomeruli were divided into four basic types: normal, hypertrophic, glomeruli with features of focal segmental glomerulosclerosis (FSGS-type), and ischemic. Compared to normal glomeruli, hypertrophic glomeruli were larger, with greater total capillary area (P=0.0005), particularly the hilar capillaries (P=0.0000), and larger capillaries in the remainder of the tuft (P=0.003), but showed no evident lesions. FSGS-type glomeruli were also larger, with larger hilar capillaries (P=0.0005), but showed an increase in ECM due to sclerotic lesions (P=0.004). The remaining capillaries showed an inverse relation with the amount of mesangial matrix, showing a spectrum of sizes from enlarged to shrunken. As anticipated, ischemic glomeruli were significantly smaller than normal ones in every parameter measured. There was a strong association between hypertrophic/FSGS-type glomeruli and hyaline arteriolosclerosis, found in 90.3% of such glomeruli, versus 29.1% for the remaining glomeruli (P=0.0001). The great majority of hyaline deposits were of the nonobstructive variety (86.2%), but some were obstructive (13.8%), particularly in FSGS-type glomeruli, consistent with a more advanced lesion.

CONCLUSIONS

We believe we have demonstrated in the aging kidney of humans the morphologic correlates of loss of autoregulation, occurring on a focal basis, with afferent arteriolar dilatation and increase in glomerular capillary size and subsequent focal segmental glomerulosclerosis. Hyaline arteriolosclerosis of the nonobstructive sort is strongly associated with these changes and may play a role in their pathogenesis.

Renoprotection by ACE inhibition or aldosterone blockade is blood pressure-dependent

Renin-angiotensin-aldosterone system blockade has been shown to protect against renal damage in salt-supplemented, stroke-prone spontaneously hypertensive rats (SHRsp). Based on intermittent tail-cuff blood pressure (BP) measurements, it has been claimed that such protection is BP-independent and mediated by a blockade of the direct tissue-damaging effects of angiotensin and/or aldosterone. BP radiotelemetry was performed for 8 weeks in approximately 10-week-old male SHRsp who received a standard diet and either tap water (n=10) or 1% NaCl to drink. Saline-drinking SHRsp were either left untreated (n=12), received enalapril (50 mg/L) in drinking fluid (n=9), or had subcutaneous implantation of time-release 200-mg pellets of aldactone (n=10). The average systolic BP (mean+/-SEM) during the final 3 weeks was significantly higher (P<0.05) in untreated saline-drinking (215+/-6 mm Hg) SHRsp but not aldactone-treated (198+/-4 mm Hg) or enalapril-treated treated SHRsp (173+/-1 mm Hg), as compared with tap water-drinking SHRsp (197+/-3 mm Hg). Histological renal damage scores at 8 weeks paralleled the BP in all groups, with an excellent correlation (r=0.8, P<0.001, n=41). Moreover, a renal damage score of >5 was only observed in SHRsp whose average systolic BP during the final 3 weeks exceeded 200 mm Hg, indicating a threshold relation with BP. These data show that protection by renin-angiotensin-aldosterone system blockade in this model is BP-dependent and mediated by preventing the severe increases in BP seen in untreated salt-supplemented SHRsp and further underscore the limitations of interpretations based on conventional tail-cuff BP measurements.

Low protein diet mediated renoprotection in remnant kidneys: Renal autoregulatory versus hypertrophic mechanisms

BACKGROUND

The mechanism of low protein diet conferred renoprotection in the ablation model remains controversial. Blockade of glomerular hypertrophy, reduced preglomerular vasodilation, and preserved autoregulation have all been postulated. The potential differential impact of calcium channel blockers on these mechanisms and glomerulosclerosis was examined.

METHODS

Rats with 5/6 renal ablation received either a 25% standard protein diet, an 8% low protein diet and a low protein diet with either verapamil or amlodipine. Renal autoregulatory and morphometric studies were performed at 3 weeks before the development of significant injury, and the assessment of glomerulosclerosis after 7 weeks of continuous blood pressure radiotelemetry in additional rats.

