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

Impaired renal autoregulation: implications for the genesis of hypertension and hypertension-induced renal injury

In summary, autoregulation of the renal vasculature provides a mechanism by which renal function is maintained relatively constant despite variations in systemic blood pressure. This system also provides a means for changes in blood pressure to occur without causing inappropriate alterations in urinary NaCl excretion. Alterations in the autoregulatory response can have clinical consequences. Increased activity of the TGF mechanism may be causally related to the development of some forms of hypertension. Decreased activity of TGF or an impaired myogenic response may help explain the increased susceptibility that certain patient groups exhibit toward hypertension-induced renal injury. The aggressive treatment of hypertension in patients with impaired renal autoregulation may be associated with an increase in the serum creatinine concentration. As long as this increase is neither excessive nor progressive, physicians should not be dissuaded from trying to achieve newly established blood pressure goals.

Comparative effects of selective T- and L-type calcium channel blockers in the remnant kidney model

We have previously reported that the dihydropyridine L-type calcium channel blockers (CCBs) have an adverse impact on glomerulosclerosis (GS) in the remnant kidney model despite significant blood pressure (BP) reduction, because of the concurrent deleterious effects on renal autoregulation. The effects of the CCB mibefradil, which is approximately 10-fold more selective for T- than L-type channels, were compared with the L-type selective amlodipine. One week after 5/6 ablation, rats were left untreated or received mibefradil or amlodipine. Systolic BP was monitored by continuous radiotelemetry. At 7 weeks, proteinuria and percent GS were quantitated. Average BP was significantly and comparably reduced after mibefradil (141+/-3 mm Hg) and amlodipine (143+/-5 mm Hg) compared with untreated rats (188+/-5 mm Hg). Despite the reduction in BP, proteinuria and percent GS in the mibefradil- or amlodipine-treated groups were not significantly different from those in the untreated rats. Excellent correlations were observed between BP and GS in each group (r=0.74 to 0.85, P<0.02). However, the slope of the relationship between GS and BP (increase in percent GS/mm Hg increase in average BP) was made significantly steeper by both mibefradil (2.7+0.6) and amlodipine (1.9+0.6) as compared with untreated rats (0.7+/-0.2; P<0.01). Thus, at any given BP elevation, greater GS was seen in mibefradil- and amlodipine-treated rats as compared with untreated rats. Additional studies performed at 3 weeks after renal ablation showed that the ability to autoregulate renal blood flow, already impaired in untreated rats, was essentially abolished by both mibefradil and amlodipine, thus providing an explanation for the shift in the slope of the relationship between BP and GS. These data indicate that CCBs with selectivity for either the T- or L-type calcium channel fail to protect against GS despite significant BP reductions because of the similar adverse effects on renal autoregulation and BP transmission.

Progression, remission, regression of chronic renal diseases

The prevalence of chronic renal disease is increasing worldwide. Most chronic nephropathies lack a specific treatment and progress relentlessly to end-stage renal disease. However, research in animals and people has helped our understanding of the mechanisms of this progression and has indicated possible preventive methods. The notion of renoprotection is developing into a combined approach to renal diseases, the main measures being pharmacological control of blood pressure and reduction of proteinuria. Lowering of blood lipids, smoking cessation, and tight glucose control for diabetes also form part of the multimodal protocol for management of renal patients. With available treatments, dialysis can be postponed for many patients with chronic nephropathies, but the real goal has to be less dialysis-in other words remission of disease and regression of structural damage to the kidney. Experimental and clinical data lend support to the notion that less dialysis (and maybe none for some patients) is at least possible.

Renal protective potential of antihypertensive drugs

The objective of this review is to discuss recent experimental and clinical data concerning the effectiveness of antihypertensive drugs in preventing or delaying renal changes caused by diabetes mellitus and hypertension and to examine possible future developments. A brief description of the mechanisms involved in the development of renal failure in diabetes and hypertension is included. Evidence is presented to show that in addition to renoprotection offered by reduction in arterial pressure, some antihypertensive agents may give more nephroprotection. This added renoprotective potential of antihypertensive agents, which are either already in use or are being developed, is discussed. The nephroprotective action of conventional antihypertensive drugs, such as beta-blockers, calcium antagonists and angiotensin-converting enzyme inhibitors is briefly reviewed. It is noted that several studies indicate that angiotensin-converting enzyme inhibitors may be more effective in preventing or retarding renal failure than other conventional drugs. The renoprotective potential of newly developed agents, such as angiotensin II Type 1 receptor antagonists, vasopeptidase inhibitors and endothelin receptor antagonists is also examined. Emphasis is placed on a possible superior renoprotective effect of combination therapy over monotherapy.

