The effect of catecholamine depletion (induced by prior treatment with reserpine) was studied in Wistar rat ventricular myocytes using whole cell voltage-clamp methods. Two calcium-independent outward currents, the transient outward potassium current (I_to) and the sustained outward potassium current (I_sus), were measured. Reserpine treatment decreased tissue norepinephrine content by 97%. Action potential duration in the isolated perfused heart was significantly increased in reserpine-treated hearts. In isolated ventricular myocytes, I_to density was decreased by 49% in reserpine-treated rats. This treatment had no effect on I_sus. The I_to steady-state inactivation-voltage relationship and recovery from inactivation remained unchanged, whereas the conductance-voltage activation curve for reserpine-treated rats was significantly shifted (6.7 mV) toward negative potentials. The incubation of myocytes with 10 µM norepinephrine for 7-10 h restored I_to, an effect that was abolished by the presence of actinomycin D. Norepinephrine (0.5 µM) had no effect on I_to. However, in the presence of both 0.5 µM norepinephrine and neuropeptide Y (0.1 µM), I_to density was restored to its control value. These results suggest that the sympathetic nervous system is involved in I_to regulation. Sympathetic norepinephrine depletion decreased the number of functional channels via an effect on the -adrenergic cascade and norepinephrine is able to restore expression of I_to channels.