Genetic ablation of IP₃R1 abolishes IP₃-induced BK_Ca channel activation in cerebral artery smooth muscle cells. (A) Western blot indicating that IP₃R1 protein (∼270 kD) is present in IP₃R1^+/+ mouse aorta and absent in IP₃R1^−/− aorta. (B) Mean data illustrating that 10 µM IP₃ activates BK_Ca channels in inside-out patches from mouse IP₃R1^+/+ cerebral artery smooth muscle cells, but not in mouse IP₃R1^−/− cerebral artery smooth muscle cells. Mean data also indicate that 1 mg/ml heparin reverses IP₃-induced BK_Ca channel activation in IP₃R1^+/+ cells. Membrane voltage was −40 mV. IP₃R1^−/−: control, n = 5; IP₃, n = 5; IP₃ plus heparin, n = 5. IP₃R1^+/+: control, n = 5; IP₃, n = 5; IP₃ plus heparin, n = 5. *, P < 0.05.