Electrophysiological and molecular remodeling in septal myocytes from fHCM patients

Purpose: Familiar Hypertrophic Cardiomyopathy (fHCM) is the most common cause of sudden death in the young. Molecular and electrophysiological mechanisms underlying ventricular hypertrophy and remodeling in fHCM, likely predisposing to fatal arrhythmias, remain largely unknown. To get insight into cellular arrhythmogenic mechanisms of fHCM, we investigated: the electrophysiological properties of freshly isolated septal myocytes, the role of serotonin 5HT2 receptors and the functional and molecular abnormalities of f-channel (HCN), taken as a marker of remodeling. Septal myocytes were enzymatically isolated from specimens obtained from patients undergoing myectomy; myocytes were patch-clamped to record action potential (AP), transient outward current (Ito), and f-current (If). Control septal myocytes were isolated from patients undergoing aortic valve replacement. Biopsies, obtained from fHCM patients, were used for relative quantifications of HCN2/4 isoforms and 5HT2 receptor gene by Real-Time PCR. Undiseased hearts not used for transplantation served as controls. Results: Duration at 90% of repolarization (APD90) is increased in fHCM septal myocytes compared to control (762±169ms vs 243±111 ms). fHCM action potential does not show the typical “notch” of phase 1 fast repolarization, clearly evident in control cells. Specific density of Ito at +50mV is reduced in fHCM septal myocytes (0.81±0.60pA/pF) compared with control myocytes (4.91±0.45pA/pF). HCN2/4 ratio was significantly increased in hypertrophied samples (6.82±4.6) relative to the controls (1.46±1.08, p<0.01). Consistently with a relative over-expression of HCN2 isoform, If activates at more negative potential in patch-clamped fHCM compared to control myocytes. 5HT2 mRNA was significantly increased in fHCM with respect to the controls (5.83±1.6 vs 1.27±0.17, p<0.05). Functional coupling of 5HT2 was confirmed by measuring the effect of 5HT2 receptor selective agonist metil-serotonin (1M) on action potential in single fHCM myocyte: it increased action potential duration, an effect which was reversed by wash-out; no effect was detected in control cells. Conclusions: fHCM is associated with typical molecular and electrophysiological alterations. These changes are consistent with those described in myocytes from other arrhythmogenic disorders, such as heart failure. 5HT2B receptors over-expression could represent a novel mechanism responsible for induction and maintenance of an altered functional phenotype in the human heart. 5HT2B receptors and f-current could represent novel targets for effective pharmacological interventions aimed to prevent and treat fHCM.