Frequency-dependent blockade of T-type Ca2+ current by efonidipine in cardiomyocytes

Voltage-gated calcium channel antagonists and traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of death and disability in the United States. Despite more than 30 years of research, no pharmacological agents have been identified that improve neurological function following TBI. However, several lines of research described in this review provide support for further development of voltage gated calcium channel (VGCC) antagonists as potential therapeutic agents. Following TBI, neurons and astrocytes experience a rapid and sometimes enduring increase in intracellular calcium ([Ca²⁺]_i). These fluxes in [Ca²⁺]_i drive not only apoptotic and necrotic cell death, but also can lead to long-term cell dysfunction in surviving cells. In a limited number of in vitro experiments, both L-type and N-type VGCC antagonists successfully reduced calcium loads as well as neuronal and astrocytic cell death following mechanical injury. In rodent models of TBI, administration of VGCC antagonists reduced cell death and improved cognitive function. It is clear that there is a critical need to find effective therapeutics and rational drug delivery strategies for the management and treatment of TBI, and we believe that further investigation of VGCC antagonists should be pursued before ruling out the possibility of successful translation to the clinic.

Mechanism underlying the block of human Cav3.2 T-type Ca2+ channels by benidipine, a dihydropyridine Ca2+ channel blocker

AIMS

Benidipine, a dihydropyridine Ca(2+) channel blocker, has been reported to block T-type Ca(2+) channels; however, the mechanism underlying this effect was unclear. In this study, we characterized the mechanism responsible for this blocking activity. Furthermore, the blocking activity was compared between two enantiomers of benidipine, (S, S)- and (R, R)-benidipine.

MAIN METHODS

Human Ca(v)3.2 (hCa(v)3.2) T-type Ca(2+) channels stably expressed in the human embryonic kidney cell line, HEK-293, were studied in whole-cell patch-clamp recordings and Ca(2+) mobilization assay.

KEY FINDINGS

In whole-cell patch-clamp recordings, benidipine blocked hCa(v)3.2 T-type Ca(2+) currents elicited by depolarization to a comparable extent as efonidipine. The block was dependent on stimulation frequency and holding potential, but not test potential. Benidipine significantly shifted the steady-state inactivation curve to the hyperpolarizing direction, but had no effect on the activation curve. Benidipine prolonged the recovery from inactivation of hCa(v)3.2 T-type Ca(2+) channels without any effect on the kinetics of activation, inactivation, or deactivation. In the Ca(2+) mobilization assay, benidipine was more potent than efonidipine in blocking Ca(2+) influx through hCa(v)3.2 T-type Ca(2+) channels. (S, S)-Benidipine was more potent than (R, R)-benidipine in blocking hCa(v)3.2 T-type Ca(2+) currents, but there was no difference in blocking the Ca(2+) influx.

SIGNIFICANCE

We have characterized the blocking activity of benidipine against hCa(v)3.2 Ca(2+) channels and revealed the difference between the two enantiomers of benidipine. The blocking action of benidipine could be mediated by stabilizing hCa(v)3.2 Ca(2+) channels in an inactivated state.

Effects of enfonidipine hydrochloride in dogs with experimental supraventricular tachyarrhythmia

It is required not to increase the ventricular rate and to preserve the ventricular systolic function in treating supraventricular tachyarrhythmia (SVTA). The objective of this study is to investigate whether or not Efonidipine hydrochloride (EH), a T and L dual type Ca(2+) channel blocker, suppresses the increasing ventricular rate without reducing the ventricular systolic function using canine SVTA models by rapid atrial pacing (RAP) method. Clinically healthy fourteen beagles were used. The 14 dogs were randomly assigned to the EH-administered group (EH group, n=7) and non-EH-administered group (control group, n=7). The EH group was orally-administered EH at 5 mg/kg SID during RAP. On the other hand, the control group was applied RAP without oral administration of EH. Duration of RAP was for 3 weeks for both groups. The ventricular rate for the EH group was significantly lower than that for the control group. The left ventricular- fractional shortening for the control group declined significantly compared to baseline. Those for the EH group did not show any changes over time and were significantly higher than the control group. The ratio between pre-ejection period and ejection for the EH group were significantly lower than those of the control group. In conclusion, the study demonstrated that EH suppresses the increasing ventricular rate without reducing the ventricular systolic function in canine SVTA model. Therefore, EH is expected to become a new treatment for canine SVTA.

