Properties of voltage-gated Ca(2+) channels in rabbit ventricular myocytes expressing Ca(2+) channel alpha(1E) cDNA

Control of Ca2+ Influx and Calmodulin Activation by SK-Channels in Dendritic Spines

The key trigger for Hebbian synaptic plasticity is influx of Ca²⁺ into postsynaptic dendritic spines. The magnitude of [Ca²⁺] increase caused by NMDA-receptor (NMDAR) and voltage-gated Ca²⁺ -channel (VGCC) activation is thought to determine both the amplitude and direction of synaptic plasticity by differential activation of Ca²⁺ -sensitive enzymes such as calmodulin. Ca²⁺ influx is negatively regulated by Ca²⁺ -activated K⁺ channels (SK-channels) which are in turn inhibited by neuromodulators such as acetylcholine. However, the precise mechanisms by which SK-channels control the induction of synaptic plasticity remain unclear. Using a 3-dimensional model of Ca²⁺ and calmodulin dynamics within an idealised, but biophysically-plausible, dendritic spine, we show that SK-channels regulate calmodulin activation specifically during neuron-firing patterns associated with induction of spike timing-dependent plasticity. SK-channel activation and the subsequent reduction in Ca²⁺ influx through NMDARs and L-type VGCCs results in an order of magnitude decrease in calmodulin (CaM) activation, providing a mechanism for the effective gating of synaptic plasticity induction. This provides a common mechanism for the regulation of synaptic plasticity by neuromodulators.

Hebbian or associative plasticity is triggered by postsynaptic Ca²⁺ influx which activates calmodulin and CaMKII. The influx of Ca²⁺ through voltage-dependent NMDA receptors and Ca²⁺ channels is regulated by Ca²⁺ -activated K⁺ channels (SK-channels) providing negative feedback regulation of postsynaptic [Ca²⁺]. Using 3-dimensional modeling of Ca²⁺ and calmodulin dynamics within dendritic spines we show that the non-linear relationship between Ca²⁺ influx and calmodulin activation endows SK-channels with the ability to “gate” calmodulin activation and therefore the induction of Hebbian synaptic plasticity. Since SK-channels are inhibited by several neuromodulator receptors including acetylcholine and noradrenaline, the gating of synaptic plasticity by SK-channels could represent a common mechanism by which neuromodulators control the induction of synaptic plasticity.

Glucose-6-phosphate dehydrogenase and NADPH redox regulates cardiac myocyte L-type calcium channel activity and myocardial contractile function

We recently demonstrated that a 17-ketosteroid, epiandrosterone, attenuates L-type Ca²⁺ currents (I_Ca-L) in cardiac myocytes and inhibits myocardial contractility. Because 17-ketosteroids are known to inhibit glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in the pentose phosphate pathway, and to reduce intracellular NADPH levels, we hypothesized that inhibition of G6PD could be a novel signaling mechanism which inhibit I_Ca-L and, therefore, cardiac contractile function. We tested this idea by examining myocardial function in isolated hearts and Ca²⁺ channel activity in isolated cardiac myocytes. Myocardial function was tested in Langendorff perfused hearts and I_Ca-L were recorded in the whole-cell patch configuration by applying double pulses from a holding potential of −80 mV and then normalized to the peak amplitudes of control currents. 6-Aminonicotinamide, a competitive inhibitor of G6PD, increased pCO₂ and decreased pH. Additionally, 6-aminonicotinamide inhibited G6PD activity, reduced NADPH levels, attenuated peak I_Ca-L amplitudes, and decreased left ventricular developed pressure and ±dp/dt. Finally, dialyzing NADPH into cells from the patch pipette solution attenuated the suppression of I_Ca-L by 6-aminonicotinamide. Likewise, in G6PD-deficient mice, G6PD insufficiency in the heart decreased GSH-to-GSSG ratio, superoxide, cholesterol and acetyl CoA. In these mice, M-mode echocardiographic findings showed increased diastolic volume and end-diastolic diameter without changes in the fraction shortening. Taken together, these findings suggest that inhibiting G6PD activity and reducing NADPH levels alters metabolism and leads to inhibition of L-type Ca²⁺ channel activity. Notably, this pathway may be involved in modulating myocardial contractility under physiological and pathophysiological conditions during which the pentose phosphate pathway-derived NADPH redox is modulated (e.g., ischemia-reperfusion and heart failure).

