Fig. 11.

(A) Activation and de-activation of muscarinic inwardly rectifying K⁺ channels is accelerated by the GTPase activating protein RGS4. The effect of a pulse of acetylcholine on the development of K⁺ currents was measured by the two electrode voltage clamp method. Top trace, K⁺ currents evoked by addition of acetylcholine (ACh) during the time-span shown, were recorded in an oocyte that that had been injected with cRNAs encoding the M2 muscarinic receptor and the Kir3.1 and Kir3.2 K⁺ channel subunits. Bottom trace, K⁺ currents evoked by acetylcholine in an oocyte that had been injected in addition with cRNA encoding mammalian RGS4. Note faster response to ACh addition and faster deactivation upon ACh wash-out. (Adapted from Doupnik et al., 1997 [98]). (B) Simulation of HR complex formation [p=f(t)] from H and R as a function of time using the formulas and rate constants shown. (C) Same as (B) but calculated for k_off values spanning from 0.01 to 30 s⁻¹. Note that at increasing k_off equilibrium is reached faster and also, that the p(eq) values decrease. (D) Same as (C) but normalizing by expressing p(t) as a function of its value at equilibrium.