Relaxant effects of NKH477, a new water‐soluble forskolin derivative, on guinea‐pig tracheal smooth muscle: the role of Ca2+‐activated K+ channels

• Published in: British journal of pharmacology 1998-02, Vol.123 (4), p.753-761
• Main Authors: Satake, K., Takagi, K., Kodama, I., Honjo, H., Toyama, J., Shibata, S.
• Format: Article
• Language: English
• Subjects: Airway smooth muscle; Animals; Biological and medical sciences; Bronchodilator Agents - pharmacology; Ca2+‐activated K+ channels; Calcium - metabolism; Colforsin - analogs & derivatives; Colforsin - pharmacology; Cyclic AMP - physiology; Guinea Pigs; histamine; Histamine - pharmacology; iberiotoxin; In Vitro Techniques; Isoproterenol - pharmacology; Male; Medical sciences; Muscle Contraction - drug effects; Muscle Relaxation - drug effects; Muscle, Smooth - drug effects; Muscle, Smooth - physiology; Nifedipine - pharmacology; NKH477; Peptides - pharmacology; Pharmacology. Drug treatments; Potassium; Potassium Channels - physiology; Respiratory system; Solubility; Trachea - drug effects; Trachea - physiology; voltage‐dependent Ca2+ channels; Water
• ISSN: 0007-1188; 1476-5381
• DOI: 10.1038/sj.bjp.0701655

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 μM histamine, NKH477 (1 nM–1 μM) caused a concentration‐dependent decrease of isometric tension, resulting in a complete relaxation at 300 nM. The EC50 for the relaxation was 32.6±4.3 nM (n=6). In the presence of 30 or 90 nM iberiotoxin (IbTX), a selective blocker of the large‐conductance Ca2+‐activated K+ (BKCa) 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 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). 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 NKH477 was unaffected by 10 μM glibenclamide, an ATP‐sensitive K+ channel blocker, or by 100 nM apamin, a blocker of small conductance Ca2+‐activated K+ channels. In muscles pretreated with 1 μM nifedipine, a blocker of the voltage‐dependent Ca2+ channel (VDC), 30–90 nM IbTX did not affect the relaxant effects of NKH477 on the histamine‐induced contraction. 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 μM). In experiments to measure the ratio of fura‐2 fluorescence signals (R340/380) as an index of the intracellular Ca2+ concentration ([Ca2+]i), the application of 100 nM NKH477 or 200 nM isoprenaline to the preparation precontracted by 3 μM histamine resulted in a decrease in [Ca2+]i in association with a decrease in tension. The reduction of [Ca2+]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 [Ca2+]i and tension were inhibited by 30 nM IbTX. The inhibitory action of IbTX was abolished in the presence of 1 μM nifedipine. These results suggest that the bronchorelaxant action of NKH477 may result, at least in part, from activation of BKCa channels, which may cause a hyperpolarization of smooth muscle cell membranes and a secondary decrease in Ca2+ influx through VDCs, leading to a decrease in [Ca2+]i. British Jou