Ca-dependent Outward Currents in Bullfrog Myocardium

抄録

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 (I_k1) and inward background current (I_b), and the reverse was noted with increases in [Ca]_o. When gk₁ was blocked with 0.3mM Ba, there were no such effects. In the presence of Ba and TTX (1.5×10^-6M), 3mM Co abolished the slow inward current (I_si) and depressed the delayed outward current (I_out). The depression was confined to the voltage range of activation of I_si. Similar results were obtained by replacement of [Ca]_o with Co in concentration of equal surface charge effects. Contrarily, isoproterenol (10^-7M) increased the I_si and I_out at the same voltage range. The activation of I_out became More rapid in the presence of isoproterenol but slowed with Co. In the presence of Ba, TTX, and Co, Ca-free EGTA (-0.3mM) Ringer further depressed the I_out, eliminating any remaining tension response. The depression, however, was more extensive, the stronger the depolarization. The activation of I_out was Markedly retarded and amplitude of the fully activated I_out diminished, yet the deactivation became More rapid. Thus, the Ca-sensitive outward current was classified into 1) Ca-linked I_k1 2) I_si-dependent I_out, and 3) I_si-independent but [Ca]_o-related I_out.