Distinct Populations of HCN Pacemaker Channels Produce Voltage-dependent and Voltage-independent Currents

Abstract

Hyperpolarization-activated HCN pacemaker channels are critical for the generation of spontaneous activity and the regulation of excitability in the heart and in many types of neurons. These channels produce both a voltage-dependent current (Ih) and a voltage-independent current (Iinst or VIC). In this study, we explored the molecular basis of the voltage-independent current. We found that for the spHCN isoform, VIC averaged ∼4% of the maximum HCN conductance that could be activated by hyperpolarization. Cyclic AMP increased the voltage-independent current in spHCN to ∼8% of maximum. In HCN2, VIC was ∼2% of the maximal current, and was little affected by cAMP. VIC in both spHCN and HCN2 was blocked rapidly both by ZD7288 (an HCN channel blocker that is thought to bind in the conduction pore) and by application of Cd2+ to channels containing an introduced cysteine in the pore (spHCN-464C or HCN2-436C). These results suggest that VIC flows through the main conduction pathway, down the central axis of the protein. We suspected that VIC simply represented a nonzero limiting open probability for HCN channels at positive voltages. Surprisingly, we found instead that the spHCN channels carrying VIC were not in rapid equilibrium with the channels carrying the voltage-dependent current, because they could be blocked independently; a single application of blocker at a depolarized potential essentially eliminated VIC with little change in Ih. Thus, VIC appears to be produced by a distinct population of HCN channels. This voltage-independent current could contribute significantly to the role of HCN channels in neurons and myocytes; VIC flowing through the channels at physiological potentials would tend to promote excitability by accelerating both depolarization and repolarization.

Calcium-dependent Binding of HCN1 Channel Protein to Hair Cell Stereociliary Tip Link Protein Protocadherin 15 CD3

“…I h itself is slow and would not be considered a candidate for the mechanosensory transduction channel on the basis of short response latencies of mechanosensory transduction in vertebrate hair cells (38), although activation of I h by stretch stimulus has recently been demonstrated, with stretch accelerating both activation and deactivation (39). I inst coded for by all of the HCN isoforms is both fast and voltage-independent and is blocked along with I h by ZD7288 (29,40). Another interesting aspect of the HCN instantaneous current is that it is enhanced by prior hyperpolarization pulses (27), with possible applicability to low frequency spontaneous oscillation of hair bundles associated with mechanosensory transduction (41).…”

Dendritic HCN2 Channels Constrain Glutamate-Driven Excitability in Reticular Thalamic Neurons

“…9). This is not altogether surprising when one considers that HCN2 channels are partially open at this potential (Chen et al, 2001a) and may generate a voltage-independent current (Proenza et al, 2002;Macri and Accili, 2004;Proenza and Yellen, 2006). Another possible explanation for this effect is poor dendritic voltage control when recording from the soma, which could result from the nonisopotentiality of RTN neurons (Destexhe et al, 1996).…”

Tanshinone IIA Selectively Enhances Hyperpolarization-Activated Cyclic Nucleotide–Modulated (HCN) Channel Instantaneous Current

“…The two components of HCN-channel currents, the instantaneous current and the slowly developing component, are supposed to flow through two distinct channel populations (22). The instantaneous current is voltageindependent and accompanies expression of HCN2 and HCN1 channels.…”

“…Interestingly, the voltage sensors and the gate molecularly failed to couple in instantaneous current-producing HCN channels (22). Figure 1C showed that in addition to the blockade of the voltage-dependent current, ZD7288 (one of the HCN-channel blockers) also blocked the inward instantaneous current at hyperpolarized potentials.…”

“…Besides acting as a pacemaker, HCN instantaneous current also functions as a regulator of resting potential and membrane resistance (22). It stabilizes the resting membrane potential because a small hyperpolarization activates HCN channels, whose inward current depolarizes the cell.…”