Development of newer calcium channel antagonists: therapeutic potential of efonidipine in preventing electrical remodelling during atrial fibrillation

Calcium channel antagonists are most frequently prescribed for the treatment of hypertension and the majority specifically inhibit the L-type Ca2+ channel. In order to prevent reflex sympathetic over activity caused by L-type calcium channel antagonists (calcium channel blockers [CCBs]), increasing attention has focused on the blockade of the T-type Ca2+ channel. The T-type Ca2+ channel is found in the kidney and can also appear in the ventricle of the heart when in failure. Therefore, the T-type Ca2+ channel is a possible new target for the treatment of nephropathy and heart failure. In clinical trials, the efficacy and safety of T-type CCBs in hypertension and chronic renal disease have been reported. It is well known that the T-type Ca2+ channel is present in the adult atrium and plays a role in the cardiac pacemaker, but recent experimental studies suggest that this current also promotes electrical remodelling of the atrium. Using efonidipine, a dual L- and T-type CCB, it has been demonstrated that atrial electrical remodelling can be diminished in dogs. Furthermore, the T-type Ca2+ channel has recently been found in the pulmonary veins, contributing to the pulmonary vein pacemaker activity and triggered activity. A variety of drugs having T-type CCB effects have been shown to be effective in the management of atrial fibrillation, suggesting that this channel may be a novel therapeutic target.

Is there a role for T-type Ca2+ channels in regulation of vasomotor tone in mesenteric arterioles?

The largest peripheral blood pressure drop occurs in terminal arterioles (<40 microm lumen diameter). L-type voltage-dependent Ca2+ channels (VDCCs) are considered the primary pathway for Ca2+ influx during physiologic activation of vascular smooth muscle cells (VSMC). Recent evidence suggests that T-type VDCCs are expressed in renal afferent and efferent arterioles, mesenteric arterioles, and skeletal muscle arterioles. T-type channels are small-conductance, low voltage-activated, fast-inactivating channels. Thus, their role in supplying Ca2+ for contraction of VSMC has been disputed. However, T-type channels display non-inactivating window currents, which may play a role in sustained Ca2+ entry. Here, we review the possible role of T-type channels in vasomotor tone regulation in rat mesenteric terminal arterioles. The CaV3.1 channel was immunolocalized in VSMC, whereas the CaV3.2 channel was predominantly expressed in endothelial cells. Voltage-dependent Ca2+ entry was inhibited by the new specific T-type blockers R(-)-efonidipine and NNC 55-0396. The effect of NNC 55-0396 persisted in depolarized arterioles, suggesting an unusually high activation threshold of mesenteric T-type channels. T-type channels were not necessary for conduction of vasoconstriction, but appear to be important for local electromechanical coupling in VSMC. The first direct demonstration of endothelial T-type channels warrants new investigations of their role in vascular biology.

