Figure 2.

IP₃-concentration dependence of the [Ca²⁺]_i sensitivity of the IP₃R. Different symbols denote data for various IP₃ concentrations as tabulated. Data points for 10 μM IP₃ were combined from experiments using various bath solutions with the same pipette [Ca²⁺]. Data points for other concentrations of IP₃ were obtained in bath solutions containing 220 nM Ca²⁺ and no ATP. The curves are Hill equation fits using Eq. 1, with K_inh varying with IP₃ concentration with values as listed in the graph, whereas P_max, K_act, H_act, and H_inh remained independent of IP₃ concentration, with the values tabulated in the graph. Numerical labels indicate calculated concentrations (in μM) of free Ca²⁺ chelator BAPTA present in pipette solutions used for the corresponding data points.