Single-channel recordings of a rapid delayed rectifier current in adult mouse ventricular myocytes: Basic properties and effects of divalent cations

The rapidly delayed rectifier current (I_Kr) has been described in ventricular myocytes isolated from many species, as well as from neonatal mice. However, whether I_Kr is present in the adult mouse heart remains controversial. We used cell-attached patch-clamp recording in symmetrical K⁺ solutions to assess the presence and behaviour of single I_Kr channels in adult mouse cardiomyocytes (mI_Kr). Of 314 patches, 158 (50.1%) demonstrated mI_Kr currents as compared with 131 (42.3%)for the I_K1 channel. Single mI_Kr channel activity was rarely observed at potentials positive to - 10 mV. The slope conductance at negative potentials was 12 pS. Upon repolarization, ensemble-averaged mI_Kr showed slow deactivation with a biexponential time course. A selective I_Kr blocker, E-4031 (1 μm), completely blocked mI_Kr channel activity. Extracellular Ca²⁺ and Mg²⁺ at physiological concentrations shifted the activation by ∼ 30 mV, accelerated deactivation kinetics, prolonged long-closed time, and reduced open probability without affecting single-channel conductance, suggesting a direct channel-blocking effect in addition to well-recognized voltage shifts. HERG subunits expressed in Chinese hamster ovary cells produced channels with properties similar to those of mI_Kr, except for the more-negative activation of the HERG channels. Despite the abundant expression of mI_Kr, single-channel events were rarely observed during action-potential clamp and 5 μm E-4031 had no detectable effect on the action potential parameters, confirming that mI_Kr plays at best a minor role in repolarization of adult mouse cardiomyocytes, probably because the modulatory effects of divalent cations prevent significant mI_Kr opening under physiological conditions.