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 IKr 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-Kl 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 mum), 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 mum 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.