Blockade of the renin-angiotensin and endothelin systems on progressive renal injury

The renin-angiotensin system (RAS) and endothelin system may both play a role in the pathogenesis of progressive renal injury. The aims of the present study were 3-fold: first, to explore the possible benefits of dual blockade of the RAS with an ACE inhibitor and an angiotensin type 1(AT1) receptor antagonist; second, to examine the relative efficacy of endothelin A receptor antagonism (ETA-RA) compared with combined endothelin A/B receptor antagonism (ETA/B-RA); and third, to assess whether interruption of both RAS and endothelin system had any advantages over single-system blockade. Subtotally nephrectomized rats were studied as a model of progressive renal injury and randomly assigned to one of the following treatments for 12 weeks: perindopril (ACE inhibitor), irbesartan (AT1 receptor antagonist), BMS193884 (ETA-RA), bosentan (ETA/B-RA), and a combination of irbesartan with either perindopril or BMS193884. Treatment with irbesartan or perindopril was associated with an improved glomerular filtration rate and reductions in blood pressure, urinary protein excretion, glomerulosclerosis, and tubular injury in association with reduced gene expression of transforming growth factor-beta(1) and matrix protein type IV collagen. The combination of irbesartan with perindopril was associated with further reductions in blood pressure and urinary protein excretion. No beneficial effects of either BMS193884 or bosentan were noted. Furthermore, the addition of BMS193884 to irbesartan did not confer any additional benefits. These findings suggest that the RAS but not the endothelin system is a major mediator of progressive renal injury after renal mass reduction and that the combination of an AT1 receptor antagonist with an ACE inhibitor may have advantages over the single agent of RAS blocker treatment.

Effect of nitrendipine and nisoldipine on renal structure and function in long-term experimental diabetes in rats

This study investigates the efficacy of late intervention with the calcium channel blockers (CCBs) nitrendipine and nisoldipine in preventing development of albuminuria and glomerular hypertrophy in experimental diabetes. Streptozotocin (STZ)-induced diabetic rats were treated with nitrendipine or nisoldipine for 6 weeks after 3 or 6 months of untreated diabetes. The CCBs were administered in the fodder in a concentration of 250 mg/kg. After 3 months of untreated diabetes, nitrendipine treatment for 6 weeks significantly reduced urinary albumin excretion (UAE; P < 0.05) and glomerular hypertrophy. Nitrendipine also prevented an increase in systemic blood pressure compared with untreated diabetes. Nisoldipine showed no significant effect on UAE or glomerular hypertrophy despite systemic blood pressures similar to those of the diabetic nitrendipine-treated group. After 6 months of untreated diabetes, treatment with nitrendipine or nisoldipine for 6 weeks did not show effects on UAE, glomerular hypertrophy, or systemic blood pressure. No effect was found on renal growth in the treatment groups, and neither nitrendipine nor nisoldipine had any effect on body weight, blood glucose level, or food intake.

Lack of evidence of blood pressure-independent protection by renin-angiotensin system blockade after renal ablation

BACKGROUND

The superiority of renin-angiotensin system (RAS) blockade in providing renoprotection has been attributed to class-specific blood pressure "(BP)-independent" mechanisms. However, the conventional BP measurement methodology on which such conclusions are based is inherently limited for an accurate assessment of the fluctuating ambient BP profiles. The present studies were undertaken to rigorously examine the relationship of renoprotection to the antihypertensive effects of RAS blockade using chronic BP radiotelemetry in the 5/6 renal ablation model.

METHODS

Rats with 5/6 renal ablation received either no treatment, the angiotensin-converting enzyme inhibitor benazepril at a dose of 25, 50, and 100 mg/L; or the angiotensin receptor antagonist losartan at a dose of 50, 120, and 180 mg/L of drinking H2O; and were followed for seven weeks.

RESULTS

Glomerulosclerosis (GS) at sacrifice (approximately 7 weeks) demonstrated a close correlation with the average systolic BP in untreated (r = 0.76, N = 20), benazepril-treated (r = 0.80, N = 33), losartan-treated (r = 0.83, N = 32), or all animals combined (r = 0.81, N = 85, P < 0.0001 for all correlations). The slope of the relationship between GS and BP (percentage of increase in GS/mm Hg increase in BP) in untreated rats (0.7 +/- 0.14) was not significantly altered by either benazepril (0.96 +/- 0.13) or losartan (0.60 +/- 0.08), indicating that RAS blockade, by either agent, resulted in renoprotection that was proportionate to the achieved BP reductions.

CONCLUSIONS

These data demonstrate that RAS blockade provides renoprotection in the rat remnant kidney model of progressive GS, primarily through "BP-dependent" and not "BP-independent" mechanisms.

Functional and structural correlates of glomerulosclerosis after renal mass reduction in the rat

Previously, it was shown that 5/6 renal mass reduction by surgical excision (RK-NX) results in a marked reduction of glomerulosclerosis (GS) at 6 wk compared with the conventional 5/6 renal ablation by infarction (RK-I) model. To determine the pathogenetic correlates of the striking differences in GS, radiotelemetrically measured BP; single nephron function; glomerular volume; the temporal expression of mRNA for renin, transforming growth factor-beta, and platelet-derived growth factor-B; and plasma renin concentration were compared between RK-NX, RK-I, and sham-operated control rats. Hypertension only developed in the RK-I model, was present at 3 d after infarction, and was correlated with both an increased expression of renin mRNA by Northern analysis and elevated plasma renin concentration. Structural (glomerular volume) and functional (single nephron blood flow and GFR) indices of the compensatory adaptive response were significantly but similarly increased in the RK-NX and RK-I rats compared with sham-operated controls, indicating that these adaptations per se are not responsible for the initiation of GS after 5/6 renal mass reduction. Glomerular capillary pressure (P(GC)) was also significantly increased in both RK-I (56 +/- 2 mmHg) and RK-NX rats (50 +/- 0.9 mmHg) compared with controls (46 +/- 0.8 mmHg, P < 0.01), but the increase was significantly greater in RK-I versus RK-NX rats (P < 0.05) consistent with the higher BP in RK-I rats. These data indicate that differences in renin probably account for the early divergence of BP (and P(GC)) responses between RK-I and RK-NX models. Transforming growth factor-beta and platelet-derived growth factor-B mRNA expression in pooled RNA from kidneys from each group showed increases at 21 d along with early evidence of glomerular injury in the RK-I group but not in the RK-NX group, consistent with their postulated roles as molecular mediators of GS, but only in rats with pathologic glomerular hypertension.

