title: MaxiK channel mediates beta2-adrenoceptor-activated relaxation to isoprenaline through cAMP-dependent and -independent mechanisms in guinea-pig tracheal smooth muscle.
abstract: 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.
authors.affiliation: Department of Chemical Pharmacology, Toho University School of Pharmaceutical Sciences, Miyama 2-2-1, Funabashi-City, Chiba 274-8510, Japan. yotanaka@phar.toho-u.ac.jp
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Tanaka
authors.forename: Yoko
authors.initials: Y
authors.lastname: Yamashita
authors.forename: Fumiko
authors.initials: F
authors.lastname: Yamaki
authors.forename: Takahiro
authors.initials: T
authors.lastname: Horinouchi
authors.forename: Koki
authors.initials: K
authors.lastname: Shigenobu
authors.forename: Katsuo
authors.initials: K
authors.lastname: Koike
mesh_terms: D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D000242:Cyclic AMP / Q000502:physiology*; D005260:Female; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000502:physiology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology*; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008297:Male; D009126:Muscle Relaxation / Q000502:physiology*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D024681:Potassium Channels, Calcium-Activated / Q000502:physiology*; D018343:Receptors, Adrenergic, beta-2 / Q000502:physiology*; D014132:Trachea / Q000187:drug effects / Q000502:physiology*
chemical_list: D000318:Adrenergic beta-Agonists; D051036:Large-Conductance Calcium-Activated Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D018343:Receptors, Adrenergic, beta-2; D000242:Cyclic AMP; D019205:GTP-Binding Protein alpha Subunits, Gs; D007545:Isoproterenol