Patterson Michael, Sneyd James, Friel David D
Department of Neurosciences, Case Western Reserve University, Cleveland, OH 44106, USA.
J Gen Physiol. 2007 Jan;129(1):29-56. doi: 10.1085/jgp.200609660.
Many models have been developed to account for stimulus-evoked [Ca(2+)] responses, but few address how responses elicited in specific cell types are defined by the Ca(2+) transport and buffering systems that operate in the same cells. In this study, we extend previous modeling studies by linking the time course of stimulus-evoked [Ca(2+)] responses to the underlying Ca(2+) transport and buffering systems. Depolarization-evoked Ca(2+) responses were studied in sympathetic neurons under voltage clamp, asking how response kinetics are defined by the Ca(2+) handling systems expressed in these cells. We investigated five cases of increasing complexity, comparing observed and calculated responses deduced from measured Ca(2+) handling properties. In Case 1, Ca(2+) responses were elicited by small Ca(2+) currents while Ca(2+) transport by internal stores was inhibited, leaving plasma membrane Ca(2+) extrusion intact. In Case 2, responses to the same stimuli were measured while mitochondrial Ca(2+) uptake was active. In Case 3, responses were elicited as in Case 2 but with larger Ca(2+) currents that produce larger and faster Ca(2+) elevations. Case 4 included the mitochondrial Na/Ca exchanger. Finally, Case 5 included ER Ca(2+) uptake and release pathways. We found that Ca(2+) responses elicited by weak stimuli (Cases 1 and 2) could be quantitatively reconstructed using a spatially uniform model incorporating the measured properties of Ca(2+) entry, removal, and buffering. Responses to strong depolarization (Case 3) could not be described by this model, but were consistent with a diffusion model incorporating the same Ca(2+) transport and buffering descriptions, as long as endogenous buffers have low mobility, leading to steep radial Ca(2+) gradients and spatially nonuniform Ca(2+) loading by mitochondria. When extended to include mitochondrial Ca(2+) release (Case 4) and ER Ca(2+) transport (Case 5), the diffusion model could also account for previous measurements of stimulus-evoked changes in total mitochondrial and ER Ca concentration.
已经开发了许多模型来解释刺激诱发的[Ca(2+)]反应，但很少有模型探讨在特定细胞类型中引发的反应是如何由在同一细胞中起作用的Ca(2+)转运和缓冲系统所定义的。在本研究中，我们通过将刺激诱发的[Ca(2+)]反应的时间进程与潜在的Ca(2+)转运和缓冲系统联系起来，扩展了先前的建模研究。在电压钳制下，研究了交感神经元中去极化诱发的Ca(2+)反应，探讨了这些细胞中表达的Ca(2+)处理系统如何定义反应动力学。我们研究了五种复杂度不断增加的情况，比较了根据测量的Ca(2+)处理特性推导的观察到的和计算出的反应。在情况1中，Ca(2+)反应由小的Ca(2+)电流引发，同时内部储存库的Ca(2+)转运受到抑制，而质膜Ca(2+)外排保持完整。在情况2中，在线粒体Ca(2+)摄取活跃时测量对相同刺激的反应。在情况3中，以与情况2相同的方式引发反应，但具有更大的Ca(2+)电流，产生更大、更快的Ca(2+)升高。情况4包括线粒体Na/Ca交换器。最后，情况5包括内质网Ca(2+)摄取和释放途径。我们发现，由弱刺激（情况1和2）引发的Ca(2+)反应可以使用一个包含测量的Ca(2+)内流、清除和缓冲特性的空间均匀模型进行定量重建。对强去极化（情况3）的反应不能用该模型描述，但与一个包含相同Ca(2+)转运和缓冲描述的扩散模型一致，只要内源性缓冲剂具有低迁移率，导致陡峭的径向Ca(2+)梯度和线粒体在空间上不均匀的Ca(2+)负载。当扩展到包括线粒体Ca(2+)释放（情况4）和内质网Ca(2+)转运（情况5）时，扩散模型也可以解释先前测量的刺激诱发的线粒体和内质网总Ca浓度变化。