RESULTS

The preserved renal autoregulation in low protein rats was abolished by both calcium channel blockers, with the impairment being either comparable to (low protein + verapamil) or greater than the standard protein rats (low protein + amlodipine). Neither calcium channel blocker blocked the inhibitory effects of low protein diet on renal blood flow, kidney weight, and glomerular volume. Results (mean +/- SE) for glomerular volume (microm-3x 10(-6)): low protein (N = 11), 1.6 +/- 0.1; low protein + verapamil (N = 10), 1.7 +/- 0.1; low protein + amlodipine (N = 12), 1.7 +/- 0.2; versus standard protein (N = 10), 2.2 +/- 0.1; P < 0.05. Only amlodipine, but not verapamil, reduced average systolic blood pressure (143 +/- 2 mm Hg versus low protein rats, 168 +/- 5 mm Hg, and standard rats, 170 +/- 6 mm Hg; P < 0.01). Nevertheless, the glomeruloprotection seen in low protein (N = 15) as compared to standard protein (N = 14) rats (9%+/- 3% versus 28%+/- 6% glomerulosclerosis; P < 0.01) was abolished in both low protein + verapamil (N = 14, 32%+/- 7%) and low protein + amlodipine rats (N = 16, 27%+/- 7%).

CONCLUSIONS

Preservation of renal autoregulation and not inhibition of hypertrophy is the critical component in low protein diet-conferred glomeruloprotection.

Autoregulation of glomerular filtration rate in patients with type 2 diabetes during isradipine therapy

OBJECTIVE

Calcium-channel blockade impairs renal autoregulation in animals. Impaired renal autoregulation leads to transmission of the systemic blood pressure (BP) into the glomerulus, resulting in capillary hypertension. Information on the impact of calcium antagonist treatment on renal autoregulation in humans is lacking. This study examines the effect of isradipine treatment on the autoregulation of the glomerular filtration rate (GFR).

RESEARCH DESIGN AND METHODS

We performed a randomized double-blind crossover study with 5 mg o.d. isradipine retard and matching placebo in 16 hypertensive patients with type 2 diabetes. Each treatment arm lasted 4 weeks. On the last day of each treatment period, GFR (single-shot 51Cr-EDTA plasma clearance technique for 4 h) was measured twice between 8:00 A.M. and 5:00 P.M., first without clonidine and then after intravenous injection of 75 micro g clonidine. BP was measured every 10 min (Takeda TM2420; A&D, Tokyo).

RESULTS

Clonidine reduced mean arterial BP (MABP) by 15 +/- 1 vs. 11 +/- 1 mmHg (means +/- SE) during placebo and isradipine treatment, respectively (P < 0.05). GFR was reduced from 102 +/- 4 to 99 +/- 4 ml. min(-1). 1.73 m(-2) with placebo (P < 0.01) and from 106 +/- 5 to 98 +/- 5 ml. min(-1). 1.73 m(-2) during treatment with isradipine (P < 0.01). Mean difference (95% CI) between changes in GFR with placebo and isradipine was -4.6 ml. min(-1). 1.73 m(-2) (-10.0 to 0.6) (P = 0.08). Six patients had a reduction in GFR >13% (exceeding the normal limit of autoregulation) combined with a complete pressure-passive vasculature (defined as DeltaMABP% < or = DeltaGFR%) during isradipine treatment versus none during the placebo treatment (P < 0.05).

CONCLUSIONS

Isradipine impairs GFR autoregulation in a sizeable proportion of hypertensive type 2 diabetic patients.

Disparate effects of angiotensin II antagonists and calcium channel blockers on albuminuria in experimental diabetes and hypertension: potential role of nephrin

OBJECTIVE AND DESIGN

To explore the effects of various antihypertensive regimes which achieve similar blood pressure control using a range of agents including the angiotensin II type 1 receptor antagonist, valsartan, as monotherapy or in combination with two subclasses of calcium channel blockers (CCBs) (the dihydropyridine, amlodipine and the phenylalkylamine, verapamil) on the progression of renal disease and the expression of the podocyte slit pore protein, nephrin in an accelerated model of diabetic nephropathy.

RESULTS

Valsartan treatment reduced systolic blood pressure as assessed by radiotelemetry (135 +/- 3 versus diabetic 153 +/- 6 mmHg) as well as retarding the increase in albumin excretion rate by approximately 50%. Combination therapy of valsartan with either amlodipine or verapamil was equally effective in reducing blood pressure to valsartan monotherapy (valsartan + amlodipine 129 +/- 4 valsartan + verapamil 133 +/- 6 mmHg;) but was not as effective at reducing albuminuria. A reduction in glomerulosclerosis was observed with valsartan monotherapy with less reduction in injury with the valsartan + amlodipine combination, despite a similar reduction in blood pressure. The decrease in nephrin, in diabetic rats was attenuated by valsartan monotherapy, but not by other treatments.

