Beta-actions of catecholamines on the K-related currents of the bullfrog atrial muscle.

Effects of catecholamines on K-related membrane currents in the bullfrog atrial muscle were studied under voltage clamp using the double sucrose-gap technique. Isoproterenol (10(-8)-10(-6) M) and norepinephrine (5 X 10(-6)-10(-5) M) induced an outward shift of the basal current level and enhanced both the background current (Ib) for hyperpolarization and the instantaneous outward current for depolarization. There was an augmentation of the slow inward current (Isi) and the delayed outward current (Ix). The augmentation of Ix was induced with or without an increase in Isi, but different kinetics were involved. In the presence of Isi, beta-agonists produced a more marked augmentation of Ix for the intermediate voltages of depolarization where Isi was enhanced, and Co eliminated these effects. In the absence of Isi with Co, beta-agonists elicited an enhancement of Ix; however, the enhancement was greater for larger and longer depolarizations. All these effects were absent in the presence of propranolol (5 X 10(-7) M). These observations indicate that beta-agonists directly increase the background K current, and enhance the delayed outward current (Ix) via two different mechanisms; one is related to the augmentation of Isi and the other is not.