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 (10-8l0_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 (Ij.The augmentation of I, was induced with or without an increase in Isi, but different kinetics were involved. In the presence of Isi, j3-agonists produced a more marked augmentation of I, for the intermediate voltages of depolarization where Isi was enhanced, and Co eliminated these effects. In the absence of Isi with Co, j3-agonists elicited an enhancement of Ig; 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 j3-agonists directly increase the background K current, and enhance the delayed outward current (Is) via two different mechanisms; one is related to the augmentation of Isi and the other is not.

Effects of Ba on the potassium-related currents were studied on the bullfrog atrial muscle under voltage clamp with double sucrosegap techniques. Ba, in a dose over 0.1 mM, abolished the anomalous rectification of the membrane by inhibiting the background current which reversed sign nearly at the K equilibrium potential (IKi). Ba, thus reducing the K-depletion current for hyperpolarizations, revealed the presence of funny inward current (If or Ih) in the proper atrial muscle. An increase in [K]o increased If, and the current showed a threshold at about -80 mV and was saturated at above -160 mV in 5 mM [K]o. The delayed outward current (Is) for depolarizations was also depressed by Ba. The depression occurred in a voltage-and time-dependent manner, manifesting an unblocking for stronger depolarizations. An analysis of the current tail, however, disclosed that low concentrations of Ba (up to 0.1 mM) inhibited the accumulation component (Ia) of the current without diminishing the next slow component of Ig (Igs). The remaining Igs showed a reversal potential of -82 mV, suggesting that this current is largely carried by potassium ions. These data clearly show that in the presence of Ba, If and Igs can be differentiated from other membrane currents in the frog atrial muscle.