A Study of the Activity of the Plasma Membrane Na/Ca Exchanger in the Cellular Environment
The Na/Ca exchanger is a large-capacity system that is supposed to be predominant in the ejection of calcium in excitable tissues. Its activity is normally evaluated in electro-physiological experiments or using Na-loaded cells or plasma membrane vesicles in experiments in which the activity is estimated by measuring the uptake of isotopic calcium. Here the activity of the exchanger has been evaluated in model cells that coexpress the exchanger together with the (protein) Ca indicator aequorin targeted to the different subcellular compartments. Exchanger isoform 1 (NCX1) has been used for experiments in which the effect of its overexpression has been evaluated on the Ca homeostasis in the cytoplasm at large, in the endoplasmic reticulum, in mitochondria, and in the layer of cytoplasm underneath the plasma membrane. The experiments have shown that, despite its very low calcium affinity, the exchanger was active in cells at rest in which calcium is around 100 nM. They have also shown that expressed NCX1 significantly reduced the cytosolic and mitochondrial calcium transients produced by the emptying of the ER calcium stores with InsP3-linked agonists. The effect of the expressed NCX1 on the calcium homeostasis in the domain underneath the plasma membrane was more complex
plasma membrane na/ca exchanger mitochondrial calcium transients cytosolic calcium transients