CONCLUSIONS

The results of this study demonstrate that despite a similar reduction in blood pressure, the addition of the CCB amlodipine to the AII antagonist failed to provide similar renoprotection to that observed with an equihypotensive regimen of valsartan as monotherapy. Furthermore, the depletion in glomerular nephrin expression in diabetic animals was only abrogated by valsartan treatment, the therapy which was most effective at retarding the development of albuminuria in this model.

Effects of efonidipine hydrochloride on renal arteriolar diameters in spontaneously hypertensive rats

Efonidipine, a calcium antagonist, has been reported to dilate not only afferent but also efferent arterioles, thereby reducing glomerular hydrostatic pressure. We investigated the effect of chronic treatment with efonidipine or lisinopril on the afferent and efferent arteriolar diameters by the vascular cast technique. Four-week-old spontaneously hypertensive rats (SHR) were divided into three groups: untreated, efonidipine (25 mg/kg/day)-treated, and lisinopril (3 mg/kg/day)-treated. At 22 weeks of age, the renal vasculatures were fixed at the maximally dilated condition. The morphometrical measurements showed that the treatments with efonidipine and lisinopril caused structural alteration of the vasculature, resulting in significantly greater efferent arteriolar diameters than in untreated SHR. In addition, lisinopril-treated rats had wider afferent lumina. The renoprotective effect of efonidipine and lisinopril might be partly due to the structurally larger efferent arteriolar lumen.

N- and L-type calcium channel antagonist improves glomerular dynamics, reverses severe nephrosclerosis, and inhibits apoptosis and proliferation in an l-NAME/SHR model

OBJECTIVE

To determine the responses of the new dihydropyridine N- and L-type calcium antagonist, cilnidipine, on systemic and renal hemodynamics, glomerular dynamics, renal function, and histopathology in an Nomega-nitro-l-arginine methylester spontaneously hypertensive rat (l-NAME/SHR) model of nephrosclerosis.

METHODS

Five groups of 20-week-old male SHR were studied using renal micropuncture techniques and histopathological analyses: group 1, control; group 2, cilnidipine (10 mg/kg per day) by gavage, for 3 weeks; group 3, l-NAME (50 mg/l) in drinking water, for 3 weeks; group 4, combination of l-NAME and cilnidipine, for 3 weeks; group 5, l-NAME for 3 weeks, followed by cilnidipine for a subsequent 3 weeks.

RESULTS

Cilnidipine significantly reduced mean arterial pressure, total peripheral resistance and renal vascular resistance, while increasing effective renal blood flow and glomerular filtration rate (P < 0.01) in l-NAME/SHR. These hemodynamic changes were associated with significantly increased single nephron glomerular filtration rate (SNGFR) and plasma flow (SNPF) and decreased afferent glomerular arteriolar resistances when cilnidipine was used alone, and with increased SNGFR and SNPF, but decreased glomerular capillary pressure, afferent and efferent arteriolar resistances, urinary protein excretion, serum creatinine and uric acid concentrations (at least P < 0.05) in l-NAME-exacerbated SHR nephrosclerosis. In addition, glomerular and arteriolar injuries were markedly reversed (both P < 0.01), and glomerular apoptosis and cellular proliferation were inhibited and associated with glomerular tuft enlargement and an increase in cell number.

CONCLUSION

Cilnidipine not only prevented, but reversed, the severe renal hemodynamic and glomerular dynamic changes, including apoptosis and glomerular cellular proliferation, in l-NAME/SHR-exacerbated nephrosclerosis. This dual-channel calcium antagonist thus exerted renoprotective pathophysiological effects in the l-NAME/SHR.

Increased genetic susceptibility to renal damage in the stroke-prone spontaneously hypertensive rat

BACKGROUND

The spontaneously hypertensive rat (SHR) develops much less renal damage than the stroke-prone strain of SHR (SHRsp) after salt-supplementation, and it has been proposed that these strains differ in their genetic susceptibility to renal damage. However, radiotelemetric BP measurements have shown that salt-supplementation results in more severe and accelerated hypertension in the SHRsp. Therefore, it is unclear whether the differences in renal damage are due to differences in BP exposure or true differences in intrinsic (genetic) renal susceptibility to hypertensive damage.

METHODS

Kidney cross transplantation was performed between the SHR and SHRsp strains in uninephrectomized recipients to allow an investigation of the susceptibility to renal damage in SHR and SHRsp kidneys maintained in the same host and exposed to the same BP profile and metabolic environment. Following transplantation, BP was radiotelemetrically monitored before and after an 8% NaCl diet given to accelerate hypertension and renal damage. Then the kidneys were removed and renal damage was assessed histologically.

