RELAXANT EFFECTS OF NKH477, A NEW WATER-SOLUBLE FORSKOLIN DERIVATIVE,ON GUINEA-PIG TRACHEAL SMOOTH-MUSCLE - THE ROLE OF CA2-ACTIVATED K+ CHANNELS()

1 Mechanisms underlying the bronchorelaxant action of NKH477, a newly developed water-soluble forskolin derivative, were investigated in gui nea-pig isolated tracheal smooth muscle. 2 In muscles precontracted wi th 3 mu M histamine, NKH477 (1 nM - 1 mu M) caused a concentration-dep endent decrease of isometric tension, resulting in a complete relaxati on at 300 nM. The EC50 for the relaxation was 32.6+/-4.3 nM (n=6). 3 I n the presence of 30 or 90 nM iberiotoxin (IbTX), a selective blocker of the large-conductance Ca2+-activated K+ (BKCa) channel, the relaxin g action of NKH477 on the histamine-induced contraction was inhibited, giving rise to a parallel shift of the concentration-response curves; the EC50 of NKH477 was increased to 131.4+/-20.4 nM at 30 nM IbTX (n= 4), and 125.3+/-12.2 nM at 90 nM IbTX (n=4). 4 Pretreatment of muscles with 30 mM tetraethylammonium (TEA) caused a similar rightward shift of the concentration-response curve to NKH477 with an increase of the EC50 to 139.8+/-18.4 nM (n=5). In contrast, the relaxing action of NKH 477 was unaffected by 10 mu M glibenclamide, an ATP-sensitive K+ chann el blocker, or by 100 nM apamin, a blocker of small conductance Ca2+-a ctivated K+ channels. 5 In muscles pretreated with 1 mu M nifedipine, a blocker of the voltage-dependent Ca2+ channel (VDC), 30-90 nM IbTX d id not affect the relaxant effects of NKH477 on the histamine-induced contraction. 6 In muscles precontracted by a K+-rich (40 mM) solution, NKH477 caused only minimal relaxation (19.8+/-1.7%, n=4) even at the highest concentration (1 mu M). 7 In experiments to measure the ratio of fura-2 fluorescence signals (R-340/380) as an index of the intracel lular Ca2+ concentration ([Ca2+](i)), the application of 100 nM NKH477 or 200 nM isoprenaline to the preparation precontracted by 3 mu M his tamine resulted in a decrease in [Ca2+](i) in association with a decre ase in tension. The reduction of [Ca2+](i) and tension by NKH477 was 4 7.0+/-5.6% and 62.8+/-7.0%, respectively (n=5), and that with isoprena line 60.6+/-7.4% and 67.4+/-6.4%, respectively (n=5). These effects of NKH477 and isoprenaline on [Ca2+](i) and tension were inhibited by 30 nM IbTX. The inhibitory action of IbTX was abolished in the presence of 1 mu M nifedipine. 8 These results suggest that the bronchorelaxant action of NKH477 may result, at least in part, from activation of BKC a channels, which may cause a hyperpolarization of smooth muscle cell membranes and a secondary decrease in Ca2+ influx through VDCs, leadin g to a decrease in [Ca2+](i).