T-type and L-type calcium channel blockers exert opposite effects on renin secretion and renin gene expression in conscious rats

Functional Relevancies of Trans-Differentiation in the Juxtaglomerular Apparatus of Rat Kidney

Glomerular filtration rate is controlled by the contractile effect of angiotensin II on afferent and efferent arterioles. The renin positivity of the afferent arterioles depends on tubuloglomerular feedback via the macula densa (MD) and short loop feedback via the afferent arteriolar endothelia. The renin-producing cells are trans-differentiated from smooth muscle cells (SMCs) of mainly the afferent arterioles, the MD cells are trans-differentiated from the neighboring tubular cells, and the high-permeability endothelial cells are trans-differentiated from normal permeability endothelial cells facing the renin-negative part of the afferent arterioles. All of the trans-differentiations depend on the activity of the renin-angiotensin system (RAS). The distribution of AT1 receptors for angiotensin II expresses the contractile effects of angiotensin II on renin-negative SMCs and the negative effect on trans-differentiation of renin-positive SMCs and MD cells. The purpose of this review is to summarize the stereological data of molecules like angiotensin II AT1 receptors, L-type calcium channels, and renin receptors in the juxtaglomerular apparatus of normal and STZ-induced diabetic rat kidneys, thus showing their functional relevancies on trans-differentiation among the juxtaglomerular apparatus’ elements.

Ion-Channel modulator TH1177 reduces glomerular injury and serum creatinine in chronic mesangial proliferative disease in rats

Background

T-type calcium channels (TTCC) are involved in mesangial cell proliferation. In acute thy-1 nephritis in the rat TTCC inhibition reduces glomerular damage and cell proliferation. This work is extended here by a study of the non-selective TTCC inhibitor TH1177 in a chronic model of proliferative glomerulonephritis (GN) including late treatment starting after the initial inflammation has resolved. The objective was to determine the effects of TH1177 in a model of chronic mesangioproliferative renal disease.

Methods

Chronic GN was induced in WKY rats by unilateral nephrectomy (day − 7) followed by day 0 injection of Ox7 thy-1 mAb. Treatment with TH1177 (10–20 mg/Kg daily IP) was started on day 2 (early treatment) or on day 14 (late treatment) and compared to vehicle-treated controls until sacrifice at day 42. Glomerular disease was assessed with a damage score, fibrosis assay, cellular counts and renal function measured by serum creatinine.

Results

Treatment with TH11777 was associated with reduced serum creatinine, less glomerular damage, reduced fibrosis and reduced glomerular cellularity. The results for early and late TH1177 treatments were essentially the same and differed significantly from vehicle.

Conclusions

The ion-channel modulator TH1177 is capable of improving glomerular outcome in chronic rat GN even when treatment starts 14 days after initiation of the disease. These data are discussed in the context of the possible targets of TH1177 including TTCC, TRP family, Stim/Orai group and other cation channels. The work supports the use of genetic models to examine the roles of individual cation channels in progressive glomerulonephritis to further define the targets of TH1177.

Deficiency of T-type Ca2+ channels Cav3.1 and Cav3.2 has no effect on angiotensin II-induced hypertension but differential effect on plasma aldosterone in mice

T-type Ca²⁺ channel Ca_v3.1 promotes microvessel contraction ex vivo. It was hypothesized that in vivo, functional deletion of Ca_v3.1, but not Ca_v3.2, protects mice against angiotensin II (ANG II)-induced hypertension. Mean arterial blood pressure (MAP) and heart rate were measured continuously with chronically indwelling catheters during infusion of ANG II (30 ng·kg^-1·min^-1, 7 days) in wild-type (WT), Ca_v3.1^-/-, and Ca_v3.2^-/- mice. Plasma aldosterone and renin concentrations were measured by radioimmunoassays. In a separate series, WT mice were infused with ANG II (100 ng·kg^-1·min^-1) with and without the mineralocorticoid receptor blocker canrenoate. Ca_v3.1^-/- and Ca_v3.2^-/- mice exhibited no baseline difference in MAP compared with WT mice, but day-night variation was blunted in both Ca_v3.1 and Ca_v3.2^-/- mice. ANG II increased significantly MAP in WT, Ca_v3.1^-/-, and Ca_v3.2^-/- mice with no differences between genotypes. Heart rate was significantly lower in Ca_v3.1^-/- and Ca_v3.2^-/- mice compared with control mice. After ANG II infusion, plasma aldosterone concentration was significantly lower in Ca_v3.1^-/- compared with Ca_v3.2^-/- mice. In response to ANG II, fibrosis was observed in heart sections from both WT and Ca_v3.1^-/- mice and while cardiac atrial natriuretic peptide mRNA was similar, the brain natriuretic peptide mRNA increase was mitigated in Ca_v3.1^-/- mice ANG II at 100 ng/kg yielded elevated pressure and an increased heart weight-to-body weight ratio in WT mice. Cardiac hypertrophy, but not hypertension, was prevented by the mineralocorticoid receptor blocker canrenoate. In conclusion, T-type channels Ca_v3.1and Ca_v3.2 do not contribute to baseline blood pressure levels and ANG II-induced hypertension. Ca_v3.1, but not Ca_v3.2, contributes to aldosterone secretion. Aldosterone promotes cardiac hypertrophy during hypertension.

