Department of Internal Medicine, Carver College of Medicine, University of Iowa, Iowa City, USA.
Circ Arrhythm Electrophysiol. 2011 Jun;4(3):379-87. doi: 10.1161/CIRCEP.110.961771. Epub 2011 Mar 15.
Catecholamines increase heart rate by augmenting the cAMP-responsive hyperpolarization-activated cyclic nucleotide-gated channel 4 pacemaker current (I(f)) and by promoting inward Na(+)/Ca(2+) exchanger current (I(NCX)) by a "Ca(2+) clock" mechanism in sinoatrial nodal cells (SANCs). The importance, identity, and function of signals that connect I(f) and Ca(2+) clock mechanisms are uncertain and controversial, but the multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is required for physiological heart rate responses to β-adrenergic receptor (β-AR) stimulation. The aim of this study was to measure the contribution of the Ca(2+) clock and CaMKII to cardiac pacing independent of β-AR agonist stimulation.
We used the L-type Ca(2+) channel agonist Bay K8644 (BayK) to activate the SANC Ca(2+) clock. BayK and isoproterenol were similarly effective in increasing rates in SANCs and Langendorff-perfused hearts from wild-type control mice. In contrast, SANCs and isolated hearts from mice with CaMKII inhibition by transgenic expression of an inhibitory peptide (AC3-I) were resistant to rate increases by BayK. BayK only activated CaMKII in control SANCs but increased L-type Ca(2+) current (I(Ca)) equally in all SANCs, indicating that increasing I(Ca) was insufficient and suggesting that CaMKII activation was required for heart rate increases by BayK. BayK did not increase I(f) or protein kinase A-dependent phosphorylation of phospholamban (at Ser16), indicating that increased SANC Ca(2+) by BayK did not augment cAMP/protein kinase A signaling at these targets. Late-diastolic intracellular Ca(2+) release and I(NCX) were significantly reduced in AC3-I SANCs, and the response to BayK was eliminated by ryanodine in all groups.
The Ca(2+) clock is capable of supporting physiological fight-or-flight responses, independent of β-AR stimulation or I(f) increases. Complete Ca(2+) clock and β-AR stimulation responses require CaMKII.
儿茶酚胺通过增强 cAMP 反应性超极化激活的环核苷酸门控通道 4 起搏电流（I(f)）和通过“钙钟”机制促进内向 Na(+)/Ca(2+)交换器电流（I(NCX)）来增加窦性结细胞（SANCs）的心率。连接 I(f)和钙钟机制的信号的重要性、身份和功能尚不确定且存在争议，但多功能钙/钙调蛋白依赖性蛋白激酶 II（CaMKII）是生理心率对β-肾上腺素能受体（β-AR）刺激的反应所必需的。本研究的目的是在不依赖β-AR 激动剂刺激的情况下测量钙钟和 CaMKII 对心脏起搏的贡献。
我们使用 L 型钙通道激动剂 Bay K8644（BayK）激活 SANC 钙钟。BayK 和异丙肾上腺素在增加野生型对照小鼠 SANCs 和 Langendorff 灌注心脏的速率方面同样有效。相比之下，通过转基因表达抑制肽（AC3-I）抑制 CaMKII 的 CaMKII 抑制的 SANCs 和分离的心脏对 BayK 引起的速率增加具有抗性。BayK 仅在对照 SANCs 中激活 CaMKII，但同等增加所有 SANCs 中的 L 型钙电流（I(Ca)），表明增加 I(Ca)是不足够的，并表明 CaMKII 激活是 BayK 引起心率增加所必需的。BayK 并未增加 I(f)或磷酸化酶 A 依赖性磷蛋白（在 Ser16）的磷酸化，表明 BayK 增加 SANC Ca(2+)并未增强这些靶标处的 cAMP/蛋白激酶 A 信号传导。AC3-I SANCs 中的晚期舒张期细胞内 Ca(2+)释放和 I(NCX)显著降低，并且在所有组中，Ryanodine 消除了对 BayK 的反应。
钙钟能够支持生理上的战斗或逃跑反应，而不依赖于β-AR 刺激或 I(f) 的增加。完整的钙钟和β-AR 刺激反应需要 CaMKII。
