The effect of tumour necrosis factor-alpha on airways smooth muscle contractility

• Main Author: Parris, J. R. M.
• Published: University of Aberdeen 1999
• Subjects: 615.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.593267

A major characteristic of asthma is a hyper-responsiveness of the bronchial smooth muscle. This hyper-responsiveness is associated with an activation of inflammatory cells, such as neutrophils, which release the cytokine TNF-α. In vivo exposure of human airways to aerosolised TNF-α produces an increase in agonist-induced contractility of the bronchial smooth muscle. The cellular mechanisms of this TNF-induced hyper-responsiveness are not yet known. The aims of this thesis are to investigate the effects of TNF-α in intact and permeabilized guinea pig airways smooth muscle to determine the mechanism of TNF-induced airway hyper-responsiveness. Smooth muscle strips from guinea pig bronchi were attached to a sensitive force transducer to measure isometric tension. Strips incubated with human recombinant TNF-α for one hour revealed an increased contractile response to carbachol compared to control strips, although TNF itself produced no change in tension. In identical experiments carried out in guinea pig portal vein, no such potentiation of the agonist-induced contractile response was observed after TNF treatment. Strips of bronchial smooth muscle were permeabilized and the intracellular stores disrupted. Permeabilized strips were incubated with TNF-α in calcium free buffer (pCa < 9.0) followed by stimulation with a sub maximally contracting calcium buffer. The calcium-activated contractile response of the permeabilized strips was significantly increased after TNF incubation, compared to controls. This potentiation of the calcium-activated contraction was also observed in permeabilized portal vein strips. These experiments suggest that TNF-α can increase the calcium sensitivity of smooth muscle excitation-contraction coupling, possibly via TNF receptor-activated signalling mechanisms. Selective pharmacological inhibitors were used in permeabilized strips to attempt to block the TNF- induced calcium sensitization. Inhibitors of protein kinase C isoforms, phospholipase A<SUB>2</SUB> and p38 mitogen-activated protein kinase had no effect on the TNF potentiation of the calcium-activated contraction. Possible involvement of p4/p44 mitogen-activated protein kinase was also investigated by examining activation of this enzyme in TNF-treated permeabilized bronchial strips. No activation was observed under the same conditions that revealed TNF-induced calcium sensitization. Further experiments by others have shown that TNF potentiates the calcium-activated myosin light chain phosphorylation. The signalling pathways responsible for potentiating the myosin light chain phosphorylation remain to be determined. This study represents a step towards unravelling the mechanisms involved in TNF-induced hyper-responsiveness to bronchoconstrictor agent observed in airways smooth muscle. Further delineation of the intracellular signalling pathways involved may identify new therapeutic targets in the treatment of asthma.