Pharmacokinetics and drug-drug interaction between enalapril, enalaprilat and felodipine extended release (ER) in healthy subjects

Since angiotensin-converting enzyme (ACE) inhibitors and calcium antagonists have complimentary mechanisms of action, enalapril, an ACE inhibitor, is used in combination with felodipine, a vascular selective dihydropyridine calcium antagonist, for the treatment of hypertension. The present study was designed to investigate the possible drug-drug interaction between these two agents in Chinese healthy subjects. A randomized, open-label, multiple-dose, 3-treatment, 3-period, 6-sequence cross-over study enrolling 12 healthy subjects (six male and six female subjects) was performed. Plasma pharmacokinetic studies were performed after 5 mg of enalapril and 5 mg of felodipine were administered alone or concomitantly twice per day for six days, and once in the morning of day seven. All 12 healthy subjects (mean [SD] age, 24.3 [2.8] years; body weight, 57.3 [5.7] kg; height, 163.2 [5.2] cm) completed all scheduled pharmacokinetic studies. Geometric mean ratios (with 90% CIs) of AUC_τ,ss and C_max,ss for enalapril administered concomitantly with felodipine vs. enalapril administered alone were 1.025 (0.80-1.25) and 1.065 (0.70-1.43), respectively. Geometric mean ratios (with 90% CIs) of AUC_τ,ss and C_max,ss for felodipine administered concomitantly with enalapril vs. felodipine administered alone were 1.14 (0.97-1.31) and 0.80 (0.65-0.95), respectively. There were no severe or serious drug-related adverse events observed during the study. Our results revealed that the co-administration of enalapril and felodipine affected the pharmacokinetics of felodipine, but not that of enalapril. Although the difference in PK parameters was statistically significant, its clinical significance may be limited, considering safety profile observed in the present study.

Functional importance of T-type voltage-gated calcium channels in the cardiovascular and renal system: news from the world of knockout mice

Over the years, it has been discussed whether T-type calcium channels Cav3 play a role in the cardiovascular and renal system. T-type channels have been reported to play an important role in renal hemodynamics, contractility of resistance vessels, and pacemaker activity in the heart. However, the lack of highly specific blockers cast doubt on the conclusions. As new T-type channel antagonists are being designed, the roles of T-type channels in cardiovascular and renal pathology need to be elucidated before T-type blockers can be clinically useful. Two types of T-type channels, Cav3.1 and Cav3.2, are expressed in blood vessels, the kidney, and the heart. Studies with gene-deficient mice have provided a way to investigate the Cav3.1 and Cav3.2 channels and their role in the cardiovascular system. This review discusses the results from these knockout mice. Evaluation of the literature leads to the conclusion that Cav3.1 and Cav3.2 channels have important, but different, functions in mice. T-type Cav3.1 channels affect heart rate, whereas Cav3.2 channels are involved in cardiac hypertrophy. In the vascular system, Cav3.2 activation leads to dilation of blood vessels, whereas Cav3.1 channels are mainly suggested to affect constriction. The Cav3.1 channel is also involved in neointima formation following vascular damage. In the kidney, Cav3.1 regulates plasma flow and Cav3.2 plays a role setting glomerular filtration rate. In conclusion, Cav3.1 and Cav3.2 are new therapeutic targets in several cardiovascular pathologies, but the use of T-type blockers should be specifically directed to the disease and to the channel subtype.

Functional and pharmacological consequences of the distribution of voltage-gated calcium channels in the renal blood vessels

