The Na+/Ca2+-exchanger: An essential component in the mechanism governing cardiac steroid-induced slow Ca2+ oscillations

Plasma membrane (PM) Na+, K+-ATPase, plays crucial roles in numerous physiological processes. Cardiac steroids (CS), such as ouabain and bufalin, specifically bind to the Na+, K+-ATPase and affect ionic homeostasis, signal transduction, and endocytosed membrane traffic. CS-like compounds, synthesized in and released from the adrenal gland, are considered a new family of steroid hormones. Previous studies showed that ouabain induces slow Ca2+ oscillations in COS-7 cells by enhancing the interactions between Na+, K+-ATPase, inositol 1,4,5-trisphosphate receptor (IP3R) and Ankyrin B (Ank-B) to form a Ca2+ signaling micro-domain. The activation of this micro-domain, however, is independent of InsP3 generation. Thus, the mechanism underlying the induction of these slow Ca2+ oscillations remained largely unclear. We now show that other CS, such as bufalin, can also induce Ca2+ oscillations. These oscillations depend on extracellular Ca2+ concentrations [Ca2+]out and are inhibited by Ni2+. Furthermore, we found that these slow oscillations are Na+out dependent, abolished by Na+/Ca2+ exchanger1 (NCX1)-specific inhibitors and markedly attenuated by NCX1 siRNA knockdown. Based on these results, a model is presented for the CS-induced slow Ca2+ oscillations in COS-7 cells.

► Cardiac steroids (bufalin, digoxin and ouabain) induce slow Ca2+ oscillations in COS7 cells.
► These oscillations depend on extracellular [Ca]2+ and [Na]+ and are inhibited by Ni2+.
► Na+/Ca2+ exchanger1 (NCX1) is expressed in COS-7 cells.
► NCX1-inhibitors and siRNA knockdown clearly attenuate the CS-induced Ca2+ oscillations.
► NCX1 is an essential factor in the mechanism generating CS-induced Ca2+ oscillations.