Transient outward K+ current, ito, in the sinoatrial node

Abstract

The transient outward K+ current, i to, is a major current in the heart. It is responsible for the early phase of repolarization (phase 1) in some regions of the heart and it can control action potential duration, e.g. Refs. [1,2]. We and others have argued that it contributes to the regional differences in electrical activity in different regions of the heart, such as across the ventricular wall [3] and in the sinoatrial (SA) node [4]. It also contributes to electrical remodelling, e.g. Ref. [5]. We have measured i to in rabbit SA node cells [6,7]. In their letter, A.O. Verkerk and A.C.G. van Ginneken point out that the recordings of i to during depolarizing pulses from rabbit SA node at a holding potential of −80 mV from our laboratory (published by Lei et al. [7]) may be contaminated by tail currents of the hyperpolarization-activated current, i f. These tail currents are the result of the deactivation of i f during depolarizing pulses. They also point out that the use of 4-AP by Lei et al. [7] to isolate i to is hampered by the non-specificity of 4-AP, including an effect of 4-AP on i f. The presence of i to in rabbit SA node cells is certain. First Nathan [8] and then later Denyer and Brown [9] and Ito and Ono [10] reported the presence of i to in rabbit SA node cells. …