Depressed transient outward potassium current density in catecholamine-depleted rat ventricular myocytes

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 muM norepinephrine for 7-10 h restored I-to, an effect that was abolished by the presence of actinomycin D. Norepinephrine (0.5 muM) had no effect on Ito. However, in the presence of both 0.5 muM norepinephrine and neuropeptide Y (0.1 muM), I-to density was restored to its control value. These results suggest that the sympathetic nervous system is involved in Ito regulation. Sympathetic norepinephrine depletion decreased the number of functional channels via an effect on the alpha-adrenergic cascade and norepinephrine is able to restore expression of I-to channels.