Elevated InsP3R expression underlies enhanced calcium fluxes and spontaneous extra-systolic calcium release events in hypertrophic cardiac myocytes

Abstract

Cardiac hypertrophy is associated with profound remodelling of Ca2+ signalling pathways. During the early, compensated stages of hypertrophy, Ca2+ fluxes may be enhanced to facilitate greater contraction, whereas as the hypertrophic heart decompensates, Ca2+ homeostatic mechanisms are dysregulated leading to decreased contractility, arrhythmia and death. Although ryanodine receptor Ca2+ release channels (RyR) on the sarcoplasmic reticulum (SR) intracellular Ca2+ store are primarily responsible for the Ca2+ flux that induces myocyte contraction, a role for Ca2+ release via the inositol 1,4,5-trisphosphate receptor (InsP3R) in cardiac physiology has also emerged. Specifically, InsP3-induced Ca2+ signals generated following myocyte stimulation with an InsP3-generating agonist (e.g. endothelin, ET-1), lead to modulation of Ca2+ signals associated with excitation-contraction coupling (ECC) and the induction of spontaneous Ca2+ release events that cause cellular arrhythmia. Using myocytes from spontaneously hypertensive rats (SHR), we recently reported that expression of the type 2 InsP3R (InsP3R2) is significantly increased during hypertrophy. Notably, this increased expression was restricted to the junctional SR in close proximity to RyRs. There, enhanced Ca2+ release via InsP3Rs serves to sensitise neighbouring RyRs causing an augmentation of Ca2+ fluxes during ECC as well as an increase in non-triggered Ca2+ release events. Although the sensitization of RyRs may be a beneficial consequence of elevated InsP3R expression during hypertrophy, the spontaneous Ca2+ release events are potentially of pathological significance giving rise to cardiac arrhythmia. InsP3R2 expression was also increased in hypertrophic hearts from patients with ischemic dilated cardiomyopathy and aortically-banded mice demonstrating that increased InsP3R expression may be a general phenomenon that underlies Ca2+ changes during hypertrophy.