Right ventricular outflow tract cardiomyocytes possess distinct calcium handling and contractile properties
Authors: Blok M , Rivaud M R , Van Wijk S W , Scholman K T , Schumacher C A , Jongbloed M R M , Brundel B J J M , Coronel R , Bosada F M , Baartscheer A , Verkerk A O , Veldkamp M W , Boukens B J D 
Published: May 2024

Abstract

Funding Acknowledgements
Type of funding sources: Other. Main funding source(s): Rembrandt Research Grant 2021

Background
The right ventricular outflow tract (RVOT) is the predominant site of origin of idiopathic ventricular tachycardia. Idiopathic ventricular tachycardias are typically sensitive to adenosine, consistent with triggered activity caused by delayed afterdepolarizations due to intracellular Ca2+ overload. The distinct properties of RVOT myocytes explaining increased susceptibility to Ca2+ overload and triggered activity are unknown.

Purpose
To identify distinct RVOT electrophysiological properties including Ca2+ handling and its molecular underpinnings.

Methods and Results
Experiments were performed using freshly isolated cardiomyocytes from the RVOT and right ventricular (RV) apex of adult rabbit hearts. To identify distinct RVOT electrophysiological properties, we employed the perforated patch clamp methodology and measured action potentials and L-type Ca2+ current. Action potentials of RVOT myocytes were significantly shorter compared to RV myocytes, particularly at lower pacing frequency. L-type Ca2+ current peak density was significantly lower in RVOT myocytes compared to RV myocytes (34.0 ± 9.1% at 0 mV). However, voltage-dependencies of L-type Ca2+ current activation and inactivation were not statistically different. Next we measured intracellular Ca2+ transients using the fluorescent Ca2+ indicator indo-1 and found that Ca2+ transient amplitudes were significantly lower in RVOT myocytes compared to RV myocytes. To investigate a relationship between RVOT myocyte Ca2+ handling and contractility, we performed unloaded sarcomere shortening measurements using the Cytocypher Multicell system. Shortening of relaxation time following increased pacing frequency (1-2 Hz) was significantly larger for RVOT myocytes compared to RV myocytes. To identify molecular factors explaining the distinct properties of RVOT myocytes, we performed RNA-seq analysis of differential gene expression using the DESeq2 method. RVOT and RV transcriptomic profiles consisted of 30 differentially expressed genes (adjusted p ≤ 0.05). One of genes significantly higher expressed by the RV was synaptotagmin (SYT1), a Ca2+ sensor and positive regulator of the cardiac L-type Ca2+ channel through inhibition of syntaxin 1A [Ref. 1, 2].

Conclusion
RVOT cardiomyocytes possess distinct electrophysiological and contractile properties, reflected by shorter action potentials, lower Ca2+ transient amplitudes, and larger shortening of relaxation time upon increasing pacing frequency. The RVOT and RV have similar but distinct gene expression signatures. Future experiments will focus on determining the role of SYT1 and other genes in regulation of the L-type Ca2+ current and Ca2+ handling in RVOT myocytes.Distinct properties of RVOT myocytes.Transcriptomic profile of the RVOT.