Functional linkage of Na + ‐Ca 2+ ‐exchanger to sarco/endoplasmic reticulum Ca 2+ pump in coronary artery: comparison of smooth muscle and endothelial cells

An increase in cytosolic Ca 2+ concentration in coronary artery smooth muscle causes a contraction but in endothelium it causes relaxation. Na + ‐Ca 2+ ‐exchanger (NCX) may play a role in Ca 2+ dynamics in both the cell types. Here, the NCX‐mediated 45 Ca 2+ uptake was compared in Na + ‐loaded pig coronary artery smooth muscle and endothelial cells. In both the cell types, this uptake was inhibited by KB‐R7943, SEA 0400 and by monensin, but not by cariporide. Prior loading of the cells with the Ca 2+ chelator BAPTA increased the NCX‐mediated 45 Ca 2+ uptake in smooth muscle but not in endothelial cells. In the presence or absence of BAPTA loading, the Na + ‐mediated 45 Ca 2+ uptake was greater in endothelial than in smooth muscle cells. In smooth muscle cells without BAPTA loading, thapsigargin diminished the NCX‐mediated 45 Ca 2+ entry. This effect was not observed in endothelial cells or in either cell type after BAPTA loading. The results in the smooth muscle cells are consistent with a limited diffusional space model in which the NCX‐mediated 45 Ca 2+ uptake was enhanced by chelation of cytosolic Ca 2+ or by its sequestration by the sarco/endoplasmic reticulum Ca 2+ pump (SERCA). They suggest a functional linkage between NCX and SERCA in the smooth muscle but not in the endothelial cells. The concept of a linkage between NCX and SERCA in smooth muscle was also confirmed by similar distribution of NCX and SERCA2 proteins when detergent‐treated microsomes were fractionated by flotation on sucrose density gradients. Thus, the coronary artery smooth muscle and endothelial cells differ not only in the relative activities of NCX but also in its functional linkage to SERCA.