FIG. 10.

Regulation of InsP₃R channel activity by ATP. A: [Ca²⁺]_i dependence of mean P_o of endogenous X-InsP₃R-1 channels in the presence of various [ATP]_free as tabulated. [Modified from Mak et al. (281).] Solid symbols represent data obtained in the absence of Mg²⁺. Open circles represent data obtained in 3 mM Mg²⁺ and 0 ATP. Open squares represent data obtained in 3 mM Mg²⁺ and 0.5 mM total [ATP], with [ATP]_free = 12 μM calculated by MaxChelator. The curves are fits using either the biphasic Hill equation (Eq. 1) to the data in [ATP]_free = 0 (blue) or 0.5 mM (black); or the activating Hill equation (see legend for Fig. 7) to the data in other [ATP]_free (10 nM < [Ca²⁺]_i < 1 μM). Biphasic Hill equation parameters fitting data in 0 [ATP]_free (blue curve) are tabulated. Inset: plot of the Hill equation parameter K_act versus [ATP]_free used. The curve is a fit to the K_act values using the modified Michaelis-Menten equation $K_{\text{act}}=K_{\text{act}}^{0\text{ATP}}+(K_{\text{act}}^{\infty \text{ATP}}-K_{\text{act}}^{0\text{ATP}}){\{1+({[\text{ATP}]}_{\text{free}}/{}_{\text{act}}{}^{\text{ATP}}K)\}}^{-1}$ with parameters as tabulated. B: [Ca²⁺]_i dependence of mean P_o of recombinant r-InsP₃R-3 channels in the presence of various [ATP]_free as tabulated. [Modified from Mak et al. (280).] Solid symbols, open circles, and open squares represent data in a similar convention as described for A. Solid curves are either biphasic Hill equation fit or activation Hill equation fit to the data in various [ATP]_free as described for A. The thick dashed blue curve is the biphasic Hill equation fit to channel P_o for X-InsP₃R-1 in the absence of ATP plotted for comparison. Parameters are tabulated for biphasic Hill equation fit to r-InsP₃R-3 channel P_o in 0 ATP (in blue), as well as those for activating Hill equation fit to r-InsP₃R-3 channel P_o in [ATP]_free = 0.3 mM (in yellow) and 0.5 mM (in black) ([Ca²⁺]_i < 1 μM). C: [Ca²⁺]_i dependence of mean P_o of endogenous X-InsP₃R-1 channels in 0 ATP in the presence of various [InsP₃] as tabulated. The curves are fits to the data using biphasic Hill equation (Eq. 1) with parameters as tabulated. [Modified from Mak et al. (279).] D: channel P_o versus [ATP]_free curves calculated for X-InsP₃R-1 channels in 10 μM InsP₃ and various [Ca²⁺]_i as labeled, using the biphasic Hill equation (Eq. 1) and the modified Michaelis-Menten equation in A.