Parapodial swim muscle in Aplysia brasiliana. I. Voltage-gated membrane currents in isolated muscle fibers

Abstract

1. We describe voltage-gated membrane currents present in single muscle fibers dissociated from the parapodia (swim appendages) of the marine gastropod mollusk Aplysia brasiliana. These muscles are utilized in swimming behavior and their activity is modulated by serotonin. It is necessary to characterize the innate membrane properties of these fibers before defining the mechanism of action of serotonin in facilitating muscle fiber responses to motoneuron input. 2. Freshly dissociated parapodial muscle fibers appear by morphological criteria to be a uniform population with an average length of 240 microns and width of 15 microns. The average resting potential of all fibers is -56 mV and the fibers contract in response to elevated extracellular K+ concentration or intracellular depolarization. 3. Muscle membrane currents were studied by single-electrode voltage clamp with the use of intracellular microelectrodes. The muscle fibers were found to fall into one of two groups, which we have classified as type I and type II, the former having two voltage-gated outward K+ currents and a small, less frequently seen Ca2+ current. Type II fibers display the same two K+ currents, a prominent Ca2+ current and, in addition, two Ca(2+)-dependent K+ currents, the latter described in a companion paper. 4. Membrane currents were characterized using 1-s voltage ramps and several voltage step protocols, including ones for analyzing K+ tail currents. Both fiber types had similar current-voltage relationships and input resistance of > or = 60 - 300 M omega. The current-voltage curves were quite flat at potentials more negative than resting potential, with no evidence of a voltage-gated, inwardly rectified (anomalous) potassium current. Outward K+ currents and a Ca2+ current were seen to appear at a threshold of near -40 mV. 5. Because type I fibers had no apparent Ca(2+)-activated K+ currents, the two voltage-gated outward K+ currents were most conveniently studied in these fibers. Compared with type II fibers, type I fibers display a relatively slowly rising total outward current with depolarization comprised of a delayed rectifier current and a transient A current (IA). These two currents were distinguished by slightly different thresholds for activation, by inactivation properties of IA, and by their partially selective sensitivity to tetraethylammonium and 4-aminopyridine. 6. Although contraction of all parapodial muscle fibers is dependent on extracellular Ca2+, an inward Ca2+ current was detected in only about one third of type I fibers, and the current was small. A similar and more prominent Ca2+ current was observed in all type II fibers and was analyzed more fully in these cells. This current had an activation threshold near -40 mV and peaked between -10 and 0 mV. It displayed little inactivation with depolarization steps of 80-200 ms, was blocked in the absence of Ca2+ or in the presence of Co2+, and was present, although not enhanced, when Ba2+ was substituted for Ca2+. This current was completely blocked by the dihydropyridine nifedipine (10 microM), and is therefore similar to an L-type Ca2+ current. 7. The voltage-gated membrane currents described in parapodial muscle fibers provide a framework for analyzing possible mechanisms by which serotonin facilitates neuromuscular output. This facilitatory mechanism will provide a better understanding of the role of serotonin in controlling locomotion.