Mechanism of voltage- and use-dependent block of class A Ca2+ channels by mibefradil

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.

Myogenic tone is impaired at low arterial pressure in mice deficient in the low-voltage-activated CaV 3.1 T-type Ca(2+) channel

AIM

Using mice deficient in the CaV 3.1 T-type Ca(2+) channel, the aim of the present study was to elucidate the molecular identity of non-L-type channels involved in vascular tone regulation in mesenteric arteries and arterioles.

METHODS

We used immunofluorescence microscopy to localize CaV 3.1 channels, patch clamp electrophysiology to test the effects of a putative T-type channel blocker NNC 55-0396 on whole-cell Ca(2+) currents, pressure myography and Ca(2+) imaging to test diameter and Ca(2+) responses of the applied vasoconstrictors, and Q-PCR to check mRNA expression levels of several Ca(2+) handling proteins in wild-type and CaV 3.1(-/-) mice.

RESULTS

Our data indicated that CaV 3.1 channels are important for the maintenance of myogenic tone at low pressures (40-80 mm Hg), whereas they are not involved in high-voltage-activated Ca(2+) currents, Ca(2+) entry or vasoconstriction to high KCl in mesenteric arteries and arterioles. Furthermore, we show that NNC 55-0396 is not a specific T-type channel inhibitor, as it potently blocks L-type and non-L-type high-voltage-activated Ca(2+) currents in mouse mesenteric vascular smooth muscle cell.

CONCLUSION

Our data using mice deficient in the CaV 3.1 T-type channel represent new evidence for the involvement of non-L-type channels in arteriolar tone regulation. We showed that CaV 3.1 channels are important for the myogenic tone at low arterial pressure, which is potentially relevant under resting conditions in vivo. Moreover, CaV 3.1 channels are not involved in Ca(2+) entry and vasoconstriction to large depolarization with, for example, high KCl. Finally, we caution against using NNC 55-0396 as a specific T-type channel blocker in native cells expressing high-voltage-activated Ca(2+) channels.

Pentobarbital inhibition of human recombinant alpha1A P/Q-type voltage-gated calcium channels involves slow, open channel block

BACKGROUND AND PURPOSE

Pre-synaptic neurotransmitter release is largely dependent on Ca(2+) entry through P/Q-type (Ca(V)2.1) voltage-gated Ca(2+) channels (PQCCs) at most mammalian, central, fast synapses. Barbiturates are clinical depressants and inhibit pre-synaptic Ca(2+) entry. PQCC barbiturate pharmacology is generally unclear, specifically in man. The pharmacology of the barbiturate pentobarbital (PB) in human recombinant alpha(1A) PQCCs has been characterized.

EXPERIMENTAL APPROACH

PB effects on macroscopic Ca(2+)(I(Ca)) and Ba(2+)(I(Ba)) currents were studied using whole-cell patch clamp recording in HEK-293 cells heterologously expressing (alpha(1A))(human)(beta(2a)alpha(2)delta-1)(rabbit) PQCCs.

KEY RESULTS

PB reversibly depressed peak current (I(peak)) and enhanced apparent inactivation (fractional current at 800 ms, r(800)) in a concentration-dependent fashion irrespective of charge carrier (50% inhibitory concentration: I(peak), 656 microM; r(800), 104 microM). Rate of mono-exponential I(Ba) decay was linearly dependent on PB concentration. PB reduced channel availability by deepening non-steady-state inactivation curves without altering voltage dependence, slowed recovery from activity-induced unavailable states and produced use-dependent block. PB (100 microM) induced use-dependent block during physiological, high frequency pulse trains and overall depressed PQCC activity by two-fold.

CONCLUSION AND IMPLICATIONS

The results support a PB pharmacological mechanism involving a modulated receptor with preferential slow, bimolecular, open channel block (K(d)= 15 microM). Clinical PB concentrations (<200 microM) inhibit PQCC during high frequency activation that reduces computed neurotransmitter release by 16-fold and is comparable to the magnitude of Ca(2+)-dependent facilitation, G-protein modulation and intrinsic inactivation that play critical roles in PQCC modulation underlying synaptic plasticity. The results are consistent with the hypothesis that PB inhibition of PQCCs contributes to central nervous system depression underlying anticonvulsant therapy and general anaesthesia.

