Isoprenaline and aminophylline relax bronchial smooth muscle by cAMP-induced stimulation of large-conductance Ca2+-activated K+ channels

Aim: To investigate whether the relaxation of bronchial smooth muscle induced by isoprenaline and aminophylline is mediated by large conductance Ca2+-activated K+ channels (BKCa) via cAMP-dependent mechanism. Methods: With isometric tension recording, the role of BKCa in relaxations of rat bronchial strips induced by isoprenaline and aminophylline was determined. With perforated patch-clamp technique, BKCa currents were observed in freshly isolated rat bronchial myocytes. Results: Tetraethylammonium 5 mmol/L, a BKCa blocker, caused a significant rightward shift in the concentration-response curves of isoprenaline and aminophylline (about 4.26-fold and 3.78-fold, respectively) in methacholine-precontracted rat bronchial strips. Isoprenaline 1 mumol/L caused a significant increase in BKCa current from (94+/-15) pA/pF to (186+/-30) pA/pF (voltage steps from -60 mV to +50 mV, n=10, P<0.01), which was partly abolished by Rp-cAMP 100 mu mol/L, a protein kinase A inhibitor. Furthermore, current-voltage relationship(I-V) curve exhibited an upward shift, and the peak current density was significantly raised (n=10, P<0.01) by ramp depolarization. from -100 mV to +100 mV. Aminophylline 1 mmol/L caused a significant increase in BKCa current from (90+/-10) pA/pF to (166+/-25) pA/pF (voltage steps from -60 mV to +50 mV, n=11, P<0.01), which was partly abolished by Rp-cAMP 100 mu mol/L. Furthermore, the I-V curve exhibited an upward shift, and the peak current density was significantly raised (n=11, P<0.01) by ramp depolarization from -100 mV to +100 mV. Conclusion: The relaxations induced by isoprenaline and aminophylline were, at least partly, mediated by cAMP-stimulation of BKCa in rat bronchial smooth muscle.