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⁺]₀ to 30 mmol/l (in the presence of [Na⁺]₀) 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⁺]₀], was 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⁺]₀. The effects of Ni²⁺ in the absence of [Na⁺]₀ confirm 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 inward v outward TI and the,preferential inhibition of inward TI by Mn²⁺ suggest involvement of multiple ionic channels in conducting TI. 4,4'-diisothiocyanatostilbene-2,2'disulfonic acid (DIDS, 10 μmol/l) and 4-acetamido-4'-isothiocyanatostilbene-2,2'disulfonic acid (SITS, 0.2 mmol/l), which block Cl^- conductance in cardiac tissues, dramatically suppressed outward TT (80 ± 9%) but to a lesser extent inward TI (20 ± 4%). Our results suggest that, in cardiac Purkinje fibres, high [Ca²⁺]₀ induced TI is conducted mainly through TI channels which fall into two different populations: cationic and anionic.