Voltage-gated Ca2+ currents are necessary for slow-wave propagation in the canine gastric antrum

Electrical slow waves determine the timing and force of peristaltic contractions in the stomach. Slow waves originate from a dominant pacemaker in the orad corpus and propagate actively around and down the stomach to the pylorus. The mechanism of slow-wave propagation is controversial. We tested whether Ca(2+) entry via a voltage-dependent, dihydropyridine-resistant Ca(2+) conductance is necessary for active propagation in canine gastric antral muscles. Muscle strips cut parallel to the circular muscle were studied with intracellular electrophysiological techniques using a partitioned-chamber apparatus. Slow-wave upstroke velocity and plateau amplitude decreased from the greater to the lesser curvature, and this corresponded to a decrease in the density of interstitial cells of Cajal in the lesser curvature. Slow-wave propagation velocity between electrodes impaling cells in two regions of muscle and slow-wave upstroke and plateau were measured in response to experimental conditions that reduce the driving force for Ca(2+) entry or block voltage-dependent Ca(2+) currents. Nicardipine (0.1-1 microM) did not affect slow-wave upstroke or propagation velocities. Upstroke velocity, amplitude, and propagation velocity were reduced in a concentration-dependent manner by Ni(2+) (1-100 microM), mibefradil (10-30 microM), and reduced extracellular Ca(2+) (0.5-1.5 mM). Depolarization (by 10-15 mM K(+)) or hyperpolarization (10 microM pinacidil) also reduced upstroke and propagation velocities. The higher concentrations (or lowest Ca(2+)) of these drugs and ionic conditions tested blocked slow-wave propagation. Treatment with cyclopiazonic acid to empty Ca(2+) stores did not affect propagation. These experiments show that voltage-dependent Ca(2+) entry is obligatory for the upstroke phase of slow waves and active propagation.

The mitochondrial Ca2+-activated K+ channel activator, NS 1619 inhibits L-type Ca2+ channels in rat ventricular myocytes

We examined the effects of the mitochondrial Ca(2+)-activated K(+) (mitoBK(Ca)) channel activator NS 1619 on L-type Ca(2+) channels in rat ventricular myocytes. NS 1619 inhibited the Ca(2+) current in a dose-dependent manner. NS 1619 shifted the activation curve to more positive potentials, but did not have a significant effect on the inactivation curve. Pretreatment with inhibitors of membrane BK(Ca) channel, mitoBK(Ca) channel, protein kinase C, protein kinase A, and protein kinase G had little effect on the Ca(2+) current and did not alter the inhibitory effect of NS 1619 significantly. The application of additional NS 1619 in the presence of isoproterenol, a selective beta-adrenoreceptor agonist, reduced the Ca(2+) current to approximately the same level as a single application of NS 1619. In conclusion, our results suggest that NS 1619 inhibits the Ca(2+) current independent of the mitoBK(Ca) channel and protein kinases. Since NS 1619 is widely used to study mitoBK(Ca) channel function, it is essential to verify these unexpected effects of NS 1619 before experimental data can be interpreted accurately.

Effects of mibefradil on uterine contractility

Mibefradil, a benzimidazolyl tetralol derivative, is a new Ca(2+) channel antagonist which is structurally distinct from other Ca(2+) channel antagonists such as nifedipine, verapamil and diltiazem. It is a very effective antihypertensive agent that is thought to achieve its action via a higher affinity block for low-voltage activated (T) than for high-voltage-activated (L) Ca(2+) channels. Nevertheless, it blocks L-type Ca(2+) channels in several tissues. In the present study, the effects of mibefradil on spontaneous rhythmic contractions and on contractions elicited by CaCl(2) (K(+)-depolarized preparations) and oxytocin (in low Ca(2+)/Ca(2+)-free solutions) were investigated on uterus strips from pregnant and non-pregnant rats. Mibefradil (10(-8)-3 x 10(-6) M) caused concentration-dependent inhibition of spontaneous contractions of uterus strips from pregnant and non-pregnant rats with the IC(50) values of 8.83 x 10(-7) M; 5.94 x 10(-7) M (amplitude) and 1.03 x 10(-6) M; 5.48 x 10(-7) M (frequency), respectively. Mibefradil (3 microM) caused a rightward shift in the concentration-response curves for CaCl(2) in K(+) (40 mM)-depolarized uterus strips taken from both pregnant and non-pregnant rats. Mibefradil (3 microM) was, however, more potent for antagonising CaCl(2) responses in uterus strips obtained from pregnant rats than in those from non-pregnant rats. Mibefradil (3 microM) had no effect on oxytocin-induced contraction in Ca(2+)-free physiological salt solution (PSS) on uterus strips from non-pregnant rats. However, it markedly inhibited oxytocin-induced contraction of pregnant rat uterus strips in Ca(2+)-free PSS. Thus, mibefradil probably antagonizes L-type Ca(2+) channels as well as interferes with the intracellular Ca(2+) release mechanism, which would be helpful in the development of a tocolytic agent.

