The Relative Contributions of Various Time-Dependent Membrane Currents to Pacemaker Activity in the Sino Atrial Node

Abstract

The successful application of the voltage clamp technique to the mammalian sino-atrial node has already given much information about the membrane currents in nodal pacemaking cells. In particular, an apparently new time-dependent current, i_f (or i_h) has been found on voltage clamping SA node cells negative to -50 mV (Brown et al., 1979; Yanagihara and Irisawa, 1980a). It now seems clear that this current, which has been shown to be a developing inward current (DiFrancesco and Ojeda, 1980) is closely related to the current i_K2 which underlies pacemaking in Purkinje fibres. I_K2, whose kinetics were first described by Noble and Tsien (1968) was for many years thought to be an outward current which decays in the pacemaker range and which is carried by potassium ions: hence its name. Recent experiments, using barium to block the background current, i_K2, in Purkinje fibres have shown that I_K2 is in reality a developing inward current which cannot be carried by potassium ions alone (see DiFrancesco, this volume). It probiably has the same ionic composition as i_f (i_h) in the SA node and is carried by both sodium and potassium ions (DiFrancesco, 1980; Brown et al., 1980). This being the case, both currents should have the same name, but since no universal agreement has yet been reached as to whether this should be i_f or i_h we here use our original nomenclature i_f.