Effect of nitrendipine on renal function in renal-transplant patients treated with cyclosporin: a randomised trial

BACKGROUND

Calcium antagonists such as nitrendipine reduce the effects of cyclosporin on renal haemodynamics, however, their long-term efficacy has not been established. We did a randomised trial to investigate the effects of nitrendipine on renal function in renal-transplant patients treated with cyclosporin.

METHODS

253 renal-transplant patients were recruited: 52 normotensive patients (diastolic blood pressure <90 mm Hg) were assigned placebo and 57 nitrendipine 5 mg twice daily; 71 hypertensive patients (diastolic blood pressure >90 to <115 mm Hg) were assigned placebo and 73 nitrendipine 10 mg twice daily. Nitrendipine was increased to 20 mg twice daily if the target diastolic blood pressure (<90 mm Hg) was not achieved. The patients were seen once a month for 24 months; blood pressure and serum creatinine concentration were recorded at each visit. Analysis was by intention to treat.

FINDINGS

63 patients were withdrawn (35 nitrendipine, 28 placebo). The mean serum creatinine concentration at baseline was slightly higher in the nitrendipine group (146.7 micromol/L [SE 4.42]) than in the placebo group (137.0 micromol/L [3.54]. At the 24-month endpoint or at dropout, serum creatinine concentration was significantly higher in the 123 patients in the placebo group than the 130 patients in the nitrendipine group (160.8 [7.1] vs 148.5 [5.3], p for effect of treatment=0.025, analysis of covariance in a two-way classification; 95% CI for difference -1.77 to -22.98). At study entry, the blood pressures of the placebo and the nitrendipine groups were almost identical. At 24 months, blood pressure was higher in the normotensive patients given a placebo than in those patients given nitrendipine. By contrast, blood-pressure values were similar in those hypertensive patients given a placebo and those given nitrendipine at the end of treatment.

INTERPRETATION

The calcium antagonist nitrendipine has no adverse effects on kidney function in renal-transplant patients with cyclosporin. The drug has a small but significant nephroprotective effect, that is independent of the drug's antihypertensive action.

Impaired autoregulation of the glomerular filtration rate in patients with nondiabetic nephropathies

BACKGROUND

The ability of the kidney to maintain constancy of the glomerular filtration rate (GFR) over a wide range of renal perfusion pressures is termed autoregulation. Defective autoregulation of GFR has been demonstrated in diabetic nephropathy. Whether this is also the case in patients with nondiabetic nephropathies is not known.

METHODS

We investigated the effect of acute lowering of blood pressure (BP) on GFR in 16 (8 males and 8 females) albuminuric subjects suffering from different nondiabetic nephropathies and in 14 (7 males and 7 females) controls matched with respect to sex, age, BP, and baseline GFR. The subjects received in random order an intravenous injection of either clonidine (150 to 225 microg) or saline (0.154 mmol/liter) within two weeks. We measured GFR ([51Cr]-EDTA), albuminuria (enzyme-linked immunosorbent assay; ELISA), and BP (Takeda TM-2420).

RESULTS

Clonidine induced similar reductions in mean arterial BP 17 (2) versus 19 (2) mm Hg [mean (SE)] in patients with nephropathy and in controls, respectively. GFR diminished in average from 89 (6) to 82 (5) ml/min/1.73 m2 (P < 0.05), and albuminuria declined from a geometric mean of 1218 (antilog SE 1.3) microg/min to 925 (1.3) in the patients with nondiabetic nephropathies (P < 0.05), whereas these variables remained unchanged in the control group. The mean difference between changes in GFR (95% confidence interval) between the nondiabetic macroalbuminuric and control subjects was 6.1 (-0.03 to 12.21) ml/min/1.73 m2 (P = 0.051).

CONCLUSION

Our study suggests that albuminuric patients with nondiabetic nephropathies frequently suffer from impaired autoregulation of GFR.

Mibefradil prevents L-NAME-exacerbated nephrosclerosis in spontaneously hypertensive rats

OBJECTIVE

To determine the potential renal protective effects of a novel calcium channel blocker mibefradil in chronic renal failure.

METHOD

We compared the long-term effects of mibefradil with an angiotensin-converting enzyme inhibitor cilazapril on blood pressure, proteinuria, renal function and histological alterations in N-nitro-L-arginine methylester (L-NAME)-treated spontaneously hypertensive rats (SHR). Three groups of SHR were studied for 45 days: group 1 (n = 14), treated with L-NAME only (50 mg/l in the drinking water); group 2 (n = 15) L-NAME plus co-treatment with mibefradil (30 mg/kg per day); group 3 (n = 15), L-NAME plus co-treatment with cilazapril (10 mg/kg per day).

