Simple allosteric reaction scheme for bimodal agonism. Reaction scheme (Chan and Young, 2009) is illustrated by a cell-free inside-out macroscopic patch recording of bimodal CNG channels with cGMP washed into and out from the bath (solid and dashed black bars, respectively). Applying cGMP elicits inward channel current with no desensitization up to 3 mM cGMP, but with 10 mM cGMP, the steady-state macroscopic conductance (white arrow) is smaller than for 3 mM. During wash-in and wash-out of the 10-mM cGMP solution, there are brief intervals with bath cGMP concentration near 3 mM giving rise to “spikes” of maximal conductance. Details of reaction scheme notation: Closed- (C) and open- (O) channel states are equilibrating rapidly compared with agonist wash-in and wash-out. Sizes of black reaction arrows indicate the favored direction of equilibrium. P_o increases with cGMP binding of up to m molecules of cGMP (pro action). Binding of the (m+1)^th cGMP molecule decreases the steady-state P_o (con action) to below the value obtained with m ligands. Experimental details of current trace: The previously characterized channel, X-fA2, incorporates the BD from fCNGA2 channel in a chimera with sequence from other CNG channel types (see Young et al., 2001, and Fig. 2). X-fA2 was expressed as homomers in Xenopus oocytes; patches were held at −40 mV. White arrows mark times where steady-state conductance, g, was measured with cGMP concentrations fixed at their nominal concentrations; for this example, g_{(10 mM cGMP)}/g_{(3 mM cGMP)} = 0.63.