Abnormal diastolic currents in ventricular myocytes from spontaneous hypertensive heart failure rats

Abstract

Abnormal diastolic currents in ventricular myocytes from spontaneous hypertensive heart failure rats. Am J Physiol Heart Circ Physiol 291: H2192-H2198, 2006. First published June 9, 2006 doi:10.1152/ajpheart.01146.2005.-Hypertension is a common cause of heart failure, and ventricular arrhythmias are a major cause of death in heart failure. The spontaneous hypertension heart failure (SHHF) rat model was used to study altered ventricular electrophysiology in hypertension and heart failure. We hypothesized that a reduction in the inward rectifier K ϩ current (IK1) and expression of pacemaker current (If) would favor abnormal automaticity in the SHHF ventricle. SHHF ventricular myocytes were isolated at 2 and 8 mo of age and during end-stage heart failure (Ն17 mo); myocytes from age-matched rats served as controls. Inward IK1 was significantly reduced at both 8 and Ն17 mo in SHHF rats compared with controls. There was a reduction in inward IK1 due to aging in the controls only at Ն17 mo. We found a significant increase in If at all ages in the SHHF rats, compared with young controls. In controls, there was an age-dependent increase in If. Action potential recordings in the SHHF rats demonstrated abnormal automaticity, which was abolished by the addition of an If blocker (10 M zatebradine). Increased If during hypertension alone or combined increases in If with reduced IK1 during the progression to hypertensive heart failure contribute to a substrate for arrhythmogenesis. aging; pacemaker current; inward rectifier potassium current; abnormal automaticity HYPERTENSION CAN LEAD TO COMPENSATORY hypertrophy in cardiac muscle. Chronically, these compensatory mechanisms can become maladaptive, and cardiomyopathy and heart failure can result (22). Experimental animal studies in hypertrophy and heart failure have shown abnormalities in ion channel function, altered expression and function of proteins involved in excitation-contraction coupling, and an increased propensity for cardiac arrhythmias (16,30,32). Clinically, ventricular arrhythmias occur in 80% of heart failure patients (11). However, the mechanisms of arrhythmogenic electrophysiological remodeling during hypertension and hypertensive heart failure are not fully elucidated.Spontaneous hypertensive heart failure (SHHF) rats develop hypertension at an early age. In contrast to spontaneously hypertensive rats (SHRs), the SHHF rats consistently develop reproducible, hypertensive heart failure in an age-dependent manner (26). Echocardiographic studies in the SHHF rat demonstrate left ventricular dysfunction (left ventricular ejection fraction Ͻ40%) at 17-18 mo of age (31). There is also evidence of progressive cardiac hypertrophy in the SHHF rats, evidenced as an increase in heart weights (36).Hypertension and heart failure can increase the pacemaker current (I f ) in the ventricular myocardium (4 -7). This can pathologically alter the diastolic phase of the action potential and enhance abnormal automaticity. In addition, aging has been shown to increase I f density i...

Down-regulation of miR-1/miR-133 Contributes to Re-expression of Pacemaker Channel Genes HCN2 and HCN4 in Hypertrophic Heart*

“…Strikingly, substantial up-regulation of I f expression has been observed in a variety of animal models of cardiac hypertrophy and heart failure and in human failing hearts as well (1-3, 9 -16). In such circumstances, I f is at least doubled in left ventricular (LV) 3 cells, depending upon severity of conditions (3,11,14), and reaches values comparable with those observed in the neonatal stage. Consistently, prominent re-expression of the I f -encoding genes, belonging to the hyperpolarization-activated cyclic nucleotide-gated channel (HCN) gene family in ventricular myocytes of hypertrophic heart has also been documented (17)(18)(19).…”

Transcriptional Control of Pacemaker Channel Genes <i>HCN2</i> and <i>HCN4</i> by Sp1 and Implications in Re-expression of These Genes in Hypertrophied Myocytes

“…Abnormally enhanced automaticity of ventricular cells may contribute to arrhythmias in hypertrophied hearts. Consistent with this notion, robust enhancement of functional I f and upregulation of HCN gene expression have been observed in a variety of animal models of cardiac hypertrophy and heart failure and in human failing hearts as well [1,[6][7][8][9][10][11][12][13][14]. Normally, I f expresses in ventricular myocytes only during fetal and neonatal life and the abundance progressively decreases toward adulthood in terms of the number of cells expressing I f and of the density of expressed I f as well [4-6, 9, 12, 15, 16].…”

“…One of the characteristic alterations of hypertrophic hearts is the re-expression or overexpression of pacemaker channel genes thereby I f current, which may contribute significantly to the enhanced abnormality and the risk of arrhythmias under such pathological conditions [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. However, the mechanisms remained poorly understood.…”

“…This results in a loss of the capacity of adult ventricular cells to generate spontaneous activity. In cardiac hypertrophy, I f is at least doubled in left ventricular (LV) cells, depending upon severity of conditions [5,13,14].…”

Pathophysiology of HCN channels

“…Furthermore, it was observed that the upregulation of I f in diseased myocytes results in the occurrence of a diastolic depolarization in ventricular cells [6,42]. Experiments with adenoviral HCN2 constructs support these finding.…”