Bath solution (0Ca²⁺ 0Mg²⁺ intracellular solution)

Reagent	Grams in 1L	Final Concentration (1X)
KOH	7.854 g	140 mM
HEPES	4.766 g	20 mM
KCl	0.1491 g	2 mM
EGTA	1.902 g	5 mM
MeSO3	13.314 g	140 mM
H2O	Add water to 1L

Use MeSO₃ to adjust pH to 7.1–7.2

^*Note: Xenoups oocytes express a large endogenous Ca²⁺-activated Cl⁻ current that may interfere with BK channel currents when the intracellular solution contains free Ca²⁺. To avoid this contamination, a Cl⁻-free solution is desired. However, 2mM KCl is included in the solution in order to maintain steady electric connection with the Ag/AgCl ground electrode.