Calcium channel blockers are widely used to treat hypertension because they inhibit voltage-gated calcium channels that mediate transmembrane calcium influx in, for example, vascular smooth muscle and cardiomyocytes. The calcium channel family consists of several subfamilies, of which the L-type is usually associated with vascular contractility. However, the L-, T- and P-/Q-types of calcium channels are present in the renal vasculature and are differentially involved in controlling vascular contractility, thereby contributing to regulation of kidney function and blood pressure. In the preglomerular vascular bed, all the three channel families are present. However, the T-type channel is the only channel in cortical efferent arterioles which is in contrast to the juxtamedullary efferent arteriole, and that leads to diverse functional effects of L- and T-type channel inhibition. Furthermore, by different mechanisms, T-type channels may contribute to both constriction and dilation of the arterioles. Finally, P-/Q-type channels are involved in the regulation of human intrarenal arterial contractility. The calcium blockers used in the clinic affect not only L-type but also P-/Q- and T-type channels. Therefore, the distinct effect obtained by inhibiting a given subtype or set of channels under experimental settings should be considered when choosing a calcium blocker for treatment. T-type channels seem to be crucial for regulating the GFR and the filtration fraction. Use of blockers is expected to lead to preferential efferent vasodilation, reduction of glomerular pressure and proteinuria. Therefore, renovascular T-type channels might provide novel therapeutic targets, and may have superior renoprotective effects compared to conventional calcium blockers.

Benidipine reduces albuminuria and plasma aldosterone in mild-to-moderate stage chronic kidney disease with albuminuria

Benidipine inhibits both L- and T-type Ca channels, and has been shown to dilate the efferent arterioles as effectively as the afferent arterioles. In this study, we conducted an open-label and randomized trial to compare the effects of benidipine with those of amlodipine on blood pressure (BP), albuminuria and aldosterone concentration in hypertensive patients with mild-to-moderate stage chronic kidney disease (CKD). Patients with BP ≥ 130/80 mm Hg, with estimated glomerular filtration rate (eGFR) of 30-90 ml min(-1) per 1.73 m(2), and with albuminuria>30 mg per g creatinine (Cr), despite treatment with the maximum recommended dose of angiotensin II receptor blockers (ARBs) were randomly assigned to two groups. Patients received either of the following two treatment regimens: 2 mg per day benidipine, which was increased up to a dose of 8 mg per day (n=52), or 2.5 mg per day amlodipine, which was increased up to a dose of 10  mg per day (n=52). After 6 months of treatment, a significant and comparable reduction in the systolic and diastolic BP was observed in both groups. The decrease in the urinary albumin to Cr ratio in the benidipine group was significantly lower than that in the amlodipine group. Although plasma renin activity was not different in the two groups, plasma aldosterone levels were significantly decreased in the benidipine group. Moreover, urinary Na/K ratio was significantly decreased in the benidipine group but remained unchanged in the serum. It may be concluded that benidipine results in a greater reduction of plasma aldosterone and albuminuria than amlodipine, and that these effects are independent of BP reduction.

Combination therapy with irbesartan and efonidipine for attenuation of proteinuria in Dahl salt-sensitive rats

Angiotensin receptor blockers (ARBs) or T- and L-type calcium channel blockers (CCBs) are useful for glomerular protection; however, the protective effects of combination therapy remain unclear. In this study, Dahl salt-sensitive rats were fed a high-salt diet and were treated daily with placebo, irbesartan (60 mg kg(-1)), efonidipine (30 mg kg(-1)), irbesartan (60 mg kg(-1))+efonidipine (30 mg kg(-1)), amlodipine (3 mg kg(-1)), or irbesartan (60 mg kg(-1))+amlodipine (3 mg kg(-1)) for 4 weeks. Significant reductions in systolic blood pressure were seen in the irbesartan-, efonidipine- and amlodipine-treated groups compared with the placebo-treated group; a further significant reduction was seen in the irbesartan+efonidipine-treated group compared with the irbesartan-treated group. Compared with the placebo-treated group, proteinuria was significantly lower in the irbesartan- and efonidipine-treated groups, but not in the amlodipine-treated group. Furthermore, a significant attenuation of proteinuria in the irbesartan+efonidipine-treated group compared with the irbesartan-treated group was observed; this effect was not observed in the irbesartan+amlodipine-treated group. The glomerulosclerosis index was significantly attenuated by all active treatments except amlodipine. The glomerulosclerosis index in the irbesartan+efonidipine-treated group, but not in the irbesartan+amlodipine-treated group, was significantly lower than that in the irbesartan-treated group. Significant attenuations of gene expressions of p22(phox), transforming growth factor-beta, monocyte chemoattractant protein-1 and collegen I were observed in the irbesartan- and efonidipine-treated groups, but not in the amlodipine-treated group. Values for these parameters were reduced to control levels in the irbesartan+efonidipine-treated group. Combination therapy with ARB and T- and L-type CCB might produce a powerful renal protective effect.

The rationale and design of the antihypertensives and vascular, endothelial, and cognitive function (AVEC) trial in elderly hypertensives with early cognitive impairment: role of the renin angiotensin system inhibition

BACKGROUND

Prior evidence suggests that the renin angiotensin system and antihypertensives that inhibit this system play a role in cognitive, central vascular, and endothelial function. Our objective is to conduct a double-blind randomized controlled clinical trial, the antihypertensives and vascular, endothelial, and cognitive function (AVEC), to compare 1 year treatment of 3 antihypertensives (lisinopril, candesartan, or hydrochlorothiazide) in their effect on memory and executive function, cerebral blood flow, and central endothelial function of seniors with hypertension and early objective evidence of executive or memory impairments.

