Milanick M A
Department of Physiology, School of Medicine, University of Missouri-Columbia 65212.
Am J Physiol. 1991 Jul;261(1 Pt 1):C185-93. doi: 10.1152/ajpcell.1991.261.1.C185.
To determine the mechanism of Na-Ca exchange, we estimated the ratio of maximum velocity to Michaelis constant for extra-cellular Ca by measuring the rate of Ca uptake at very low extracellular Ca. In a Ping-Pong mechanism, one set of sites alternatively transports Ca and Na. In a sequential mechanism, Ca and Na sites are both filled during part of the transport cycle. In each set of experiments, two intracellular Na concentrations were studied. The Ca uptake rate (at low Ca) increased as Na increased; this is consistent with a sequential model, as has been found in other cells. We also examined the alternative hypothesis that the exchanger followed Ping-Pong kinetics and that the red blood cells had a submembrane pool for Ca that limited mixing with the cytosol. In these experiments Ca pump activity was monitored by measuring ATP hydrolysis. This model was disproven by experiments that indicated that greater than 80% of the Ca that entered the cell became bound to EGTA and less than 20% resulted in Ca efflux by the Ca pump.
为了确定钠钙交换的机制，我们通过测量极低细胞外钙浓度下的钙摄取速率，估算了细胞外钙的最大速度与米氏常数的比值。在乒乓机制中，一组位点交替转运钙和钠。在顺序机制中，钙和钠位点在部分转运周期中同时被占据。在每组实验中，研究了两种细胞内钠浓度。（在低钙情况下）钙摄取速率随钠浓度的增加而增加；这与顺序模型一致，正如在其他细胞中所发现的那样。我们还检验了另一种假设，即交换体遵循乒乓动力学，并且红细胞有一个限制与胞质溶胶混合的钙的亚膜池。在这些实验中，通过测量ATP水解来监测钙泵活性。表明进入细胞的钙超过80%与乙二醇双四乙酸（EGTA）结合，而通过钙泵导致钙外流的不到20%，这些实验反驳了该模型。
