Identical Unitary Current Amplitude and Ca²⁺ Block of Cardiac Na Channel before and during β-Adrenergic Stimulation

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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 &mgr;M 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.