RESULTS

Both mibefradil and cilazapril attenuated the increased systolic blood pressure, and prevented the development of proteinuria and the decreased creatinine clearance (Ccr) seen at day 42 in the group treated with L-NAME alone. Notably, mibefradil had similar effects to cilazapril on proteinuria and Ccr, despite a reduced antihypertensive effect All animals receiving mibefradil co-treatment remained alive throughout the experiment, whereas the mortality rate was 43% in SHR treated with L-NAME alone. Both mibefradil and cilazapril completely prevented renal structural damage as assessed by scoring glomerular, tubulo-interstitial and vascular lesions.

CONCLUSIONS

Our data show that mibefradil prevented the development of hypertension and proteinuria, renal functional impairment and nephrosclerosis, and also improved animal survival. The renal protective effects of mibefradil were at least equivalent to those of an ACE inhibitor in this animal model of chronic renal failure.

Calcium-channel blockers and the progression of renal disease

Effective blood pressure (BP) reduction is now generally recognized as a clinically proven strategy to retard the seemingly inexorable downhill progression of patients with diabetic and nondiabetic chronic renal disease. Although calcium-channel blockers (CCBs) are effective antihypertensive agents, the available experimental and clinical data are quite contradictory as to whether BP reduction achieved with CCBs provides the expected renoprotection. Blockade of the "L" type, voltage-gated Ca channels that mediate the BP reduction also concurrently impairs renal autoregulatory responses of the preglomerular vasculature. Because these renal autoregulatory resistance changes provide the primary protection against the transmission of systemic hypertension to the renal microvasculature, the adverse effects of CCBs on renal autoregulation counteract the beneficial effects on BP reduction. The degree of renoprotection achieved, therefore, depends on the balance between these two opposing effects. The data also indicate that there are probably important and clinically relevant differences between the classes of CCBs, with the dihydropyridine (DHP) CCBs most likely to have consistent deleterious effects on renal autoregulation. However, the available data also indicate that the adverse effects of DHP CCBs are not likely to be observed if BP is lowered well into the normotensive range, possibly through the use of combination therapies. Even when used as adjunctive therapy, close monitoring may be advisable to ensure BP normalization and the absence of any untoward effects on proteinuria and renal function.

Renal protection and antihypertensive drugs: current status

The renal protective effect of antihypertensive drugs is linked to 2 mechanisms. First, reduction in blood pressure (BP) is a fundamental prerequisite common to all antihypertensive drugs. The exact definition of the level to which BP should be reduced remains to be established, although there is some evidence that BP should be reduced below 130/85 mm Hg in patients with diabetic and nondiabetic nephropathies and below 125/75 mm Hg in patients with nondiabetic nephropathies and proteinuria >1 g/day. However, available data suggest that tight BP control (BP<140/80 mm Hg) can reduce the risk of cardiovascular complications in hypertensive patients with type 2 diabetes mellitus (non-insulin-dependent diabetes mellitus; NIDDM). Secondly, intrarenal actions on mechanisms such as glomerular hypertension and hypertrophy, proteinuria, mesangial cell proliferation, mesangial matrix production and probably endothelial dysfunction, which can cause and/or worsen renal failure, are relevant for the renal protective action of some drug classes. ACE inhibitors possess such properties and also seem to lower proteinuria more than other antihypertensive drugs, despite a similar BP lowering effect. Calcium antagonists likewise exert beneficial intrarenal effects, but with some differences among subclasses. It remains to be evaluated whether angiotensin II-receptor antagonists can exert intrarenal effects and antiproteinuric actions similar to those of ACE inhibitors. While primary prevention of diabetic nephropathy is still an unsolved problem. there is convincing evidence that in patients with type 1 (insulin-dependent diabetes mellitus; IDDM) or 2 diabetes mellitus and incipient nephropathy ACE inhibitors reduce urinary albumin excretion and slow the progression to overt nephropathy. Similar effects have been reported with some long-acting dihydropyridine calcium antagonists, although less consistently than with ACE inhibitors. In patients with diabetic overt nephropathy, ACE inhibitors and nondihydropyridine calcium antagonists are particularly effective in reducing proteinuria and both drugs can slow the decline in glomerular filtration rate more successfully than other antihypertensive treatment. Available data in patients with nondiabetic nephropathies indicate that ACE inhibitors can be beneficial, principally in patients with significant proteinuria, in slowing the progression of renal failure. However, it is still unclear whether this beneficial effect of ACE inhibitors is particularly evident in patients with mild and/or more advanced renal failure and whether calcium antagonists possess a similar nephroprotective effect. Overall, data from clinical trials thus seem to indicate that ACE inhibitors and possibly calcium antagonists should be preferred in the treatment of patients with diabetic and nondiabetic nephropathies. However, further information is needed to understand renal protection.

Class differences in the effects of calcium channel blockers in the rat remnant kidney model

BACKGROUND

Controversy persists as to the existence of class differences between calcium channel blockers (CCBs) in their ability to provide renoprotection and as to potential mechanisms involved.

METHODS

Rats with 5/6 renal ablation were left untreated or received diltiazem, verapamil, or felodipine after the first week, and the relationship between continuous radiotelemetrically measured blood pressure (BP) and glomerulosclerosis (GS) was assessed at seven weeks. Additionally, the effects of these CCBs on renal autoregulation and hypertrophy were examined at three weeks after renal ablation.

