Time course of Ca2+-dependent K+ and Cl- currents in single smooth muscle cells of guinea-pig trachea

The time course of two types of Ca2+-dependent currents were compared in single smooth muscle cells freshly isolated from guinea-pig trachea. When the pipette solution contained mainly 140 mM KCl, depolarization from -60 mV to 0 mV evoked an initial inward current followed by an outward current which consisted of transient (I-to) and sustained components. In addition, a long-lasting inward tail current (I-tail) was occasionally observed after the repolarization to -60 mV. Although I-to often occurred repetitively during depolarization, the first I-to reached the peak of similar to 50 ms after the start of depolarization and had the largest amplitude in most cells examined. The amplitude of I-tail increased with the increase in depolarization period up to about 500 ms. Pharmacological analyses indicate that I-to and I-tail are Ca2+-dependent K+ and Cl- currents (I-K-Ca and I-Cl-Ca), respectively, and suggest that not only Ca2+-influx through Ca2+ channels but also subsequent Ca2+ release from stores contributes to activate these currents. Spontaneous transient outward and inward currents, I-K-Ca and I-Cl-/Ca, respectively, were simultaneously recorded at -40 mV. In over 80% of the spontaneous current events, outward and inward currents coupled one to one and always occurred in this order. Puff-application of 10 mM caffeine also induced I-K-Ca and I-Cl-Ca, in this order at -40 mV. When caffeine was applied twice with various intervals, the current amplitude in the second application depended upon the period of the interval. The recovery of I-Cl-Ca during the interval was faster than that of I-K-Ca. The results indicate that the activation and decay time courses of I-Cl-Ca are slower but its recovery is faster than those of I-K-Ca.