Transmembrane action potentials were recorded from fibers of the atrioventricular region of the rabbit heart.
Displacement of the recording microelectrode from the A.V. node towards the distal segment of the bundle of His revealed that the rate of rise and amplitude of the action potentials increased gradually along the bundle. According to these characteristics and to the site from which the action potentials were recorded, they were classified in three groups: upper His (UH), middle His (MH) and lower His (LH).
The effects of some specific blocking agents of the ionic fluxes were studied on the electrical activity of the fibers of the bundle of His. Tetrodotoxin (5 μg/ml) and high KCl (18.9 and 27.0 mM/l) were used to abolish the fast excitatory current, and MnCl2 (5 mM/l) to block the slow inward current.
The present results indicate that both the fast and the slow inward currents participate in the generation of the action potentials of the bundle of His. The proportion of each current varies, depending on the segment of the bundle of His in which the action potentials were originated. In the cells of the upper bundle the upstroke of the action potentials is determined mainly by the activation of the slow inward current with a negligible contribution from the fast sodium current. Along the bundle of His the participation of the fast inward current increases progressively.
Heart Electrical Activity Bundle of His Transmembrane Currents Sodium and Calcium Ions