Identical unitary current amplitude and Ca2+ block of cardiac Na channel before and during β-adrenergic stimulation

Abstract

We examined the possibility of Ca2+ permeation through cardiac Na channels ("slip mode conductance") by an analysis of the voltage-dependent block of Na channels by Ca2+. A Ca2+ block of Na channels was evident in rat and guinea pig ventricular myocytes during cell-attached single channel recordings with a physiological ionic environment (140 mM Na+ and 1 to 10 mM Ca2+ in the pipette solution). Increasing external Ca2+ concentration ([Ca2+]o) in the pipette solution reduced the unitary current amplitude predominantly at negative potentials. With [Ca2+]o>1 mM, unitary current amplitude did not increase at potentials negative to -40 mV in spite of augmented driving forces. The application of 5 μM isoproterenol potentiated the single channel activity elicited by depolarizing pulses from the holding potential of -120 mV, indicating that the channels in the patch under examination were modified by protein kinase A (PKA) stimulation. Increased activity was also confirmed with veratridine-modified Na channels, where channel openings were markedly prolonged. In either case, isoproterenol-induced potentiation neither reduced nor altered the properties of Ca2+ block of cardiac Na channels, as evidenced by the stable unitary current amplitudes at potential levels from -60 to -20 mV. These results indicate that interactions among Na+, Ca2+, and the channel molecule were not modified with respect to permeation properties. They therefore argue against the "slip mode" concept of classical cardiac Na channel if a general concept of ion permeation through "multi-ion pores" is applicable to determine the ionic selectivity of Na channels.