METHODS/DESIGN

The overall experimental design of the AVEC trial is a 3-arm double blind randomized controlled clinical trial. A total of 100 community eligible individuals (60 years or older) with hypertension and early cognitive impairment are being recruited from the greater Boston area and randomized to lisinopril, candesartan, or hydrochlorothiazide ("active control") for 12 months. The goal of the intervention is to achieve blood pressure control defined as SBP < 140 mm Hg and DBP < 90 mm Hg. Additional antihypertensives are added to achieve this goal if needed. Eligible participants are those with hypertension, defined as a blood pressure 140/90 mm Hg or greater, early cognitive impairment without dementia defined (10 or less out of 15 on the executive clock draw test or 1 standard deviation below the mean on the immediate memory subtest of the repeatable battery for the assessment of neuropsychological status and Mini-Mental-Status-exam >20 and without clinical diagnosis of dementia or Alzheimer's disease). Individuals who are currently receiving antihypertensives are eligible to participate if the participants and the primary care providers are willing to taper their antihypertensives. Participants undergo cognitive assessment, measurements of cerebral blood flow using Transcranial Doppler, and central endothelial function by measuring changes in cerebral blood flow in response to changes in end tidal carbon dioxide at baseline (off antihypertensives), 6, and 12 months. Our outcomes are change in cognitive function score (executive and memory), cerebral blood flow, and carbon dioxide cerebral vasoreactivity.

DISCUSSION

The AVEC trial is the first study to explore impact of antihypertensives in those who are showing early evidence of cognitive difficulties that did not reach the threshold of dementia. Success of this trial will offer new therapeutic application of antihypertensives that inhibit the renin angiotensin system and new insights in the role of this system in aging.

TRIAL REGISTRATION

Clinicaltrials.gov NCT00605072.

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.

Comparison of the effects of efonidipine and amlodipine on aldosterone in patients with hypertension

To prevent cardiovascular disease, targeting aldosterone synthesis and release may be clinically important. Aldosterone production in the adrenal gland is mediated mainly by the T-type calcium channel in vitro. Efonidipine inhibits both L- and T-type Ca channels. To compare the effects of efonidipine on neurohumoral factors with those of amlodipine, an L-type Ca channel blocker, we studied 40 essential hypertensive outpatients. Forty patients who had been administered amlodipine for more than 1 year were treated with efonidipine for 6 months in place of amlodipine. Substituting efonidipine for amlodipine had no significant effect on clinic systolic blood pressure or the plasma levels of brain natriuretic peptide, norepinephrine or active renin. However, the heart rate was significantly decreased (72.0+/-1.3 vs. 69.8+/-1.3 beats/min, p<0.01) and the plasma aldosterone level was also significantly decreased after efonidipine treatment (97.7+/-7.9 vs. 79.7+/-5.6 pg/mL, p<0.0001). Changes in the aldosterone level correlated with the baseline value before the replacement of amlodipine by efonidipine (r=-0.769, p<0.0001). These findings indicate that at the effective antihypertensive doses of efonidipine and amlodipine, efonidipine significantly decreases heart rate and plasma aldosterone level compared with those under amlodipine treatment in hypertensive patients.

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.

New Generation Calcium Channel Blockers in Hypertensive Treatment

During a couple of decades, a number of antihypertensive drugs have been developed, and the choice of hypertension treatment has been expanded. Among antihypertensive drugs, calcium channel blockers, which inhibit L-type voltage-gated calcium channels, are potent vasodilators, and have been used as a first- or second-line drug. Dihydropyridine-class calcium channel blockers are categorized into three generations according to the length of activity, and long-acting calcium channel blockers cause less activation of sympathetic nervous system, and are reported to offer beneficial action compared with short-action agents. Furthermore, novel types of calcium channel blockers have been developed that possess the blocking action on other calcium channel subtypes (T- and N-type), and exert agent-specific action apart from their class effects, such as the effects on heart rate and renin/aldosterone release. These additional benefits conferred by T/N-type calcium channel blockade are anticipated to provide organ protective actions in the treatment of hypertension, in addition to the blood pressure-lowering effect of L-type calcium channel blockade. In conclusion, novel calcium channel blockers with sustained activity and T/N-type calcium channel blocking action could provide more beneficial effects than classical blockers, and may expand the clinical utility of these agents.

