Ca-dependent outward currents in bullfrog myocardium.

Properties of Ca-dependent outward current were studied in bullfrog atrial muscle using double sucrose-gap techniques. Decreasing [Ca]o inhibited the instantaneous outward current (Ik1) and inward background current (Ib), and the reverse was noted with increases in [Ca]o. When gk1 was blocked with 0.3 mM Ba, there were no such effects. In the presence of Ba and TTX (1.5 X 10(-6)M), 3 mM Co abolished the slow inward current (Isi) and depressed the delayed outward current (Iout). The depression was confined to the voltage range of activation of Isi. Similar results were obtained by replacement of [Ca]o with Co in concentration of equal surface charge effects. Contrarily, isoproterenol (10(-7) M) increased the Isi and Iout at the same voltage range. The activation of Iout became more rapid in the presence of isoproterenol but slowed with Co. In the presence of Ba, TTX, and Co, Ca-free EGTA (approximately 0.3 mM) Ringer further depressed the Iout, eliminating any remaining tension response. The depression, however, was more extensive, the stronger the depolarization. The activation of Iout was markedly retarded and amplitude of the fully activated Iout diminished, yet the deactivation became more rapid. Thus, the Ca-sensitive outward current was classified into 1) Ca-linked Ik1, 2) Isi-dependent Iout, and 3) Isi-independent but [Ca]o-related Iout.