Cellular regulation of sodium-calcium exchange.

Na(+)/Ca(2+) exchange activity was studied in transfected Chinese hamster ovary (CHO) cells expressing the wild-type cardiac exchanger (NCX1.1) or mutants created by site-directed mutagenesis. The activity of the wild-type exchanger, but not exchanger mutants deficient in Ca(2+)-dependent activation, was inhibited by sphingolipids such as ceramide and sphingosine. We propose that sphingolipids interfere with the regulatory activation of exchange activity by Ca(2+) and suggest that this interaction provides a means for monitoring and regulating diastolic Ca(2+) levels in beating cardiac myocytes. Exchange activity in CHO cells was also linked, through a poorly understood feedback mechanism, to Ca(2+) accumulation within internal stores such as the endoplasmic reticulum and the mitochondria. Finally, the F-actin cytoskeleton was shown to modulate exchange activity through interactions involving the exchanger's central hydrophilic domain. We conclude that regulation of exchange activity in intact cells involves multiple interactions with various lipid species, cytosolic Ca(2+), organellar Ca(2+) stores, and the cytoskeleton.