Weiran Liu

Pacing-induced heart failure causes a reduction of delayed rectifier potassium currents along with decreases in calcium and transient outward currents in rabbit ventricle

OBJECTIVE Heart failure in patients and in animal models is associated with action potential prolongation of the ventricular myocytes. Changes in several membrane currents have been already demonstrated to underlie this prolongation. However, information on the two components (I(Kr) and I(Ks)) of the delayed rectifier potassium current (I(K)) in rapid pacing induced heart failure is lacking. METHODS AND RESULTS Action potentials and whole-cell currents, I(K), I(to1), I(K1), and I(Ca-L) were recorded in apical myocytes of left ventricle from 10 rabbits subjected to left ventricular pacing at 350-380 beats/min for 3-4 weeks and 10 controls with sham operation. Action potential duration at 90% repolarization (APD(90)) was prolonged in myocytes from failing hearts compared to controls at both cycle lengths of 333 and 1000 ms. Both E-4031-sensitive and -resistant components of I(K) (I(Kr), I(Ks)) in myocytes from failing hearts were significantly less than those of control hearts; tail current densities of I(Kr) and I(Ks) following depolarization to +50 mV were 0.62+/-0.05 vs. 0.96+/-0.12 pA/pF (P<0.05), and 0.27+/-0.08 vs. 0.52+/-0.08 pA/pF (P<0.05), respectively. There was no significant difference between control and failing myocytes in the voltage- and time-dependence of activation of total I(K), I(Kr) and I(Ks). The peak of L-type Ca(2+) current (I(Ca-L)) was significantly reduced in myocytes from failing hearts (at +10 mV, -9.29+/-0.52 vs. -12.28+/-1.63 pA/pF, P<0.05), as was the Ca(2+)-independent transient outward current (I(to1); at +40 mV, 4.8+/-0.9 vs. 9.6+/-1.3 pA/pF, P<0.05). Steady state I-V curve for I(K1) was similar in myocytes from failing and control hearts. CONCLUSIONS Decrease of I(K) (both I(Kr) and I(Ks)) in addition to reduced I(to1), may underly action potential prolongation at physiological cycle length and thereby contribute to arrhythmogenesis in heart failure.

Heterogeneous distribution of the two components of delayed rectifier K+ current: a potential mechanism of the proarrhythmic effects of methanesulfonanilideclass III agents

OBJECTIVE To elucidate the regional difference of the K+ current blocking effects of methanesulfonanilide class III agents. METHODS Regional differences in action potential duration (APD) and E-4031-sensitive component (IKr) as well as -insensitive component (IKs) of the delayed rectifier K+ current (IK) were investigated in enzymatically isolated myocytes from apical and basal regions of the rabbit left ventricle using the whole-cell clamp technique. RESULTS At 1 Hz stimulation, APD was significantly longer in the apex than in the base (223.1 +/- 10.6 vs. 182.7 +/- 14.5 ms, p < 0.05); application of 1 microM E-4031 caused more significant APD prolongation in the apex than in the base (32.5 +/- 6.4% vs. 21.0 +/- 8.8%, p < 0.05), resulting in an augmentation of regional dispersion of APD. In response to a 3-s depolarization pulse to +40 mV from a holding potential of -50 mV, both IK tail and IKs tail densities were significantly smaller in apical than in basal myocytes (IK: 1.56 +/- 0.13 vs. 2.09 +/- 0.21 pA/pF, p < 0.05; IKs: 0.40 +/- 0.15 vs. 1.43 +/- 0.23, p < 0.01), whereas IKr tail density was significantly greater in the apex than in the base (1.15 +/- 0.13 vs. 0.66 +/- 0.11 pA/pF, p < 0.01). The ratio of IKs/IKr for the tail current in the apex was significantly smaller than that in the base (0.51 +/- 0.21 vs. 3.09 +/- 0.89; p < 0.05). No statistical difference was observed in the voltage dependence as well as activation and deactivation kinetics of IKr and IKs between the apex and base. Isoproterenol (1 microM) increased the time-dependent outward current of IKs by 111 +/- 8% during the 3-s depolarizing step at +40 mV and its tail current by 120 +/- 9% on repolarization to the holding potential of -50 mV, whereas it did not affect IKr. CONCLUSIONS The regional differences in IK, in particular differences in its two components may underlie the regional disparity in APD, and that methanesulfonanilide class III antiarrhythmic agents such as E-4031 may cause a greater spatial inhomogeneity of ventricular repolarization, leading to re-entrant arrhythmias.