Calcium Homeostasis in Human Placenta: Role of Calcium‐Handling Proteins

The human placenta is a transitory organ, representing during pregnancy the unique connection between the mother and her fetus. The syncytiotrophoblast represents the specialized unit in the placenta that is directly involved in fetal nutrition, mainly involving essential nutrients, such as lipids, amino acids, and calcium. This ion is of particular interest since it is actively transported by the placenta throughout pregnancy and is associated with many roles during intrauterine life. At term, the human fetus has accumulated about 25-30 g of calcium. This transfer allows adequate fetal growth and development, since calcium is vital for fetal skeleton mineralization and many cellular functions, such as signal transduction, neurotransmitter release, and cellular growth. Thus, there are many proteins involved in calcium homeostasis in the human placenta.

Pathophysiological significance of T-type Ca2+ channels: role of T-type Ca2+ channels in renal microcirculation

Since conventional Ca(2+) antagonists, with predominant blockade of L-type voltage-dependent Ca(2+) channels, elicit preferential dilation of afferent arterioles, they might ostensibly aggravate glomerular hypertension. Recently, novel Ca(2+) antagonists, with inhibitory action on L-/T-type Ca(2+) channels, have been reported to dilate both afferent and efferent arterioles. The present review attempted to characterize the renal action of these Ca(2+) antagonists and evaluated the consequences following the treatment with these agents. In contrast to conventional Ca(2+) antagonists (e.g., nifedipine), novel antagonists (e.g., benidipine, efonidipine) potently dilated afferent and efferent arterioles; their action on efferent arterioles appeared to be mediated by the T-type Ca(2+) channel blockade, probably through the inhibition of the intracellular Ca(2+) release. The comparison of the anti-proteinuric action in subtotally nephrectomized rats showed that efonidipine exerted more prominent action than nifedipine. Furthermore, Ca(2+) antagonists with T-type Ca(2+) inhibitory action inhibited renin/aldosterone release and proinflammatory process. Finally, patients with chronic renal disease given a 48-week efonidipine treatment showed reduced proteinuria, and this effect was seen even when mean arterial blood pressure failed to become less than 100 mmHg. Collectively, T-type Ca(2+) channel blockade provides beneficial action in renal injury. Various mechanisms serve to protect against renal injury, including systemic/glomerular hemodynamic action and non-hemodynamic mechanisms.

T-type calcium channel trigger p21ras signaling pathway to ERK in Cav3.1-expressed HEK293 cells

We constructed a new cell line which stably expressed Cav3.1 and Kir2.1 subunits in HEK293 cells (HEK293/Cav3.1/Kir2.1) in order to investigate the unknown cellular signaling pathways of T-type voltage-dependent calcium channels. The new cell line has a stable resting membrane potential and can activate T-type Ca(2+) channels by KCl-mediated depolarization. We showed that Cav3.1 activation resulted in the level of p21(ras)-GTP in the cells being rapidly decreased during the first 2 min, and then recovering between 2 min and 15 min. The kinetics of ERK activation following Cav3.1 stimulation was also investigated. ERK activation was decreased from 2 min to 5 min after KCl stimulation, which means that Cav3.1 activation reduced ERK activity in the very early stages of activation. In addition, similar results for Cav3.1 activation were also shown in the case of Sos1, Grb2, and Shc, which means that Cav3.1 activation triggers p21(ras) and that this signal is transferred to ERK by Sos1, Grb2, and Shc.

Pathophysiological Significance of T-type Ca2+ Channels: T-type Ca2+ Channels and Drug Development

T-type Ca(2+) channels are present in cardiovascular, neuronal, and endocrine systems; and they are now receiving attention as novel therapeutic targets. Many drugs and compounds non-specificaly block T-type Ca(2+) channels. Certain dihydropyridine compounds, such as efonidipine, have blocking activity on both L-type and T-type Ca(2+) channels which possibly underlies their excellent clinical profiles such as minimum reflex tachycardia and renal protection. Selective inhibitors of T-type Ca(2+) channels, such as non-hydrolyzable mibefradil and R(-)-efonidipine, are powerful pharmacological tools for further studies and may lead to the development of novel therapeutic strategies.

Cellular mechanism of renin release

In this review we aim to give a comprehensive overview over the current knowledge of the cellular control of renin release. We hereby focus on the inhibitory effects of calcium on the exocytosis of renin. After a short introduction into general aspects of the regulation of renin release, including a brief summary on the role of the second messengers cAMP and cGMP, we will discuss parts of the literature on the effects of calcium on the renin system together with recent studies from our laboratory, investigating putative calcium influx and extrusion pathways of juxtaglomerular cells. Finally, as the precise mechanisms by which calcium inhibits the exocytosis of renin are far from being understood, we will present some hypotheses on the intracellular events being involved in the suppression of renin release by calcium.

