We have investigated the effect of the potassium channel opener, NIP-121, on contraction elicited by melittin (a phospholipase A2 activator) in epithelium-intact and epithelium-denuded trachea isolated from guinea-pigs. The effects of NIP-121 were compared with those of isoprenaline, aminophylline and hydrocortisone. 2. In the presence of the cyclo-oxygenase inhibitor, indomethacin (5 microM), melittin (3 micrograms ml-1) caused time-dependent contraction. The melittin-induced contractile response was significantly augmented by removal of the epithelium and was concentration-dependently and completely inhibited by the phospholipase A2 (PLA2) inhibitor, p-bromophenacyl bromide (BPB 10-100 microM), and the lipoxygenase inhibitor, phenidone (10-300 microM). Similar inhibition of the melittin response by BPB (10 microM) and phenidone (10 microM) was observed in the epithelium-denuded trachea. 3. Concentration-dependent inhibition of the melittin response was induced by preincubation with NIP-121 (0.03 and 0.1 microM), isoprenaline (0.001 and 0.01 microM), aminophylline (30 and 100 microM) and hydrocortisone (100 and 300 microM). The effect of NIP-121 was abolished by glibenclamide (1 microM). 4. The inhibitory effect of NIP-121 on the melittin response was greatly reduced by removing the epithelium while that of the isoprenaline, aminophylline or hydrocortisone was not changed. 5. The inhibition of the melittin response by these drugs was similar to their inhibition of the contraction elicited by a low concentration (3 nM) of leukotriene D4 (LTD4) in the epithelium-intact trachea. Inhibition of the LTD4 response by NIP-121 was not observed in the epithelium-denuded trachea. However, higher concentrations of NIP-121 (0.3 and 1 microM) did inhibit LTD4-induced contractions of epithelium-denuded trachea.6. These findings suggest that melittin causes epithelium-dependent contraction of the guinea-pig isolated trachea which is mediated by products of lipoxygenase activity. NIP-121 may inhibit the melittin response by activating glibenclamide-sensitive potassium channels, which appear to be epithelium-dependent (an indirect effect of NIP-121 apart from its direct effect on the airway smooth muscle) while isoprenaline, aminophylline and hydrocortisone act directly to relax the trachealis smooth muscle.
