Voltage dependence of the coupling of Ca(2+) sparks to BK(Ca) channels in urinary bladder smooth muscle

Large-conductance Ca(2+)-dependent K(+) (BK(Ca)) channels play a critical role in regulating urinary bladder smooth muscle (UBSM) excitability and contractility. Measurements of BK(Ca) currents and intracellular Ca(2+) revealed that BK(Ca) currents are activated by Ca(2+) release events (Ca(2+) sparks) from ryanodine receptors (RyRs) in the sarcoplasmic reticulum. The goals of this project were to characterize Ca(2+) sparks and BK(Ca) currents and to determine the voltage dependence of the coupling of RyRs (Ca(2+) sparks) to BK(Ca) channels in UBSM. Ca(2+) sparks in UBSM had properties similar to those described in arterial smooth muscle. Most Ca(2+) sparks caused BK(Ca) currents at all voltages tested, consistent with the BK(Ca) channels sensing approximately 10 microM Ca(2+). Membrane potential depolarization from -50 to -20 mV increased Ca(2+) spark and BK(Ca) current frequency threefold. However, membrane depolarization over this range had a differential effect on spark and current amplitude, with Ca(2+) spark amplitude increasing by only 30% and BK(Ca) current amplitude increasing 16-fold. A major component of the amplitude modulation of spark-activated BK(Ca) current was quantitatively explained by the known voltage dependence of the Ca(2+) sensitivity of BK(Ca) channels. We, therefore, propose that membrane potential, or any other agent that modulates the Ca(2+) sensitivity of BK(Ca) channels, profoundly alters the coupling strength of Ca(2+) sparks to BK(Ca) channels.