Relaxant effects of NKH477, a new water‐soluble forskolin derivative, on guinea‐pig tracheal smooth muscle: the role of Ca 2+ ‐activated K + channels

Mechanisms underlying the bronchorelaxant action of NKH477, a newly developed water‐soluble forskolin derivative, were investigated in guinea‐pig isolated tracheal smooth muscle. In muscles precontracted with 3 μ M histamine, NKH477 (1 n M –1 μ M ) caused a concentration‐dependent decrease of isometric tension, resulting in a complete relaxation at 300 n M . The EC 50 for the relaxation was 32.6±4.3 n M ( n =6). In the presence of 30 or 90 n M iberiotoxin (IbTX), a selective blocker of the large‐conductance Ca 2+ ‐activated K + (BK Ca ) channel, the relaxing action of NKH477 on the histamine‐induced contraction was inhibited, giving rise to a parallel shift of the concentration‐response curves; the EC 50 of NKH477 was increased to 131.4±20.4 n M at 30 n M IbTX ( n =4), and 125.3±12.2 n M at 90 n M IbTX ( n =4). Pretreatment of muscles with 30 m M tetraethylammonium (TEA) caused a similar rightward shift of the concentration‐response curve to NKH477 with an increase of the EC 50 to 139.8±18.4 n M ( n =5). In contrast, the relaxing action of NKH477 was unaffected by 10 μ M glibenclamide, an ATP‐sensitive K + channel blocker, or by 100 n M apamin, a blocker of small conductance Ca 2+ ‐activated K + channels. In muscles pretreated with 1 μ M nifedipine, a blocker of the voltage‐dependent Ca 2+ channel (VDC), 30–90 n M IbTX did not affect the relaxant effects of NKH477 on the histamine‐induced contraction. In muscles precontracted by a K + ‐rich (40 m M ) solution, NKH477 caused only minimal relaxation (19.8±1.7%, n =4) even at the highest concentration (1 μ M ). In experiments to measure the ratio of fura‐2 fluorescence signals (R 340/380 ) as an index of the intracellular Ca 2+ concentration ([Ca 2+ ] i ), the application of 100 n M NKH477 or 200 n M isoprenaline to the preparation precontracted by 3 μ M histamine resulted in a decrease in [Ca 2+ ] i in association with a decrease in tension. The reduction of [Ca 2+ ] i and tension by NKH477 was 47.0±5.6% and 62.8±7.0%, respectively ( n =5), and that with isoprenaline 60.6±7.4% and 67.4±6.4%, respectively ( n =5). These effects of NKH477 and isoprenaline on [Ca 2+ ] i and tension were inhibited by 30 n M IbTX. The inhibitory action of IbTX was abolished in the presence of 1 μ M nifedipine. These results suggest that the bronchorelaxant action of NKH477 may result, at least in part, from activation of BK Ca channels, which may cause a hyperpolarization of smooth muscle cell membranes and a secondary decrease in Ca 2+ influx through VDCs, leading to a decrease in [Ca 2+ ] i .British Journal of Pharmacology (1998) 123 , 753–761; doi: 10.1038/sj.bjp.0701655