RESULTS

Although an excellent linear correlation was observed between the average BP levels and GS in all groups (r = 0.75 to 0.84, P < 0.01), significant protection was not achieved with any of the CCBs, but for different reasons. The antihypertensive effects of diltiazem were not sustained beyond the second week. Verapamil significantly reduced the average BP (144 +/- 4 mm Hg vs. 181 +/- 8 in untreated rats) but shifted the slope of the relationship between BP and GS (increase in percentage GS/mm Hg increase in average systolic BP) to the left (x intercept 121 vs. 144 mm Hg for untreated rats, P < 0.01) so that GS was not reduced. Felodipine also significantly reduced the average BP (144 +/- 3 mm Hg) and shifted the slope to the left (x intercept 123 mm Hg), but additionally made the slope steeper (2.3 +/- 0.5 vs. 0.82 +/- 0.2 in untreated rats). Because of these differing effects on the relationship between BP and GS, the rank order of GS for any given BP elevation was as follows: felodipine > verapamil > diltiazem = untreated. Felodipine, but not verapamil or diltiazem, caused additional impairment of the already impaired renal autoregulation in untreated rats, thereby explaining its adverse effects on GS. By contrast, the adverse effects of verapamil on GS were attributable to the greater amplitude of BP fluctuations that was observed in the verapamil-treated rats such that for any given average BP, these rats were exposed to greater peak pressures as compared with the other groups. None of the CCBs had a significant effect on glomerular hypertrophy.

CONCLUSIONS

These class differences between CCBs in their relative impact on systemic BP profiles, renal autoregulation, and glomerular pressure transmission may have clinically significant implications and may account for the variable glomeruloprotection that has been observed with these agents in both experimental models and in humans.

Renal afferent and efferent arteriolar dilation by nilvadipine: studies in the isolated perfused hydronephrotic kidney

Although calcium antagonists are believed to exert preferential vasodilator action on the renal preglomerular afferent arteriole, we recently demonstrated that efonidipine, a novel calcium antagonist, vasodilates both afferent and efferent arterioles. Nilvadipine also is reported to increase renal blood flow and reduce filtration fraction, suggesting indirectly afferent and efferent arteriolar vasodilation. No direct investigation, however, has been conducted examining the renal microvascular action of nilvadipine. We therefore characterized the renal microvascular reactivity to nilvadipine, by using the isolated perfused rat hydronephrotic kidney. The administration of angiotensin II (0.3 nM) caused marked vasoconstriction of afferent (from 13.5 +/- 0.6 to 9.2 +/- 0.6 microm, p < 0.01, n = 6) and efferent arterioles (from 11.5 +/- 1.0 to 7.4 +/- 0.7 microm, p < 0.01; n = 5). The subsequent addition of nilvadipine (10 nM, 100 nM, and 1 microM) caused 37 +/- 5%, 91 +/- 4%, and 95 +/- 8% reversal of afferent arteriolar constriction, respectively. Similarly, efferent arterioles manifested 59 +/- 12% reversal by 1 microM nilvadipine. Thus unlike nifedipine, which we previously reported to cause modest efferent arteriolar dilation (21 +/- 1% reversal at 1 microM), nilvadipine possesses the greater ability to dilate efferent arterioles (p < 0.01 vs. nifedipine), although both antagonists cause similar magnitudes of afferent arteriolar vasodilation. Variable effects on the efferent arteriole suggest the heterogeneity in the calcium antagonist with regard to the renal microvascular action of this agent.

Effects of antihypertensive therapy on progression of diabetic nephropathy

There is a clear relationship between hypertension and the microvascular complications of diabetes. Genetic predisposition to hypertension has been correlated to the risk of diabetic nephropathy in type I diabetes, and hypertension is a well known risk factor for developing nephropathy in patients with type II diabetes. Multiple studies have emphasized the importance of hypertension on renal disease progression, and blood pressure control with conventional antihypertensive drugs slows the rate of renal function loss in diabetic nephropathy. Furthermore, evidence of the role of renin-angiotensin system (RAS) on progression of renal damage has focused much interest on the therapeutic action of the RAS blockade. In patients with type I diabetes, blocking the RAS with angiotensin converting enzyme (ACE) inhibitors prevents progression from microalbuminuria to overt nephropathy, and in overt nephropathy decreases the gradual loss of renal function beyond its blood pressure lowering effect. Less clinical information is available in type II diabetic nephropathy, but our experience and some recent studies suggest that ACE inhibitors also have a renoprotective action in type II diabetes. The role of calcium channel blockers in diabetic nephropathy is not clear. Several short-term studies with the first generation dihydropyridine calcium antagonists showed a lower effect on urinary albumin excretion and a more rapid progression to renal failure than with ACE inhibitors. However, other calcium channel blockers, particularly of the non-dihydropyridine type, have been shown to have a beneficial effect on diabetic nephropathy, decreasing proteinuria and slowing progression.

