Unit of Cardiac Physiology, Division of Cardiovascular Sciences, Faculty of Biology Medicine and Health, Manchester Academic Health Science Centre, University of Manchester, Manchester, UK.
Manchester University NHS Foundation Trust, Manchester, UK.
J Physiol. 2022 Jun;600(11):2637-2650. doi: 10.1113/JP282168. Epub 2022 Mar 20.
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.
心室性心律失常可导致心力衰竭（HF）患者死亡。触发因素是 Ca 波的出现，Ca 波激活钠-钙交换（NCX）电流，导致延迟后去极化和触发动作电位。当肌浆网（SR）Ca 含量达到阈值时，会出现 Ca 波，通常通过提高细胞外 Ca 来在实验中诱导 Ca 波，尽管引起 Ca 波的机制尚不清楚，这也是本研究的重点。在电压钳制的心室肌细胞中测量了来自对照绵羊和快速起搏产生 HF 的绵羊的细胞内 Ca。通过在 Ca 波后应用咖啡因（10 mM）来确定 SR Ca 含量的阈值，并整合 Ca 波和咖啡因诱导的 NCX 电流。与对照相比，HF 细胞中更易出现 Ca 波，而 HF 细胞中 SR Ca 含量的增加较小，因为阈值较低。提高细胞外 Ca 对通过 L 型 Ca 电流 I 的总流入没有影响，但增加了 NCX 的流出。对肌浆膜通量的分析表明，在稳定状态下，Ca 波期间持续存在大量背景 Ca 内流，以维持 Ca 外流。用 Gd 或 TRPC6 抑制剂 BI 749327 可降低 Ca 波频率和背景 Ca 内流。这些药物还可阻断 Mn 内流。抑制连接蛋白半通道、TRPC1/4/5、L 型通道或 NCX 对背景内流没有影响。总之，通过 TRPC6 通道的背景 Ca 内流，提高细胞外 Ca 可诱导 Ca 波。HF 中 Ca 波出现的倾向更大，是由于背景内流增加和 SR 内容物阈值降低所致。关键点：心力衰竭是一种致心律失常状态，心律失常是死亡的主要原因。在细胞水平上，导致延迟后去极化的 Ca 波是心律失常的关键触发因素。当肌浆网（SR）Ca 过载时，Ca 波就会出现。我们研究了提高细胞外 Ca 导致 Ca 波的机制，以及 HF 中如何改变这种机制。我们证明，通过 TRPC6 通道的新型肌浆膜背景 Ca 内流导致 SR Ca 过载和 Ca 波。HF 中 Ca 波出现的倾向更大，是由于背景内流增加，SR 内容物阈值降低所致。本研究的结果突出了心力衰竭中 Ca 波可能产生的一种新机制，为进一步的研究和新的治疗靶点提供了依据。
