Pharmacology of Hyperpolarization-Activated Cyclic Nucleotide-Gated (HCN) Channels - Ion Channels and Their Inhibitors - page 37

3 Basic Biophysical Properties of HCN Channel Subtypes

An important feature of HCN channels is their activation by hyperpolarization.
Generally, h-currents activate with hyperpolarizing steps to potentials negative to

50

to 70 mV. Unlike most other voltage-gated currents, I h does not inactivate.

Activation of I h is preceded by a delay, resulting in a typical sigmoidal time course
of onset. Several mechanisms have been described in the literature to explain the
processes of I h activation and deactivation that follow this delay [ 19 ]. The kinetic
features of I h require complex multi-state kinetic modeling based on the existence
of distinct “delaying” and proper “gating” processes [ 20 , 21 ]. Depending on the cell
type, the kinetics of activation of the current are quite variable; these differences
could reflect the diverse intrinsic activation properties of distinct HCN channel
isoforms underlying the current and/or the experimental conditions, or even the
cellular microenvironment of the HCN-channel [ 22 ].
All four HCN channel types (HCN1-4) display the principal biophysical
properties of the native pacemaker current. Nevertheless, the biophysical properties
vary according to experimental parameters and also diverge depending on the
expression system or cell type. In general, the different isoforms differ from each
other with respect to their voltage dependence and their degree of cAMP-dependent
modulation. HCN2 has a more negative activation threshold than HCN1 and HCN4
[ 17 , 20 , 23 ]. The kinetics for voltage-dependent activation vary between the HCN
channel subtypes (see Fig. 1c ).
HCN1 is the fastest channel (time constant of 25-300 ms) depending on the
voltage values employed [ 9 , 24 ], while HCN4 is the slowest channel [ 25 - 27 ],
displaying time constant values between a few hundred milliseconds at 140 mV

up to several seconds at 70 mV. HCN2 and HCN3 activate with kinetics that

range between those for HCN1 and HCN4 [ 9 , 20 , 23 ].
Evidence for the ion permeation properties of pacemaker channels is derived
from experiments on Purkinje fibers and on isolated rabbit SA node myocytes [ 28 ,
29 ]. Ionic substitution experiments identified Na + and K + ions as carriers of the
cardiac pacemaker f-current, with an Na + /K + permeability ratio of 0.27 [ 28 , 30 ].
Accordingly, HCN channels are more permeable to K + than to Na + (with perme-
ability ratios of about 4:1 (see [ 16 ] for review). Despite this preference for K +
conductance, h-channels carry an inward Na + current under physiological
conditions. Moreover, the global conductance of the pacemaker current increases
with external K + concentration [ 28 ]. It has been reported that HCN channels also
display a very low permeability to Ca 2+ [ 31 , 32 ].
An ongoing discussion concerns the value of the single channel conductance of
pacemaker channels [ 10 ]. Native channels recorded in isolated rabbit SAN
myocytes have a very small single channel conductance estimated to be only
1 pS (see Fig. 2a )[ 29 ]. However, single channel currents recorded by Michels
and coworkers [ 33 ] do not appear to be the same as those reported previously by
DiFrancesco [ 29 ]. One difference is in the conductance, which for native pace-
maker channels is nearly 20-fold higher than that previously reported. The