Lado M G, Sheu S S, Fozzard H A
Am J Physiol. 1982 Jul;243(1):H133-7. doi: 10.1152/ajpheart.1982.243.1.H133.
One hypothesis to explain increased contraction strength with increased stimulation rate in heart muscle requires that stimulation increase intracellular Na+ activity(aiNa). This is proposed to increase intracellular Ca2+ activity (aiCa) via the Na-Ca exchange mechanism. Several indirect studies have supported the idea that aiNa is increased with stimulation, and more recently we have directly demonstrated aiNa elevation with Na+-sensitive microelectrodes. We now report aiCa to be elevated after trains of stimuli at different rates in sheep cardiac Purkinje strands. The resting level of aiCa in six strands was 93 +/- 15 (SE) nM, and it increased to 162 +/- 20 nM after stimulation at 3 Hz. The recovery of aiCa was exponential, and the time constants of 80-120 s were similar to those previously found for aiNa. A positive force-frequency relation was found in sheep Purkinje fibers that correlated with the increased aiCa. The results support the hypothesis that the Na-Ca exchange system plays an important role in the force-frequency phenomenon.
一种解释心肌收缩强度随刺激频率增加而增强的假说认为，刺激会增加细胞内钠离子活性（aiNa）。这被认为是通过钠钙交换机制来增加细胞内钙离子活性（aiCa）。一些间接研究支持了aiNa随刺激增加的观点，最近我们利用对钠离子敏感的微电极直接证明了aiNa的升高。我们现在报告，在不同频率的刺激串作用于绵羊心脏浦肯野纤维束后，aiCa会升高。六条纤维束中aiCa的静息水平为93±15（标准误）nM，在3Hz刺激后升高至162±20 nM。aiCa的恢复呈指数形式，80 - 120秒的时间常数与之前发现的aiNa的时间常数相似。在绵羊浦肯野纤维中发现了正性的力-频率关系，这与aiCa的增加相关。这些结果支持了钠钙交换系统在力-频率现象中起重要作用这一假说。
