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 toxicity in patients with overdose. We characterized the tonic and use-dependent amitriptyline block of human cardiac (hill) Na⁺ channels expressed in human embryonic kidney cells under voltage-clamp conditions. Our results show that, near the therapeutic plasma concentration of 1 μM, amitriptyline is an effective use-dependent blocker of hill Na⁺ channels during repetitive pulses (~55% block at 5 Hz). The tonic block for resting and for inactivated hill channels by amitriptyline (0.1-100 μM) yielded IC₅₀ values (50% inhibitory concentration) of 24.8 ± 2.0 (n = 9) and 0.58 ± 0.03 μM (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 μM. The time constants of recovery from the inactivated-state amitriptyline block in hill wild-type and hH1-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 significantly increases the IC₅₀ values for resting and inactivated states of amitriptyline, but the increase is much more pronounced with the hH1-F1760K mutation. Because these two residues were proposed to form a part of the local anesthetic binding site, we conclude that amitriptyline and local anesthetics interact with a common binding site. Furthermore, at therapeutic concentrations, the ability of amitriptyline to act as a potent use-dependent blocker of Na⁺ channels may, in part, explain its analgesic actions.