Sustained beta-adrenergic stimulation increased L-type Ca2+ channel expression in cultured quiescent ventricular myocytes

The abundance of voltage-gated L-type Ca2+ channels is altered by beta-adrenergic receptor (beta-AR) stimulation and by an elevation of the intracellular Ca2+ concentration in cardiac myocytes. In whole animal, chronic beta-AR stimulation or pacing heart results in various changes in the abundance of the channel, but it reduces the beta-AR responsiveness of the L-type channel. Because beta-AR stimulation facilitates the L-type calcium channels, it is difficult in the whole animal to study the effects of beta-AR and Ca2+ influx on the upregulation of the L-type channel independently of each other, which makes the culture of nonbeating adult myocytes an attractive model. We found that culturing quiescent adult rabbit ventricular myocytes with isoproterenol (ISO, 2 microM) for 72 h or more caused a significant increase in the expression of mRNA coding for the L-type channel alpha(1C) subunit by approximately twofold as compared to time-matched controls, and it was followed by a 1.8-fold increase in the Ca2+ current density at 96 h. Somewhat surprisingly, an acute application of 1 microM ISO increased the current amplitude even in ISO-treated cells. The increase in the current density, induced by sustained beta-AR stimulation, was blocked by a beta-AR antagonist, propranolol (10 microM), but not by a Ca2+ antagonist, nitrendipine (10 microM). In addition, the effects were reproduced by forskolin (10 microM), but not by a Ca2+ agonist, Bay-K 8644 (2 microM). Taken together, these results suggest that sustained beta-AR stimulation upregulates L-type channel expression, but does not alter the beta-AR responsiveness of the channel in quiescent myocytes.

Leukemia Inhibitory Factor Activates Cardiac L-Type Ca 2+ Channels via Phosphorylation of Serine 1829 in the Rabbit Ca v 1.2 Subunit

We have previously reported that leukemia inhibitory factor (LIF) gradually increased cardiac L-type Ca 2+ channel current ( I CaL ), which peaked at 15 minutes in both adult and neonatal rat cardiomyocytes, and this increase was blocked by the mitogen-activated protein kinase kinase inhibitor PD98059. This study investigated the molecular basis of LIF-induced augmentation of I CaL in rodent cardiomyocytes. LIF induced phosphorylation of a serine residue in the α 1c subunit (Ca v 1.2) of L-type Ca 2+ channels in cultured rat cardiomyocytes, and this phosphorylation was inhibited by PD98059. When constructs encoding either a wild-type or a carboxyl-terminal–truncated rabbit Ca v 1.2 subunit were transfected into HEK293 cells, LIF induced phosphorylation of the resultant wild-type protein but not the mutant protein. Cotransfection of constitutively active mitogen-activated protein kinase kinase also resulted in phosphorylation of the Ca v 1.2 subunit in the absence of LIF stimulation. In in-gel kinase assays, extracellular signal–regulated kinase phosphorylated a glutathione S -transferase fusion protein of the carboxyl-terminal region of Ca v 1.2 (residues 1700 through 1923), which contains the consensus sequence Pro-Leu-Ser-Pro. A point mutation within this consensus sequence, which results in a substitution of alanine for serine at residue 1829 (S1829A), was sufficient to abolish the LIF-induced phosphorylation. LIF increased I CaL in HEK cells transfected with wild-type Ca v 1.2 but not with the mutated version. These results provide direct evidence that LIF phosphorylates the serine residue at position 1829 of the Ca v 1.2 subunit via the actions of extracellular signal–regulated kinase and that this phosphorylation increases I CaL in cardiomyocytes.

Decoding of synaptic voltage waveforms by specific classes of recombinant high-threshold Ca(2+) channels

