Block of human heart hH1 sodium channels by amitriptyline

Amitriptyline is a tricyclic antidepressant used to treat major depression and various neuropathic pain syndromes. This drug also causes cardiac toxic ity in patients with overdose. We characterized the tonic and use-dependent amitriptyline block of human cardiac (hH1) Na+ channels expressed in human embryonic kidney cells under voltage-clamp conditions. Our results show th at, near the therapeutic plasma concentration of 1 mu M, amitriptyline is a n effective use-dependent blocker of hH1 Na+ channels during repetitive pul ses (similar to 55% block at 5 Hz). The tonic block for resting and for ina ctivated hH1 channels by amitriptyline (0.1-100 mu M) yielded IC50 values ( 50% inhibitory concentration) of 24.8 +/- 2.0 (n = 9) and 0.58 +/- 0.03 mM (n = 7), respectively. Substitution of phenylalanine with lysine at the hH1 -F1760 position, a putative binding site for local anesthetics, eliminates the use-dependent block by amitriptyline at 1 mM. The time constants of rec overy from the inactivated-state amitriptyline block in hH1 wild-type and h H1-F1760K mutant channels are 8.0 +/- 0.5 (n = 6) and 0.45 +/- 0.07 s (n = 6), respectively. A substitution at either hH1-F1760K or hH1-Y1767K signifi cantly increases the IC50 values for resting and inactivated states of amit riptyline, but the increase is much more pronounced with the hH1-F1760K mut ation. Because these two residues were proposed to form a part of the local anesthetic binding site, we conclude that amitriptyline and local anesthet ics interact with a common binding site. Furthermore, at therapeutic concen trations, the ability of amitriptyline to act as a potent use-dependent blo cker of Na+ channels may, in part, explain its analgesic actions.