Generation of Sodium-Permeable Cav1.3 Channel: Insights into the Molecular Basis for the Sustained Inward Current in Cardiac Pacemaker Cells

Abstract

The sustained inward current (Ist) is a novel pacemaker current identified in spontaneously active sinoatrial and atrioventricular node cells of rabbits, guinea-pigs, rats and mice. Because Ist is activated and produces an inward current over the entire range of the slow diastolic depolarization, its contribution to the pacemaker activity has been suggested. However, due to the absence of specific blockers and unidentified molecular determinants, it is still difficult to directly investigate the significance of Ist in cardiac automaticity. Although Ist is a Na+ current, its pharmacological properties are qualitatively identical with those of L-type Ca2+ current (ICa,L). In the present study, we generated a Na+-permeable CaV1.3 channel by a substitution of key glutamate residue in the Ca2+-selective filter of the pore with lysine (E1160K). The CaV1.3-E1160K channel expressed in HEK cells evoked an inward current carried by Na+ even in the presence of extracellular Ca2+ (1.8 mM). The Na+ current was characterized by a slow inactivation kinetics, a low activation threshold (∼−60 mV) and sensitivity to ICa,L blockers such as nifedipine, diltiazem and Cd2+. These properties of the CaV1.3-E1160K current were very similar to those of Ist, suggesting that an ICa,L channel variant with altered ion selectivity may mediate Ist. Besides, application of the CaV1.3-E1160K channel to the biological pacemaker would be an intriguing approach to understand the impact of Ist on cardiac pacemaking.