Bers D M
Division of Biomedical Sciences, University of California, Riverside 92521-0121.
Ann N Y Acad Sci. 1991;639:375-85. doi: 10.1111/j.1749-6632.1991.tb17326.x.
During normal relaxation in rabbit, guinea-pig, and rat ventricular muscle, the Na-Ca exchange system competes with the SR Ca pump, with the former being responsible for about 20-30% of the Ca removal from the cytoplasm. Ca extrusion via Na-Ca exchange is Em-sensitive, whereas Ca uptake by the SR is not. Neither the sarcolemmal Ca-ATPase pump nor mitochondrial Ca uptake appear to contribute significantly to the decline of [Ca]i during relaxation. Furthermore, the diastolic efflux of Ca from cardiac muscle cells appears to be primarily attributable to Na-Ca exchange and not the sarcolemmal Ca-ATPase pump. In rabbit ventricle Ca entry via Na-Ca exchange is favored thermodynamically during much of a normal twitch contraction and Ca extrusion occurs primarily between beats. In rat ventricle Ca efflux via Na-Ca exchange occurs during the contraction and net Ca influx may occur between beats. This fundamental difference in Ca fluxes during the cardiac cycle in rat versus rabbit ventricle may be a simple consequence of the shorter action potential duration and higher aNai in rat ventricle (due to the effects of Em and [Na] and [Ca] gradients on Na-Ca exchange).
在兔、豚鼠和大鼠心室肌正常舒张过程中，钠钙交换系统与肌浆网钙泵相互竞争，前者负责约20% - 30%的胞质钙清除。通过钠钙交换进行的钙外流对膜电位（Em）敏感，而肌浆网的钙摄取则不然。在舒张过程中，肌膜钙ATP酶泵和线粒体钙摄取似乎对胞内钙浓度（[Ca]i）的下降均无显著贡献。此外，心肌细胞舒张期的钙外流似乎主要归因于钠钙交换，而非肌膜钙ATP酶泵。在兔心室，在正常单次收缩的大部分时间里，通过钠钙交换的钙内流在热力学上占优势，钙外流主要发生在心动周期的搏动之间。在大鼠心室，通过钠钙交换的钙外流发生在收缩期，净钙内流可能发生在搏动之间。大鼠和兔心室在心动周期中钙通量的这种根本差异，可能仅仅是由于大鼠心室动作电位持续时间较短以及钠内流（aNai）较高（这是由于膜电位、[Na]和[Ca]梯度对钠钙交换的影响）所致。