Protective effect of T-type calcium channel blocker in histamine-induced paw inflammation in rat

The aim of this paper was to investigate the protective effect of the T-type calcium channel blocker in a model of acute local inflammation (histamine-induced). The intraplantar injection of histamine elicited an inflammatory response that was characterized by a time-dependent increase in paw oedema and neutrophil infiltration in paw tissue. The maximal increase in paw volume was observed at 90 min after histamine administration (maximal paw volume: 0.97 +/- 0.07). In addition, polymorphonuclear leucocyte (PNL) number was markedly increased in the histamine-treated paw tissue (144 +/- 25.56). However, histamine-induced paw oedema was significantly reduced in a dose-dependent manner by treatment with mibefradil (given at 10, 25, 50, and 100 mg x kg(-1)) at 30, 60, 90, 120, 150, 180 min after injection of histamine. Mibefradil treatment also caused a significant reduction of the polymorphonuclear leucocyte number in the paw tissue. Our findings support the view that mibefradil exerts antiinflammatory effects.

On the role of Ca(2+)- and voltage-dependent inactivation in Ca(v)1.2 sensitivity for the phenylalkylamine (-)gallopamil

L-type calcium channels (Ca(v)1.m) inactivate in response to elevation of intracellular Ca(2+) (Ca(2+)-dependent inactivation) and additionally by conformational changes induced by membrane depolarization (fast and slow voltage-dependent inactivation). Molecular determinants of inactivation play an essential role in channel inhibition by phenylalkylamines (PAAs). The relative impacts, however, of Ca(2+)-dependent and voltage-dependent inactivation in Ca(v)1.2 sensitivity for PAAs remain unknown. In order to analyze the role of the different inactivation processes, we expressed Ca(v)1.2 constructs composed of different beta-subunits (beta(1a)-, beta(2a)-, or beta(3)-subunit) in Xenopus oocytes and estimated their (-)gallopamil sensitivity by means of the two-microelectrode voltage clamp with either Ba(2+) or Ca(2+) as charge carrier. Ca(v)1.2 consisting of the beta(2a)-subunit displayed the slowest inactivation and the lowest apparent sensitivity for the PAA (-)gallopamil. A significantly higher apparent (-)gallopamil-sensitivity with Ca(2+) as charge carrier was observed for all 3 beta-subunit compositions. The kinetics of Ca(2+)-dependent inactivation and slow voltage-dependent inactivation were not affected by drug. The higher sensitivity of the Ca(v)1.2 channels for (-)gallopamil with Ca(2+) as charge carrier results from slower recovery (tau(rec,Ca) approximately 15 seconds versus tau(rec,Ba) approximately 3 to 5 seconds) from a PAA-induced channel conformation. We propose a model where (-)gallopamil promotes a fast voltage-dependent component in Ca(v)1.2 inactivation. The model reproduces the higher drug sensitivity in Ca(2+) as well as the lower sensitivity of slowly inactivating Ca(v)1.2 composed of the beta(2a)-subunit.

Molecular determinants of inactivation in voltage-gated Ca2+ channels

Evolution has created a large family of different classes of voltage-gated Ca2+ channels and a variety of additional splice variants with different inactivation properties. Inactivation controls the amount of Ca2+ entry during an action potential and is, therefore, believed to play an important role in tissue-specific Ca2+ signalling. Furthermore, mutations in a neuronal Ca2+ channel (Ca(v)2.1) that are associated with the aetiology of neurological disorders such as familial hemiplegic migraine and ataxia cause significant changes in the process of channel inactivation. Ca2+ channels of a given subtype may inactivate by three different conformational changes: a fast and a slow voltage-dependent inactivation process and in some channel types by an additional Ca2+-dependent inactivation mechanism. Inactivation kinetics of Ca2+ channels are determined by the intrinsic properties of their pore-forming alpha1-subunits and by interactions with other channel subunits. This review focuses on structural determinants of Ca2+ channel inactivation in different parts of Ca2+ channel alpha1-subunits, including pore-forming transmembrane segments and loops, intracellular domain linkers and the carboxyl terminus. Inactivation is also affected by the interaction of the alpha1-subunits with auxiliary beta-subunits and intracellular regulator proteins. The evidence shows that pore-forming S6 segments and conformational changes in extra- (pore loop) and intracellular linkers connected to pore-forming segments may play a principal role in the modulation of Ca2+ channel inactivation. Structural concepts of Ca2+ channel inactivation are discussed.

