Figure 6. Ap₄A and ATP compete for the same site to inhibit K_ATP channels.

A, Ap₄A inhibited cardiac K_ATP currents in a concentration-dependent manner and application of 20 μM oleoyl-CoA greatly reduced Ap₄A inhibition. B, concentration-response curves for Ap₄A inhibition of WT, R50E and K185Q channels, and WT channels after oleoyl-CoA (A); fits to standard Hill equations with K_i(Ap4A) values of 10 ± 1 μM (WT), 438 ± 77 μM (R50E) and 200 ± 35 μM (K185Q). C, inhibition of K_ATP currents by 16 μM Ap₄A in the presence and absence of 40 μM ATP is plotted as means ± S.E.M. The dashed line represents the expected theoretical value for current inhibition by 16 μM Ap₄A in the presence of 40 μM ATP assuming direct competition between Ap₄A and ATP. To obtain this value, the amount of Ap₄A inhibition (in the absence of ATP) was used to calculate the ATP concentration (14.5 μM) necessary to produce the same inhibition using a concentration-response curve as in Fig. 1C. Accordingly, the theoretical value represents the expected increase in ATP inhibition produced by a rise in the ATP concentration from 40 μM to (40 + 14.5) μM. D, inhibition of K_ATP currents by 333 μM CoA in the presence and absence of 40 μM Ap₄A is plotted as means ± S.E.M. The dashed line represents the expected theoretical value for current inhibition by 333 μM CoA in the presence of 40 μM Ap₄A assuming direct competition between CoA and Ap₄A (the theoretical value was calculated as described in C).