Developmental changes of Ca2+ handling in mouse ventricular cells from early embryo to adulthood

Transplant of immature cardiomyocytes is recently attracting a great deal of interest as a new experimental strategy for the treatment of failing hearts. Full understanding of normal cardiomyogenesis is essential to make this regenerative therapy feasible. We analyzed the molecular and functional changes of Ca(2+) handling proteins during development of the mouse heart from early embryo at 9.5 days postcoitum (dpc) through adulthood. From the early to the late (18 dpc) embryonic stage, mRNAs estimated by the real time PCR for ryanodine receptor (type 2, RyR2), sarcoplasmic reticulum (SR) Ca(2+) pump (type 2, SERCA2) and phospholamban (PLB) increased by 3-15 fold in the values normalized to GAPDH mRNA, although Na(+)/Ca(2+) exchanger (type 1, NCX1) mRNA was unchanged. After birth, there was a further increase in the mRNAs for RyR2, SERCA2 and PLB by 18-33 fold, but a 50% decrease in NCX1 mRNA. The protein levels of RyR2, SERCA2, PLB and NCX1, which were normalized to total protein, showed qualitatively parallel developmental changes. L-type Ca(2+) channel currents (I(Ca-L)) were increased during the development (1.3-fold at 18 dpc, 2.2-fold at adult stage, vs. 9.5 dpc). At 9.5 dpc, the Ca(2+) transient was, unlike adulthood, unaffected by the SR blockers, ryanodine (5 microM) and thapsigargin (2 microM), and also by a blocker of the Ca(2+) entry via Na(+)/Ca(2+) exchanger, KB-R 7943 (1 microM). The Ca(2+) transient was abolished after application of nisoldipine (5 microM). These results indicate that activator Ca(2+) for contraction in the early embryonic stage depends almost entirely on I(Ca-L).

β-Adrenergic modulation of L-type Ca2+-channel currents in early-stage embryonic mouse heart

Little information is available concerning the modulation of cardiac function by β-adrenergic agonists in early-stage embryonic mammalian heart. We have examined the effects of isoproterenol (Iso) on the spontaneous beating rate and action potential (AP) configuration in embryonic mouse hearts at 9.5 days postcoitum (dpc), just 1 day after they started to beat. Iso (3 μM) increased the spontaneous beating rate in whole hearts, dissected ventricles, and isolated ventricular myocytes. In ventricular myocytes, Iso also increased the slope of the pacemaker potential and the action potential duration but decreased the maximum upstroke velocity. In whole cell voltage-clamp experiments, the Ca2+-channel currents were measured as Ba2+ currents ( I Ba). In 9.5-dpc myocytes, I Ba was enhanced significantly from -4.7 ± 0.9 to -6.7 ± 1.2 pA/pF (by 52.4 ± 14.8%, n = 10) after the application of Iso. Propranolol (3 μM) reversed the effect of Iso. Forskolin (For, 10 μM) produced an increase in I Ba by 95.5 ± 18.8% ( n = 8). In ventricular myocytes at a late embryonic stage (18 dpc), 3 μM Iso caused an appreciably greater increase in I Ba from -6.2 ± 0.5 to -14.5 ± 2.2 pA/pF (by 137.8 ± 33.0%, n = 8), whereas the increase in I Ba by 10 μM For (by 120.0 ± 23.0%, n = 7) was comparable to that observed in the early stage (9.5 dpc). These results indicate that the L-type Ca2+-channel currents are modulated by β-adrenergic receptors in the embryonic mouse heart as early as 9.5 dpc, probably via a cAMP-dependent pathway.

Developmental changes of the ultrarapid delayed rectifier K+ current in rat ventricular myocytes.

Abstract Recent studies have shown the presence of a 4-aminopyridine (4-AP) sensitive, ultrarapid delayed rectifier K+ current (IK,ur) in adult human atria, but an apparent absence of this current in adult human ventricles. The present experiment was designed to investigate the postnatal changes of IK,ur in rat ventricular myocytes. The presence of IK,ur was evaluated with the use of a low concentration of 4-AP (50 μM). In 3-day-old newborns, the channel activity of rapidly activating outward current was predominantly the transient outward current (Ito). IK,ur could be recorded in a small number of 3-day-old cells lacking Ito (16%). In 10-day-old and adult rat ventricular myocytes, almost all cells expressed Ito and its current density increased significantly with age. The fraction of cells expressing IK,ur dramatically decreased with age and no IK,ur-like component could be detected in the adult cells. These findings first demonstrate that the expression of IK,ur is regulated developmentally in mammalian ventricular myocytes.