Microcirculatory hyporesponsiveness in lipopolysaccharide-induced inflammation is endothelial cell dependent but calcium independent

Background

Two hallmarks of sepsis are decreased vascular resistance and hyporeactivity to vasoconstrictor agents in the microcirculation. Whether the impairment in vasoreactivity is caused by defects in the signal transduction pathway inherent to endothelial cells or smooth muscle cells remains unclear.

Objective

The aim of this study was to determine whether impaired vasoreactivity during LPS-induced inflammation is associated with altered Ca²⁺sensitivity of contractile proteins in small mesenteric resistance arteries (SMRA).

Methods

LPS (15 mg/kg) or sterile water was injected intraperitoneally into mice. SMRAs were harvested 18 hours after injection. The arterioles (~190–220 μm) were mounted on a pressure myograph, superfused with MOPS buffer at 37°C, and loaded with fura-2. The arteriolar diameter and global intracellular Ca²⁺ were measured concurrently using light microscopy and a photomultiplier system. Concentration-response curves to phenylephrine (PE) (10^-9 to 10^-4 M) were conducted. In all experiments, n = 4–6; *P < 0.05 indicates statistical significance.

Results

LPS treatment resulted in hyporesponsiveness to PE as demonstrated with an increase in EC₅₀ (0.7 ± 0.2 μM vs 1.9 ± 1.0 μM*), a decrease in maximal contractile response (E_max 35 ± 6% vs 19 ± 9%*) and a reduction in Ca²⁺ sensitivity (Fig. 1, left). Removal of the endothelium resulted in a near-normal response to PE in LPS-treated mice (EC₅₀ 0.9 ± 0.2 μM vs 1.0 ± 0.6 μM and E_max 43 ± 2% vs 38 ± 5%). Interestingly, Ca²⁺ sensitivity remained decreased in SMRAs from LPS-treated mice (Fig. 1, right).

Figure 1

abstract P96

Conclusion

In small resistance arteries, LPS-induced inflammation results in endothelial cell dependent hyporesponsiveness to vasoconstrictors in association with, but independent of, a decreased Ca²⁺ sensitivity within the smooth muscle cells. This suggests that, in the microcirculation, endothelial cells moderate contractility function without affecting calcium sensitivity within the smooth muscle cells.