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

<jats:p>Mice with smooth muscle (SM)-specific knockout of Na<jats:sup>+</jats:sup>/Ca<jats:sup>2+</jats:sup>exchanger type-1 (NCX1<jats:sup>SM−/−</jats:sup>) 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 NCX1<jats:sup>SM−/−</jats:sup>mice generated with Cre recombinase. Mean blood pressure (BP) was 6–10 mmHg lower in NCX1<jats:sup>SM−/−</jats:sup>mice than in wild-type (WT) controls. Vasoconstriction was studied in isolated, pressurized mesenteric small arteries from WT and NCX1<jats:sup>SM−/−</jats:sup>mice and in heterozygotes with a global null mutation (NCX1<jats:sup>Fx/−</jats:sup>). Reduced NCX1 activity was manifested by a marked attenuation of responses to low extracellular Na<jats:sup>+</jats:sup>concentration, nanomolar ouabain, and SEA0400. Myogenic tone (MT, 70 mmHg) was reduced by ∼15% in NCX1<jats:sup>SM−/−</jats:sup>arteries and, to a similar extent, by SEA0400 in WT arteries. MT was normal in arteries from NCX1<jats:sup>Fx/−</jats:sup>mice, which had normal BP. Vasoconstrictions to phenylephrine and elevated extracellular K<jats:sup>+</jats:sup>concentration were significantly reduced in NCX1<jats:sup>SM−/−</jats:sup>arteries. Because a high extracellular K<jats:sup>+</jats:sup>concentration-induced vasoconstriction involves the activation of L-type voltage-gated Ca<jats:sup>2+</jats:sup>channels (LVGCs), we measured LVGC-mediated currents and Ca<jats:sup>2+</jats:sup>sparklets in isolated mesenteric artery myocytes. Both the currents and the sparklets were significantly reduced in NCX1<jats:sup>SM−/−</jats:sup>(vs. WT or NCX1<jats:sup>Fx/−</jats:sup>) 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 Ca<jats:sup>2+</jats:sup>sparklets between NCX1<jats:sup>SM−/−</jats:sup>and control myocytes, suggesting that LVGC expression was normal in NCX1<jats:sup>SM−/−</jats:sup>myocytes. Bay K 8644 did not, however, eliminate the difference in myogenic constriction between WT and NCX1<jats:sup>SM−/−</jats:sup>arteries. We conclude that, under physiological conditions, NCX1-mediated Ca<jats:sup>2+</jats:sup>entry contributes significantly to the maintenance of MT. In NCX1<jats:sup>SM−/−</jats:sup>mouse artery myocytes, the reduced Ca<jats:sup>2+</jats:sup>entry via NCX1 may lower cytosolic Ca<jats:sup>2+</jats:sup>concentration and thereby reduce MT and BP. The reduced LVGC activity may be the consequence of a low cytosolic Ca<jats:sup>2+</jats:sup>concentration.</jats:p>