Figure 4. Calcium response and drug response of Ca²⁺-activated K⁺ channels (BK) of flexor digitorum brevis (FDB) and soleus (Sol) fibers.

(A–B) Currents were recorded during voltage steps from 0 mV holding potential, prepulse of −70 mV, to −80 mV and +80 mV (Vm) in excised macropatches with 150 mM KCl on both sides of the membrane patches and in the presence of increasing calcium concentrations in the bath. Upward deflection in the current record indicates outward currents recorded at +30 mV or +80 mV (Vm); downward deflection indicates inward currents recorded at −60 mV (Vm). C, closed channel level; O, open channel level. The currents were leak-subtracted. (A) The represented BK channel current traces of FDB and Sol fibers were obtained by an ensemble average of 16 and 18 sweeps (1 sweep/voltage membrane/patch) from 16 and 18 patches for each FDB and Sol muscle, respectively. Channel currents were recorded in the FDB and Sol fibers using pipettes of 1.9±0.6 MOhm and 2.1±0.4 MOhm resistance, respectively. The Ca²⁺ response of the channels was evaluated at +30 mV (Vm) in the presence of increasing concentration of internal Ca²⁺ ions from 0 M to 1.3x10⁻⁶M and 10⁻⁵M. The recorded currents were than normalized to I BKmax which is the maximal BK current recorded at +80 mV (Vm) in the presence of 10⁻⁵M concentration of free Ca²⁺ ions. A maximal BK channel activation of 80% was observed in the FDB fibers in these patches in the presence of 10⁻⁵ M concentration of Ca²⁺ ions, whereas no more than 15% activation of the BK channel current was observed in Sol fibers in the presence of 1.3x10⁻⁶M and 10⁻⁵ M concentrations of Ca²⁺ ions indicating that the BK channel population in FDB fibers was more sensitive to Ca²⁺ ions than those of Sol fibers. (B) BK channel current response to resveratrol (RESV) (2x10⁻⁴M) and bendroflumethiazide (BFT) (2x10⁻⁴M) recorded in fibers from FDB and Sol muscles. Single channel currents were recorded in the FDB and Sol fibers using pipettes of 3.2 MOhm and 3.4 MOhm resistance, respectively. Single channel transitions were observed in the FDB and Sol fibers at +30 mV (Vm) in the presence of 10⁻⁵ M free Ca²⁺ ions. The N Popen was higher in the Sol fiber as compared with that recorded in the FDB fiber, the number of functional channels in the patches were 3 in the FDB fiber and 5 in the Sol fiber. No difference was observed in the single channel current between the FDB and Sol fibers. The application of RESV and BFT increased the N Popen of the channel without affecting single channel current. In the FDB fiber, BFT was more effective as BK opener than RESV, whereas RESV was more effective than BFT in the Sol fiber.