Characterization of BK Channels Cloned from Mouse Sinoatrial Node Cells

Abstract

BK currents are well-established modulators of neural and smooth muscle excitability and recently we reported that BK currents alter the excitability of mouse sinoatrial node (SAN) cells, the predominant cardiac pacemaking cells (Lai et al. 2014, AJP- Heart & Circ Physiol). Because of the relatively small amplitude of native BK currents in SANCs, and the inability to record native BK currents under different Ca2+ conditions, we characterized currents produced by two BK channel variants cloned from SAN cells (BKVYR and BKQEERL). BK currents were recorded from inside-out patches from HEK293T cells in symmetrical K+. Analysis of the conductance-voltage relationships revealed similar V1/2s between BKVYR and BKQEERL currents (approximately 142 mV, 55 mV, and −27 mV at 1, 10, and 100 μM Ca2+, respectively). To gain insight on the mechanism underlying the role of BK currents in the SAN action potential, we also characterized BKVYR and BKQEERL currents at nominal, 1 μM, and 50 μM Ca2+ in physiological K+. Under physiological ionic conditions, BKVYR and BKQEERL currents began to activate at voltages near the maximum diastolic potential (MDP), approximately −60 mV, when the intracellular Ca2+ was 50 μM. By comparison, we previously reported that the native SAN cell BK currents activate at potentials positive to the MDP. These data suggest that native SAN BK currents require a significant elevation in intracellular Ca2+ to achieve meaningful BK current activation at physiological membrane potentials, and suggest close functional coupling of BK channels with Ca2+ sources in SAN cells.