Ambulatory blood pressure monitoring and active renin in menopausal women treated with amlodipine and hormone replacement therapy

The aim of this study was to follow up the effect of an 8-week treatment with amlodipine given alone or in combination with hormone replacement therapy (HRT) on blood pressure and active renin in postmenopausal women with mild to moderate arterial hypertension using both conventional clinical blood pressure measurements and ambulatory blood pressure monitoring. Twenty-nine hypertensive menopausal women were divided randomly into two groups according to the treatment regimens: amlodipine and amlodipine plus HRT. The combination with HRT led to normalization of 24-h and daytime systolic and diastolic blood pressure. In contrast to the group treated with amlodipine alone, where a significant fall only of systolic night-time blood pressure was observed, in the group treated with amlodipine plus HRT both systolic and diastolic night-time blood pressure decreased significantly. Active renin did not change significantly after treatment in both groups. Triglycerides decreased significantly and high-density lipoprotein-cholesterol increased significantly only after amlodipine treatment. There were no significant differences in serum total cholesterol and low-density lipoprotein-cholesterol after HRT plus amlodipine. In conclusion, amlodipine is effective in reducing blood pressure in postmenopausal women. The maintenance of a normal circadian blood pressure pattern was influenced by HRT.

Amlodipine at high dose increases preproendothelin-1 expression in the ventricles and aorta of normotensive rats

BACKGROUND

High doses of dihydropyridine calcium channel blockers can activate the sympathetic nervous system and the renin-angiotensin system. Both noradrenaline and angiotensin II stimulate preproendothelin-1 gene expression, yet the effects of high doses of dihydropyridines on preproendothelin-1 expression in vivo remain unknown.

OBJECTIVES

To investigate the effects of high doses of dihydropyridines on preproendothelin-1 expression in the ventricles and aorta of normotensive rats.

METHODS

Sprague-Dawley rats were treated with amlodipine 5 or 20 mg/kg per day (Amlo 5 or Amlo 20) in drinking water for 5 days or 5 weeks. Systolic blood pressure and heart rate were measured by tail-cuff plethysmography. Gene expression was examined by reverse transcriptase polymerase chain reaction.

RESULTS

Amlo 5 increased heart rate during the first week only and had no effect on blood pressure and ventricular weight and gene expression. Amlo 20 reduced blood pressure transiently and increased heart rate consistently. It did not change relative left ventricular weight (corrected for body weight) after 5 days, but increased it after 5 weeks; it increased relative right ventricular weight at both time points. Aorta weight (mg/mm) was decreased after 5 weeks of treatment with both dosages of amlodipine. Preproendothelin-1 mRNA levels were increased by Amlo 20 in the ventricles and aorta and, concomitantly, renin mRNA was increased in the kidney. Less consistently, interleukin-6 mRNA also increased in ventricles, whereas cardiotrophin-1 mRNA remained unchanged. The sensitivity of isolated aorta to the contractile effect of noradrenaline was decreased by Amlo 5, but not by Amlo 20.

CONCLUSIONS

In Sprague-Dawley rats, high-dose amlodipine, while promoting neurohormonal activation, induced overexpression of preproendothelin-1 mRNA in the ventricles and aorta. Endothelin-1 overexpression could contribute to the lack of inhibitory effect of high-dose amlodipine on ventricular mass in normotensive rats.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Efonidipine hydrochloride: a dual blocker of L- and T-type ca(2+) channels

T-type Ca(2+) channels have properties different from those of the L-type and are involved in cardiac pacemaking and regulation of blood flow, but not in myocardial contraction. Efonidipine is an antihypertensive and antianginal drug with dihydropyridine structure that was recently found to block both L- and T-type Ca(2+) channels. In isolated myocardial and vascular preparations, efonidipine has potent negative chronotropic and vasodilator effects but only a weak negative inotropic effect. In experimental animals and patients, reduction of blood pressure by the drug was accompanied by no or minimum reflex tachycardia leading to improvement of myocardial oxygen balance and maintenance of cardiac output. Efonidipine increased glomerular filtration rate without increasing intraglomerular pressure. By relaxing both the afferent and efferent arterioles, efonidipine markedly reduced proteinuria. Thus, efonidipine, an L- and T-type dual Ca(2+) channel blocker, appears to have an ideal profile as an antihypertensive and antianginal drug with organ-protective effects in the heart and kidney.

