Knockout of Na+/Ca2+ exchanger in smooth muscle attenuates vasoconstriction and L-type Ca2+ channel current and lowers blood pressure

Abstract

Mice with smooth muscle (SM)-specific knockout of Na+/Ca2+ exchanger type-1 (NCX1SM−/−) and the NCX inhibitor, SEA0400, were used to study the physiological role of NCX1 in mouse mesenteric arteries. NCX1 protein expression was greatly reduced in arteries from NCX1SM−/− mice generated with Cre recombinase. Mean blood pressure (BP) was 6–10 mmHg lower in NCX1SM−/− mice than in wild-type (WT) controls. Vasoconstriction was studied in isolated, pressurized mesenteric small arteries from WT and NCX1SM−/− mice and in heterozygotes with a global null mutation (NCX1Fx/−). Reduced NCX1 activity was manifested by a marked attenuation of responses to low extracellular Na+ concentration, nanomolar ouabain, and SEA0400. Myogenic tone (MT, 70 mmHg) was reduced by ∼15% in NCX1SM−/− arteries and, to a similar extent, by SEA0400 in WT arteries. MT was normal in arteries from NCX1Fx/− mice, which had normal BP. Vasoconstrictions to phenylephrine and elevated extracellular K+ concentration were significantly reduced in NCX1SM−/− arteries. Because a high extracellular K+ concentration-induced vasoconstriction involves the activation of L-type voltage-gated Ca2+ channels (LVGCs), we measured LVGC-mediated currents and Ca2+ sparklets in isolated mesenteric artery myocytes. Both the currents and the sparklets were significantly reduced in NCX1SM−/− (vs. WT or NCX1Fx/−) myocytes, but the voltage-dependent inactivation of LVGCs was not augmented. An acute application of SEA0400 in WT myocytes had no effect on LVGC current. The LVGC agonist, Bay K 8644, eliminated the differences in LVGC currents and Ca2+ sparklets between NCX1SM−/− and control myocytes, suggesting that LVGC expression was normal in NCX1SM−/− myocytes. Bay K 8644 did not, however, eliminate the difference in myogenic constriction between WT and NCX1SM−/− arteries. We conclude that, under physiological conditions, NCX1-mediated Ca2+ entry contributes significantly to the maintenance of MT. In NCX1SM−/− mouse artery myocytes, the reduced Ca2+ entry via NCX1 may lower cytosolic Ca2+ concentration and thereby reduce MT and BP. The reduced LVGC activity may be the consequence of a low cytosolic Ca2+ concentration.