Progress in the Intracellular Clock that Drives the Heart's Pacemaker

LCS对起搏器功能的研究产生了这样的想法，如本文所述，SANC的正常自律性是内部钙离子时钟和表面膜离子通道时钟相互夹带的结果。总之，兔SANC的钙钟是由高基础水平的cAMP驱动的。在SANC中不存在b-ARs的结构性激活。相反，SANC表达被Ca~(2+)激活的AC亚型(AC1和8)，允许Ca~(2+)释放作为前馈系统来控制SANC跳动：AC的Ca~(2+)激活增加cAMP，后者激活PKA，导致Ca~(2+)循环蛋白的磷酸化和Ca~(2+)时钟的加快。SANC内显著升高的基础PDE活性作为刹车，限制cAMP水平，cAMP依赖的钙循环蛋白的磷酸化程度调节LCRs的特征，并将基础搏动频率维持在远低于最大水平。 换句话说，依赖于PKA的磷酸化调节了肌质网钙离子循环的动力学，也决定了钙激活的钙通道的结构活性，进而增加了cAMP和PKA，从而决定了LCRs的周期性，进而决定了自发的SARC周期长度。阶段性抑制PKA活性导致钙循环蛋白磷酸化阶段性降低，钙时钟阶段性减慢(LCR周期延长)，自发搏动频率阶段性降低。 基于我们提供的广泛的数据库，我们建议对整个DD形状提出一种新的、相对简单的解释，这是心脏起搏器领域自发现以来的一个主要问题。钙离子时钟通过DD加速启动后续AP，从而启动每个起搏器周期。MDP，随后是早期的慢DD，实际上是后电位的组成部分，即从先前AP恢复离子通道。 在成人和胚胎起搏器细胞中，钙离子和有节律性的自发钙离子释放在启动和调节正常心脏自律性中起关键作用的证据也提供了重新结合起搏器和心室细胞研究的关键32，35。简化论者分别阐明了心室肌细胞收缩强度和起搏细胞自发搏动频率是如何调节的，这两种方法的成就现在趋于一致：通过心室肌细胞和起搏细胞共同的蛋白质循环进入SR和离开SR的Ca~(2+)循环可以描述为Ca~(2+)时钟。钙离子时钟和表膜离子通道时钟的相互夹带调节这两种细胞的占空比。因此，在我们的研究的基础上，我们提出了心脏匀浆和心脏匀浆的一般理论。

LCS studies of pacemaker function have generated the idea that, as reviewed here, normal automaticity of SANC results from the mutual entrainment of an internal Ca2+ clock and surface membrane ion channel clock. In short, The Ca2+ clock in rabbit SANC is driven by high basal level of cAMP. Constitutive activation of b-ARs is not present within SANC. Rather, SANC express AC isoforms that are Ca2+ activated (AC1&8), permitting Ca2+ releases to act as a feed forward system to control SANC beating: Ca2+ activation of AC increases cAMP which activates PKA resulting in phosphorylation of Ca2+ cycling proteins and an acceleration of the Ca2+ clock. A markedly elevated basal PDE activity within SANC acts as brake to limit the cAMP level and the extent of cAMP-dependent phosphorylation of Ca2+ cycling proteins regulates LCRs characteristics and maintains basal beating rate well below maximum level. In other words, PKA dependent phosphorylation regulates kinetics of SR Ca2+ cycling and also determines the constitutive activity of ACs (activated by Ca2+), which, in turn, increases cAMP and PKA and thus determines the periodicity of LCRs, which in turn, determines the spontaneous SANC cycle length. Graded inhibition of PKA activity results in graded reduction of Ca2+ cycling protein phosphorylation, graded slowing of the Ca2+ clock (prolongation of LCR period) and graded reduction of the spontaneous beating rate 4.As noted, evidence regarding control of basal cAMP mediated PKA dependent Ca2+ cycling protein phosphorylation that drives spontaneous beating of SANC has begun to emerge. Based on our extensive database presented here, we suggest a new, relatively simple interpretation of the entire DD shape, a major problem of the cardiac pacemaker field since its discovery. The Ca2+ clock initiates each pacemaker cycle by igniting a subsequent AP via DD acceleration. The MDP, followed by early slow DD, are in fact components of an afterpotential, i.e. recovery of ion channels from previous AP. The evidence for the crucial role of Ca2+ and rhythmic spontaneous Ca2+ releases in the initiation and regulation of normal cardiac automaticity both in adult and embryonic pacemaker cells also provides the key that reunites pacemaker and ventricular cell research 32, 35. The separate achievements of reductionist approaches to elucidate how strength of contraction of ventricular myocytes and the spontaneous beating rate of pacemaker cells are regulated now converge: Ca2+ cycling into and from the SR by proteins common to both ventricular myocytes and pacemaker cells can be portrayed as Ca2+ clock. The mutual entrainment of the Ca2+ clock and surface membrane ion channel clocks regulate the duty cycle of both cell types. THUS, BASED UPON OUR STUDIES, WE PROPOSE A GENERAL THEORY OF CARDIAC INOTROPY AND CHRONOTROPY.