Molecular determinants of inactivation in voltage-gated Ca2+ channels

Evolution has created a large family of different classes of voltage-gated Ca2+ channels and a variety of additional splice variants with different inactivation properties. Inactivation controls the amount of Ca2+ entry during an action potential and is, therefore, believed to play an important role in tissue-specific Ca2+ signalling. Furthermore, mutations in a neuronal Ca2+ channel (Ca(v)2.1) that are associated with the aetiology of neurological disorders such as familial hemiplegic migraine and ataxia cause significant changes in the process of channel inactivation. Ca2+ channels of a given subtype may inactivate by three different conformational changes: a fast and a slow voltage-dependent inactivation process and in some channel types by an additional Ca2+-dependent inactivation mechanism. Inactivation kinetics of Ca2+ channels are determined by the intrinsic properties of their pore-forming alpha1-subunits and by interactions with other channel subunits. This review focuses on structural determinants of Ca2+ channel inactivation in different parts of Ca2+ channel alpha1-subunits, including pore-forming transmembrane segments and loops, intracellular domain linkers and the carboxyl terminus. Inactivation is also affected by the interaction of the alpha1-subunits with auxiliary beta-subunits and intracellular regulator proteins. The evidence shows that pore-forming S6 segments and conformational changes in extra- (pore loop) and intracellular linkers connected to pore-forming segments may play a principal role in the modulation of Ca2+ channel inactivation. Structural concepts of Ca2+ channel inactivation are discussed.

High affinity interaction of mibefradil with voltage-gated calcium and sodium channels

Mibefradil is a novel Ca(2+) antagonist which blocks both high-voltage activated and low voltage-activated Ca(2+) channels. Although L-type Ca(2+) channel block was demonstrated in functional experiments its molecular interaction with the channel has not yet been studied. We therefore investigated the binding of [(3)H]-mibefradil and a series of mibefradil analogues to L-type Ca(2+) channels in different tissues. [(3)H]-Mibefradil labelled a single class of high affinity sites on skeletal muscle L-type Ca(2+) channels (K(D) of 2.5+/-0.4 nM, B(max)=56.4+/-2.3 pmol mg(-1) of protein). Mibefradil (and a series of analogues) partially inhibited (+)-[(3)H]-isradipine binding to skeletal muscle membranes but stimulated binding to brain L-type Ca(2+) channels and alpha1C-subunits expressed in tsA201 cells indicating a tissue-specific, non-competitive interaction between the dihydropyridine and mibefradil binding domain. [(3)H]-Mibefradil also labelled a heterogenous population of high affinity sites in rabbit brain which was inhibited by a series of nonspecific Ca(2+) and Na(+)-channel blockers. Mibefradil and its analogue RO40-6040 had high affinity for neuronal voltage-gated Na(+)-channels as confirmed in binding (apparent K(i) values of 17 and 1.0 nM, respectively) and functional experiments (40% use-dependent inhibition of Na(+)-channel current by 1 microM mibefradil in GH3 cells). Our data demonstrate that mibefradil binds to voltage-gated L-type Ca(2+) channels with very high affinity and is also a potent blocker of voltage-gated neuronal Na(+)-channels. More lipophilic mibefradil analogues may possess neuroprotective properties like other nonselective Ca(2+)-/Na(+)-channel blockers.

Determinants of voltage-dependent inactivation affect Mibefradil block of calcium channels

The voltage gated calcium channel family is a major target for a range of therapeutic drugs. Mibefradil (Ro 40-5967) belongs to a new chemical class of these molecules which differs from other Ca2+ antagonists by its ability to potently block T-type Ca2+ channels. However, this molecule has also been shown to inhibit other Ca2+ channel subtypes. To further analyze the mechanism governing the Ca2+ channel-Mibefradil interaction, we examined the effect of Mibefradil on various recombinant Ca2+ channels expressed in mammalian cells from their cloned cDNAs, using Ca2+ as the permeant ion at physiological concentration. Expression of alpha1A, alpha1C, and alpha1E in tsA 201 cells resulted in Ca2+ currents with functional characteristics closely related to those of their native counterparts. Mibefradil blocked alpha1A and alpha1E with a Kd comparable to that reported for T-type channels, but had a lower affinity (approximately 30-fold) for alpha1C. For each channel, inhibition by Mibefradil was consistent with high-affinity binding to the inactivated state. Modulation of the voltage-dependent inactivation properties by the nature of the coexpressed beta subunit or the alpha1 splice variant altered block at the Mibefradil receptor site. Therefore, we conclude that the tissue and sub-cellular localization of calcium channel subunits as well as their specific associations are essential parameters to understand the in vivo effects of Mibefradil.

