IP3 Receptor-mediated Ca Release Facilitates RyR-Ca Release To Cause Inotropy And Arrhythmogenicity In Mouse Ventricular Myocytes

Endothelin (ET-1)-induced IP3R-dependent Ca release in atrial myocytes is well established. ET-1-induced positive inotropy and arrhythmogenicity have also been shown in ventricular myocytes from rat and rabbit. Here we used transgenic mice overexpressing IP3R2 and IP3 sponge to study ET-1-induced inotropy and arrhythmogenicity in ventricular myocytes. 100 nM ET-1 (14 min) increased ventricular myocyte Ca transients (ΔCa) in IP3R2OX by 37% (0.5 Hz, fluo4 AM, n=11) vs WT and IP3 sponge ventricle (-6±10 %). ET-1 increased spontaneous calcium transient (SCT) frequency in IP3R2OX ventricles compared with WT and IP3 sponge (10±4 vs 1±1 SCT/min). Similar ΔCa results were obtained using the Ca indicator indo-1 (41±9 vs 19±12 % IP3R2OX vs WT, n=6). We found no differences between IP3R2OX (n=17) and WT (n=10) ventricular myocytes in control (0.5 Hz) ΔCa (350±70 vs 330±28 nM Cai) or SR Ca content as assessed with 10 mM caffeine (1368±273 vs 1385±78 nM Cai). Basal Ca spark frequency in saponin-permeabilized ventricular myocytes did not differ between IP3R2OX and WT (19±4 vs 16±4 sparks x s-1 x (100 μm)-1). However, direct application of 10 μM IP3 produced higher increase in Ca spark frequency on IP3R2OX (35 %, n=4) than WT (18 %). After 3 min of IP3 application SR Ca content was depleted to 80 % of control. These data suggest that IP3R-dependent inotropy is mediated via IP3R-dependent Ca release that facilitates Ca release through ryanodine receptors. This IP3-dependent inotropy is associated with spontaneous Ca release activity which may be arrhythmogenic (inducing action potentials). We conclude that IP3R expression and IP3-dependent Ca release play an important role in the generation of ventricular arrhythmias.