Release and actions of inhibitory prostaglandins from canine tracheal epithelium
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This study evaluated the sources and actions of inhibitory prostanoids on canine tracheal smooth muscle to determine if these accounted for epithelium derived relaxing factor activity. Concentration-response curves of canine tracheal smooth muscle were generated to carbachol in the presence and absence of the epithelium and (or) indomethacin, a cyclooxygenase inhibitor. Removal of the epithelium or addition of indomethacin (10(-5) M) in the presence of the epithelium shifted the curve significantly leftward compared with epithelial-intact tissue. Furthermore, addition of indomethacin to epithelial-denuded tissue produced the greatest shift in the curve. Removal of the epithelium increased contractility in response to electrical-field stimulation at intermediate frequencies compared with epithelium-intact tissues. The addition of indomethacin to epithelium-intact tissue also increased the contractile responses. Removal of the epithelium in the presence of indomethacin further increased responses. Radioimmunoassay of muscle bath fluid indicated that the inhibitory prostanoids prostaglandin I2 (PGI2) and prostaglandin E2 (PGE2) were released. PGI2 was released from the epithelium as well as from a nonepithelial source. PGE2 was released from the epithelium in response to electrical-field stimulation. The release of PGE2 was greatly reduced by the addition of indomethacin (10(-5) M), but not by the addition of omega-conotoxin (GVIA), an N-type Ca2+ channel antagonist, nor by the addition of tetrodotoxin, a Na+ channel blocker. Both toxins abolished contractions to electrical-field stimulation. We conclude that the inhibitory prostanoids PGI2 and PGE2 are released, along with a nonprostanoid factor from epithelium, and modulate airway smooth muscle contractility to stimulation of cholinergic nerves and muscarinic agonists. PGE2 is released from epithelium by electrical-field stimulation independent of nerve function.
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