Tanaka Yoshio, Yamashita Yoko, Yamaki Fumiko, Horinouchi Takahiro, Shigenobu Koki, Koike Katsuo
Department of Chemical Pharmacology, Toho University School of Pharmaceutical Sciences, Miyama 2-2-1, Funabashi-City, Chiba 274-8510, Japan.
J Smooth Muscle Res. 2003 Dec;39(6):205-19. doi: 10.1540/jsmr.39.205.
We examined the contribution of large-conductance, Ca(2+)-sensitive K+ (MaxiK) channel to beta2-adrenoceptor-activated relaxation to isoprenaline in guinea-pig tracheal smooth muscle focusing on the role for cAMP in the coupling between beta2-adrenoceptor and MaxiK channel. Isoprenaline-elicited relaxation was confirmed to be mediated through beta2-type of adrenoceptor since the response was antagonized in a competitive fashion by a beta2-selective adrenoceptor antagonist butoxamine with a pA2 value of 6.56. Isoprenaline-induced relaxation was significantly potentiated by a selective inhibitor of cyclic AMP-specific phosphodiesterase, Ro-20-1724 (0.1-1 microM). cAMP-dependent mediation of MaxiK channel in the relaxant response to isoprenaline was evidenced since the potentiated response to isoprenaline by the presence of Ro-20-1724 (1 microM) was inhibited by the channel selective blocker, iberiotoxin (IbTx, 100 nM). This concept was supported by the finding that the relaxation to a membrane permeable cAMP analogue, 8-bromo-cAMP (1 mM), was susceptible to the inhibition by IbTx. On the other hand, isoprenaline-induced relaxation was not practically diminished by an adenylyl cyclase inhibitor SQ 22,536 (100 microM). However, isoprenaline-induced relaxation in the presence of SQ 22,536 was suppressed by IbTx. Characteristics of isoprenaline-induced relaxant response, i.e., impervious to SQ 22,536 but susceptible to IbTx, were practically mimicked by cholera toxin (CTX, 5 microg/ml), an activator of adenylyl cyclase coupled-heterotrimeric guanine nucleotide-binding regulatory protein Gs. These findings indicate that in guinea-pig tracheal smooth muscle: 1) MaxiK channel substantially mediates beta2-adrenoceptor-activated relaxation; 2) both cAMP-dependent and -independent mechanisms underlie the functional coupling between beta2-adrenoceptor and MaxiK channel to induce muscle relaxation; and 3) direct regulation of MaxiK channel by Gs operates in cAMP-independent coupling between beta2-adrenoceptor and this ion channel.
我们研究了大电导钙敏感钾通道（MaxiK通道）在豚鼠气管平滑肌中对β2肾上腺素能受体激活的异丙肾上腺素舒张反应的贡献，重点关注环磷酸腺苷（cAMP）在β2肾上腺素能受体与MaxiK通道偶联中的作用。异丙肾上腺素引起的舒张反应被证实是通过β2型肾上腺素能受体介导的，因为该反应被β2选择性肾上腺素能受体拮抗剂布托沙明以竞争性方式拮抗，其pA2值为6.56。环磷酸腺苷特异性磷酸二酯酶的选择性抑制剂Ro-20-1724（0.1 - 1微摩尔）显著增强了异丙肾上腺素诱导的舒张反应。由于通道选择性阻滞剂iberiotoxin（IbTx，100纳摩尔）抑制了Ro-20-1724（1微摩尔）存在时对异丙肾上腺素增强的反应，因此证明了cAMP依赖性介导的MaxiK通道在对异丙肾上腺素的舒张反应中的作用。这一概念得到了以下发现的支持：对膜通透性cAMP类似物8-溴-cAMP（1毫摩尔）的舒张反应易受IbTx抑制。另一方面，腺苷酸环化酶抑制剂SQ 22,536（100微摩尔）实际上并未减弱异丙肾上腺素诱导的舒张反应。然而，IbTx抑制了在SQ 22,536存在下异丙肾上腺素诱导的舒张反应。异丙肾上腺素诱导的舒张反应的特征，即对SQ 22,536不敏感但对IbTx敏感，实际上被霍乱毒素（CTX，5微克/毫升）模拟，霍乱毒素是腺苷酸环化酶偶联的异三聚体鸟嘌呤核苷酸结合调节蛋白Gs的激活剂。这些发现表明，在豚鼠气管平滑肌中：1）MaxiK通道在很大程度上介导了β2肾上腺素能受体激活的舒张反应；2）cAMP依赖性和非依赖性机制均是β2肾上腺素能受体与MaxiK通道之间功能性偶联以诱导肌肉舒张的基础；3）Gs对MaxiK通道的直接调节作用于β2肾上腺素能受体与该离子通道之间的cAMP非依赖性偶联。
