Figure 5.

Chimera X-α is less efficiently activated by cGMP than is X-β, and exhibits desensitization at high cGMP concentrations. (A, top traces) Macroscopic current traces from homomeric X-α channels (different patch than that shown in Fig. 4 A), elicited by indicated concentrations of agonist. Channels desensitize in 30 mM cGMP and recover to the activation level of 3 mM cGMP during the slow washout of agonist (dotted portion of open bars). (bottom traces) Currents during voltage steps at indicated steady-state cGMP concentrations (tested as in Fig. 1). (B1–B3) Excerpts of single X-α channel recordings at indicated potentials and steady-state cGMP concentrations (same patch as in Fig. 4, B1–B4). P_open values from excerpt in 3 mM cGMP are 0.676 at +80 mV and 0.462 at −80 mV; values from excerpt in 30 mM cGMP (collected at steady-state after onset of desensitization) are 0.131 at +80 mV and 0.125 at −80 mV. (C) Mean dose–response data for X-α activation by cAMP (closed circles) and cGMP (open circles) at −100 mV, compiled from six patches expressing macroscopic currents; graph is normalized with P_max,cAMP = 0.49, the mean from single-channel measurements (materials and methods). Lines show Hill equation fits for cAMP (solid) and cGMP (dashed; excluding data >3 mM); see Table for values of K_1/2 and h. Using fitted values for I_max, the ratio I_max,cAMP/I_max,cGMP is 1.405 ± 0.034. For comparison, X-β dose–response curves for cAMP (solid line) and cGMP (dashed line), taken from Fig. 1C and normalized as in Fig. 4 C, are shown.