Short- and Long-Term Amiodarone Treatments Regulate Cav3.2 Low-Voltage-Activated T-type Ca2+ Channel through Distinct Mechanisms

Low-voltage-activated T-type Ca2+ channels have been recognized recently in the mechanisms underlying atrial arrhythmias. However, the pharmacological effects of amiodarone on the T-type Ca2+ channel remain unclear. We investigated short- and long-term effects of amiodarone on the T-type (Cav 3.2) Ca2+ channel. The Cav3.2 alpha1H subunit derived from human heart was stably transfected into cells [human embryonic kidney (HEK)-Cav3.2] cultured with or without 5 muM amiodarone. Patch-clamp recordings in the conventional whole-cell configuration were used to evaluate the actions of amiodarone on the T-type Ca2+ channel current (ICa.T). Amiodarone blockade of ICa.T occurred in a dose- and holding potential-dependent manner, shifting the activation and the steady-state inactivation curves in the hyperpolarization direction, when amiodarone was applied immediately to the bath solution. However, when the HEK-Cav3.2 cells were incubated with 5 microM amiodarone for 72 h, ICa.T density was significantly decreased by 31.7+/-2.3% for control,-93.1+/-4.3 pA/pF (n=8), versus amiodarone,-56.5+/-3.2 pA/pF (n=13), P<0.001. After the prolonged administration of amiodarone, the activation and the steady-state inactivation curves were shifted in the depolarization direction by -7.1 (n=41) and -5.5 mV (n=37), respectively, and current inactivation was significantly delayed [time constant (tau): control, 13.3+/-1.1 ms (n=6) versus amiodarone, 39.6+/-5.5 ms (n=6) at -30 mV, P<0.001)]. Nevertheless, short-term inhibitory effects of amiodarone on the modified T-type Cav3.2 Ca2+ channel created by long-term amiodarone treatment were functionally maintained. We conclude that amiodarone exerts its short- and long-term inhibitory actions on ICa.T via distinct blocking mechanisms.

Actions of Mibefradil, Efonidipine and Nifedipine Block of Recombinant T- and L-Type Ca2+ Channels with Distinct Inhibitory Mechanisms

We compared detailed efficacy of efonidipine and nifedipine, dihydropyridine analogues, and mibefradil using recombinant T- and L-type Ca2+ channels expressed separately in mammalian cells. All these Ca2+ channel antagonists blocked T-type Ca2+ channel currents (I(Ca(T))) with distinct blocking manners: I(Ca(T)) was blocked mainly by a tonic manner by nifedipine, by a use-dependent manner by mibefradil, and by a combination of both manners by efonidipine. IC50s of these Ca2+ channel antagonists to I(Ca(T)) and L-type Ca2+ channel current (I(Ca(L))) were 1.2 micromol/l and 0.14 nmol/l for nifedipine; 0.87 and 1.4 micromol/l for mibefradil, and 0.35 micromol/l and 1.8 nmol/l for efonidipine, respectively. Efonidipine, a dihydropyridine analogue, showed high affinity to T-type Ca2+ channel.

Inhibitory Effect of Efonidipine on Aldosterone Synthesis and Secretion in Human Adrenocarcinoma (H295R) Cells

Targeting aldosterone synthesis and/or release represents a potentially useful approach to the prevention of cardiovascular disease. Aldosterone production is stimulated by angiotensin II (Ang II) or extracellular K+ and is mediated mainly by Ca2+ influx into adrenal glomerulosa cells through T-type calcium channels. We therefore examined the effects of efonidipine, a dual T-type/L-type Ca2+ channel blocker, on aldosterone secretion in the H295R human adrenocarcinoma cell line; 100 nmol/L Ang II and 10 mmol/L K+ respectively increased aldosterone secretion from H295R cells 12-fold and 9-fold over baseline. Efonidipine dose-dependently inhibited both Ang II- and K+-induced aldosterone secretion, and nifedipine, an L-type Ca2+ channel blocker, and mibefradil, a relatively selective T-type channel blocker, similarly inhibited Ang II- and K+-induced aldosterone secretion, but were much less potent than efonidipine. Efonidipine also lowered cortisol secretion most potently among these drugs. Notably, efonidipine and mibefradil also significantly suppressed Ang II- and K+-induced mRNA expression of 11-beta-hydroxylase and aldosterone synthase, which catalyze the final two steps in the aldosterone synthesis, whereas nifedipine reduced only K+-induced enzyme expression. These findings suggest that efonidipine acts via T-type Ca2+ channel blockade to significantly reduce aldosterone secretion, and that this effect is mediated, at least in part, by suppression of 11-beta-hydroxylase and aldosterone synthase expression.