RESULTS

In the SHR recipients, the SHRsp donor kidneys exhibited more hypertensive damage than the contralateral native SHR kidneys, but histologic evidence of mild cellular immunologic rejection also was observed that could have facilitated the increased renal damage. However, even in SHRsp recipients, the native SHRsp kidneys exhibited twice the damage seen in the contralateral transplanted SHR kidneys.

CONCLUSION

These data unequivocally demonstrate that the SHRsp kidneys are intrinsically more susceptible than the SHR kidneys to renal damage when exposed to exactly the same BP and metabolic environment.

Effects of antihypertensive therapy on intrarenal angiotensin and bradykinin levels in experimental renal insufficiency

BACKGROUND

Whereas angiotensin converting enzyme inhibitors and angiotensin type 1 receptor antagonists have beneficial effects in the remnant model of renal failure, calcium channel blockers do not consistently improve renal disease in this model. This study examined whether these different means of blood pressure reduction have different effects on renal levels of angiotensin (Ang) and bradykinin peptides.

METHODS

Rats subjected to five-sixths nephrectomy were divided into groups with similar hypertension and proteinuria at 4 to 5 weeks. They then received either no treatment, or enalapril, losartan or nifedipine for 2 weeks. Following repeat measurements of proteinuria and blood pressure, Ang II and bradykinin peptides were measured in the remnant kidney and renin, Ang II, and aldosterone were measured in the plasma.

RESULTS

All three drugs had equivalent blood pressure-lowering effects. Enalapril and losartan reduced proteinuria but nifedipine did not. Reduction of proteinuria in rats treated with enalapril and losartan was associated with a reduction in Ang II levels in both the peri-infarct and intact portions of the remnant kidney. By contrast, nifedipine increased Ang II levels in the intact portion of the remnant kidney. Losartan reduced bradykinin levels in the peri-infarct portion of the remnant kidney while enalapril reduced bradykinin levels in the intact portion of the remnant kidney. Nifedipine had no effect on intrarenal bradykinin levels.

CONCLUSIONS

The differential effects of enalapril, losartan and nifedipine on proteinuria and intrarenal Ang II and bradykinin levels suggest that the ability of an antihypertensive to decrease proteinuria may depend on its ability to decrease kidney Ang II and bradykinin levels.

Long-term renal consequences of hypertension for normal and diseased kidneys

Substantial evidence indicates that the adverse effects of hypertension on the kidney depend on the degree to which systemic blood pressure elevations are transmitted to the renal microvasculature. Such blood pressure transmission and consequent susceptibility to hypertensive renal damage is markedly exacerbated in states characterized by preglomerular vasodilation and an impairment of the normally protective renal autoregulatory mechanisms, e.g. diabetes or chronic renal disease. Moreover, this pathophysiology gives rise to the prediction that prevention of hypertension-induced barotrauma will require blood pressure to be reduced well into the normotensive range in such patients, as is being recognized in the currently recommended blood pressure goals. Agents that block the renin-angiotensin system should be preferred as initial therapy as they may provide additional risk reductions and minimize the potassium and magnesium depletion associated with the diuretic use usually necessary to achieve the lower blood pressure targets. The current failure to achieve optimal blood pressure reductions may be contributing not only to the still escalating incidence of end-stage renal disease in diabetic patients, but also to their greatly increased cardiovascular morbidity and mortality.

Autoregulated glomerular filtration rate during candesartan treatment in hypertensive type 2 diabetic patients

BACKGROUND

Impaired autoregulation of the glomerular filtration rate (GFR) implies disturbances in the downstream transmission of the systemic blood pressure into the glomerulus, leading to capillary hypertension or hypotension dependent of the level of blood pressure. The impact on renal autoregulation of different antihypertensive drugs in animals has been elucidated, whereas information in humans is lacking.

METHODS

A randomized, double-blind crossover study with candesartan cilexetil 16 mg o.d. and placebo was performed in 17 hypertensive type 2 diabetic patients without nephropathy. Each treatment arm lasted four weeks. On the last day, GFR (single shot [51Cr] EDTA plasma clearance technique for 4 hours) was measured twice between 8 a.m. and 5 p.m., first without clonidine and then after an intravenous injection of clonidine 75 microg. Blood pressure (Takeda TM2420, A&D, Tokyo, Japan) was measured every ten minutes, and the urinary albumin excretion rate (UAER) was measured by ELISA during each GFR determination.