Effects of antihypertensive drugs on autoregulation of RBF and glomerular capillary pressure in SHR

The relationship between systemic blood pressure and glomerular capillary pressure (Pgc) in spontaneously hypertensive rats (SHR) during treatment with antihypertensive drugs is still unclear. The effects of an angiotensin-converting enzyme inhibitor (enalapril), two calcium channel antagonists (nifedipine and verapamil), and an alpha1-receptor blocker (doxazosin) on renal blood flow (RBF) autoregulation, Pgc, and renal segmental resistances were therefore studied in SHR. Recordings of RBF autoregulation were done before and 30 min after intravenous infusion of the different drugs, and Pgc was thereafter measured with the stop-flow technique. When the mean arterial pressure (MAP) was reduced to approximately 120 mmHg by infusions of doxazosin or enalapril, the lower pressure limit of RBF autoregulation was reduced significantly. Nifedipine or verapamil abolished RBF autoregulation. Doxazosin did not change Pgc (43.6 +/- 1.4 vs. 46.7 +/- 1.5 mmHg in controls, P > 0.5), enalapril lowered (41.3 +/- 0.8 mmHg, P < 0.01), and the calcium channel antagonists increased Pgc [53.7 +/- 1.4 mmHg (nifedipine) and 54.8 +/- 1.2 mmHg (verapamil), P < 0.01]. When MAP was reduced to approximately 85 mmHg by drugs, Pgc was reduced to 43.3 +/- 1.7 mmHg after nifedipine (P > 0.2 vs. control), whereas Pgc after enalapril was 38.5 +/- 0.5 mmHg (P < 0.05 vs. control). Enalapril reduced Pgc mainly by reducing efferent resistance. During treatment with calcium channel antagonists, Pgc became strictly dependent on MAP. Monotherapy with nifedipine may increase Pgc and by this mechanism accelerate glomerulosclerosis if a strict blood pressure control is not obtained.

Differential effects of calcium channel blockers on size selectivity of proteinuria in diabetic glomerulopathy

BACKGROUND

Calcium channel blockers (CCBs) are known to have differential effects on both changes in proteinuria as well as progression of diabetic nephropathy. No clinical study, however, has evaluated whether the differential antiproteinuric effects of CCBs may be explained by their effect on glomerular membrane permeability. We, therefore, tested the hypothesis that certain subclasses of CCBs reduce proteinuria by changing size selectivity of the glomerular membrane, hence changing its permeability.

METHODS

Twenty-one patients with type 2 diabetes and the presence of nephropathy with hypertension were randomized to receive either diltiazem CD or nifedipine GITS after baseline data for mean systolic and diastolic pressure, urinary protein excretion, glomerular filtration rate, renal plasma flow, neutral dextran and IgG clearances were obtained. Glomerular filtration rate, renal plasma flow, neutral dextran and IgG clearance were measured every three months, arterial pressure and heart rate every month. Patients were followed for 21 months.

RESULTS

At 21 months, both patient groups had similar levels of blood pressure control, however, only the diltiazem group had a change in proteinuria (4+/-10%delta, nifedipine vs. -57+/-18%delta, diltiazem; P < 0.001) with improvement in glomerular size selectivity and change in IgG clearance.

CONCLUSIONS

These data support the hypothesis that CCBs that provide sustained reductions in proteinuria do so, in part, by improving glomerular size permselectivity.

Mechanosensitive cation channels mediate afferent arteriolar myogenic constriction in the isolated rat kidney

1. In order to assess ionic mechanisms mediating renal afferent arteriolar myogenic constriction, experiments were performed using isolated perfused hydronephrotic rat kidneys. 2. Increasing pressure progressively constricted the afferent arteriole (-0.26 +/- 0.02% mmHg-1, n = 21, r = 0.97). Gadolinium (10 microM), a mechanosensitive cation channel blocker, abolished this myogenic constriction. However, high potassium media (30 mM) constricted the afferent arteriole in the presence of gadolinium. 3. Lowering extracellular sodium concentration gradually attenuated afferent arteriolar myogenic constriction. In the perfusate containing 50 mM sodium, the myogenic response was arrested. 4. Afferent arteriolar myogenic constriction was prevented in calcium-free perfusate or by the L-type calcium channel blocker diltiazem (10 microM). 5. Our present findings provide evidence that increasing pressure gates mechanosensitive cation channels on the afferent arteriole, thereby eliciting membrane depolarization and activating voltage-dependent calcium channels.

Antihypertensive treatment of patients with proteinuric renal diseases: risks or benefits of calcium channel blockers?

In patients with proteinuric renal diseases the rate of progression of renal insufficiency is determined by the level of blood pressure and proteinuria. It has been demonstrated that strict blood pressure control with angiotensin converting enzyme (ACE)-inhibitors or beta-blockers, aimed at reaching values below 130/80 mm Hg, attenuates the deterioration of renal function. In general, the beneficial effects of these drugs are reflected in a parallel lowering of proteinuria. Calcium channel blockers are effective antihypertensive drugs, however, their safety in patients with proteinuric renal diseases and renal insufficiency may be questioned because of reported untoward effects on urinary protein excretion. The present review discusses the potential benefits and risks of calcium channel blockers (CCBs) in the treatment of patients with renal diseases. To this end we have evaluated the effects of these drugs in animal models of progressive renal injury. In these animal models adverse effects of CCBs have been reported which are attributed to an impairment of autoregulation. In patients with proteinuria, the dihydropyridine CCBs do not lower proteinuria despite a reduction of blood pressure. Studies on the effects on the course of renal function are limited, however, the available data do suggest that this class of CCBs may be less advantageous than other antihypertensive drugs, thus arguing against the use of these agents as first-line drugs in patients with proteinuric renal diseases. Information on the effects of the non-dihydropyridine CCBs is limited to a small number of studies in patients with diabetic renal disease. Although the data suggest that these classes of CCBs might be more beneficial, more studies are needed, particularly in patients with non-diabetic renal diseases, before founded conclusions can be reached.