A comparison between calcium channel blocking drugs with different potencies for T- and L-type channels in preventing atrial electrical remodeling

Calcium overload plays a key role in the development of atrial electrical remodeling. The effect of an L-type Ca channel blocker in preventing this remodeling has been reported to be short lasting, partly due to down-regulation of this channel and persisting Ca entry through the T-type Ca channel. To prove if efonidipine, a dual L- and T-type Ca channel blocker exerts a greater effect than an L-type Ca channel blocker verapamil, 21 dogs underwent rapid atrial pacing at 400 bpm for 14 days, pretreatment with efonidipine in 7 (E), verapamil in 7 (V), and none in 7 (C). We measured the atrial effective refractory period (ERP) serially during 14 days of rapid pacing. In response to rapid pacing, ERP decreased progressively in C. In contrast, in E and V, ERP remained greater than ERP in C (P < 0.01) on days 2 through 7. However, on the 14th day, ERP in V decreased to the level seen in C, whereas ERP in E remained significantly longer than ERPs in C or V (P < 0.01). The blockade L-type Ca channel alone is not sufficient, but the addition of a T-type Ca channel blockade shows a more sustained effect to prevent atrial electrical remodeling.

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.

The R(−)-Enantiomer of Efonidipine Blocks T-type but Not L-type Calcium Current in Guinea Pig Ventricular Myocardium

In guinea pig ventricular cardiomyocytes, the R(-)-enantiomer of efonidipine concentration-dependently blocked T-type Ca2+ current with 85% inhibition at 1 microM. In contrast, R(-)-efonidipine (1 microM) had no effect on the L-type Ca2+ current and Ca2+ transient in cardiomyocytes and contractile force in papillary muscles. Thus, R(-)-efonidipine is a highly selective blocker of the T-type Ca2+ current in native myocardia.

Clinical efficacy of efonidipine hydrochloride, a T-type calcium channel inhibitor, on sympathetic activities

Dihydropyridine Ca antagonists cause reflex tachycardia related to their hypotensive effects. Efonidipine hydrochloride has inhibitory effects on T-type Ca channels, even as it inhibits reflex tachycardia. In the present study, the influence of efonidipine hydrochloride on heart rate and autonomic nervous function was investigated. Using an electrocardiogram and a tonometric blood pressure measurement, autonomic nervous activity was evaluated using spectral analysis of heart rate/systolic blood pressure variability. Three protocols were used: (1) a single dose of efonidipine hydrochloride was administered orally to healthy subjects with resting heart rate values of 75 beats/min or more (high-HR group) and to healthy subjects with resting heart rate values less than 75 beats/min (low-HR group); (2) efonidipine hydrochloride was newly administered to untreated patients with essential hypertension, and autonomic nervous activity was investigated after a 4-week treatment period; and (3) patients with high heart rate values (>/=75 beats/min) who had been treated with a dihydropyridine L-type Ca channel inhibitor for 1 month or more were switched to efonidipine hydrochloride and any changes in autonomic nervous activity were investigated. In all protocols, administration of efonidipine hydrochloride decreased the heart rate in patients with a high heart rate, reduced sympathetic nervous activity, and enhanced parasympathetic nervous activity. In addition, myocardial scintigraphy with (123)I-metaiodobenzylguanidine showed significant improvement in the washout rate and H/M ratio of patients who were switched from other dihydropyridine Ca antagonists to efonidipine hydrochloride. Efonidipine hydrochloride inhibits increases in heart rate and has effects on the autonomic nervous system. It may be useful for treating hypertension and angina pectoris, and may also have a cardiac protective function.

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.