Isoform-specific regulation of mood behavior and pancreatic beta cell and cardiovascular function by L-type Ca 2+ channels

Ca(v)1.2 and Ca(v)1.3 L-type Ca(2+) channels (LTCCs) are believed to underlie Ca(2+) currents in brain, pancreatic beta cells, and the cardiovascular system. In the CNS, neuronal LTCCs control excitation-transcription coupling and neuronal plasticity. However, the pharmacotherapeutic implications of CNS LTCC modulation are difficult to study because LTCC modulators cause cardiovascular (activators and blockers) and neurotoxic (activators) effects. We selectively eliminated high dihydropyridine (DHP) sensitivity from Ca(v)1.2 alpha 1 subunits (Ca(v)1.2DHP-/-) without affecting function and expression. This allowed separation of the DHP effects of Ca(v)1.2 from those of Ca(v)1.3 and other LTCCs. DHP effects on pancreatic beta cell LTCC currents, insulin secretion, cardiac inotropy, and arterial smooth muscle contractility were lost in Ca(v)1.2DHP-/- mice, which rules out a direct role of Ca(v)1.3 for these physiological processes. Using Ca(v)1.2DHP-/- mice, we established DHPs as mood-modifying agents: LTCC activator-induced neurotoxicity was abolished and disclosed a depression-like behavioral effect without affecting spontaneous locomotor activity. LTCC activator BayK 8644 (BayK) activated only a specific set of brain areas. In the ventral striatum, BayK-induced release of glutamate and 5-HT, but not dopamine and noradrenaline, was abolished. This animal model provides a useful tool to elucidate whether Ca(v)1.3-selective channel modulation represents a novel pharmacological approach to modify CNS function without major peripheral effects.

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.

Extra- and intracellular action of quaternary devapamil on muscle L-type Ca(2+)-channels

1. The quaternary derivative of the potent verapamil-analogue, (-)-D888, (qD888, 4-cyano-4-(3,4-dimethoxyphenyl)-N-[2-(3-methoxy phenyl)ethyl]-N,N,5-trimethyl-1-hexanaminium) was synthesized as a novel membrane-impermeable probe to study the localization of phenylalkylamine (PAA) effector domains on L-type Ca2+ channels. Channel block by qD888 was investigated in smooth muscle-like (A7r5) cells after extra- and intracellular application by use of the whole-cell configuration of the patch clamp technique. 2. Extracellularly applied qD888 inhibited Sr2+ (Isr) (IC50 = 90 microM) and Na+ (IC50 = 27 microM) inward currents through L-type Ca(2+)-channels mainly in a resting-state-dependent manner. Structurally closely related quaternary PAAs (e.g. D890) were ineffective after extracellular application. 3. QD888 also blocked Isr from the cytoplasmic side, as did other quaternary PAAs (D890, D575). Intracellular block was clearly dependent on channel opening, which resulted in pronounced use-dependence. 4. We conclude that qD888 blocks L-type Ca2+ channels not only from the intracellular side, via interaction with the classical PAA binding domain, but also from the extracellular channel surface. The properties of Ca2+ channel block together with previous biochemical and structural data suggest that extracellular block may be mediated by a site that also confers tonic block by quaternary benzothiazepines.

Benzothiazepine binding domain of purified L-type calcium channels: direct labeling using a novel fluorescent diltiazem analogue

We have synthesized a series of N-propylamino-substituted benzazepinones (NPSBs) as specific probes for the benzothiazepinone (BTZ) binding domain of muscle L-type calcium channels (LTCCs). NPSBs were identified which possess high affinity for the channel after purification. We synthesized a fluorescent NPSB, DMBODIPY-BAZ, as the first benz(othi)azepinone derivative known to reversibly label partially purified LTCCs. DMBODIPY-BAZ binds to the partially purified channel with high affinity (Kd = 25 nM, Bmax = 580 pmol/mg of protein). Fluorescence resonance energy transfer (FRET) occurred between tryptophan residues of the channel protein and the DMBODIPY fluorophore upon specific drug binding. FRET was exploited to allow highly time-resolved detection of specific drug binding kinetics. We found that the dissociation half-life (t1/2) of DMBODIPY-BAZ decreased with the concentration of an unlabeled competitor, which indicates ligand-induced accelerated dissociation. In contrast, t1/2 was concentration-dependently increased by the dihydropyridine (DHP) (+)-isradipine. These kinetic properties of DMBODIPY-BAZ indicate that a high-affinity BTZ binding domain also exists on purified LTCCs. NPSBs represent novel tools to provide further insight into the molecular pharmacology of the BTZ binding domain on LTCCs.