Two types of voltage-gated K⁺ currents in dissociated heart ventricular muscle cells of the snail Lymnaea stagnalis

We have used a combination of current-clamp and voltage-clamp techniques to characterize the electrophysiological properties of enzymatically dissociated Lymnaea heart ventricle cells. Dissociated ventricular muscle cells had average resting membrane potentials of -55 ± 5 mV. When hyperpolarized to potentials between -70 and -63 mV, ventricle cells were capable of firing repetitive action potentials (8.5 ± 1.2 spikes/min) that failed to overshoot 0 mV. The action potentials were either simple spikes or more complex spike/plateau events. The latter were always accompanied by strong contractions of the muscle cell. The waveform of the action potentials were shown to be dependent on the presence of extracellular Ca²⁺ and K⁺ ions. With the use of the single-electrode voltage-clamp technique, two types of voltage-gated K⁺ currents were identified that could be separated by differences in their voltage sensitivity and time-dependent kinetics. The first current activated between -50 and -40 mV. It was relatively fast to activate (time-to-peak; 13.7 ± 0.7 ms at +40 mV) and inactivated by 53.3 ± 4.9% during a maintained 200-ms depolarization. It was fully available for activation below -80 mV and was completely inactivated by holding potentials more positive than -40 mV. It was completely blocked by 5 mM 4-aminopyridine (4-AP) and by concentrations of tetraethylammonium chloride (TEA) > 10 mM. These properties characterize this current as a member of the A-type family of voltage-dependent K⁺ currents. The second voltage-gated K⁺ current activated at more depolarized potentials (-30 to -20 mV). It activated slower than the A-type current (time-to-peak; 74.1 ± 3.9 ms at +40 mV) and showed little inactivation (6.2 ± 2.1%) during a maintained 200-ms depolarization. The current was fully available for activation below -80 mV with a proportion of the current still available for activation at potentials as positive as 0 mV. The current was completely blocked by 1-3 mM TEA. These properties characterize this current as a member of the delayed rectifier family of voltagedependent K⁺ currents. The slow activation rates and relatively depolarized activation thresholds of the two K⁺ currents are suggestive that their main role is to contribute to the repolarization phase of the action potential.