Voltage-independent calcium channels mediate lipopolysaccharide-induced hyporeactivity to endothelin-1 in the rat aorta

The roles of intracellular calcium concentration ([Ca2+]i) and Ca2+ sensitization in lipopolysaccharide (LPS)-induced vascular smooth muscle (VSM) hyporesponsiveness are incompletely understood. To investigate these roles, contraction responses to endothelin-1 (ET-1) and 80 mM KCl; relaxation responses to nifedipine; the expression levels of mRNAs of ET-1 and its receptors (ETA or ETB); the expression levels of protein kinase C (PKC) and phosphorylation of Rho kinase (ROKα), CPI-17, and myosin phosphatase target subunit-1 (MYPT1); and changes in aortic VSM cell [Ca2+]i were measured in LPS-treated aortic rings from male Wistar rats (250–300 g). LPS (10 μg/ml, 20 h) decreased contraction induced by ET-1 (0.3–100 nM) or 80 mM KCl. LPS-induced hypocontractility was not observed in the absence of external Ca2+, but LPS-treated aorta remained hypocontractile on subsequent stepwise restoration of extracellular Ca2+ (0.01–10 mM). Vascular relaxation to nifedipine; mRNA expression levels of ET-1, ETA, or ETB; protein expression levels of PKC; and phosphorylation levels of ROKα, CPI-17, and MYPT1 were not affected by LPS. In isolated aortic VSM cells, ET-1 caused a transient initial increase in [Ca2+]i, followed by a maintained tonic increase in [Ca2+]i, which was decreased by LPS pretreatment and was dependent on external Ca2+. Subsequent restoration of extracellular Ca2+ increased [Ca2+]i, but this increase was lower in the LPS-treated group. This difference in response to extracellular Ca2+ addition was not affected by diltiazem, but was abolished by SKF-96365. Therefore, LPS induces hyporeactivity to ET-1 in rat aorta that depends on external Ca2+ influx through non-voltage-operated Ca2+ channels, but not on ET-1 receptor expression or Ca2+ sensitization.