Studies suggest that the preferential role of L-type voltage-sensitive Ca(2+) channels (VSCCs) in coupling strong synaptic stimulation to transcription is due to their selective activation of local chemical events. However, it is possible that selective activation of the L-type channel by specific voltage waveforms also makes a contribution. To address this issue we have examined the response of specific Ca(2+) channel types to simulated complex voltage waveforms resembling those encountered during synaptic plasticity (gamma and theta firing frequency). L-, P/Q- and N-type VSCCs (alpha1C, alpha1A, alpha1B/beta1B/alpha2delta, respectively) were all similarly activated by brief action potential (AP) waveforms or sustained step depolarization. When complex waveforms containing large excitatory postsynaptic potentials (EPSPs), APs and spike accommodation were applied under voltage clamp we found that the integrated L-type VSCC current was approximately three times larger than that produced by the P/Q- or N-type Ca(2+) channels (gamma frequency 1 s stimulation). For P/Q- or N-type channels the complex waveforms led to a smaller current than that expected from the response to a simple 1 s step depolarization to 0 or +20 mV. EPSPs present in the waveforms favoured the inactivation of P/Q- and N-type channels. In contrast, activation of the L-type channel was dependent on both EPSP- and AP-mediated depolarization. Expression of P/Q-type channels with reduced voltage-dependent inactivation (alpha1A/beta2A/alpha2delta) or the use of hyperpolarized intervals between AP stimuli greatly increased their response to complex voltage stimuli. We propose that in response to complex synaptic voltage waveforms P/Q- and N-type channels can undergo selective voltage-dependent inactivation leading to a Ca(2+) current mediated predominantly by L-type channels.

Stimulation of interleukin-12 production in mouse macrophages via activation of p38 mitogen-activated protein kinase by alpha2-adrenoceptor agonists

Interleukin-12 is a cytokine primarily produced by monocytes and macrophages. It plays an essential role in the development of cell-mediated immunity and stimulates T helper type 1 (Th1) immune responses. This study was designed to determine if alpha(2)-adrenoceptor agonists are involved in the induction of interleukin-12 production by macrophages. alpha(2)-adrenoceptor agonists such as clonidine, guanfacine, and oxymetazoline significantly induced interleukin-12 secretion and interleukin-12 mRNA expression by macrophages in a concentration-dependent manner. Moreover, stimulation of alpha(2)-adrenoceptor by their agonists triggered the activation of the p38 mitogen-activated protein kinase (MAPK) signaling pathway. Inhibitors of p38 MAPK prevented the stimulatory effects of alpha(2)-adrenoceptor agonists on IL-12 production. Yohimbine and 2-(2,3-dihydro-2-methoxy-1,4-benzodioxin-2-yl)4,5-dihydro-1H-imidazole (RX821002), alpha(2)-adrenoceptor antagonists, significantly blocked agonist-induced interleukin-12 production and p38 MAPK activation, indicating that the effects of the agonists were mediated through alpha(2)-adrenoceptor. In addition, protein kinase C (PKC) inhibitors, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) and chelerythrine, significantly inhibited guanfacine-induced interleukin-12 production and p38 MAPK in a concentration-dependent manner. These findings show that alpha(2)-adrenoceptor agonists induce interleukin-12 production in mouse macrophages via a PKC/p38 MAPK signaling pathway and suggest that the effect of alpha(2)-adrenoceptor agonists on interleukin-12 secretion may be a new and novel means of augmenting cell-mediated immune responses.

Identification of the calcium channel alpha 1E (Ca(v)2.3) isoform expressed in atrial myocytes

Antisense oligonucleotides targeting the calcium channel alpha 1E (Ca(v)2.3) subunit significantly inhibit the insulin-like growth factor-1 (IGF-1)-stimulated increase in low voltage-activated (LVA) (T-type) calcium current in cultured rat atrial myocytes [Proc. Natl. Acad. Sci. U.S.A. 94(1997) 14936]. As part of a continuing effort to understand the regulation of LVA current expression in the heart, we have identified the specific alpha 1E isoform that is expressed in atrial tissue. Through reverse transcription-polymerase chain reaction (RT-PCR), nine overlapping partial clones spanning the entire coding region of the cardiac alpha 1E mRNA were obtained. The predominate isoform in atrial tissue was identified and found to be highly homologous to the alpha 1E isoform previously isolated from kidney and the islets of Langerhans [Eur. J. Biochem. 257(1998) 274]. The expression of alpha 1E in the heart occurs specifically in cardiac myocytes and not in smooth muscle or fibroblasts as demonstrated by RT-PCR performed on isolated atrial myocytes and by in situ hybridization.

