Abstract
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 microm), completely blocked mI(Kr) channel activity. Extracellular Ca(2+) and Mg(2+) at physiological concentrations shifted the activation by approximately 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 microm 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.