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

Mice with smooth muscle (SM)-specific knockout of Na⁺/Ca ²⁺ exchanger type-1 (NCX1^SM-/-) 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 ^SM-/- mice generated with Cre recombinase. Mean blood pressure (BP) was 6-10 mmHg lower in NCX1^SM-/- mice than in wild-type (WT) controls. Vasoconstriction was studied in isolated, pressurized mesenteric small arteries from WT and NCX1^SM-/- mice and in heterozygotes with a global null mutation (NCX1^Fx/-). 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 NCX1^SM-/- arteries and, to a similar extent, by SEA0400 in WT arteries. MT was normal in arteries from NCX1^Fx/- mice, which had normal BP. Vasoconstrictions to phenylephrine and elevated extracellular K⁺ concentration were significantly reduced in NCX1^SM-/- 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 NCX1 ^SM-/- (vs. WT or NCX1^Fx/-) 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 NCX1^SM-/- and control myocytes, suggesting that LVGC expression was normal in NCX1^SM-/- myocytes. Bay K 8644 did not, however, eliminate the difference in myogenic constriction between WT and NCX1^SM-/- arteries. We conclude that, under physiological conditions, NCX1-mediated Ca²⁺ entry contributes significantly to the maintenance of MT. In NCX1^SM-/- 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.