Pacemaker channels produce an instantaneous current

Spontaneous rhythmic activity in mammalian heart and brain depends on pacemaker currents (I-h), which are produced by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Here, we report that the mouse HCN2 pacemaker channel isoform also produced a large instantaneous current (I-inst(HCN2)) in addition to the well characterized, slowly activating I-h. I-inst(HCN2) was specific to expression of HCN2 on the plasma membrane and its amplitude was correlated with that of I-h. The two currents had similar reversal potentials, and both were modulated by changes in intracellular Cl- and cAMP. A mutation in the S4 domain of HCN2 (S306Q) decreased I-h but did not alter I-inst(HCN2), and instantaneous currents in cells expressing either wild type HCN2 or mutant S306Q channels were insensitive to block by Cs+. Co-expression of HCN2 with the accessory subunit, MiRP1, decreased I-h and increased I-inst(HCN2), suggesting a mechanism for modulation of both currents in vivo. These data suggest that expression of HCN channels may be accompanied by a background conductance in native tissues and are consistent with at least two open states of HCN channels: I-inst(HCN2) is produced by a Cs+-open state; hyperpolarization produces an additional Cs+-sensitive open state, which results in I-h.