Recombinant expression of the plasma memembrane Na+/Ca2+ exchanger affects local and global Ca2+ homeostasis in Chinese hamster ovary cells

The cardiac type Na+/Ca2+ exchanger (NCX1) has been transiently expressed in Chinese hamster ovary cells, which do not contain an endogenous exchanger, together with aequorin chimeras that are targeted to different intracellular compartments to investigate intracellular Ca2+ homeostasis. The expression of NCX decreased the endoplasmic reticulum Ca2+ concentration, [Ca2+](er), in resting cells, showing that the exchanger was operative under these conditions. It induced a greater reduction in the height of the mitochondrial and cytosolic Ca2+ transients in agonist-stimulated cells than would have been expected from the [Ca2+](er) decrease. It also had a major effect on the sub-plasma membrane Ca2+ concentration, [Ca2+](pm): after a transient [Ca2+](pm) rise induced by the activation of capacitative Ca2+ influx, [Ca2+](pm) settled to a value about 3-fold higher than in controls. The sustained [Ca2+](pm) increase after the transient was due to the operation of the exchanger, either directly by operating in the Ca2+ entry mode, or indirectly by removing the Ca2+ inhibition on the capacitative Ca2+ influx channels.