RESULTS

Candesartan induced a mean (SE) reduction in mean arterial blood pressure (MABP) of 6 (2) mm Hg (P < 0.02) and had a tendency to reduce UAER (P = 0.07), while GFR remained unchanged (95 vs. 93 mL/min/1.73 m2). Clonidine reduced MABP with 17 (2) versus 16 (1) mm Hg during placebo versus candesartan 16 mg o.d., respectively (NS). GFR diminished in average from 95 (3) to 92 (4) mL/min/1.73 m2 with placebo (NS), and from 93 (3) to 89 (4) mL/min/1.73 m2 during treatment with candesartan (NS). The mean difference (95% CI) in the changes in GFR between the examination with placebo and with candesartan was 0.1 (-5.5 to 5.8) mL/min/1.73 m2 (NS).

CONCLUSION

Candesartan reduces blood pressure without adversely altering the preserved ability to autoregulate GFR in hypertensive type 2 diabetic patients without nephropathy.

Vasopeptidase inhibition attenuates the progression of renal injury in subtotal nephrectomized rats

BACKGROUND

Vasopeptidase inhibitors are a new class of cardiovascular compounds that inhibit both angiotensin-converting enzyme (ACE) and neutral endopeptidase (NEP). The aim of the present study was to explore the effects of omapatrilat, a vasopeptidase inhibitor, on renal function and pathology in subtotally nephrectomized (STNx) rats.

METHODS

STNx rats were randomized to four groups and treated for 12 weeks: no treatment (N = 14); omapatrilat at a low dose of 10 mg/kg (L, N = 12) and at a high dose of 40 mg/kg (H, N = 10); or an ACE inhibitor, fosinopril, at a dose of 10 mg/kg (N = 12). Sham-operated rats were used as control animals (N = 12).

RESULTS

Elevated blood pressure in STNx rats (174 +/- 9 mm Hg) was reduced by omapatrilat in a dose-dependent manner (L, 121 +/- 3 mm Hg; H, 110 +/- 3 mm Hg) and by fosinopril (149 +/- 5 mm Hg). Proteinuria in STNx rats (246 +/- 73 mg/day) was reduced by treatment with fosinopril (88 +/- 21 mg/day) and was normalized by treatment with omapatrilat (L, 30 +/- 4 mg/day; H, 20 +/- 2 mg/day vs. control 25 +/- 1 mg/day). Decreased glomerular filtration rates, elevated plasma urea and creatinine and glomerulosclerosis, and tubulointerstitial fibrosis were ameliorated by omapatrilat and fosinopril to a similar degree. Compared with fosinopril, omapatrilat treatment was associated with increased plasma renin activity and decreased renal ACE and NEP binding in a dose-dependent manner.

CONCLUSION

These findings suggest that vasopeptidase inhibition may provide a useful strategy for the treatment of progressive renal disease.

Renal blood flow dynamics and arterial pressure lability in the conscious rat

It is not known whether renal blood flow (RBF) is still autoregulated when the kidney is exposed to large transient blood pressure (BP) fluctuations such as those occurring spontaneously in conscious sinoaortic baroreceptor-denervated (SAD) rats. In this study, BP and RBF were simultaneously recorded in 8 SAD rats (2 weeks before study) and 8 baroreceptor-intact rats during approximately 3 hours of spontaneous activity. The kidney used for RBF recordings was denervated to prevent the interference of changes in renal sympathetic tone with autoregulatory mechanisms. In intact rats, RBF variability (coefficient of variation 9.1+/-0.8%) was larger (P<0.02) than BP variability (5.9+/-0.2%). This was mainly because of slow changes in RBF that were unrelated to BP and also to a prominent oscillation of RBF of approximately 0.25-Hz frequency. Autoregulatory patterns were identified at frequencies <0.1 Hz and provided a modest attenuation of BP fluctuations. In SAD rats, RBF variability (12.4+/-1.6%) was lower (P<0.02) than BP variability (18.2+/-1.1%). Autoregulation powerfully attenuated BP changes <0.1 Hz (normalized transfer gain 0.21+/-0.02 in the 0.0015- to 0.01-Hz frequency range) but at the expense of an oscillation located at approximately 0.05 Hz that possibly reflected the operation of the tubuloglomerular feedback. Large transient hypertensive episodes were not translated into RBF changes in SAD rats. We conclude that autoregulatory mechanisms have an ample capacity to protect the kidney against spontaneous BP fluctuations in the conscious rat. This capacity is not fully used under normal conditions of low BP variability.