ZD7288 blockade of I(h) channels requires channel opening and is mediated by residues in the pore-lining S6 region that are guarded by an intracellular activation gate.
Hyperpolarization-activated cation currents (I(h)) are key determinants of repetitive electrical activity in heart and nerve cells. The bradycardic agent ZD7288 is a selective blocker of these currents. We studied the mechanism for ZD7288 blockade of cloned I(h) channels in excised inside-out patches. ZD7288 blockade of the mammalian mHCN1 channel appeared to require opening of the channel, but strong hyperpolarization disfavored blockade. The steepness of this voltage-dependent effect (an apparent valence of approximately 4) makes it unlikely to arise solely from a direct effect of voltage on blocker binding. Instead, it probably indicates a differential affinity of the blocker for different channel conformations. Similar properties were seen for ZD7288 blockade of the sea urchin homologue of I(h) channels (SPIH), but some of the blockade was irreversible. To explore the molecular basis for the difference in reversibility, we constructed chimeric channels from mHCN1 and SPIH and localized the structural determinant for the reversibility to three residues in the S6 region likely to line the pore. Using a triple point mutant in S6, we also revealed the trapping of ZD7288 by the closing of the channel. Overall, the observations led us to hypothesize that the residues responsible for ZD7288 block of I(h) channels are located in the pore lining, and are guarded by an intracellular activation gate of the channel.
Shin et al. (Tue,) conducted a other in Hyperpolarization-activated cation currents. ZD7288 was evaluated on Mechanism of ZD7288 blockade and reversibility. ZD7288 blockade of I(h) channels requires channel opening and is mediated by residues in the pore-lining S6 region that are guarded by an intracellular activation gate.