Parapodial swim muscle in Aplysia brasiliana. II. Ca(2+)-dependent K+ currents in isolated muscle fibers and their blockade by chloride substitutes.

1. We describe here the properties of two Ca(2+)-dependent K+ currents found in type II muscle fibers dissociated from the parapodia (swim appendages) of the marine snail Aplysia brasiliana. 2. Type II parapodial muscle fibers display three voltage-dependent currents that are also seen in type I fibers, a delayed rectifier current [IK(V)], a transient A current (IA), and a prominent L-type Ca2+ current. In addition, type II fibers also have two outward K+ currents, a transient, inactivating one and a slower, noninactivating one [IK(Ca,t) and IK(Ca,s), respectively], that are Ca2+ dependent. The expression of these currents in normal type II fibers generally produces a waveform of total outward current that is faster to peak than the total outward current seen in response to voltage steps in type I fibers and that does not inactivate at the end of an 80-ms voltage step. 3. Both IK(Ca,t) and IK(Ca,s) are absent when external Ca2+ is eliminated or when extracellular Ca2+ concentration ([Ca2+]o) is substituted with 10 mM Co2+ or Ba2+. Their threshold for activation is around -40 mV. IK(Ca,t) peaks rapidly and then inactivates, but IK(Ca,s) rises slowly and does not inactivate for as long as 200 ms. Both currents, like IK(V) and IA, are sensitive to tetraethylammonium and 4-aminopyridine and are not readily separated from either the voltage-gated currents or from one another by these pharmacological agents. 4. Tail current analysis from depolarized voltage steps in varying (K+]o demonstrates that these currents are carried by K+ ions and not by Cl-. 5. An unexpected finding, however, is that these Ca(2+)-dependent K+ currents are blocked by standard Cl- ion substitutes, such as methanesulfonate, isethionate, and propionate. IK(Ca,s) is slightly more sensitive to these Cl- substitutes than is IK(Ca,t). The chloride blocker 4,4'-diisothiocyantastilbene-2,2'disulfonic acid also partially blocked the Ca(2+)-dependent K+ currents. 6. The presence of these Ca(2+)-dependent K+ currents in type II fibers may contribute to a more rapid repolarization following depolarization-induced contractions. In contrast to type I fibers, which have smaller calcium current and no Ca(2+)-activated K+ currents, type II muscle cells may function more like "fast" fibers and relax more rapidly.