Extent of Kir2.1 PIP₂ activation is controlled by CD-I stability. (A) Depletion of endogenous PIP₂ with CaCl₂ (10 mM) eliminates channel activity, which is reactivated by application of exogenous PIP₂. (B) After CaCl₂ exposure, channel activity was measured at varying concentrations of short-chain PIP₂ (DiC8-PIP₂), and the maximum response was determined at the end of each recording with PIP₂. (C) DiC8-PIP₂ dose–response relationship fit with the Hill equation (Eq. 2). Data are presented as means ± SEM; *, P < 0.05 relative to WT means (Student’s t test).