Heteromeric CNGA3+CNGB3 channels are inhibited by PIP₃ or PIP₂. (A) Representative current traces for heteromeric CNGA3+CNGB3 channels, elicited by subsaturating concentrations of cGMP (5 µM), before (black) and after (red) 1 µM PIP₃ or 10 µM PIP₂ application for ∼5 min. (B) Representative dose–response relationships for the activation of heteromeric CNGA3+CNGB3 channels by cGMP before and after PIP₃ or PIP₂ application. The shadings indicate the approximate physiological range of intracellular cGMP levels within vertebrate photoreceptors in the dark. The Hill parameters for each representative patch were: K_1/2 cGMP = 16.1 µM, h = 2.05 before PIP₃; K_1/2 cGMP = 26.5 µM, h = 2.0 after PIP₃; K_1/2 cGMP = 12.8 µM, h = 1.8 before PIP₂; and K_1/2 cGMP = 24.5 µM, h = 1.6 after PIP₂. (C) Plot of percentage increase in K_1/2 cGMP for heteromeric A3+B3 channels at different concentrations of PIP₃ (closed squares), PIP₂ (closed circles), and natural PIP₂ (open circles; n = 4–5 for each PIP_n concentration). The PIP_n concentration–response relationships were fitted with the Hill equation, and the Hill parameters (K_1/2 and h) for each phosphoinositide species were as follows: 0.58 µM and 1.4 for PIP₃ (diC8-PIP₃), 3.44 µM and 1.5 for PIP₂ (diC8-PIP₂), and 2.84 µM and 1.4 for natural PIP₂. Error bars indicate mean ± SEM.