Comparison of L-type and mixed L- and T-type calcium channel blockers on kidney injury caused by deoxycorticosterone-salt hypertension in rats

The efficiency of calcium channel blockers (CCBs) in the treatment of chronic renal disease (CRD) is controversial. In this study, we investigated whether combined T- and L-type CCBs, using mibefradil (30 mg/kg/d), provided superior protection versus traditional L-type voltage-gated CCBs, using amlodipine (10 mg/kg/d), in the deoxycorticosterone acetate (DOCA)-salt model of high glomerular blood pressure (P(GC)) and rapidly developing kidney damage. After 4 to 5 weeks of DOCA-salt, amlodipine did not reduce proteinuria (protein, 341 +/- 90 versus 482 +/- 54 mg/24 h; P = not significant) or degree of glomerular damage (20% +/- 4% versus 28% +/- 6% damaged glomeruli; P = not significant) compared with untreated rats. Conversely, mibefradil reduced proteinuria and glomerular damage versus untreated DOCA-salt rats (protein, 244 +/- 75 mg/24 h; P < 0.02; damaged glomeruli, 11% +/- 3%; P < 0.05). Both CCBs had similar antihypertensive actions, returning blood pressure to the untreated sham value. Of note, P(GC) also was reduced by a similar extent (and to the sham value) with both mibefradil (58 +/- 2 mm Hg; P < 0.001) and amlodipine (61 +/- 2 mm Hg; P < 0.005) versus untreated DOCA-salt rats (70 +/- 1 mm Hg). This study shows that combined T- and L-type CCBs provide superior protection against CRD in the DOCA-salt model compared with L-type CCBs alone. However, this protection was not hemodynamic because similar systemic and glomerular antihypertensive responses occurred with both mibefradil and amlodipine. Although mibefradil was withdrawn from the market because of adverse drug interactions not associated with CCBs, other mixed channel blockers may provide an alternative or adjunctive therapy to angiotensin-converting enzyme inhibition in CRD.

Divergent renal vasodilator action of L- and T-type calcium antagonists in vivo

OBJECTIVE

To assess the in-vivo action on the renal microvasculature of the calcium antagonists nifedipine (L-type blocker), efonidipine (L/T-type blocker), and mibefradil (predominant T-type blocker).

DESIGN

An intravital needle-type charge-coupled device (CCD) camera videomicroscope was introduced to visualize the renal microcirculation directly in vivo.

METHODS

In anesthetized mongrel dogs, nifedipine (0.01-1 mg/kg per min), efonidipine (0.033-0.33 mg/kg per min), or mibefradil (0.01-1 mg/kg per min) was infused intravenously after the insertion of a CCD probe into the kidney. Renal microvascular responses to calcium antagonists were directly evaluated, with concomitant observation of renal clearance.

RESULTS

Each calcium antagonist caused modest vasodepressor action without affecting heart rate. Nifedipine (1 mg/kg per min, n = 9) increased renal plasma flow (RPF) (14 +/- 4%, P < 0.05) and glomerular filtration rate (GFR) (19 +/- 5%, P < 0.05), and tended to increase the filtration fraction (5 +/- 2% increment, P = 0.07). Efonidipine (0.33 mg/kg per min, n = 9), however, had no effect on filtration fraction, with 14 +/- 6% increments in RPF (P < 0.05) and 14 +/- 7% increments in GFR (P = 0.08). Rather, mibefradil (1 mg/kg per min, n = 9) elicited 6 +/- 2% decreases in filtration fraction (P < 0.05), with slight increments in RPF (6 +/- 3%) and no changes in GFR. In direct in-vivo microvasculature observations, nifedipine caused predominant (22 +/- 2%) dilatation of afferent arterioles (from 15.5 +/- 0.4 to 18.9 +/- 0.4 microm, n = 5), compared with that of efferent arterioles (10 +/- 2%; from 11.0 +/- 0.4 to 12.1 +/- 0.3 microm). In contrast, efonidipine caused a similar magnitude of vasodilatation (16 +/- 4%) compared with 18 +/- 2%; n = 6), and mibefradil caused greater dilatation of efferent arterioles (20 +/- 4%, n = 7) than that of afferent arterioles (13 +/- 4%).

CONCLUSIONS

There exists marked heterogeneity in action of nifedipine, efonidipine and mibefradil on the renal microvascular in canine kidneys in vivo. Furthermore, our current observations suggest an important contribution of T-type calcium channel activity to efferent arteriolar tone in vivo.

