Decreased pacemaker activity in aged sinoatrial node

心脏变时性和变异性的一般理论是如何应用于衰老的？ 我们早期和最近的研究都为理解人类和动物模型中与年龄相关的心肌收缩能力和心率恶化提供了开创性的观点。我们在巴尔的摩老龄化纵向研究(BLSA)中对健康参与者的研究表明，随着年龄的增长，在应对分级运动压力时，提高心率和减少收缩末期容量的能力发生了显著变化。(值得注意的是，只有后者，而不是前者，可以通过身体锻炼来改善。) 我们随后证明，人类的收缩能力和心率的缺陷部分是由于cAMP-PKA的AR刺激反应减少所致。 1.可伸缩性 在分离的大鼠心肌上，我们已经直接证明了钙循环和对-AR刺激的收缩反应的减少。在无-AR刺激时，钙瞬变和收缩的松弛时间延长，这可能是由于SERCA2表达减少所致。AP也明显延长，这是由于L型钙电流和钾电流的增龄性变化。在单个Vm中，除了表面膜离子通道的众多特性外，我们还发现Vm的钙时钟随年龄而降低，表现为兴奋-钙释放-收缩耦合过程的恢复时间延长，这是由于受L型钙电流的激活，通过RyRs释放SR钙的恢复时间延长。我们还发现，由于AP的延长，来自旧心脏的心肌细胞的肌浆网钙负荷和钙瞬变幅度得以保留。 2.心率 我们在兔SANC中发现的通过偶时钟系统调节心率的发现，最终(25年后)在阐明与年龄相关的变时性储备减少的潜在细胞机制方面取得了进展。具体地说，我们的概念性突破认为，基础起搏细胞自律性的调节需要受PDE和磷酸酶活性调节的Ca-PKA-CaMKII信号，而受体的刺激只是延长了这种调节，这导致了我们假设，参与调节随年龄增长的固有心率的机制恶化的机制，以及那些调节心率储备的机制可能涉及SANC的钙时钟的放缓，并且至少在某些方面，与调节Vm中钙循环和收缩的机制的年龄相关的恶化有关。 对照组与10 M IBMX孵育后，16位点的磷蛋白(PLB)磷酸化(PLB/总PLB)免疫标记无明显变化，青年组增加了2.20.2倍，而老年组则无明显增加。 固有的SR钙循环及其对PDE抑制的反应随着年龄的增长而下降，这是至少部分解释与年龄相关的IHR下降的候选机制。五年来，我们一直试图分离出表现出稳定的正常自律性的单个小鼠窦房结细胞，但失败了。最近，我们取得了一些成功。我们的初步结果表明：单个SARC的自发AP放电频率随年龄的增长而下降；AP对AP的最大RyR钙释放通量(以dCa/dt max为指标)随年龄的增长而下降； 我们的结果提示，衰老小鼠体内cAMP-PKA-Ca~(2+)信号转导途径存在缺陷。这种缺陷可能是由于：1)钙循环蛋白的数量或功能减少，如肌质网钙泵、磷蛋白(PLB)、RyRs。和/或2。)由于cAMP介导的PKA依赖的磷酸化增加，降低了钙循环蛋白如PLB的磷酸化。3.)这些缺陷可能解释了为什么年老的心脏不如年轻的心脏跳动得快。未来的计划是：1)用免疫印迹法检测青年和老年SANS细胞中的钙循环蛋白密度；2)直接检测cAMP-PKA-钙通路的下游信号，即PKA依赖的蛋白磷酸化和磷酸酶活性。

How does the General Theory of Cardiac Chronotropy and Inotropy Apply to Aging? Both our earlier and very recent studies have contributed seminal perspectives toward understanding age-associated deterioration of both cardiac contractility and heart rate in both humans, and in animal models. Our studies of healthy participants in the Baltimore Longitudinal Study of Aging (BLSA) demonstrated substantial age-associated changes in the ability to increase heart rate and reduce end systolic volume in response to graded exercise stress. (Of note, only the latter, but not the former, can be improved by physical conditioning.) We subsequently demonstrated that deficits both in contractility and heart rate in humans are due, in part, to reduced -AR stimulation response of cAMP-PKA. 1. Contractility In isolated rat cardiac ventricular muscle, we had directly demonstrated a reduction in the Ca2+ cycling and contractile response to -AR stimulation. The relaxation time of the Ca2+ transient and contraction in the absence of -AR stimulation were prolonged, that we traced to a reduced expression of SERCA2. The AP was also markedly prolonged, due to age-associated changes in L-type Ca2+ and K+ currents. In single VM, in addition to numerous properties of surface membrane ion channels, we documented an age-associated reduction in the VM Ca2+ clock, manifest as a prolonged time for restitution of the excitation- Ca2+ release-contraction coupling process, was due to a prolonged restitution time for SR Ca2+ release via RyRs in response to activation by an L-type Ca2+ current. We also showed that SR Ca2+ loading and the amplitude of the Ca2+ transient were preserved in myocytes from the old heart, by virtue of the prolonged AP. 2. Heart rate Our discovery of heart rate regulation by a coupled-clock system in rabbit SANC enabled progress, finally (after 25 years), on the elucidation of potential cellular mechanisms of the age-associated reduction in chronotropic reserve. Specifically, our conceptual breakthrough that regulation of basal pacemaker cell automaticity requires Ca-PKA-CaMKII signaling regulated by PDE and phosphatase activities, and that stimulation of receptors merely extended this regulation, led us to hypothesize that mechanisms involved in the deterioration of mechanisms that regulate of intrinsic heart rate that accompanies advancing age and that those regulating heart rate reserve may involve a slowing of the Ca2+ clock of SANC and linked, in some respects, at least, to the age-associated deterioration of mechanisms that regulate Ca2+ cycling and contractility in VM. Phospholamban (PLB) phosphorylation at Ser16 (PLB/total PLB) immunolabeling in control did not differ with age, increased 2.20.2 fold after incubation with 10 M IBMX in young, but did not increase in old SANC. The intrinsic SR Ca2+ cycling and its response to PDE inhibition decline with aging, and are candidate mechanisms to explain, in part at least, the age-associated decrease in IHR. We had failed for five years in our attempts to isolate single mouse sinoatrial node cells that exhibited stable normal automaticity. Recently, we have met with some success. Our initial results indicate that: the spontaneous AP firing rate of single SANC declines with age; the maximum RyR Ca release flux in response to an AP (indexed as d Ca/dt max) declines with age; and the kinetics of relaxation of the Ca2+ transient become reduced with age. Our results suggest that the intrinsic cAMP-PKA-Ca2+ signaling is deficient in the aged mice. This deficiency may result from: 1.)reduced amount or function of Ca2+ cycling proteins e.g. SR Ca2+ pump, phospholamban (PLB), RyRs. and/or 2.) reduced phosphorylation of Ca2+ cycling proteins e.g.PLB, in response to an increase in cAMP-mediated-PKA dependent phosphorylation. 3.) These deficiencies may explain why the aged heart cannot beat as fast as the young heart. Future plans are: 1) to employ western blotting in cells from young and old SANs to measure Ca2+ cycling protein density; 2) to directly measure the downstream signaling of cAMP-PKA- Ca2+ pathway i.e. PKA-dependent protein phosphorylation and phosphatase activity.