Na+-K+ pump activation inhibits endothelium-dependent relaxation by activating the forward mode of Na+/Ca2+ exchanger in mouse aorta

Department of Physiology and Medical Research Institute, College of Medicine, Ewha Women's University, Seoul, Republic of Korea Submitted 2 September 2004 ; accepted in final form 24 June 2005 The effect of Na + -K + pump activation on endothelium-dependent relaxation (EDR) and on intracellular Ca 2+ concentration ([Ca 2+ ] i ) was examined in mouse aorta and mouse aortic endothelial cells (MAECs). The Na + -K + pump was activated by increasing extracellular K + concentration ([K + ] o ) from 6 to 12 mM. In aortic rings, the Na + ionophore monensin evoked EDR, and this EDR was inhibited by the Na + /Ca 2+ exchanger (NCX; reverse mode) inhibitor KB-R7943. Monensin-induced Na + loading or extracellular Na + depletion (Na + replaced by Li + ) increased [Ca 2+ ] i in MAECs, and this increase was inhibited by KB-R7943. Na + -K + pump activation inhibited EDR and [Ca 2+ ] i increase (K + -induced inhibition of EDR and [Ca 2+ ] i increase). The Na + -K + pump inhibitor ouabain inhibited K + -induced inhibition of EDR. Monensin (>0.1 µM) and the NCX (forward and reverse mode) inhibitors 2'4'-dichlorobenzamil (>10 µM) or Ni 2+ (>100 µM) inhibited K + -induced inhibition of EDR and [Ca 2+ ] i increase. KB-R7943 did not inhibit K + -induced inhibition at up to 10 µM but did at 30 µM. In current-clamped MAECs, an increase in [K + ] o from 6 to 12 mM depolarized the membrane potential, which was inhibited by ouabain, Ni 2+ , or KB-R7943. In aortic rings, the concentration of cGMP was significantly increased by acetylcholine and decreased on increasing [K + ] o from 6 to 12 mM. This decrease in cGMP was significantly inhibited by pretreating with ouabain (100 µM), Ni 2+ (300 µM), or KB-R7943 (30 µM). These results suggest that activation of the forward mode of NCX after Na + -K + pump activation inhibits Ca 2+ mobilization in endothelial cells, thereby modulating vasomotor tone. extracellular potassium; sodium-potassium pump; forward mode of sodium/calcium exchanger; endothelial cells; intracellular calcium Address for reprint requests and other correspondence: S. H. Suh, Dept. of Physiology, College of Medicine, Ewha Women's Univ., 911–1 Mok-6-dong, Yang Chun-gu, Seoul, Republic of Korea 158–710 (e-mail: shsuh{at}ewha.ac.kr )