Methoxamine inhibits transient outward potassium current through alpha1A-adrenoceptors in rat ventricular myocytes

Kv4.2 phosphorylation by PKA drives Kv4.2 - KChIP2 dissociation, leading to Kv4.2 out of lipid rafts and internalization

Sympathetic regulation of the Kv4.2 transient outward potassium current is critical for the acute electrical and contractile response of the myocardium under physiological and pathological conditions. Previous studies have suggested that KChIP2, the key auxiliary subunit of Kv4 channels, is required for the sympathetic regulation of Kv4.2 current densities. Of interest, Kv4.2 and KChIP2, and key components mediating acute sympathetic signaling transduction are present in lipid rafts, which are profoundly involved in regulation of I_to densities in rat ventricular myocytes. However, little is known about the mechanisms of Kv4.2-raft association and its connection with acute sympathetic regulation. With the aid of high-resolution fluorescent microscope, we demonstrate that KChIP2 assists Kv4.2 localization in lipid rafts in HEK293 cells. Moreover, PKA-mediated Kv4.2 phosphorylation, the downstream signaling event of acute sympathetic stimulation, induced dissociation between Kv4.2 and KChIP2, resulting in Kv4.2 shifting out of lipid rafts in KChIP2-expressed HEK293．The mutation that mimics Kv4.2 phosphorylation by PKA similarly disrupted Kv4.2 interaction with KChIP2 and also decreased the surface stability of Kv4.2. The attenuated Kv4.2-KChIP2 interaction was also observed in native neonatal rat ventricular myocytes (NRVMs) upon acute adrenergic stimulation with phenylephrine (PE). Furthermore, PE accelerated internalization of Kv4.2 in native NRVMs, but disruption of lipid rafts dampens this reaction. In conclusion, KChIP2 contributes to targeting Kv4.2 to lipid rafts. Acute adrenergic stimulation induces Kv4.2 - KChIP2 dissociation, leading to Kv4.2 out of lipid rafts and internalization, reinforcing the critical role of Kv4.2-lipid raft association in the essential physiological response of I_to to acute sympathetic regulation.

Adrenergic regulation of the rapid component of delayed rectifier K+ currents in guinea pig cardiomyocytes

BACKGROUND

Guinea pig ventricular cardiomyocytes display the rapid component of the delayed rectifier potassium current (Ikr) that contributes to ventricular repolarization and promotes stress-induced arrhythmias. Adrenergic stimulation favors ventricular arrhythmogenesis but its effects on Ikr are poorly understood.

METHODS

Adrenergic modulation of Ikr was studied in isolated guinea pig ventricular cardiomyocytes using whole-cell patch clamping.

RESULTS

We found that the Ikr amplitude was reduced to 0.66±0.02 and 0.62±0.03 in response to 0.1 µM phenylephrine (PE), an α1AR agonist, and 10 µM isoproterenol (ISO), a βAR agonist, respectively. The effect of PE can be blocked by the selective α1A-adrenoceptor antagonist 5-methylurapidil, but not by the α1B-adrenoceptor antagonist chloroethylclonidine or α1D-adrenoceptor antagonist BMY7378. Additionally, the effect of ISO can be blocked by the β1-selective AR antagonist CGP-20712A, but not by the β2-selective AR antagonist ICI-118551. Although PE and ISO was continuously added to cells, ISO did not decrease the current to a greater extent when cells were first given PE. In addition, PE's effect on Ikr was suppressed by β1AR stimulation.

CONCLUSIONS

Ikr can by regulated by both the α1 and β ARs system, and that in addition to direct regulation by each receptor system, crosstalk may exist between the two systems.

