LEVCROMAKALIM-INDUCED AND ISOPRENALINE-INDUCED RELAXATION OF HUMAN ISOLATED AIRWAYS - ROLE OF THE EPITHELIUM AND OF K+ CHANNEL ACTIVATION

In this study we have investigated the mechanism of action of levcroma kalim and isoprenaline in human isolated airways with respect to the K + channels they activate and the possibility that these smooth muscle relaxants activate K+ channels on the airway epithelium. Mechanical re moval of the epithelial layer (mean percentage of epithelium present 2 0 +/- 3%, n = 20 tissues) did not affect the relaxation responses to l evcromakalim or isoprenaline, either in terms of maximal relaxation or sensitivity. Whilst having no effect on isoprenaline-induced relaxati on, studied from basal tone, the ATP-sensitive K+ channel blocker BRL 31660 (10, 30 and 50 mu M) reduced relaxation responses induced (from basal tone) by levcromakalim from 74 +/- 6% (of the maximal response t o isoprenaline) to 48 +/- 12% (n = 7), 9 +/- 9% (n=4) and 0 (n=4), res pectively. Charybdotoxin, a blocker of high conductance Ca2+-activated Kf channels, at concentrations of 30 and 100 nM, had no effect on eit her levcromakalim- or isoprenaline-induced relaxation responses and ye t charybdotoxin was active at K+ channels in outside-out patches of hi ppocampal granule cells. Moreover, tetraethylammonium (10 mM) inhibite d neither isoprenaline- nor levcromakalim-induced relaxation. This stu dy has demonstrated that the relaxation responses elicited in human br onchus to isoprenaline and levcromakalim are likely to be the result o f direct effects on the smooth muscle with no contribution from epithe lial receptors or K-Ca channels. The actions of levcromakalim appear t o be mediated only via activation of K-ATP channels. Further, we have made the important observation that, under the experimental conditions of our study, isoprenaline does not activate the K-Ca channel to prod uce relaxation in human bronchus.