Toshihiko Kaku

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 µmol/l and 0.14 nmol/l for nifedipine; 0.87 and 1.4 µmol/l for mibefradil, and 0.35 µmol/l and 1.8 nmol/l for efonidipine, respectively. Efonidipine, a dihydropyridine analogue, showed high affinity to T-type Ca²⁺ channel.

Voltage-Dependent and Frequency-Independent Inhibition of Recombinant Cav3.2 T-Type Ca2+ Channel by Bepridil

Effects of bepridil on the low voltage-activated T-type Ca2+ channel (CaV3.2) current stably expressed in human embryonic kidney (HEK)-293 cells were examined using patch-clamp techniques. Bepridil potently inhibited ICa,T with a markedly voltage-dependent manner; the IC50 of bepridil was 0.4 µmol/l at the holding potential of –70 mV, which was 26 times as potent as that at –100 mV (10.6 µmol/l). Steady-state inactivation curve (8.4 ± 1.7 mV) and conductance curve (5.9 ± 1.9 mV) were shifted to the hyperpolarized potential by 10 µmol/l bepridil. Bepridil exerted the tonic blocking action but not the use-dependent block. Bepridil had no effect on the recovery from inactivation of T-type Ca2+ channels. Thus, high efficacy of bepridil for terminating atrial fibrillation and atrial flutter may be considered to be attributed, at least in a part, to the T-type Ca2+ channel-blocking actions.

17β-Estradiol Modulates Expression of Low-Voltage-Activated CaV3.2 T-Type Calcium Channel via Extracellularly Regulated Kinase Pathway in Cardiomyocytes

T-type Ca(2+) channel current (I(Ca,T)) plays an important role for spontaneous pacemaker activity and is involved in the progression of structural heart diseases. Estrogens are of importance for the regulation of growth and differentiation and function in a wide array of target tissues, including those in the cardiovascular system. The aim of this study was to elucidate the short-term and long-term effects of 17beta-estradiol (E(2)) on I(Ca,T) in cardiomyocytes. We employed in vivo and in vitro techniques to clarify E(2)-mediated modulation of heart rate (HR) in ovariectomized rats and I(Ca,T) in cardiomyocytes. Ovariectomy increased HR and E(2) supplement reduced HR in ovariectomized rats. Slowing of E(2)-induced HR was consistent with the deceleration of automaticity in E(2)-treated neonatal cardiomyocytes. Short-term application of E(2) did not have significant effects on I(Ca,T), whereas in cardiomyocytes treated with 10 nm E(2) for 24 h, estrogen receptor-independent down-regulation of peak I(Ca,T) and declination of Ca(V)3.2 mRNA were observed. Expression of a cardiac-specific transcription factor Csx/Nkx2.5 was also suppressed by E(2) treatment for 24 h. On the other hand, expression of Ca(V)3.1 mRNA was unaltered by E(2) treatment in this study. An ERK-1/2, 5 inhibitor, PD-98059, abolished the effects of E(2) on I(Ca,T) and Ca(V)3.2 mRNA as well as Csx/Nkx2.5 mRNA. These findings indicate that E(2) decreases Ca(V)3.2 I(Ca,T) through activation of ERK-1/2, 5, which is mediated by the suppression of Csx/Nkx2.5-dependent transcription, suggesting a genomic effect of E(2) as a negative chronotropic factor in the heart.

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 α1H subunit derived from human heart was stably transfected into cells [human embryonic kidney (HEK)-Cav3.2] cultured with or without 5 μM 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 μM 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 steadystate 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 (τ): 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.

Lysophosphatidylcholine augments Ca(v)3.2 but not Ca(v)3.1 T-type Ca(2+) channel current expressed in HEK-293 cells.

Lysophosphatidylcholine (LPC) has been shown to induce electrophysiological disturbances to arrhythmogenesis. However, the effects of LPC on the low-voltage-activated T-type Ca2+ channels in the heart are not understood yet. We found that LPC increases the T-type Ca2+ channel current (ICa.T) in neonatal rat cardiomyocytes. To further investigate the underlying modulatory mechanism of LPC on T-type Ca2+ channels, we utilized HEK-293 cells stably expressing α1G and α1H subunits (HEK-293/α1G and HEK-293/α1H), by use of patch-clamp techniques. A low concentration of LPC (10 µmol/l) significantly increased Cav3.2 ICa.T (α1H) that were similar to those observed in neonatal rat cardiomyocytes. Activation and steady-state inactivation curves were shifted in the hyperpolarized direction by 5.1 ± 0.2 and 4.6 ± 0.4 mV, respectively, by application of 10 µmol/l LPC. The pretreatment of cells with a protein kinase C inhibitor (chelerythrine) attenuated the effects of LPC on ICa.T (α1H). However, the application of LPC failed to modify Cav3.1 (α1G) ICa.T at concentrations of 10–50 µmol/l. In conclusion, these data demonstrate that extracellularly applied LPC augments Cav3.2 ICa.T (α1H) but not Cav3.1 ICa.T (α1G) in a heterologous expression system, possibly by modulating protein kinase C signaling.

Rescue of pulmonary hypertension with an oral sulfonamide antibiotic sulfisoxazole by endothelin receptor antagonistic actions.

Pulmonary hypertension (PH) is a disease of unknown etiology that ultimately causes right ventricle heart failure with a lethal outcome. An increase in circulating endothelin (ET)-1 levels may contribute to disease progression. This study aimed to examine the possible effects of an orally active ET receptor antagonist, sulfisoxazole (SFX), for the rescue of PH, right ventricular hypertrophy, and eventual right ventricular failure. PH rats (single injection of monocrotaline [MCT]) were treated with an ET antagonist, SFX, an orally active sulfonamide antibody. Effects of SFX on PH rats were assessed in terms of survival rate, pulmonary artery blood pressure (PABP), autonomic nerve activity, and atrial natriuretic peptide (ANP) concentration in right ventricular myocytes and plasma. SFX did not change systemic blood pressure, however, it significantly suppressed the elevation of PABP. SFX maintained the derangement of autonomic nerve control, blunted an increase in ANP in myocytes and plasma, and significantly improved survival in right heart failure and/or related organs dysfunction in PH rats. The ET antagonistic action of the antimicrobial agent, SFX, was experimentally confirmed for treatment of PH in rats.

Nonapeptide Hormones Oxytocin and Vasopressin Distinctly Regulate CaV1.2 L-type Calcium Channel Expression in Cardiomyocytes

Background: The neurohypophyseal hormones oxytocin (OT) and [Arg8]‐vasopressin (AVP) have recently been implicated in cardiac function. This study was designed to investigate actions of OT and AVP on regulation of the voltage‐gated Ca2+ channel in cardiomyocytes.