Central role of PKCα in isoenzyme-selective regulation of cardiac transient outward current Ito and Kv4.3 channels

The transient outward current I(to) is an important determinant of the early repolarization phase. I(to) and its molecular basis Kv4.3 are regulated by adrenergic pathways including protein kinase C. However, the exact regulatory mechanisms have not been analyzed yet. We here analyzed isoenzyme specific regulation of Kv4.3 and I(to) by PKC. Kv4.3 channels were expressed in Xenopus oocytes and currents were measured with double electrode voltage clamp technique. Patch clamp experiments were performed in isolated rat cardiomyocytes. Unspecific PKC stimulation with PMA resulted in a reduction of Kv4.3 current. Similar effects could be observed after activation of conventional PKC isoforms by TMX. Both effects were reversible by pharmacological inhibition of the conventional PKC isoenzymes (Gö6976). In contrast, activation of the novel PKC isoforms (ingenol) did not significantly affect Kv4.3 current. Whereas TMX-induced PKC activation was not attenuated inhibition of PKCβ, inhibition of PKCα with HBDDE prevented inhibitory effects of both PMA and TMX. Accordingly, stimulatory effects of PMA and TMX could be mimicked by the α-isoenzyme selective PKC activator iripallidal. Further evidence for the central role of PKCα was provided with the use of siRNAs. We found that PKCα siRNA but not PKCβ siRNA abolished the TMX induced effect. In isolated rat cardiomyocytes, PMA dependent I(to) reduction could be completely abolished by pharmacologic inhibition of PKCα. In summary we show that PKCα plays a central role in protein kinase C dependent regulation of Kv4.3 current and native I(to). These results add to the current understanding of isoenzyme selective ion channel regulation by protein kinases.

alpha1-Adrenoceptors stimulate a Galphas protein and reduce the transient outward K+ current via a cAMP/PKA-mediated pathway in the rat heart

alpha(1)-Adrenoceptor stimulation prolongs the duration of the cardiac action potentials and leads to positive inotropic effects by inhibiting the transient outward K(+) current (I(to)). In the present study, we have examined the role of several protein kinases and the G protein involved in I(to) inhibition in response to alpha(1)-adrenoceptor stimulation in isolated adult rat ventricular myocytes. Our findings exclude the classic alpha(1)-adrenergic pathway: activation of the G protein G(alphaq), phospholipase C (PLC), and protein kinase C (PKC), because neither PLC, nor PKC, nor G(alphaq) blockade prevents the alpha(1)-induced I(to) reduction. To the contrary, the alpha(1)-adrenoceptor does not inhibit I(to) in the presence of protein kinase A (PKA), adenylyl cyclase, or G(alphas) inhibitors. In addition, PKA and adenylyl cyclase activation inhibit I(to) to the same extent as phenylephrine. Finally, we have shown a functional coupling between the alpha(1)-adrenoceptor and G(alphas) in a physiological system. Moreover, this coupling seems to be compartmentalized, because the alpha(1)-adrenoceptor increases cAMP levels only in intact cells, but not in isolated membranes, and the effect on I(to) disappears when the cytoskeleton is disrupted. We conclude that alpha(1)-adrenoceptor stimulation reduces the amplitude of the I(to) by activating a G(alphas) protein and the cAMP/PKA signaling cascade, which in turn leads to I(to) channel phosphorylation.

Metabolic disturbances in shock, and the role of ATP-MgCl2 and sex steroids

Hemorrhage following accidental injuries is a common cause of death in the industrialized world. Moreover, the impact of elective surgery and solid organ transplantation sometimes results in low flow conditions similar to those seen following hemorrhagic shock. A shortage in O(2) availability, or hypoxia, leads to sequential changes in cell metabolism and morphology, including inflammatory responses and the expression of hypoxia-inducible transcription factor-1, which controls the cellular adaptation to hypoxia. These endogenous adaptive responses show that O(2) deprivation is not an unforeseen event for cells. The purpose of this review article is to discuss the pathophysiologic principles of shock and the metabolic alterations that cells undergo during low flow conditions. Moreover, the rationale for therapeutic intervention by administering ATP-MgCl(2) and sex steroids following shock and trauma will also be discussed.