Inadequate ischaemia-selectivity limits the antiarrhythmic efficacy of mibefradil during regional ischaemia and reperfusion in the rat isolated perfused heart

1. Mibefradil was compared with (+/-)-verapamil for effects on ischaemia- and reperfusion-induced ventricular fibrillation (VF), and the role of ischaemia-selective L-channel block was examined. Langendorff perfused rat hearts (n=12/group) were used. 2. Neither drug at up to 100 nM reduced the incidence of VF during 30 min regional ischaemia. 300 and 600 nM (+/-)-verapamil abolished VF (P<0. 05); mibefradil was effective only at 600 nM (P<0.05). Reperfusion-induced VF incidence was reduced only by 600 nM (+/-)-verapamil (P<0.05). Both drugs at >/=100 nM increased coronary flow (P<0.05) with a similar potency and maximum effectiveness. 3. In separate hearts perfused with Krebs' solution containing 3 mM K+ (the same as that used for arrhythmia studies) neither drug at up to 600 nM affected ventricular contractility. With K+ raised to 6 mM, (+/-)-verapamil >/=30 nM reduced developed pressure (P<0.05); mibefradil did so only at 600 nM (P<0.05). With K+ raised to 10 mM the effects of (+/-)-verapamil were further increased (P<0.05) and mibefradil became active at >/=100 nM (P<0.05). Likewise both drugs impaired diastolic relaxation, with raised K+ exacerbating the effects and (+/-)-verapamil being more potent and its effects more greatly exacerbated by K+. In contrast, when K+ was normal (3 mM), coronary flow was increased by each drug at >/=30 nM (P<0.05) indicating a marked vascular : myocardial selectivity. 4. In conclusion, mibefradil differed from (+/-)-verapamil in its myocardial effects only in terms of its lower potency. As mibefradil is the more potent T-channel blocker, the T-channel is unlikely to represent the molecular target for these effects. The K+ elevations that occur in the ischaemic milieu determine the ability of both drugs to block myocardial L-channels; this is sufficient to account for the drugs' actions on VF. Neither drug possesses sufficient selectivity for ischaemic myocardium versus blood vessels to permit efficacy (VF suppression without marked vasodilatation) and so inappropriate hypotension is likely to preclude the safe use of mibefradil (or similar analogue) in VF suppression, and explains the lack of clinical effectiveness of (+/-)-verapamil.

Inactivation determinant in the I-II loop of the Ca2+ channel alpha1-subunit and beta-subunit interaction affect sensitivity for the phenylalkylamine (-)gallopamil

1. The role of calcium (Ca2+) channel inactivation in the molecular mechanism of channel block by phenylalkylamines (PAAs) was analysed in a PAA-sensitive rabbit brain class A Ca2+ channel mutant (alpha1A-PAA). Use-dependent barium current (IBa) inhibition of alpha1A-PAA by (-)gallopamil and Ca2+ channel recovery from inactivation and block were studied with two-microlectrode voltage clamp after expression of alpha1A-PAA and auxiliary alpha2-delta- and beta1a- or beta2a-subunits in Xenopus oocytes. 2. Mutation Arg387Glu (alpha1A numbering) in the intracellular loop connecting domains I and II of alpha1A-PAA slowed the inactivation kinetics and reduced use-dependent inhibition (100 ms test pulses at 0.2 Hz from -80 to 20 mV) of the resulting mutant alpha1A-PAA/R-E/beta1a channels by 100 microM (-)gallopamil (53 +/- 2 %, alpha1A-PAA/beta1a vs. 31 +/- 2 %, alpha1A-PAA/R-E/beta1a, n >= 4). This amino acid substitution simultaneously accelerated the recovery of channels from inactivation and from block by (-)gallopamil. 3. Coexpression of alpha1A-PAA with the beta2a-subunit reduced fast IBa inactivation and induced a substantial reduction in use-dependent IBa inhibition by (-)gallopamil (25 +/- 4 %, alpha1A-PAA/beta2a; 13 +/- 1 %, alpha1A-PAA/R-E/beta2a). The time constant of recovery from block at rest was not significantly affected. 4. These results demonstrate that changes in channel inactivation induced by Arg387Glu or beta2a-alpha1-subunit interaction affect the drug-channel interaction.