Mechanism of the antiischemic effect of mibefradil, a selective T calcium channel blocker in dogs: comparison with amlodipine

Effect of hypernatremia on injury caused by energy deficiency: role of T-type Ca2+ channel

Hypernatremia exerts multiple cellular effects, many of which could influence the outcome of an ischemic event. To further evaluate these effects of hypernatremia, isolated neonatal cardiomyocytes were chronically incubated with medium containing either normal (142 mM) or elevated sodium (167 mM) and then transferred to medium containing deoxyglucose and the electron transport chain inhibitor amobarbital. Chronic hypernatremia diminished the degree of calcium accumulation and reactive oxygen species generation during the period of metabolic inhibition. The improvement in calcium homeostasis was traced in part to the downregulation of the Ca(V)3.1 T-type calcium channel, as deficiency in the Ca(V)3.1 subtype using short hairpin RNA or treatment with an inhibitor of the Ca(V)3.1 variant of the T-type calcium channel (i.e., diphenylhydantoin) attenuated energy deficiency-mediated calcium accumulation and cell death. Although hyperosmotically stressed cells (exposed to 50 mM mannitol) had no effect on T-type calcium channel activity, they were also resistant to death during metabolic inhibition. Both hyperosmotic stress and hypernatremia activated Akt, suggesting that they initiate the phosphatidylinositol 3-kinase/Akt cytoprotective pathway, which protects the cell against calcium overload and oxidative stress. Thus hypernatremia appears to protect the cell against metabolic inhibition by promoting the downregulation of the T-type calcium channel and stimulating cytoprotective protein kinase pathways.

Mibefradil (Ro 40-5967): the first selective T-type Ca2+ channel blocker

Mibefradil is a novel Ca2+ antagonist acting on both L- and T-type Ca2+ channels, with a ten-fold selectivity for T-type Ca2+ channels. It belongs to a chemical class different from other Ca2+ antagonists (tetralol derivative), and binds to a new receptor site on the L-type Ca2+ channel, where it does not affect dihydropyridine (DHP) binding but appears to overlap the verapamil and fantofarone sites. In vitro and in vivo studies indicate that mibefradil has a high selectivity for the coronary vasculature over the peripheral vasculature and the myocardium. It has no relevant negative inotropic effects in various animal models, in normotensive patients, and patients with hypertension or angina pectoris. Instead, treatment with mibefradil slightly decreases heart rate and improves cardiac function. Clinical studies confirm that mibefradil is an effective antihypertensive and anti-ischaemic drug, which may be beneficial in the treatment of heart failure. Its excellent pharmacological and safety profile combined with high bioavailability makes it a promising new drug. Many of the unique pharmacological properties of mibefradil may derive from its selective block of T-type Ca2+ channels.

Molecular pharmacology of high voltage-activated calcium channels

Voltage-gated calcium channels are key sources of calcium entry into the cytosol of many excitable tissues. A number of different types of calcium channels have been identified and shown to mediate specialized cellular functions. Because of their fundamental nature, they are important targets for therapeutic intervention in disorders such as hypertension, pain, stroke, and epilepsy. Calcium channel antagonists fall into one of the following three groups: small inorganic ions, large peptide blockers, and small organic molecules. Inorganic ions nonselectively inhibit calcium entry by physical pore occlusion and are of little therapeutic value. Calcium-channel-blocking peptides isolated from various predatory animals such as spiders and cone snails are often highly selective blockers of individual types of calcium channels, either by preventing calcium flux through the pore or by antagonizing channel activation. There are many structure-activity-relation classes of small organic molecules that interact with various sites on the calcium channel protein, with actions ranging from selective high affinity block to relatively nondiscriminatory action on multiple calcium channel isoforms. Detailed interactions with the calcium channel protein are well understood for the dihydropyridine and phenylalkylamine drug classes, whereas we are only beginning to understand the molecular actions of some of the more recently discovered calcium channel blockers. Here, we provide a comprehensive review of pharmacology of high voltage-activated calcium channels.

Role of T channels in cardiovascular function

Although two types of Ca2+ channels are found to occur in the cardiovascular system, very little is known about one of them, primarily because a pharmacological blocking agent has been lacking. The enigmatic transient (T)-type Ca2+ channel has finally been recognized by a selective Ca2+ antagonist. The novel tetralol Ca2+ antagonist, mibefradil, is a selective T-type Ca2+ channel blocker that produces effective vasodilatation with additional inhibitory actions on blood vessel wall and left ventricular thickening. The availability of a blocking agent has begun to reveal the significance of T-type Ca2+ channel signals. Selective T-type Ca2+ channel blockade characteristics include vascular selectivity, freedom from negative cardiac inotropism, consistent and predictable reduction in heart rate, reduction in subendothelial proliferation, and increased survival of severely hypertensive and heart failure animal models. Mibefradil increases coronary blood flow without increasing myocardial oxygen consumption, and by decreasing heart rate and thus time spent in diastole, improves subendocardial perfusion. Improved perfusion of the myocardial wall and lowered heart rate appear to normalize the underlying pathophysiological factors, improve heart failure, and provide long-term protection. Thus, T-type Ca2+ channel blockade offers significant new cardiovascular protective benefits, even in the presence of critical pathophysiological elements (i.e. increased heart rate and neurohumors in the presence of decreased ejection fraction and contractility) found in heart failure.

