Endotoxin-induced vascular hyporesponsiveness could potentially involve alterations of vascular smooth muscle (VSM) myoplasmic free calcium (Cam) mobilization mechanisms. Contractile function and Cam(fura-2 microfluorometry) regulation were evaluatedin vitrousing coronary (COR) and mesenteric (MES) artery preparations (100–250 μm inner diameter) isolated from guinea pigs 16 h after intraperitoneal (i.p.) injection of either saline (control; CON) orEscherichia coliendotoxin lipopolysaccharide (LPS; 4 mg/kg). Concentration-response relationships to K+(5–100 mM) were significantly enhanced in both COR and MES arteries isolated from LPS-treated animals. In contrast, contractile responses to prostaglandin F2α(PGF2α; 1–100 μM) were markedly impaired in COR and MES arteries from LPS-treated animals, while endothelin-1 (ET; 1–100 nM)-mediated contractile responses of these arteries were enhanced at the maximal dose (100 nM). In COR arteries, PGF2α(1–100 μM) and ET (1–100 nM) produced biphasic increases in Camin both CON and LPS groups. No significant differences were observed in either the initial transient peak or secondary sustained Camresponses between groups, suggesting a lack of effect of LPS upon intracellular Ca2+release or Ca2+influx mechanisms in COR arteries. Exposure of MES arteries to PGF2αand ET produced concentration-dependent increases in Camin both groups. However, Camresponses of MES arteries lacked initial peak responses, suggesting potential differences in Cammobilization between COR and MES arteries. Camresponses to K+(80 mM) and PGF2α(1–100 μM) were similar in MES arteries from both groups; however, ET-mediated increases in Camwere significantly blunted in LPS compared with CON MES arteries. Thus, endotoxemia produced differential effects upon depolarization (K+) and receptor (PGF2α, ET)-mediated contractile responses in both COR and MES arteries. Reductions in VSM Cammobilization appear unlikely as a mechanism for LPS-induced impairment of contractile function of COR and MES arteries; other mechanisms (i.e., decreased Ca2+sensitivity of contractile proteins) may be involved in. effects of LPS upon VSM function of COR and MES arteries.