Lee C O, Levi A J
Department of Physiology, Cornell University Medical College, New York, New York 10021.
Ann N Y Acad Sci. 1991;639:408-27. doi: 10.1111/j.1749-6632.1991.tb17329.x.
Intracellular sodium was estimated in ventricular myocytes using the new Na-sensitive fluorescent indicator SBFI. Membrane potential and contraction were also measured simultaneously. Using an in situ calibration method, we found that intracellular sodium activity (aiNa) was 2.9 mM in quiescent rabbit cells. When the digitalis analogue strophanthidin inhibited the Na-K pump of myocytes with action potentials (APs), changes of contraction and aiNa were dissociated in time. There was also marked hysteresis between contraction and aiNa. When strophanthidin was applied to the same myocytes under voltage-clamp conditions, temporal dissociation between contraction and aiNa was dramatically reduced. This suggests that much of the dissociation and hysteresis was due the change in AP shape with strophanthidin. A small amount of residual hysteresis still existed even with voltage-clamp, and this persisted when the pump was blocked by removal of external potassium as an alternative method. We suggest that a gradient of sodium concentration from the subsarcolemmal space to the bulk cytoplasm might be responsible for hysteresis. Whereas SBFI probably signals the average Na level of the cytoplasm, subsarcolemmal Na may control Ca influx and contraction via Na-Ca exchange.
利用新型钠敏感荧光指示剂SBFI对心室肌细胞内的钠进行了评估。同时还测量了膜电位和收缩情况。采用原位校准方法，我们发现静息兔细胞内的钠活性（aiNa）为2.9 mM。当洋地黄类似物毒毛花苷抑制具有动作电位（AP）的心肌细胞的钠钾泵时，收缩和aiNa的变化在时间上出现分离。收缩和aiNa之间也存在明显的滞后现象。当在电压钳制条件下将毒毛花苷应用于相同的心肌细胞时，收缩和aiNa之间的时间分离显著减少。这表明，大部分的分离和滞后现象是由于毒毛花苷导致的动作电位形状改变所致。即使在电压钳制条件下，仍存在少量残余滞后现象，当通过去除细胞外钾作为另一种方法阻断泵时，这种现象仍然存在。我们认为，从肌膜下间隙到细胞质整体的钠浓度梯度可能是导致滞后现象的原因。虽然SBFI可能反映的是细胞质中钠的平均水平，但肌膜下的钠可能通过钠钙交换控制钙内流和收缩。
