Hysteresis in the Voltage Dependence of HCN Channels

Abstract

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) ion channels are important for rhythmic activity in the brain and in the heart. In this study, using ionic and gating current measurements, we show that cloned spHCN channels undergo a hysteresis in their voltage dependence during normal gating. For example, both the gating charge versus voltage curve, Q(V), and the conductance versus voltage curve, G(V), are shifted by about +60 mV when measured from a hyperpolarized holding potential compared with a depolarized holding potential. In addition, the kinetics of the tail current and the activation current change in parallel to the voltage shifts of the Q(V) and G(V) curves. Mammalian HCN1 channels display similar effects in their ionic currents, suggesting that the mammalian HCN channels also undergo voltage hysteresis. We propose a model in which HCN channels transit between two modes. The voltage dependence in the two modes is shifted relative to each other, and the occupancy of the two modes depends on the previous activation of the channel. The shifts in the voltage dependence are fast (τ ≈ 100 ms) and are not accompanied by any apparent inactivation. In HCN1 channels, the shift in voltage dependence is slower in a 100 mM K extracellular solution compared with a 1 mM K solution. Based on these findings, we suggest that molecular conformations similar to slow (C-type) inactivation of K channels underlie voltage hysteresis in HCN channels. The voltage hysteresis results in HCN channels displaying different voltage dependences during different phases in the pacemaker cycle. Computer simulations suggest that voltage hysteresis in HCN channels decreases the risk of arrhythmia in pacemaker cells.

Use Dependent Attenuation of Rat HCN1-Mediated Ih in Intact HEK293 Cells

“…It also markedly accelerated the activation time course of I h (from τ act-40mV = 386.9 ± 52.6 ms to τ act-40mV = 282 ± 31.1 ms, n = 10, p < 0.05, paired t test). This is consistent with a voltage dependent hysteresis as described for certain HCN channels [11]. The holding currents remained comparable ( I holdini = 52.9 ± 12.4 pA. vs .…”

“…Consistent with that model, activating mouse HCN1 channels facilitates I h by depolarizing its voltage dependence as revealed in intact frog oocytes (by recordings in 2 electrode voltage clamp) [11]. This depolarizing shift might be pronounced under certain conditions (up to +60 mV) preventing arrhythmic firing in model cells of sino-atrial node [11]. We previously found that in intact mammalian cells (HEK293) (by recordings in cell-attached mode) strong hyperpolarizations led to a reduction of maximum rat (r)HCN1 mediated I h [12].…”

“…Whereas in Xenopus oocytes HCN1 activation induced an up to +60 mV depolarizing shift in voltage dependent activation [11] we found a pre-pulse dependent amplitude reduction of HCN1 mediated I h in intact mammalian HEK293 cells [12]. …”

“…Nevertheless, an opening transition of HCN channels might be allosterically coupled to conformational changes in the cyclic nucleotide binding domain (CNBD) and therewith enhance the cAMP sensitivity of the channels open state as predicted by a six state cyclic allosteric model [10]. Consistent with that model, activating mouse HCN1 channels facilitates I h by depolarizing its voltage dependence as revealed in intact frog oocytes (by recordings in 2 electrode voltage clamp) [11]. This depolarizing shift might be pronounced under certain conditions (up to +60 mV) preventing arrhythmic firing in model cells of sino-atrial node [11].…”

Dynamic memory of a single voltage-gated potassium ion channel: A stochastic nonequilibrium thermodynamic analysis

“…[4][5][6][7][8][9][10][11] A fascinating feature of ion channels is their bistable behavior reflected in the hysteretic conductance under time-varying voltage showing physiologically functional bio-molecular memory. [12][13][14][15][16] Theoretical analyses have shown that comparable time scales of the external perturbation and system's natural relaxation give rise to such kind of behavior in model systems 17 as well as in ion channels. 18,19 Recently, nonequilibrium response spectroscopic technique 20,21 with periodic voltage protocol is used to study the ion channel kinetics in out-of-equilibrium situations.…”