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Departments of 1 Molecular Physiology and Biophysics and 2 Pharmacology, University of Vermont College of Medicine, Burlington, Vermont 05405
Large-conductance Ca2+-dependent K+
(BKCa) channels play a critical role in regulating urinary
bladder smooth muscle (UBSM) excitability and contractility.
Measurements of BKCa currents and intracellular
Ca2+ revealed that BKCa currents are activated
by Ca2+ release events (Ca2+ sparks) from
ryanodine receptors (RyRs) in the sarcoplasmic reticulum. The goals of
this project were to characterize Ca2+ sparks and
BKCa currents and to determine the voltage dependence of
the coupling of RyRs (Ca2+ sparks) to BKCa
channels in UBSM. Ca2+ sparks in UBSM had properties
similar to those described in arterial smooth muscle. Most
Ca2+ sparks caused BKCa currents at all
voltages tested, consistent with the BKCa channels sensing
~10 µM Ca2+. Membrane potential depolarization from
50 to
20 mV increased Ca2+ spark and BKCa
current frequency threefold. However, membrane depolarization over this
range had a differential effect on spark and current amplitude, with
Ca2+ spark amplitude increasing by only 30% and
BKCa current amplitude increasing 16-fold. A major
component of the amplitude modulation of spark-activated
BKCa current was quantitatively explained by the known
voltage dependence of the Ca2+ sensitivity of
BKCa channels. We, therefore, propose that membrane potential, or any other agent that modulates the Ca2+
sensitivity of BKCa channels, profoundly alters
the coupling strength of Ca2+ sparks to
BKCa channels.
guinea pig; sarcoplasmic reticulum; ryanodine receptor; iberiotoxin; thapsigargin; large-conductance Ca2+-activated K+ channels
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