Hyperpolarization-activated HCN channels are modulated by direct binding of cyclic nucleotides. For HCN2 channels, cAMP shifts the voltage dependence for activation, with relatively little change in the maximal conductance. By contrast, in spHCN channels, cAMP relieves a rapid inactivation process and produces a large increase in maximum conductance. Our results suggest that these two effects of cAMP represent the same underlying process. We also find that spHCN inactivation occurs not by closure of a specialized inactivation gate, as for other voltage-dependent channels, but by reclosure of the same intracellular gate opened upon activation. Effectively, the activation gate exhibits a "desensitization to voltage," perhaps by slippage of the coupling between the voltage sensors and the gate. Differences in the initial coupling efficiency could allow cAMP to produce either the inactivation or the shift phenotype by strengthening effective coupling: a shift would naturally occur if coupling is already strong in the absence of cAMP.
Shin, K. S., Maertens, C., Proenza, C., Rothberg, B. S., & Yellen, G. (2004). Inactivation in HCN channels results from reclosure of the activation gate: Desensitization to voltage. Neuron, 41(5), 737–744. https://doi.org/10.1016/S0896-6273(04)00083-2