Beneficial effects of the T- and L-type calcium channel antagonist, mibefradil, against exercise-induced myocardial stunning in dogs

Abnormalities in calcium homeostasis such as calcium overload have been shown to participate in the pathogenesis of myocardial stunning. The goal of this study was to investigate the effects of mibefradil, a mixed T- and L-type calcium channels antagonist on exercise-induced ischemia (i.e., high-flow ischemia). Nine dogs were permanently instrumented to measure left ventricular wall thickening (Wth) and coronary blood flow (Doppler). Infusion of saline or mibefradil (30 and 40 microg/kg/min, i.v., for 20 min) was started 10 min before exercise (10 min, 14 km/h; slope, 13%) and stopped at its end. Circumflex coronary artery stenosis (pneumatic occluders) was set up 5 min before exercise to suppress exercise-induced increase in mean coronary blood flow without simultaneously affecting Wth at rest. Mibefradil (30 microg/kg/min) was also administered at the beginning of the recovery period in a subset of four dogs. During exercise with saline, Wth was dramatically reduced (-77 +/- 7%; p < 0.05) and recovered only after 24 h. Mibefradil at both doses significantly limited tachycardia during exercise (211 +/- 7 and 210 +/- 5 beats/min vs. 240 +/- 8 beats/min for mibefradil, 30 microg/kg/min, mibefradil, 40 microg/kg/min, and saline, respectively) but exerted no negative inotropic effects. Mibefradil at both doses significantly reduced the intensity of myocardial stunning and the time to recovery in Wth (3 h). Administration of mibefradil at the beginning of the recovery period did not protect against myocardial stunning. Administration of a mixed T- and L-type calcium channel antagonists before ischemia confers cardioprotection against exercise-induced myocardial stunning. This may potentially be related to the limitation of exercise-induced tachycardia and/or the prevention of calcium overload.

Calcium channel blockers in veterinary medicine

Calcium channel blockers are becoming increasingly popular in veterinary medicine for the treatment of systemic hypertension, cardiac arrhythmias, and hypertrophic cardiomyopathy. Calcium is vital to many cellular functions and thus stringent regulation of intracellular calcium concentrations is required. Pharmacologic manipulation of the regulatory mechanisms has the potential to alter cellular function in all body systems. In human medicine, calcium channel blockers are being evaluated for, among other things, use in treating glaucoma, deep vein thrombosis, and pulmonary hypertension, in renal transplantation, and for prevention of reperfusion injury. The potentially beneficial effects of these drugs have often been overshadowed by adverse effects including hypotension, inappetence, bradycardia, conduction abnormalities, and decreased cardiac output. With the introduction of sustained-release formulations (diltiazem) and 2nd generation calcium channel blockers (amlodipine) many of these effects have been attenuated or eliminated. This paper will review the functions of calcium and the calcium channels as well as discussing the classes and current and potential uses of the various calcium channel blockers.

Different effects of calcium antagonists on fluid filtration of large arteries and albumin permeability in spontaneously hypertensive rats

OBJECTIVE

To compare the effects of chronic administration of two dihydropyridines, nifedipine and amlodipine, and the non-dihydropyridine Ca2+ antagonist mibefradil on fluid filtration of large arteries and extravasation of albumin in spontaneously hypertensive rats.

METHODS

Spontaneously hypertensive rats aged 2 months were randomly allocated to oral treatment once a day with 30 mg/kg mibefradil (n=12), 100 mg/kg nifedipine (n=12), 20 mg/kg amlodipine (n=12) or placebo (n=12) for 1 month. Instantaneous blood pressure of rats under pentobarbital anaesthesia was recorded at the end of the treatment Fluid filtration across the carotid arterial wall was determined in situ in the isolated carotid artery. Extravasation of 25 mg/kg Evans Blue dye that had been injected intravenously was used to assess whole vascular permeability to albumin after chronic treatment with mibefradil.

RESULTS

Similar reductions in mean arterial pressure were obtained in all Ca2+ antagonist-treated rats. Heart rate was similar in rats in control, nifedipine and amlodipine groups but was significantly lower in mibefradil-treated rats (by 19%, P< 0.001). Fluid filtration across the carotid wall was greater in all Ca2+ antagonist-treated animals. However, fluid filtration was significantly less in mibefradil-treated rats than it was in nifedipine-treated, and amlodipine-treated rats. Furthermore, administration of mibefradil did not significantly modify extravasation of albumin in all tested tissues (pancreas, testis, spleen, lung, kidney, intestine, liver, skeletal muscle) except for cardiac and brain tissues, in which the permeability of albumin was increased by 24 and 33%, respectively, compared with values for the control group (P < 0.05).

