Endogenous PIP₂ activates native BK currents in the presence of blockers of PLC-mediated PIP₂ downstream products. Perforated patch recordings from two (A–E and F–J) freshly isolated cerebral artery myocytes bathed in physiological saline solution (PSS; composition in Materials and methods). Total outward K⁺ currents were recorded in the continuous presence of 5 mM 4-AP and 0.1 mM niflumic acid. Bath application of 2 μM Ro31-8220 and 0.2 μM thapsigargin increases mean outward current by 95% (B vs. A, and G vs. F). Subsequent inhibition of PI3 kinase by 5 nM wortmannin further increases current by 184% from control (C). Inhibition of PLC by 25 μM U73122 drastically increases current (D), likely due to buildup of PIP₂ in the membrane. The current is blocked by 0.3 μM paxilline (E and H), indicating it is mediated by BK channels. Preapplication of paxilline, a selective BK channel blocker, prevents both wortmannin (I) and U73122 (J) actions (n = 5–6).