Na+/Ca2+ exchanger overexpression in smooth muscle augments cytosolic Ca2+ in femoral arteries of living mice

Abstract

Plasma membrane Na⁺/Ca²⁺ exchanger-1 (NCX1) helps regulate the cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) in arterial myocytes. NCX1 mediates both Ca²⁺ entry and exit and tends to promote net Ca²⁺ entry in partially constricted arteries. Mean blood pressure (telemetry) is elevated by ≈10 mmHg in transgenic (TG) mice that overexpress NCX1 specifically in smooth muscle. We tested the hypothesis that NCX1 overexpression mediates Ca²⁺ gain and elevated [Ca²⁺]_CYT in exposed femoral arteries that also express the Ca²⁺ biosensor exogenous myosin light chain kinase. [Ca²⁺]_CYT and the NCX1-dependent (SEA0400-sensitive) component, ≈15% of total basal constriction in controls, were increased in TG arteries, but constrictions to phenylephrine and ANG II were comparable in TG and control arteries. Normalized phenylephrine dose-response curves and constriction to 30 and 300 ng/kg iv ANG II were virtually identical in control and TG arteries. ANG II-evoked constrictions, superimposed on elevated basal tone, accounted for the larger blood pressure responses to ANG II in TG arteries. TG and control mouse arteries fit the same pCa-constriction relationship over a wide range of pCa (≈125-500 nM). Vasodilation to acetylcholine, normalized to passive diameter, was also comparable in TG and control arteries, implying normal endothelial function. TG artery Na⁺ nitroprusside (nitric oxide donor)-induced dilations were, however, shifted to lower Na⁺ nitroprusside concentrations, indicating that TG myocyte vasodilator mechanisms were augmented. Maximum arterial dilation was comparable in TG and control mice, although passive diameter was ≈6-7% smaller in TG mice. The changes in TG arteries were apparently largely functional rather than structural, despite the congenital hypertension. NEW & NOTEWORTHY Smooth muscle Na⁺/Ca²⁺ exchanger-1 transgene overexpression (TG mice) increases femoral artery basal cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) and tone in vivo and raises blood pressure. Arterial constriction to phenylephrine and angiotensin II are normal but superimposed on the augmented basal [Ca²⁺]_CYT and tone (constriction) in TG mouse arteries. Similar effects in resistance arteries would explain the elevated blood pressure. Acetylcholine-induced vasodilation is unimpaired, implying a normal endothelium, but TG arteries are hypersensitive to sodium nitroprusside.

Keywords: SEA0400, sodium nitroprusside; arterial tone; vasodilation.