Diabetic hypertensive patients: improving their prognosis

Diabetes is a devastating disease with multiple adverse effects on the vasculature. Moreover, hypertension is a prerequisite for patients with diabetes to progress to end-stage renal disease and to develop cardiovascular complications. Adequate control of blood glucose and blood pressure are the two most important factors that predict a favorable renal outcome. Recent studies have also shown that some classes of antihypertensive medications, such as the angiotensin-converting enzyme (ACE) inhibitors, may be ideal initial agents to control blood pressure in the hypertensive diabetic patient and thus to preserve renal function. In addition, nondihydropyridine calcium-channel blockers have been shown to retard the decline in renal function in patients with non-insulin-dependent diabetes mellitus (NIDDM) nephropathy who have lost at least 50% of their renal function. Retrospective analyses demonstrate that a reduction in blood pressure, especially to levels of <130/85 mg Hg in diabetic patients, retards the progression of renal disease. Reduction in arterial pressure to these low levels is probably more important than the agents used to achieve this goal. Because many of these patients require more than one medication to achieve these lower levels of arterial pressure, it is clear that fixed-dose combinations of such agents will both improve the likelihood of achieving a given blood pressure goal as well as medication compliance.

Nephroprotective effect of antihypertensive drugs in essential hypertension

BACKGROUND

Effective antihypertensive treatment has prevented target-organ involvement in hypertension, markedly reducing morbidity and mortality from strokes, coronary heart disease, cardiac failure, and hypertensive emergencies. However, the incidence of hypertension-related end-stage renal disease continues to increase, suggesting that therapeutic reduction in arterial pressure by itself is not sufficient to prevent the development of hypertensive renal failure.

OBJECTIVE

To examine experimental and clinical data concerning the protective effect of reduction of arterial pressure on the progression of hypertension-related renal disease, and the evidence indicating that some antihypertensive agents may afford more nephroprotection, over and above that attributable to reduction of arterial pressure.

RESULTS

Results of numerous studies clearly indicate that adequate control of arterial pressure, irrespective of the antihypertensive agent used, slowed the progression of renal disease. Results of some studies suggest that lowering arterial pressure below the level that is usually considered adequate has an additional beneficial effect by slowing the progression of renal injury.

CONCLUSION

Results of a number of studies evaluating nephroprotective effects of various drugs and regimens have indicated that certain agents, most notably angiotensin converting enzyme inhibitors and their combination with calcium antagonists, afford more protection than do others at similar levels of reduction of arterial pressure. Results of still other studies suggest that certain agents that exert greater nephroprotection are more efficient at controlling arterial pressure. Therefore, further data are needed before any final conclusion can be drawn. However, it is clear that, in order to establish nephroprotection in patients with essential hypertension, the problem should not be further complicated by additional comorbid diseases such as diabetes mellitus.

Calcium antagonists and the progression of chronic renal failure

End-stage renal disease, which signifies irreversible renal failure, constitutes a major and growing public health problem worldwide. The striking increase in end-stage renal disease has catalyzed clinical and investigative focus on pharmacologic interventions to retard progression to this condition. Increasing evidence indicates that some classes of antihypertensive medications may confer a greater effect than others in slowing progression of renal disease despite similar levels of blood pressure reduction. Substantive data indicate that angiotensin converting enzyme inhibition preferentially retards the progression of renal disease, primarily by protecting the injured kidney from hemodynamically mediated glomerular damage. Newer studies suggest that calcium antagonists also have diverse properties, which are independent of their renal microcirculatory effects that might afford renal protection.

Bosentan prevents preglomerular alterations during angiotensin II hypertension

The present study was performed to characterize structurofunctional alterations of preglomerular vessels during chronic angiotensin II (Ang II)-induced hypertension (Ang II group: 400 ng x kg[-1] x min[-1], 10 days) and to assess the role of endothelin-1 in rats receiving Ang II and the mixed receptor antagonist bosentan (Ang II+B group: 30 mg x kg[-1] x d[-1], 10 days). Systolic blood pressure rose by 56+/-3 and 54+/-6 mm Hg in Ang II and Ang II+B rats, respectively. Albuminuria increased similarly in both Ang II-treated groups, reflecting glomerular barrier dysfunction. Preglomerular vessels were isolated after HCI maceration and comprised arcuate arteries and their branches, interlobular arteries (ILA), and afferent arterioles (AA). In the Ang II group, focal vascular lesions affected 36+/-6%, 20+/-5%, and 4+/-1% of arcuate arterial branches, ILA, and AA, respectively. They were characterized by 74% increased media thickness and accumulation of Sudan black-positive (SB+) lipid droplets, and media cell proliferation was documented through immunohistochemistry. The occurrence of SB+ lesions was strikingly reduced with bosentan. Autoregulatory responses (AR) were assessed along ILA and AA with the use of blood-perfused juxtamedullary nephron preparations. AR were elicited by raising blood perfusion pressure from 60 to 160 mm Hg and quantified through videomicroscopy as pressure-induced constrictions. AR were inhibited in Ang II-treated rats along ILA and AA; Ang II-induced AR changes were prevented by bosentan. Maximal relaxation induced by Mn2+ revealed equal basal tone in Ang II-treated, Ang II+B-treated, and control vessels. Chronic Ang II-induced hypertension is therefore associated with the development of SB+ lesions and selective impairment of AR in juxtamedullary nephrons. Endothelin-1 likely mediates the structurofunctional alterations of preglomerular vasculature during Ang II hypertension.

