Biphasic Response of Action Potential Duration to Sudden Sympathetic Stimulation in Anesthetized Cats

The potassium current carried by TREK-1 channels in rat cardiac ventricular muscle

We studied the potassium current flowing through TREK-1 channels in rat cardiac ventricular myocytes. We separated the TREK-1 current from other current components by blocking most other channels with a blocker cocktail. We tried to inhibit the TREK-1 current by activating protein kinase A (PKA) with a mixture of forskolin and isobutyl-methylxanthine (IBMX). Activation of PKA blocked an outwardly rectifying current component at membrane potentials positive to -40 mV. At 37 °C, application of forskolin plus IBMX reduced the steady-state outward current measured at positive voltages by about 52 %. Application of the potassium channel blockers quinidine or tetrahexylammonium also reduced the steady-state outward current by about 50 %. Taken together, our results suggest that the increase in temperature from 22 to 37 °C increased the TREK-1 current by a factor of at least 5 and that the average density of the TREK-1 current in rat cardiomyocytes at 37 °C is about 1.5 pA/pF at +30 mV. The contribution of TREK-1 to the action potential was assessed by using a dynamic patch clamp technique. After subtraction of simulated TREK-1 currents, action potential duration at 50 or 90 % repolarisation was increased by about 12 %, indicating that TREK-1 may be functionally important in rat ventricular muscle. During sympathetic stimulation, inhibition of TREK-1 channels via PKA is expected to prolong the action potential primarily in subendocardial myocytes; this may decrease the transmural dispersion of repolarisation and thus may serve to prevent the occurrence of arrhythmias.

Modulation of ventricular repolarization in patients with transient left ventricular apical ballooning: a case control study

OBJECTIVE

Even though diffuse T wave inversion and prolongation of the QT interval in the surface electrocardiogram (ECG) have been consistently reported in patients with transient stress-induced left ventricular apical ballooning (AB), ventricular repolarization has not yet been systematically investigated in this clinical entity.

BACKGROUND

AB, an emerging syndrome that mimics acute ST-segment elevation myocardial infarction (MI), is characterized by reversible left ventricular wall motion abnormalities in the absence of obstructive coronary heart disease and significant QT interval prolongation.

METHODS

We prospectively enrolled 22 consecutive patients (21 women, median age 65 years) with transient left ventricular AB. A total of 22 age-, gender-, body-mass-index-, and left-ventricular-function-matched patients with acute anterior ST-segment elevation MI undergoing successful direct percutaneous coronary intervention for a proximal occlusion of the LAD, as well as 22 healthy volunteers served as control groups. Beat-to-beat QT interval and QT interval dynamicity were determined from 24-hour Holter ECGs, recorded on the third day after hospital admission.

RESULTS

There were no significant differences in baseline clinical characteristics, except higher peak enzyme release in MI patients. Compared with MI patients, AB patients exhibited significantly prolonged mean QT intervals and rate-corrected QT intervals (QT: 418 +/- 37 vs 384 +/- 33 msec, P < 0.01; QTcBazett: 446 +/- 40 vs 424 +/- 35 msec, P < 0.05; QTcFridericia: 437 +/- 35 vs 412 +/- 31 msec, P < 0.05). Mean RR intervals tended to be higher in AB patients, without reaching statistical significance (877 +/- 96 vs 831 +/- 102 msec, P = NS). The linear regression slope of QT intervals plotted against RR intervals was significantly flatter in AB patients at both day- and nighttime (QT/RR slopeday: 0.18 +/- 0.04 vs 0.22 +/- 0.06, P < 0.01; QT/RR slopenight: 0.12 +/- 0.03 vs 0.17 +/- 0.05, P < 0.01).

CONCLUSION

The present study is the first to demonstrate significant differences of QT interval modulation in patients with transient left ventricular AB and acute ST-segment elevation MI. Even though transient AB is associated with a significant QT interval prolongation, rate adaptation of ventricular repolarization (i.e., QT dynamicity) is not significantly altered, suggesting a differential effect of autonomic nervous activity on the ventricular myocardium in transient AB and in acute MI.