Cardiovascular risk, renal hypertensive damage, and effects of amlodipine treatment in transgenic TGR(mREN2)27 rats

Transgenic rats (TGRs) TGR(mREN2)27 are characterized by fulminant hypertension, an inverse circadian blood pressure rhythm, and severe hypertensive target organ damage. In the present study, we evaluated cardiovascular risk factors, renal function, and urinary protein loss in transgenic rats before and after treatment with the calcium channel blocker amlodipine. Amlodipine was injected intraperitoneally in a dose of 5 mg/kg/day, either once daily at 8.00 h or twice daily in divided doses at 8.00 and 20.00 h. Untreated TGRs and Sprague-Dawley rats served as hypertensive and normotensive controls, respectively. Before and after 5 weeks of treatment, rats were placed in metabolic cages for sampling of urine. Prior to treatment, urinary excretion rates of protein, albumin, and Ca2+ were significantly higher in TGRs than in Sprague-Dawley controls. Urinary excretion of protein and albumin was reduced by 5 weeks of amlodipine treatment, whereas the excretion of Ca2+ was not affected. The reductions in renal proteinuria and albuminuria by amlodipine treatment were significantly correlated with the treatment-induced decrease in blood pressure. These findings indicate that blood pressure itself is an important contributor to albumin loss by the kidney in renin-dependent hypertension of TGRs.

Role of renal nerves in the stimulation of the renin system by reduced renal arterial pressure

The aim of this study was to determine the role of renal innervation in the prolonged stimulation of renin secretion and renin synthesis accompanying renal artery stenosis. Male Sprague-Dawley rats, in which the left kidney had been denervated or sham denervated 4 days earlier, received a left renal artery clip (ID 0.2 mm). Plasma renin activity and renin mRNA were assayed 1, 2, or 4 days after clipping. The stimulation of both plasma renin activity and renin mRNA was blunted markedly in the rats with the denervated clipped kidney. The typical suppression of renin mRNA in the intact right kidney, however, was not different between rats with sham-denervated or denervated left kidneys, nor was the increase of blood pressure in response to renal artery clipping different between the experimental groups. To test whether the suppression of renin mRNA in the contralateral kidney was related to the increase of blood pressure, another group of rats with denervated clipped left kidneys was treated additionally with the T-type calcium channel blocker mibefradil (15 mg. kg(-1). d(-1)). Despite blood pressure normalization by mibefradil, plasma renin activities and renin mRNA levels in the clipped denervated kidneys and in the intact right kidneys remained unchanged. These findings suggest that renal nerves are responsible for marked background stimulation of both renin secretion and renin mRNA expression, which is normally masked by the inhibitory effect of renal perfusion pressure on the renin system. Renal nerve activity is therefore an important determinant of the gain of renin stimulation during reduced renal arterial pressure.

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.

Contrasting effects of selective T- and L-type calcium channel blockade on glomerular damage in DOCA hypertensive rats

Mibefradil and amlodipine are calcium antagonists with different channel selectivities. Mibefradil blocks both L- and T-type calcium channels; although in the usual pharmacological doses, it predominantly blocks the T-type channels. In contrast, amlodipine selectively blocks L-type channels. The goal of the present study was to assess whether this differential selectivity would result in different effects on end-organ damage in experimental hypertension. For this purpose, deoxycorticosterone acetate (DOCA)-salt hypertensive rats were treated either with equipotent doses of mibefradil or amlodipine (30 mg. kg(-1). d(-1) as food admix). Despite the fact that both drugs decreased systolic arterial pressure to the same extent (140+/-5 mm Hg in the mibefradil group and 144+/-3 mm Hg in the amlodipine group versus 225+/-5 mm Hg in the untreated-DOCA group), only mibefradil decreased proteinuria (35. 5+/-6.5 versus 103.3+/-14.1 mg/24 h in untreated DOCA-salt animals) and prevented glomerular lesions. Both drugs, however, prevented the occurrence of vascular renal lesions. To elucidate the mechanism responsible for this difference, we evaluated in an additional series of experiments the effects of mibefradil and amlodipine on plasma and renal renin concentrations, as well as the effects of the addition of enalapril, an ACE inhibitor, given on top of both drugs on proteinuria. Amlodipine, in contrast to mibefradil, markedly stimulated the plasma (17.8+/-2.6 ng Ang I. mL(-1). h(-1) in the amlodipine group versus 3.9+/-0.4 ng Ang I. mL(-1). h(-1) in the mibefradil group and 3.2+/-0.3 ng Ang I. mL(-1). h(-1) in the untreated-DOCA group) and renal (2.42+/-0.37 ng Ang I. mL(-1). h(-1) in the amlodipine group versus 0.36+/-0.04 ng Ang I. mL(-1). h(-1) in the mibefradil group and 0.26+/-0.08 ng Ang I. mL(-1). h(-1) in the untreated-DOCA group) renin concentrations. Stimulation of the renin-angiotensin system could explain the absence of a renal protective effect of amlodipine. This was also suggested by the fact that enalapril given in addition to amlodipine could decrease proteinuria. In conclusion, T-type channel blockade by mibefradil decreases blood pressure without stimulation of the renin-angiotensin system and therefore prevents most of the glomerular damage in DOCA hypertensive rats.