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

ABSTRACT: Ventricular arrhythmias can cause death in heart failure (HF). A trigger is the occurrence of Ca(2+) waves which activate a Na(+)‐Ca(2+) exchange (NCX) current, leading to delayed after‐depolarisations and triggered action potentials. Waves arise when sarcoplasmic reticulum (SR) Ca(2+) content reaches a threshold and are commonly induced experimentally by raising external Ca(2+), although the mechanism by which this causes waves is unclear and was the focus of this study. Intracellular Ca(2+) was measured in voltage‐clamped ventricular myocytes from both control sheep and those subjected to rapid pacing to produce HF. Threshold SR Ca(2+) content was determined by applying caffeine (10 mM) following a wave and integrating wave and caffeine‐induced NCX currents. Raising external Ca(2+) induced waves in a greater proportion of HF cells than control. The associated increase of SR Ca(2+) content was smaller in HF due to a lower threshold. Raising external Ca(2+) had no effect on total influx via the L‐type Ca(2+) current, I (Ca‐L), and increased efflux on NCX. Analysis of sarcolemmal fluxes revealed substantial background Ca(2+) entry which sustains Ca(2+) efflux during waves in the steady state. Wave frequency and background Ca(2+) entry were decreased by Gd(3+) or the TRPC6 inhibitor BI 749327. These agents also blocked Mn(2+) entry. Inhibiting connexin hemi‐channels, TRPC1/4/5, L‐type channels or NCX had no effect on background entry. In conclusion, raising external Ca(2+) induces waves via a background Ca(2+) 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(2+) waves resulting in delayed after‐depolarisations are a key trigger of arrhythmias. Ca(2+) waves arise when the sarcoplasmic reticulum (SR) becomes overloaded with Ca(2+). We investigate the mechanism by which raising external Ca(2+) causes waves, and how this is modified in heart failure. We demonstrate that a novel sarcolemmal background Ca(2+) influx via the TRPC6 channel is responsible for SR Ca(2+) overload and Ca(2+) waves. The increased propensity for Ca(2+) 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(2+) waves may arise in heart failure, providing a basis for future work and novel therapeutic targets.