Pathophysiological significance of T-type Ca2+ channels: properties and functional roles of T-type Ca2+ channels in cardiac pacemaking

Species difference in the contribution of T-type calcium current to cardiac pacemaking as revealed by r(-)-efonidipine

The contribution of the T-type Ca2+ current to cardiac pacemaking was examined in isolated right atrial tissue from the mouse, guinea pig, and rabbit using a specific blocker, R(-)-efonidipine. At 10(-6) M, R(-)-efonidipine produced negative chronotropy, which was prominent in the mouse and small but significant in the guinea pig. No effect was observed in the rabbit. Microelectrode recordings revealed that R(-)-efonidipine significantly prolongs the pacemaker (phase 4) depolarization of the sinoatrial-node action potential in the mouse and guinea pig. These results provide the first pharmacological evidence that the contribution of T-type Ca2+ current to cardiac pacemaking differs among experimental animal species.

The missing link in the mystery of normal automaticity of cardiac pacemaker cells

Earlier studies of the initiating event of normal automaticity of the heart's pacemaker cells, inspired by classical quantitative membrane theory, focused upon ion currents (IK, I f) that determine the maximum diastolic potential and the early phase of the spontaneous diastolic depolarization (DD). These early DD events are caused by the prior action potential (AP) and essentially reflect a membrane recovery process. Events following the recovery process that ignite APs have not been recognized and remained a mystery until recently. These critical events are linked to rhythmic intracellular signals initiated by Ca2+ clock (i.e., sarcoplasmic reticulum [SR] cycling Ca2+). Sinoatrial cells, regardless of size, exhibit intense ryanodine receptor (RyR), Na+/Ca2+ exchange (NCX)-1, and SR Ca2+ ATPase-2 immunolabeling and dense submembrane NCX/RyR colocalization; Ca2+ clocks generate spontaneous stochastic but roughly periodic local subsarcolemmal Ca2+ releases (LCR). LCRs generate inward currents via NCX that exponentially accelerate the late DD. The timing and amplitude of LCR/I NCX-coupled events control the timing and amplitude of the nonlinear terminal DD and therefore ultimately control the chronotropic state by determining the timing of the I CaL activation that initiates the next AP. LCR period is precisely controlled by the kinetics of SR Ca2+ cycling, which, in turn, are regulated by 1) the status of protein kinase A-dependent phosphorylation of SR Ca2+ cycling proteins; and 2) membrane ion channels ensuring the Ca2+ homeostasis and therefore the Ca2+ available to Ca2+ clock. Thus, the link between early DD and next AP, missed in earlier studies, is ensured by a precisely physiologically regulated Ca2+ clock within pacemaker cells that integrates multiple Ca2+-dependent functions and rhythmically ignites APs during late DD via LCRs-I NCX coupling.

Changes in T-type calcium channel and its subtypes in overactive detrusor of the rats with partial bladder outflow obstruction

AIMS

To investigate the activity of the T-type calcium channel (TCC) and the expression of its subtypes in overactive detrusor (OD) myocytes in rats after partial bladder outflow obstruction (PBOO).

METHODS

Thirteen male Wistar rats with OD after PBOO (OD group) and eight sham-operated rats (control group) were studied. The two groups were compared regarding the expression of TCC subtype genes by reverse transcription-polymerase chain reaction (RT-PCR) and the TCC kinetics and cell action potential by whole-cell patch-clamp.

RESULTS

The time course and density of the current were significantly higher in the OD cells than those in the control detrusor. Whole-cell patch-clamp analysis showed that the activation of TCCs in detrusor myocytes in the OD group was faster than the control group, but inactivation was almost the same in both groups, suggesting a significant enhancement of the Ca(2+) "window" current in the OD group. Patch-clamp recording of action potentials in the OD cells indicated an increase in excitability and a decrease in the repolarization interval. RT-PCR assay showed an abnormal expression of alpha1G subtype in the OD cells.

CONCLUSIONS

TCCs could be one of the crucial factors for the abnormal excitation in OD cells. The development of OD after PBOO presumably relates to the increase in TCC current in the bladder cells, the enhancement of the Ca(2+) "window" current for Ca(2+) inflow, the prolongation of the intracellular calcium oscillations, and the acceleration of the cell depolarization.

Transcription factors Csx/Nkx2.5 and GATA4 distinctly regulate expression of Ca2+ channels in neonatal rat heart

The cardiac transcription factors Csx/Nkx2.5 and GATA4 play important roles in vertebrate heart development. Although mutations of Csx/Nkx2.5 or GATA4 are associated with various congenital heart diseases, their mechanism of action on cardiomyocyte function is not completely elucidated. In this study, we therefore investigated the actions of these transcription factors on the electrophysiological features and expression of ion channels in cardiomyocytes. Genes for transcription factors Csx/Nkx2.5 and GATA4 were transfected into rat neonatal cardiomyocytes by adenoviral infection. Action potentials, L-, T-type Ca(2+) channels and hyperpolarization-activated cation current (I(h)) of rat neonatal myocytes were recorded by patch clamp technique after adenoviral infection. Expression of ion channels was confirmed by real-time PCR. In Csx/Nkx2.5 overexpression myocytes, the spontaneous beating rate was markedly increased with an up-regulation of the Ca(v)3.2 T-type Ca(2+) channel, while in GATA4 overexpression myocytes, the T-type Ca(2+) channel was unchanged. On the other hand, the L-type Ca(2+) channel was down-regulated by both Csx/Nkx2.5 and GATA4 overexpression; the level of Ca(v)1.3 mRNA was dramatically decreased by Csx/Nkx2.5 overexpression. These results indicate that Csx/Nkx2.5 and GATA4 play important roles on the generation of pacemaker potentials modulating voltage-dependent Ca(2+) channels in the neonatal cardiomyocyte.

The emergence of a general theory of the initiation and strength of the heartbeat

Sarcoplasmic reticulum (SR) Ca(2+) cycling, that is, the Ca(2+) clock, entrained by externally delivered action potentials has been a major focus in ventricular myocyte research for the past 5 decades. In contrast, the focus of pacemaker cell research has largely been limited to membrane-delimited pacemaker mechanisms (membrane clock) driven by ion channels, as the immediate cause for excitation. Recent robust experimental evidence, based on confocal cell imaging, and supported by numerical modeling suggests a novel concept: the normal rhythmic heart beat is governed by the tight integration of both intracellular Ca(2+) and membrane clocks. In pacemaker cells the intracellular Ca(2+) clock is manifested by spontaneous, rhythmic submembrane local Ca(2+) releases from SR, which are tightly controlled by a high degree of basal and reserve PKA-dependent protein phosphorylation. The Ca(2+) releases rhythmically activate Na(+)/Ca(2+) exchange inward currents that ignite action potentials, whose shape and ion fluxes are tuned by the membrane clock which, in turn, sustains operation of the intracellular Ca(2+) clock. The idea that spontaneous SR Ca(2+) releases initiate and regulate normal automaticity provides the key that reunites pacemaker and ventricular cell research, thus evolving a general theory of the initiation and strength of the heartbeat.

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.