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The acute vascular inflammatory dysfunction associated with endotoxaemia may reflect an imbalance between matrix metalloproteinases (MMPs) and their natural inhibitors (TIMPs), induced by the endotoxin. This possibility was tested in rat aortic tissue.Tone induced by phenylephrine in aortic rings was measured after exposure in vitro to ambient lipopolysaccharide (LPS) or the proinflammatory cytokine interleukin-1β (IL-1β) for 6h, with or without MMP inhibitors (doxycycline or GM6001). Gelatinase and MMP activities, TIMP proteins and contractility were measured in aortae taken from rats 6h after receiving LPS in vivo.Inhibition of MMP prevented the loss of phenylephrine–induced tone in aortic rings after LPS or IL-1β. IL-1β also increased release of MMP-2 activity from aortic tissue. In aortae exposed in vivo to LPS, net gelatinase, MMP-9 activities and TIMP-1 protein levels were increased, whereas TIMP-4 was reduced. These aortae were hypocontractile to both phenylephrine and KCl. Hypocontractility was partially reversed by doxycycline ex vivo.MMP inhibitors ameliorate vascular hyporeactivity induced by either LPS or IL-1β in vitro. LPS in vivo alters the balance between MMPs and TIMPs, contributing to vascular dysfunction which is partially reversed by MMP inhibitors. Vascular MMPs are activated as a result of LPS or IL-1β-induced stress and contribute to the hyporeactivity of blood vessels to vasoconstrictors.