Effects of Metoprolol on Sympathetic Remodeling and Electrical Remodeling at Infarcted Border Zone after Myocardial Infarction in Rabbits

BACKGROUND

The findings of sympathetic remodeling and its electrophysiological implications force us to rerecognize the drugs presently used. The aim of this study was to investigate the effects of metoprolol on sympathetic remodeling and electrical remodeling at the infarcted border zone (IBZ) after myocardial infarction (MI).

METHODS

Forty rabbits were randomly assigned into two groups: MI group (n = 20), ligation of the anterior descending coronary; Metoprolol group (n = 20), ligation of the anterior descending coronary and administration of oral metoprolol 5 mg/kg/day. Eight weeks after surgery, transmural dispersion of repolarization (TDR) at baseline, TDR and difference of TDR (deltaTDR) during sympathetic nerve stimulation were measured at the IBZ. The distribution and densities of growth associated protein 43 and tyrosine hydroxylase positive nerves at the IBZ were detected with immunohistochemical techniques.

RESULTS

The study was completed in the 36 surviving animals (18 rabbits in each group). The densities of growth associated protein 43 and tyrosine hydroxylase positive nerves in the Metoprolol group (2,550 +/- 554 and 1,779 +/- 458 microm2/mm2, respectively) were lower than in the MI group (3,217 +/- 589 and 2,616 +/- 528 microm2/mm2, respectively; both p < 0.01). TDR at baseline, TDR and deltaTDR during sympathetic nerve stimulation were shorter in the Metoprolol group than in the MI group (p < 0.01 for all).

CONCLUSION

Metoprolol can inhibit sympathetic remodeling and electrical remodeling at the IBZ after MI. The association of metoprolol with improved electrical remodeling may be partly related to the inhibition of sympathetic remodeling.

Effects of the ??2-Adrenergic/DA2-Dopaminergic Agonist CHF-1024 in Preventing Ventricular Arrhythmogenesis and Myocyte Electrical Remodeling, in a Rat Model of Pressure-Overload Cardiac Hypertrophy

Cardiac hypertrophy induces morpho-functional myocardial alterations favoring arrhythmogenesis, especially under specific conditions such as sympathetic stimulation. We analyzed whether the dopaminergic agent CHF-1024, given its effect in decreasing adrenergic drive and collagen deposition in hypertrophied hearts, can also reduce arrhythmia vulnerability. Eighty-one male Wistar rats with intrarenal aortic coarctation and 18 control animals were studied. Fifty-eight banded animals were treated with CHF-1024 at four different doses (6, 2, 0.67, or 0.067 mg/Kg/die). One month after aortic ligature, spontaneous and sympathetic-induced ventricular arrhythmic events (VAEs) were telemetrically recorded in conscious animals. After sacrifice, membrane capacitance (Cm) and action potential duration (APD) were measured in isolated left ventricular myocytes (patch-clamp). In all groups, spontaneous VAEs were negligible whereas they significantly increased during sympathetic activation (stress exposure). Banded untreated animals showed a higher number of stress-induced VAEs, longer action potentials, and larger values of Cm and cell width as compared with control group. The treatment with CHF-1024 exhibited an antiarrhythmic effect, abolished APD prolongation, and reduced cell width at all doses. The lowest dose also prevented Cm increase. In conclusion, we demonstrated that in this model of pressure-overload hypertrophy CHF-1024 reduces arrhythmogenesis and causes a recovery of cell excitable properties toward a normal phenotype.

Correlation of alpha-skeletal actin expression, ventricular fibrosis and heart function with the degree of pressure overload cardiac hypertrophy in rats

We have analysed alterations of alpha-skeletal actin expression and volume fraction of fibrosis in the ventricular myocardium and their functional counterpart in terms of arrhythmogenesis and haemodynamic variables, in rats with different degrees of compensated cardiac hypertrophy induced by infra-renal abdominal aortic coarctation. The following coarctation calibres were used: 1.3 (AC1.3 group), 0.7 (AC0.7) and 0.4 mm (AC0.4); age-matched rats were used as controls (C group). One month after surgery, spontaneous and sympathetic-induced ventricular arrhythmias were telemetrically recorded from conscious freely moving animals, and invasive haemodynamic measurements were performed in anaesthetized animals. After killing, subgroups of AC and C rats were used to evaluate in the left ventricle the expression and spatial distribution of alpha-skeletal actin and the amount of perivascular and interstitial fibrosis. As compared with C, all AC groups exhibited higher values of systolic pressure, ventricular weight and ventricular wall thickness. AC0.7 and AC0.4 rats also showed a larger amount of fibrosis and upregulation of alpha-skeletal actin expression associated with a higher vulnerability to ventricular arrhythmias (AC0.7 and AC0.4) and enhanced myocardial contractility (AC0.4). Our results illustrate the progressive changes in the extracellular matrix features accompanying early ventricular remodelling in response to different degrees of pressure overload that may be involved in the development of cardiac electrical instability. We also demonstrate for the first time a linear correlation between an increase in alpha-skeletal actin expression and the degree of compensated cardiac hypertrophy, possibly acting as an early compensatory mechanism to maintain normal mechanical performance.

