Differential regulation of Ca2+-activated K+ channels by β-adrenoceptors in guinea pig urinary bladder smooth muscle

Abstract

Stimulation of β-adrenoceptors contributes to the relaxation of urinary bladder smooth muscle (UBSM) through activation of large-conductance Ca2+-activated K+ (BK) channels. We examined the mechanisms by which β-adrenoceptor stimulation leads to an elevation of the activity of BK channels in UBSM. Depolarization from −70 to +10 mV evokes an inward L-type dihydropyridine-sensitive voltage-dependent Ca2+ channel (VDCC) current, followed by outward steady-state and transient BK current. In the presence of ryanodine, which blocks the transient BK currents, isoproterenol, a nonselective β-adrenoceptor agonist, increased the VDCC current by ∼25% and the steady-state BK current by ∼30%. In the presence of the BK channel inhibitor iberiotoxin, isoproterenol did not cause activation of the remaining steady-state K+ current component. Decreasing Ca2+ influx through VDCC by nifedipine or depolarization to +80 mV suppressed the isoproterenol-induced activation of the steady-state BK current. Unlike forskolin, isoproterenol did not change significantly the open probability of single BK channels in the absence of Ca2+ sparks and with VDCC inhibited by nifedipine. Isoproterenol elevated Ca2+ spark (local intracellular Ca2+ release through ryanodine receptors of the sarcoplasmic reticulum) frequency and associated transient BK currents by ∼1.4-fold. The data support the concept that in UBSM β-adrenoceptor stimulation activates BK channels by elevating Ca2+ influx through VDCC and by increasing Ca2+ sparks, but not through a Ca2+-independent mechanism. This study reveals key regulatory molecular and cellular mechanisms of β-adrenergic regulation of BK channels in UBSM that could provide new targets for drugs in the treatment of bladder dysfunction.