Transient Inward Current is Conducted through Two Types of Channels in Cardiac Purkinje Fibres

Two-microelectrode voltage-clamp technique was applied and a number of experimental manoeuvres were used to determine the charge-carrying systems of the arrhythmogenic transient inward current (TI) in rabbit cardiac Purkinje fibres. Increasing [Ca2+]oto 30 mmol/l (in the presence of [Na+]o) induced TI with a clear-cut reversal potential around −23 mV. This observation suggests that the TI is conducted through an ionic channel. Nickel chloride (2.5 mmol/l) which blocks Na+/Ca2+exchange current greatly decreased peak inward TI (57±3%) but significantly increased peak outward TI (86.4±9.6%), which suggests that Na+Ca2+exchange is not the charge-carrying system for TI. In experiments in which TI was induced when [Na+]owas replaced by either N-methyl-D-glucamine, choline, or sucrose (to eliminate Na+Ca2+exchange), Ni2+still decreased inward TI and increased outward TI. There was no significant difference between the effects of Ni2+in the presence and absence of [Na+]o. The effects of Ni2+in the absence of [Na+]oconfirm that Ni2+-induced attenuation of inward TI is not mediated by the Na+Ca2+exchange, but rather through an inhibition of TI channels. Acute exposure to Mn2+(5 mmol/l) almost abolished inward TI at a time when outward TI just showed a slight decrease. A third divalent cation, Cd2+(0.25–1.0 mmol/l), strongly suppressed both inward and outward TI at the same time. The opposite effects of Ni2+on inwardvoutward TI and the preferential inhibition of inward TI by Mn2+suggest involvement of multiple ionic channels in conducting TI. 4,4-diisothiocyanatostilbene-2,2disulfonic acid (DIDS, 10μmol/l) and 4-acetamido-4-isothiocyanatostilbene-2,2disulfonic acid (SITS, 0.2 mmol/l), which block Cl−conductance in cardiac tissues, dramatically suppressed outward TI (80±9%) but to a lesser extent inward TI (20±4%). Our results suggest that, in cardiac Purkinje fibres, high [Ca2+]oinduced TI is conducted mainly through TI channels which fall into two different populations: cationic and anionic.