Interaction of background Ca 2+ influx, sarcoplasmic reticulum threshold and heart failure in determining propensity for Ca 2+ waves in sheep heart

Ventricular arrhythmias can cause death in heart failure (HF). A trigger is the occurrence of Ca waves which activate a Na -Ca exchange (NCX) current, leading to delayed after-depolarisations and triggered action potentials. Waves arise when sarcoplasmic reticulum (SR) Ca content reaches a threshold and are commonly induced experimentally by raising external Ca , although the mechanism by which this causes waves is unclear and was the focus of this study. Intracellular Ca was measured in voltage-clamped ventricular myocytes from both control sheep and those subjected to rapid pacing to produce HF. Threshold SR Ca content was determined by applying caffeine (10 mM) following a wave and integrating wave and caffeine-induced NCX currents. Raising external Ca induced waves in a greater proportion of HF cells than control. The associated increase of SR Ca content was smaller in HF due to a lower threshold. Raising external Ca had no effect on total influx via the L-type Ca current, I , and increased efflux on NCX. Analysis of sarcolemmal fluxes revealed substantial background Ca entry which sustains Ca efflux during waves in the steady state. Wave frequency and background Ca entry were decreased by Gd or the TRPC6 inhibitor BI 749327. These agents also blocked Mn entry. Inhibiting connexin hemi-channels, TRPC1/4/5, L-type channels or NCX had no effect on background entry. In conclusion, raising external Ca induces waves via a background Ca influx through TRPC6 channels. The greater propensity to waves in HF results from increased background entry and decreased threshold SR content. KEY POINTS: Heart failure is a pro-arrhythmic state and arrhythmias are a major cause of death. At the cellular level, Ca waves resulting in delayed after-depolarisations are a key trigger of arrhythmias. Ca waves arise when the sarcoplasmic reticulum (SR) becomes overloaded with Ca . We investigate the mechanism by which raising external Ca causes waves, and how this is modified in heart failure. We demonstrate that a novel sarcolemmal background Ca influx via the TRPC6 channel is responsible for SR Ca overload and Ca waves. The increased propensity for Ca waves in heart failure results from an increase of background influx, and a lower threshold SR content. The results of the present study highlight a novel mechanism by which Ca waves may arise in heart failure, providing a basis for future work and novel therapeutic targets.