Acetylcholine-induced endothelium-dependent vascular smooth muscle relaxation in nitroglycerin-tolerant isolated rat aorta

Amorphous nanosilica particles evoke vascular relaxation through PI3K/Akt/eNOS signaling

There have been several reported studies on the distribution and/or toxicity of nanosilica particles. However, the influence of these particles on blood vessels through which they are distributed is poorly understood. Hence, we investigated the effects of nano- and micromaterials on blood vessel shrinkage and relaxation. Nanosilica particles with diameters of 70 nm (nSP70) were used as the nanomaterial, and particles of 300 and 1000 nm (nSP300 and mSP1000, respectively) were used as micromaterials. A rat thoracic aorta was used as the test blood vessel. The nano- and micromaterials had no effect on vessel shrinkage. Of the nano- and micromaterials tested, only nSP70 strongly evoked vascular relaxation. Vascular relaxation evoked by nSP70 was almost completely inhibited by the phosphoinositide 3-kinase (PI3K) inhibitor wortmannin. In addition, the selective nitric oxide synthesis inhibitor NG-nitro-l-arginine methyl ester, which inhibits endothelial nitric oxide synthase (eNOS) downstream of PI3K signaling, inhibited vascular relaxation evoked by nSP70. In an analysis using bovine aortic endothelial cells (bAECs), nSP70 phosphorylated protein kinase B (AKT) and eNOS acted downstream of PI3K signaling. PI3K inhibition by wortmannin reduced AKT and eNOS phosphorylation. These results demonstrated that 70-nm amorphous nanosilica particles evoked vascular relaxation through PI3K/Akt/eNOS signaling. Moreover, it was suggested that nanomaterials, in general, control or disrupt vascular function by activating a known signal cascade.

Acetylcholine-induced AMP-activated protein kinase activation attenuates vasoconstriction through an LKB1-dependent mechanism in rat aorta

Numerous studies of acetylcholine (ACh)-induced endothelium-dependent relaxation in arteries have been reported since the original description by Furchgott and Zawadzki (1980). ACh also produces endothelium-independent relaxation. However, it is still unknown whether ACh-induced AMP-activated protein kinase (AMPK) activation can attenuate vasoconstriction in endothelium-denuded rat aorta. Here, we investigated whether ACh may exert a regulatory effect for vascular tone via AMPK activation and its underlying mechanism in vascular smooth muscle cells (VSMCs). Western blotting showed that ACh dose- and time-dependently increased LKB1 and AMPK phosphorylation in VSMCs. The ACh-induced activation of AMPK required muscarinic receptors in VSMCs. LKB1 and AMPK activation by ACh inhibited myosin light-chain kinase (MLCK) and phosphorylated myosin light chain (p-MLC) expression in VSMCs. In addition, a tension study showed the inhibitory effect of ACh-induced AMPK activation on phenylephrine-mediated contraction in endothelium-denuded rat aorta. These data suggest that the ACh-induced activation of AMPK may attenuate vasoconstriction via LKB1-AMPK-dependent mechanism in endothelium-denuded rat aorta.

Lack of critical involvement of endothelial nitric oxide synthase in vascular nitrate tolerance in mice

We examined the direct involvement of endothelial nitric oxide (eNOS) in nitrate tolerance using eNOS knockout (eNOS (-/-)) and wild-type (eNOS (+/+)) mice. Animals were treated with either nitroglycerin (NTG, 20 mg kg(-1)s.c. 3 x daily for 3 days) or vehicle (5% dextrose, D5W), and nitrate tolerance was assessed ex vivo in isolated aorta by vascular relaxation studies and cyclic GMP accumulation. Western blot was performed to determine NOS expression after NTG treatment. In both the eNOS (-/-) and (+/+) mice, the EC(50) from NTG concentration-response curve was increased by approximately 3 fold, and vascular cyclic GMP accumulation was similarly decreased after NTG pretreatment. Vascular tolerance did not lead to changes in eNOS protein expression in eNOS (+/+) mice. These results indicate that vascular nitrate tolerance was similarly induced in eNOS (-/-) and (+/+) mice, suggesting that eNOS may not be critically involved in nitrate tolerance development in mice.