Predictors of renal and cardiovascular mortality in patients with non-insulin-dependent diabetes: a brief overview of microalbuminuria and insulin resistance

Both microalbuminuria and insulin resistance are present at some stage in the natural history of non-insulin-dependent diabetes mellitus (NIDDM). Microalbuminuria predicts both progression to endstage renal disease and an increase in cardiovascular mortality compared to diabetic patients without microalbuminuria. Conversely, microalbuminuria is not a strong predictor of either renal or cardiovascular mortality in hypertensive nondiabetic subjects. This difference in risk may relate to the presence of glycated albumin in patients with diabetes. Glycation of albumin occurs because of persistent hyperglycemia. Glycated albumin is directly toxic to both renal and vascular tissue through stimulation of reactive oxygen species by both renal and immune protective cells. Blunting the rise in microalbuminuria with either aggressive blood glucose control or angiotensin-converting enzyme (ACE) inhibition, early in the course of the disease, markedly reduces renal mortality. In contrast to microalbuminuria, which is a reflection of renal injury, insulin resistance is a genetically determined problem that directly relates to peripheral glucose utilization. In most cases, insulin resistance is phenotypically expressed as diabetes as a result of environmental factors such as obesity. Insulin resistance is associated with an increased risk for development of both hypertension and NIDDM as well as atherosclerosis. Diabetic or hypertensive subjects with insulin resistance have an increased risk of cardiovascular but not renal mortality. Sustained weight loss is the best way to reduce insulin resistance and arterial pressure. Additionally, alpha blockers, more than other antihypertensive agents reduce insulin resistance. This class of drugs, however, has not been shown to reduce either microalbuminuria or overall cardio-renal mortality.

Dynamic autoregulation and renal injury in Dahl rats

The Dahl salt-sensitive (Dahl S) rat develops hypertension and renal injuries when challenged with a high salt diet and has been considered to be a model of chronic renal failure. Renal injuries appear very early in life compared with the spontaneously hypertensive rat (SHR). During the course of hypertension, a gradual impairment of autoregulatory control of renal blood flow might expose the glomerular circulation to periods of elevated pressure, resulting in renal injuries in Dahl S rats. Dynamic autoregulatory capacity was assessed in Dahl S and Dahl salt-resistant (Dahl R) rats, SHR, and Sprague-Dawley rats by inducing broad-band fluctuations in the arterial blood pressure and simultaneously measuring renal blood flow. Dynamic autoregulation was estimated by the transfer function using blood pressure as the input and renal blood flow as the output. Renal morphological injuries were evaluated in Dahl S rats and SHR and were scored semiquantitatively. Dynamic autoregulation was efficient and comparable in the low-frequency range (<0.015 Hz) in Dahl R rats, SHR, and Sprague-Dawley rats. The response in Dahl S rats depended strongly on the initiation time of the high salt diet. Autoregulation was preserved during a low salt diet and in rats exposed to a late-onset hypertension of short duration, only partly preserved if the late-onset hypertension was of a longer duration, and abolished in early-onset hypertension. All Dahl S rats on a high salt diet showed severe morphological changes in the kidney. In conclusion, autoregulatory capacity in the kidney of Dahl S rats is gradually impaired when rats are rendered hypertensive with a high salt diet. Renal morphological injuries develop before loss of dynamic autoregulation. Impaired autoregulation appears to be the result, not the cause, of the process that ultimately leads to renal failure in the Dahl S rat.

Combined effects of an angiotensin converting enzyme inhibitor and a calcium antagonist on renal injury

BACKGROUND

Angiotensin converting enzyme inhibitors have uniformly been shown to prevent the development both of proteinuria and of glomerulosclerosis in rats with a remnant kidney. Conversely, dihydropyridine calcium antagonists (DCA) have failed to demonstrate such a benefit in spite of causing an equivalent reduction in blood pressure.

OBJECTIVE

To test the hypothesis that concomitant administration of an angiotensin converting enzyme inhibitor and a DCA would lead to a smaller increase both in proteinuria and in glomerulosclerosis relative to that caused by administration of a DCA alone at similar levels of blood pressure.

METHODS

Experiments were carried out using Sprague-Dawley rats that had been subjected to five-sixths renal ablation. Animals were allocated randomly to one of four groups: control (no treatment), amlodipine (A rats), benazepril (B rats), or a combination of benazepril and amlodipine (B + A rats). We implanted intraperitoneal sensors for telemetric monitoring of the animal's blood pressure. Other parameters measured at baseline included proteinuria and inulin clearance. After approximately 7 weeks all of the parameters were remeasured and animals killed for morphologic assessment of the kidney.

RESULTS

The B + A rats had lower levels of proteinuria than did the rats in group A (21 +/- 12 mg/day for B + A rats versus 59 +/- 24 mg/day for A rats, P < 0.05). The degree of glomerulosclerosis in the B + A rats was also reduced markedly compared with that in A rats (12 +/- 4% for B + A rats versus 43 +/- 12% for A rats, P < 0.05). Moreover, the results on proteinuria and glomerulosclerosis of B + A rats were similar to those for B rats. These differences could not be explained totally in terms of differences in blood pressure control (144 +/- 12 mmHg in A rats versus 132 +/- 13 mmHg in B + A rats, NS).

CONCLUSION

The results were consistent with the observation that a combination of benzepril and amlodipine provides additional protection against renal injury compared with that provided by amlodipine alone. The mechanism for this benefit is not known.