Gender differences in autonomic modulation of ventricular repolarization in humans

BACKGROUND

Gender differences in the incidence of ventricular arrhythmias have been reported and torsades de pointes associated with long QT syndrome are more common in women than men. Although increased sympathetic tone has an important role in vulnerability to arrhythmia, little is currently known regarding gender differences in the dynamic electrophysiological response to sympathetic stimulation. Therefore, we investigated whether there is a gender difference in humans with respect to the dynamic response of ventricular repolarization to beta-adrenergic stimulation and to autonomic blockade.

METHODS

Twelve-lead ECGs were continuously recorded during isoproterenol infusion (protocol 1) and autonomic blockade with propranolol and atropine infusion (protocol 2) in 24 healthy volunteers (12 men, 23 +/- 2 years; 12 women, 23 +/- 5 years). QT (QTc) intervals were measured at the baseline and at a heart rate of 75, 100, and 120 beats/min.

RESULTS

(1) The morphology of the T wave dynamically and transiently changed to bifid or biphasic during the acute phase of isoproterenol infusion. The incidence of these morphologic changes was higher in women than men (P < 0.05). (2) The QTc interval was initially prolonged and then shortened in both men and women during isoproterenol administration. However, QTc prolongation was significantly greater in women (0.44 +/- 0.02 to 0.55 +/- 0.03 sec) than men (0.42 +/- 0.03 to 0.51 +/- 0.04 sec; P < 0.05). (3) The QTc interval was significantly prolonged under autonomic blockade and the intrinsic QTc interval was longer in women than men (P < 0.05).

CONCLUSION

While sympathetic stimulation and autonomic blockade modulated the dynamics of ventricular repolarization in both sexes, it was more pronounced in women. This gender difference may partially account for the susceptibility of women to arrhythmogenesis.

Effect of Nifekalant, a Class III Anti-Arrhythmic Agent, on Ca2+ Waves in Rat Intact Trabeculae

BACKGROUND

Nifekalant, a class III anti-arrhythmic agent, has been used clinically at serum concentrations of 1-10 micromol/L in patients with ventricular arrhythmias. However, the effect of nifekalant on triggered arrhythmias has not yet been established.

METHODS AND RESULTS

Trabeculae were dissected from the right ventricles of 16 rat hearts. The force was measured using a silicon strain gauge, the membrane potential using ultra-compliant microelectrodes, and the regional intracellular Ca2+ ([Ca2+]i) using electrophoretically microinjected fura-2 and an image intensified CCD camera at a sarcomere length of 2.1 microm. Rapid cooling contractures (RCCs) were measured to estimate the Ca2+ content in the sarcoplasmic reticulum. Ca2+ waves and aftercontractions were measured after the induction of reproducible Ca2+ waves. Nifekalant at 1, 10 and 250 micromol/L increased significantly the action potential duration, the peak [Ca2+]i, the developed force and the amplitude of RCCs in a concentration-dependent manner (stimulus interval = 2 s, [Ca2+]o = 0.7 mmol/L, 26.0+/-0.2 degrees C). Nifekalant at 10 and 250 micromol/L increased significantly the velocity of Ca2+ waves with an enhancement of the aftercontractions (stimulus interval = 0.5 s for 7.5 s, [Ca2+]o = 1.8+/-0.1 mmol/L, 22.3+/-0.5 degrees C).

CONCLUSIONS

Nifekalant, even at a therapeutic concentration, can increase muscle contraction, but may worsen triggered arrhythmias because of the acceleration of Ca2+ waves under Ca2+-overloaded conditions.