Epiandrosterone, a metabolite of testosterone precursor, blocks L-type calcium channels of ventricular myocytes and inhibits myocardial contractility

The dehydroepiandrosterone metabolite epiandrosterone (EPI) inhibits the pentose phosphate pathway (PPP) and dilates isolated blood vessels pre-contracted by partial depolarization. We found that EPI (10-100 microM) also dose-dependently decreases left-ventricular developed pressure (LVDP), the rate of myocardial contraction (+d p /d t), and the pressure rate product (PRP); at 100 microM EPI, LVDP (131+/-9 vs 34+/-7 mmHg), +d p /dt (1515+/-94 vs 542+/-185 mmHg/s), and PRP (37870+/-2471 vs 9498+/-2375 HR x mmHg/min) were all significantly (P<0.05) reduced. EPI also elevated CPP in isolated hearts, decreased levels of myocardial NADPH and nitrite, and dose-dependently relaxed rat aortic rings pre-contracted with KCl. Electrophysiological analysis of single ventricular myocytes using whole cell clamp showed EPI to dose-dependently (100 n M-100 microM) and reversibly inhibit L-type channel currents carried by Ba2+ (IBa) (IC50=42+/-6 microM) by as much as 50%. At 30 microM, EPI shifted the steady-state inactivation curve to more negative potentials (V50=-26.6 mV vs -38.0 mV), thereby accelerating the decay of IBa during depolarization. These results suggest that EPI may act as a L-type Ca2+ channel antagonist with properties similar to those of 1,4-dihydropyridine (DHP) Ca2+ channel blockers.

Ca(v)2.3 (alpha1E) Ca2+ channel participates in the control of sperm function

To know the function of the Ca2+ channel containing alpha(1)2.3 (alpha1E) subunit (Ca(v)2.3 channel) in spermatozoa, we analyzed Ca2+ transients and sperm motility using a mouse strain lacking Ca(v)2.3 channel. The averaged rising rates of Ca2+ transients induced by alpha-D-mannose-bovine serum albumin in the head region of Ca(v)2.3-/- sperm were significantly lower than those of Ca(v)2.3+/+ sperm. A computer-assisted sperm motility assay revealed that straight-line velocity and linearity were greater in Ca(v)2.3-/- sperm than those in Ca(v)2.3+/+ sperm. These results suggest that the Ca(v)2.3 channel plays some roles in Ca2+ transients and the control of flagellar movement.

Analysis of Ca(2+) currents in spermatocytes from mice lacking Ca(v)2.3 (alpha(1E)) Ca(2+) channel

In mammalian male germ-line cells, low-voltage-activated (LVA) Ca(2+) current has been identified and its electrophysiological properties have been studied. To investigate whether alpha(1)2.3 (alpha(1E)) subunit of the voltage-dependent Ca(2+) channel codes for the LVA current, whole-cell patch clamp and following reverse transcription-polymerase chain reaction (RT-PCR) experiments were performed in pachytene spermatocytes from Ca(v)2.3+/+ and Ca(v)2.3-/- mice. Whole-cell current in acutely dissociated pachytene spermatocytes from Ca(v)2.3+/+ and Ca(v)2.3-/- mice displayed a typical profile of LVA Ca(2+) currents and kinetics with no significant differences. Single-cell RT-PCR revealed the expression of Cacna1g in the pachytene spermatocytes from Ca(v)2.3+/+ and Ca(v)2.3-/- mice in which LVA Ca(2+) currents were actually recorded. These results suggest that the Ca(v)2.3 channel makes no detectable contribution to the LVA Ca(2+) current in the pachytene spermatocyte. Instead, Ca(v)3 family such as Ca(v)3.1 may be the likely candidates responsible for the LVA currents in pachytene spermatocytes.

Intact LTP and fear memory but impaired spatial memory in mice lacking Ca(v)2.3 (alpha(IE)) channel

To investigate the functional roles of the Ca(v)2.3 (alpha(1E)) channel in hippocampal CA1 pyramidal neurons, we studied in vitro synaptic properties and in vivo behaviors of the Ca(v)2.3 gene deficient mice. The Ca(v)2.3 channel mRNA was identified in the hippocampal formation of the wild-type mouse by in situ hybridization. The basic excitatory synaptic transmission and long-term potentiation by theta-burst stimulation were intact in CA1 region of Ca(v)2.3-/- mice. We performed two forms of behavioral tests to examine the hippocampus-dependent function, i.e., emotional and spatial learning tests. The Ca(v)2.3-/- mice were able to establish and maintain fear memories. Although general improvement in the performance of Morris water maze test was seen in Ca(v)2.3-/- mice, they displayed an obvious impairment in the probe test. These results suggest that the Ca(v)2.3 channel plays some role in formation of the accurate spatial memory but not of the fear memory.