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

The roles of intracellular calcium concentration ([[[Ca.sup.2+].sub.i]) and [Ca.sup.2+] 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 KC1; relaxation responses to nifedipine; the expression levels of mRNAs of ET-1 and its receptors ([ET.sub.A] or [ET.sub.B]); the expression levels of protein kinase C (PKC) and phosphorylation of Rho kinase (ROK[alpha], CPI-17, and myosin phosphatase target subunit-1 (MYPT1); and changes in aortic VSM cell [[[Ca.sup.2+].sub.i] were measured in LPS-treated aortic rings from male Wistar rats (250-300 g). LPS (10 [micro]g/ml, 20 h) decreased contraction induced by ET-1 (0.3-100 nM) or 80 mM KC1. LPS-induced hypocontractility was not observed in the absence of external [Ca.sup.2+], but LPS-treated aorta remained hypocontractile on subsequent stepwise restoration of extracellular [Ca.sup.2+] (0.01-10 mM). Vascular relaxation to nifedipine; mRNA expression levels of ET-I, [ET.sub.A], or [ET.sub.B]; protein expression levels of PKC; and phosphorylation levels of ROK[alpha], CPI-17, and MYPT1 were not affected by LPS. In isolated aortic VSM cells, ET-1 caused a transient initial increase in [[[Ca.sup.2+].sub.i], followed by a maintained tonic increase in [[[Ca.sup.2+].sub.i], which was decreased by LPS pretreatment and was dependent on external [Ca.sup.2+]. Subsequent restoration of extracellular [Ca.sup.2+] increased [[[Ca.sup.2+].sub.i], but this increase was lower in the LPS-treated group. This difference in response to extracellular [Ca.sup.2+] 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 [Ca.sup.2+] influx through non-voltage-operated [Ca.sup.2+] channels, but not on ET-1 receptor expression or [Ca.sup.2+] sensitization. calcium sensitization; sepsis; vascular smooth muscle