Fig. 1.

Original recordings illustrating the inhibitory effect of isoproterenol on the contractility of urinary bladder smooth muscle (UBSM) strips isolated from wild-type (WT) and large-conductance Ca²⁺-activated K⁺ (BK) channel α-subunit knockout (KO) mice. Isoproterenol (0.1 nM–1 μM) inhibited in a concentration-dependent manner both phasic and tonic contractions of WT UBSM strips (top trace), WT UBSM strips pretreated with the BK channel inhibitor iberiotoxin (IBTX; 200 nM; middle trace), and KO UBSM strips (bottom trace). The overall contractility was increased in the presence of IBTX and in the absence of functional BK channel (BK-KO mice). Note that isoproterenol was less effective when the BK channel was inhibited with IBTX but had similar potency in the BK-KO compared with WT mice. Phasic and tonic contractions were induced by addition of 20 mM K⁺ in the bath solution. Dotted lines indicate the initial baseline level, which corresponds to the muscle tone.