In canine ventricular myocytes, the transient outward current comprises 4-aminopyridine-sensitive Ito1 and manganese-sensitive Ito2, with Ito1 showing significantly faster kinetics than in other tissues.
Canine ventricular myocytes possess two distinct components of transient outward current (Ito1 and Ito2) with Ito1 exhibiting significantly faster kinetics than observed in other cardiac tissues.
Repolarization during phase 1 of cardiac action potential is important in that it may influence both impulse conduction in partially depolarized tissue and action potential duration. Thus, it is important to know the properties and regulation of the underlying currents. In about 50% of canine ventricular myocytes, the actin potential displays a phase 1 of fast repolarization and a prominent notch between phase 1 and the plateau. A transient outward current is responsible for both. This current is composed of two components: one (Ito1) blocked by 4-aminopyridine and the other (Ito2) blocked by manganese. In the present study, we characterized each of the components in isolation from the other. Both had an activation threshold between -30 and -20 mV. At the same voltage, Ito1 was larger than Ito2 and had a shorter time to peak. The peak current-voltage relationship for Ito1 was almost linear, but that for Ito2 was bell-shaped. Ito1 decayed during sustained depolarization with a single exponential time course: tau less than 30 msec at all voltages. It recovered from inactivation with a voltage-dependent time course: tau = 70 msec at -90 mV and 720 msec at -40 mV. Ito2 was augmented by elevating Ca2+o or by isoproterenol. It was inhibited by caffeine, ryanodine, or a preceding transient inward current, suggesting that it was activated by intracellular calcium released from sarcoplasmic reticulum. We conclude that Ito1 and Ito2 in canine ventricle are similar to those described for many other cardiac tissues, but the kinetics of Ito1 are significantly faster than in other tissues.
Tseng et al. (1989) studied Canine ventricular myocytes. Electrophysiological characterization of Ito1 and Ito2 was evaluated on Properties and regulation of transient outward current components (Ito1 and Ito2). In canine ventricular myocytes, the transient outward current comprises 4-aminopyridine-sensitive Ito1 and manganese-sensitive Ito2, with Ito1 showing significantly faster kinetics than in other tissues.