Interaction of iberiotoxin with beta-adrenoceptor agonists and sodium nitroprusside on guinea pig trachea

Abstract

Airway smooth muscle plasma membranes contain a variety of functional K+ channels. In particular, there is a predominance of Ca(2+)-activated K+ channels (maxi-K). Inhibition of these K+ channels has been postulated to account for the ability of charybdotoxin (ChTX) to produce contraction of airway smooth muscle and to modify the relaxant effects of beta-adrenoceptor agonists and sodium nitroprusside (SNP). Iberiotoxin (IbTX) is more selective and more potent than ChTX at blocking maxi-K channels. In this study, pharmacological experiments were performed on guinea pig trachea to determine whether IbTX produced effects similar to ChTX. The concentration-response curves to salbutamol were markedly affected by IbTX, with a > 60-fold rightward shift being produced with 20 nM IbTX. The maximal relaxation to salbutamol was reduced to 49.3 +/- 0.9, 22.3 +/- 4.7, and 15.0 +/- 2.7% of control maximum in the presence of 20, 60, and 180 nM IbTX, respectively. Similar to salbutamol, the maximal relaxation to SNP was reduced to 80 +/- 1.6, 19 +/- 1.7, and 12 +/- 2.1% of control maximum in the presence of 20, 60, and 180 nM IbTX, respectively. IbTX (180 nM) failed to produce a significant alteration of relaxation to the ATP-dependent K+ channel agonist BRL-34915. Exposure of tissues to K(+)-rich medium (80 mM) inhibited responses to salbutamol > or = SNP > isoproterenol. These results confirm and extend our earlier observations that maxi-K channels may be involved in regulating tone and relaxation of carbachol-contracted guinea pig tracheal smooth muscle. This mechanism is of particular importance for beta 2-adrenoceptor- and SNP-induced relaxation.

Activation of endogenous GABAA channels on airway smooth muscle potentiates isoproterenol-mediated relaxation

“…To illustrate that GABA A channel augmentation of isoproterenol-mediated relaxation occurs independent of relaxation achieved due to opening of the maxi K ϩ channel, we performed experiments under conditions of iberiotoxin blockade of the maxi K ϩ channel. Iberiotoxinmediated inhibition of GP smooth muscle relaxation after ␤-agonist treatment closely correlated with work by other investigators (14). Yet, under these conditions, we observed a significant improvement in isoproterenol-mediated relaxation with the addition of muscimol, suggesting the prorelaxant effects we observed are not due to nonspecific effects mediated through the maxi-K channel.…”

“…In addition, there is evidence that ␤-adrenoceptor-mediated cAMP independent pathways can also activate maxi-K channels (19,36). The significance of the maxi-K channel to functional relaxation in the airway with ␤-agonists is further substantiated by evidence that selective inhibition with iberiotoxin of the maxi-K channel greatly limits the degree of ASM relaxation achievable by ␤-adrenoceptor agonist treatment (14). Activation of the maxi-K channel results in egress of potassium and a resultant hyperpolarization, which can limit the free intracellular Ca 2ϩ pool by its effect on L-type voltage-gated Ca 2ϩ channels.…”

“…Hyperpolarization of the ASM cell membrane is in part responsible for the potent relaxant effects observed with ␤ 2 -adrenoceptor agonists (18) via opening of large-conductance calcium-activated potassium channels (maxi-K Ca ; Refs. 14,15,20,35). Thus hyperpolarization via inward flux of chloride through GABA A channels and opening of K Ca channels by ␤-adrenoceptor agonists share mechanistic overlap.…”

Beyond the dogma: novel β2-adrenoceptor signalling in the airways

“…These channels, which are composed of four subunits, are abundant in ASM and are a substrate for PKA [89,[91][92][93]. Furthermore, their role in regulating ASM contractility is suggested from the finding that pharmacological blockade of these channels with charybdotoxin, and the more selective iberiotoxin, prevents hyperpolarisation and b 2 -adrenoceptor-mediated relaxation [93][94][95][96]. …”

.BETA.1-Adrenoceptor-mediated relaxation with isoprenaline and the role of MaxiK channels in guinea-pig esophageal smooth muscle

“…The mechanisms by which such stimulation generates smooth muscle relaxation are still to be established. However, recent physiological and pharmacological studies have shown evidence for a significant role for K + channels in β-adrenoceptor-mediated smooth muscle relaxation (Kume et Correspondence to: Yoshio Tanaka, Ph.D., Department of Chemical Pharmacology, Toho University School of Pharmaceutical Sciences, Miyama 2-2-1, Funabashi-City, Chiba 274-8510, Japan Phone: +81-47-472-1435+81-47-472- Fax: +81-47-472-1448+81-47-472- e-mail: yotanaka@phar.toho-u.ac.jp al., 1989Jones et al, 1990;Huang et al, 1993;Jones et al, 1993;Scornik et al, 1993;Kume et al, 1994;Torphy, 1994;Johnson, 1998;Nara et al, 1998;Horinouchi et al, 2003;Tanaka et al, 2003). Particular attention has been given to the roles of large-conductance, Ca…”