FIG. 7.

[Ca²⁺]_i and [InsP₃] regulation of InsP₃R channel activity. A: [Ca²⁺]_i dependence of mean P_o of endogenous X-InsP₃R-1 channels (solid symbols) in various [InsP₃] as tabulated. [Modified from Mak et al. (282).] Each data point in this and subsequent P_o versus [Ca²⁺ ]_i plots is the average of channel P_o from at least 4 experiments using the same ligand concentrations. The curves are least-squares fit of the data points using the biphasic Ca²⁺ regulation Hill equation (Eq. 1) with parameters as tabulated. The large open circles represent P_o for recombinant rat InsP₃R-1 channels in various [Ca²⁺]_i in saturating 10 μM InsP₃. [Modified from Boehning et al. (42).] Inset: plot of K_inh derived from the biphasic Hill equation fit of P_o data versus [InsP₃] used. The curve is the least-squares fit of the K_inh values using the activation Hill equation $K_{\text{inh}}=K_{\text{inh}}^{\infty }{\{1+{({}_{\text{inh}}{}^{{\text{IP}}_3}K/[{\text{InsP}}_3])}^{{}_{\text{inh}}{}^{{\text{IP}}_3}H}\}}^{-1}$ with parameters as tabulated. B: [Ca²⁺]_i dependence of mean P_o of recombinant r-InsP₃R-3 channels in various [InsP₃] as tabulated. [Modified from Mak et al. (283).] Data points and fitted curves are obtained as described for A. C: [Ca²⁺]_i dependence of mean P_o of endogenous InsP₃R channels from Sf9 cells in various [InsP₃] as tabulated. [Modified from Ionescu et al. (196).] Data points, fitted curves, and inset graph are obtained as described for A. D: [Ca²⁺]_i dependence of mean P_o of X-InsP₃R-1 InsP₃R channels that have been exposed to bath solution with very low [Ca²⁺]_i (< 5 nM) for a few minutes before the patch-clamp experiments, in various [InsP₃] as tabulated. The curves are least-squares fits to the data using activation Hill equation P_o = P_Hill{1 + (K_act/[Ca²⁺]_i)^H_act}⁻¹ with parameters as tabulated. [Modified from Mak et al. (286).]