CONCLUSION

These results indicate that Ca2+ antagonists increase fluid filtration through large arteries from spontaneously hypertensive rats. That the lower fluid filtration in mibefradil-treated rats was associated with no change in extravasation of albumin in most tissues and especially in skeletal muscle suggests that vascular permeability in hypertensive rats was impaired less by mibefradil treatment than it was by dihydropyridine Ca2+ antagonist treatments.

Discovery and main pharmacological properties of mibefradil (Ro 40-5967), the first selective T-type calcium channel blocker

PROPERTIES OF MIBEFRADIL: Mibefradil is a novel calcium channel antagonist with structural and pharmacological characteristics clearly distinct from those of classical calcium antagonists. It is a potent vasodilator with a high selectivity for the coronary vasculature over the peripheral vasculature and the myocardium. Most importantly, this compound can relax vascular muscle and slow the heart rate without reducing cardiac contractility. In addition, it does not stimulate neurohormonal reflexes and it exhibits a good pharmacological profile characterized by a long duration of action.

MECHANISM OF ACTION

The mechanism of action of mibefradil is characterized by the selective blockade of transient, low-voltage-activated (T-type) calcium channels over long-lasting, high-voltage-activated (L-type) calcium channels, which is probably responsible for many of its unique properties. CLINICAL USE OF MIBEFRADIL: Although calcium antagonists are mainly used for the treatment of hypertension and angina pectoris, there is strong preclinical evidence that mibefradil may also be beneficial in the treatment of congestive heart failure.

Mibefradil (Ro 40-5967) blocks multiple types of voltage-gated calcium channels in cultured rat spinal motoneurones

The actions of the novel calcium (Ca2+) channel antagonist mibefradil (Ro 40-5967), a selective T-type channel blocker in myocardium, were investigated in embryonic rat spinal motoneurones maintained in culture. Whole-cell currents were recorded with the patch-clamp technique. Motoneurones displayed transient, low-voltage-activated (LVA) and, more sustained, high-voltage-activated (HVA) Ca2+ currents. The LVA currents were small and preferentially blocked by amiloride and low doses of nickel. Most of the HVA Ca2+ current flowed through N-type Ca2+ channels, while L-, and P/Q-type channels represented a smaller fraction. Mibefradil caused a rapid and reversible dose-dependent block of inward Ca2+ channel currents. Inhibition was nearly complete at 10 microM, suggesting mibefradil blockade of all subclasses of Ca2+ channels. The IC50 was approximately 1.4 microM on currents measured at 0 mV, from a holding potential of -90 mV. Inhibition of LVA Ca2+ current occurred over the same contraction range. Slow tail currents induced by the dihydropyridine agonist Bay K 8644 were also blocked by mibefradil, although with a slightly lower potency (IC50 = 3.4 microM). These broad inhibitory effects of mibefradil on Ca2+ influx were also supported by the strong inhibition of depolarization-induced intracellular calcium transients, measured from Indo-1 loaded motoneurones imaged with confocal microscopy. We conclude that mibefradil has potent blocking effects on Ca2+ channels in mammalian motoneurones. We hypothesize that therapeutic and pharmacological effects of mibefradil may involve actions on Ca2+ channels other than type T.

Inhibition by mibefradil, a novel calcium channel antagonist, of Ca(2+)- and volume-activated Cl- channels in macrovascular endothelial cells

1. We have studied the effects of mibefradil, a novel calcium antagonist, on the resting potential and ion channel activity of macrovascular endothelial cells (calf pulmonary artery endothelial cells, CPAE). The patch clamp technique was used to measure ionic currents and the Fura-II microfluorescence technique to monitor changes in the intracellular Ca2+ concentration, [Ca2+]i. 2. Mibefradil (10 microM) hyperpolarized the membrane potential of CPAE cells from its mean control value of -26.6 +/- 0.6 mV (n = 7) to -59.8 +/- 1.7 mV (n = 6). A depolarizing effect was observed at higher concentrations (-13.7 +/- 0.6 mV, n = 4, 30 microM mibefradil). 3. Mibefradil inhibited Ca(2+)-activated Cl- currents, ICl,Ca, activated by loading CPAE cells via the patch pipette with 500 nM free Ca2+ (Ki = 4.7 +/- 0.18 microM, n = 8). 4. Mibefradil also inhibited volume-sensitive Cl- currents, ICl,vol, activated by challenging CPAE cells with a 27% hypotonic solution (Ki = 5.4 +/- 0.22 microM, n = 6). 5. The inwardly rectifying K+ channel, IRK, was not affected by mibefradil at concentrations up to 30 microM. 6. Ca2+ entry activated by store depletion, as assessed by the rate of [Ca2+]i-increase upon reapplication of 10 mM extracellular Ca2+ to store-depleted cells, was inhibited by 17.6 +/- 6.5% (n = 8) in the presence of 10 microM mibefradil. 7. Mibefradil inhibited proliferation of CPAE cells. Half-maximal inhibition was found at 1.7 +/- 0.12 microM (n = 3), which is similar to the concentration for half-maximal block of Cl- channels. 8. These actions of mibefradil on Cl- channels and the concomitant changes in resting potential might, in addition to its effect on T-type Ca2+ channels, be an important target for modulation of cardiovascular function under normal and pathological conditions.