Abstract
We examined the contribution of large-conductance, Ca2+-sensitive K+ (MaxiK) channel to β2-adrenoceptor-activated relaxation to isoprenaline in guinea-pig tracheal smooth muscle focusing on the role for cAMP in the coupling between β2-adrenoceptor and MaxiK channel. Isoprenaline-elicited relaxation was confirmed to be mediated through β2-type of adrenoceptor since the response was antagonized in a competitive fashion by a β2-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 μM). 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 μM) 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 μM). 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 μg/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 β2-adrenoceptor-activated relaxation; 2) both cAMP-dependent and -independent mechanisms underlie the functional coupling between β2-adrenoceptor and MaxiK channel to induce muscle relaxation; and 3) direct regulation of MaxiK channel by Gs operates in cAMP-independent coupling between β2-adrenoceptor and this ion channel.
