Characteristics of a transient inward current that causes delayed afterdepolarizations in atrial cells of the canine coronary sinus.

Delayed afterdepolarizations and triggered activity occur in atrial cells of the canine coronary sinus in response to catecholamines. We studied the properties of the membrane current that causes the afterdepolarizations with the two-microelectrode voltage clamp technique in small preparations (about 0.5 x 1 mm). At a holding potential of -50 mV a transient inward current (TI) occurred after repolarization from a depolarizing step to between -40 and -20 mV in the absence of catecholamines. When the depolarizing pulse was made more positive or its duration increased the amplitude of the TI current increased and it reached peak amplitude faster. The current-voltage relationship of the TI current was studied by changing the voltage to which the membrane was repolarized after a depolarizing clamp pulse of fixed amplitude and duration. At repolarization levels positive to -30 mV there were current fluctuations without a distinct TI current. As the repolarization voltage was made more negative, a TI current occurred and its time to peak increased monotonically. The TI current amplitude increased and reached a maximum amplitude at around -60 to -70 mV, and then declined at more negative repolarization voltages. Norepinephrine increased the TI current while simultaneously augmenting the slow inward current. Elevating [Ca]0 increased the TI current amplitude. Caffeine (2 mM) increased the TI current amplitude, while caffeine (4 mM) increased and then decreased the current amplitude. The dependence of the TI current on the voltage and duration of the activating depolarizing step in these atrial cells are qualitatively similar to those of the TI current associated with digitalis toxicity in Purkinje fibers and ventricular muscle, although there are some quantitative differences. There is no distinct TI reversal in these atrial cells, similar to TI in ventricular muscle but dissimilar to TI in Purkinje fibers.