Role of protein kinase C in alpha(1)-adrenergic regulation of a(Na)(i) in guinea pig ventricular myocytes

The acute effects of hydrocortisone on cardiac electrocardiography, action potentials, intracellular calcium, and contraction: The role of protein kinase C

Hydrocortisone exerts adverse effects on various organs, including the heart. This study investigated the still unclear effects of hydrocortisone on electrophysiological and biochemical aspects of cardiac excitation-contraction coupling. In guinea pigs' hearts, hydrocortisone administration reduced the QT interval of ECG and the action potential duration (APD). In guinea pig ventricular myocytes, hydrocortisone reduced contraction and Ca²⁺ transient amplitudes. These reductions and the effects on APD were prevented by pretreatment with the protein kinase C (PKC) inhibitor staurosporine. In an overexpression system of Xenopus oocytes, hydrocortisone increased hERG K⁺ currents and reduced Kv1.5 K⁺ currents; these effects were negated by pretreatment with staurosporine. Western blot analysis revealed dose- and time-dependent changes in PKCα/βII, PKCε, and PKCγ phosphorylation by hydrocortisone in guinea pig ventricular myocytes. Therefore, hydrocortisone can acutely affect cardiac excitation-contraction coupling, including ion channel activity, APD, ECG, Ca²⁺ transients, and contraction, possibly via biochemical changes in PKC.

Effects of bisindolylmaleimide PKC inhibitors on p90RSK activity in vitro and in adult ventricular myocytes

1 Bisindolylmaleimide inhibitors of protein kinase C (PKC), such as GF109203X and Ro31-8220, have been used to investigate the roles of PKC isoforms in many cellular processes in cardiac myocytes, but these agents may also inhibit p90RSK activity. 2 In in vitro kinase assays utilising 50 microM [ATP], GF109203X and Ro31-8220 inhibited p90RSK isoforms (IC50 values for inhibition of RSK1, RSK2 and RSK3, respectively, were 610, 310 and 120 nM for GF109203X, and 200, 36 and 5 nM for Ro31-8220) as well as classical and novel PKC isoforms (IC50 values for inhibition of PKCalpha and PKCepsilon, respectively, were 8 and 12 nM for GF109203X, and 4 and 8 nM for Ro31-8220). 3 At physiological [ATP] (5 mM), both GF109203X and Ro31-8220 exhibited reduced potency as inhibitors of RSK2, PKCalpha and PKCepsilon (IC50 values of 7400, 310 and 170 nM, respectively, for GF109203X, and 930, 150 and 140 nM, respectively, for Ro31-8220), with the latter agent retaining its relatively greater potency. 4 To determine the effects of GF109203X and Ro31-8220 on p90RSK activity in cultured adult rat ventricular myocytes (ARVM), phosphorylation of the eukaryotic elongation factor 2 kinase (eEF2K) at Ser366, a known p90RSK target, was used as the index of such activity. Adenoviral expression of a constitutively active form of mitogen-activated protein kinase (MAPK) or extracellular signal-regulated kinase (ERK) kinase 1 (MEK1) was used to induce PKC-independent p90RSK activation and downstream phosphorylation of eEF2K. 5 eEF2K phosphorylation was abolished by U0126 (1 microM), a selective inhibitor of MEK1, and was significantly reduced by GF109203X at > or =3 microM and by Ro31-8220 at > or =1 microM. At 1 microM, both agents inhibited PMA-induced PKC activity in ARVM. 6 These data show that GF109203X and Ro31-8220 inhibit various isoforms of PKC and p90RSK in vitro and in intact ARVM, with the former agent exhibiting relatively greater selectivity for PKC.

Block of HERG Human K+ Channel and IKr of Guinea Pig Cardiomyocytes by Chlorpromazine

Chlorpromazine, a commonly used antipsychotic drug, has been known to induce QT prolongation and torsades de pointes, which can cause sudden death. We studied the effects of chlorpromazine on the human ether-a-go-go-related gene (HERG) channel expressed in Xenopus oocytes and on delayed rectifier K current of guinea pig ventricular myocytes. Application of chlorpromazine showed a dose-dependent decrease in the amplitudes of steady-state currents and tail currents of HERG. The decrease became more pronounced at increasingly positive potential, suggesting that the blockade of HERG by chlorpromazine is voltage dependent. IC50 for chlorpromazine block of HERG current was progressively decreased according to depolarization: IC50 values at -30, 0, and +30 mV were 10.5, 8.8, and 4.9 microM, respectively. The block of HERG current during the voltage step increased with time starting from a level 89% of the control current. In guinea pig ventricular myocytes, bath application of 2 and 5 microM chlorpromazine at 36 degree C blocked rapidly activating delayed rectifier K current (IKr) by 31 and 83%, respectively. How-ever, the same concentrations of chlorpromazine failed to significantly block slowly activating delayed rectifier K current (IKs). Our findings suggest that the arrhythmogenic side effect of chlorpromazine is caused by blockade of HERG and rapid component of delayed rectifier K current rather than by blockade of the slow component.