Comparative mechanisms of 4-aminopyridine-resistant I(to) in human and rabbit atrial myocytes

Abstract

The cardiac transient outward current (I(to)) has been shown in several species to consist of two components: 1) a 4-aminopyridine (4-AP)-sensitive component (I(to1)) and 2) a 4-AP-resistant component (I(to2)). In rabbits, I(to2) is a Ca2+dependent Cl- current [I(Cl(Ca))]; similar mechanisms have been suggested to underlie I(to2) in human atrium. We used whole cell patch- clamp techniques to define the mechanism of I(to2) (defined as the component resistant to 5 mM 4-AP) in human atrial myocytes, with parallel experiments performed in rabbit atrial cells. In rabbit atrium, I(to2) activated more slowly than I(to1) and had a bell-shaped current-voltage (I-V) relation. Ryanodine suppressed a component of I(to) with properties similar to I(to2) in the rabbit, and a similar component recorded with pipette K+ replaced by Cs+ was suppressed by the substitution of methanesulfonate for Cl- in the superfusate. In human cells, a 4-AP-resistant I(to2) was recorded at a depolarizing pulse frequency of 1 Hz, but not at 0.1 Hz. I(to2) activated rapidly and inactivated earlier than I(to1), whereas its I-V relation was linear like that of I(to1). Ryanodine had no effect on human atrial I(to). When K+-free pipette solutions were used, no I(to) was recorded in 30 human atrial myocytes, and external Cl- replacement with methanesulfonate failed to reveal an I(to). In 13 human myocytes, isoproterenol increased I(Ca) but failed to activate an I(to) compatible with I(Cl(Ca)). Whereas caffeine suppressed human atrial I(to), it also suppressed I(to1) [in the presence of 200 μM Cd2+ to block I(Ca) and 5 mM intracellular ethylene glycol-bis(β- aminoethyl ether)-N,N,N',N'-tetraacetic acid to buffer intracellular Ca2+] in both human and rabbit atrium, indicating an action unrelated to Ca2+- triggered Ca2+ release. In conclusion, we were unable to demonstrate the presence of I(Cl(Ca)) in human atrial myocytes, and the 4-AP-resistant component of I(to) appeared to be due to 4-AP unblocking.