CHARACTERIZATION OF THE MEMBRANE ION CURRENTS OF A MODEL MOLLUSCAN MUSCLE, THE ACCESSORY RADULA CLOSER MUSCLE OF APLYSIA-CALIFORNICA .3. DEPOLARIZATION-ACTIVATED CA CURRENT

1. The accessory radula closer (ARC) muscle of Aplysia californica and its innervation is a model preparation for the study of the neural an d cellular mechanisms of behavioral plasticity. Much of the plasticity is due to modulation of contractions of the muscle by a variety of ne urotransmitters and peptide cotransmitters. Preliminary to investigati ng the cellular mechanisms of this modulation, we have characterized t he major membrane ion currents present in the unmodulated ARC muscle a nd their likely roles in normal contraction. We have studied single di ssociated but functionally intact ARC muscle fibers under voltage clam p. This is the last of three papers describing this work. In the first paper we characterized two currents prominent at hyperpolarized volta ges, a classical inwardly rectifying K current and a Cl current induce d by elevated intracellular Cl-. In the second paper we examined two l arge outward K currents activated at more depolarized voltages, an ''A '' current and a delayed rectifier. 2. In this paper, we describe an i nward depolarization-activated Ca current that underlies and is normal ly completely masked by the K currents and is revealed when they are b locked. 3. The Ca current begins to activate above -40 or -30 mV. It i s fully available for activation at voltages more negative than -60 mV . It activates in milliseconds, then inactivates relatively slowly wit h maintained depolarization. The current is larger and inactivates slo wer when it is carried by Ba2+ rather than Ca2+. The inactivation is c urrent rather than voltage dependent. The current is blocked by Co2+, Cd2+, and with a characteristic time dependence, by the dihydropyridin e Ca-channel antagonist nifedipine. 4. These properties of the current characterize it as a high-threshold, L-type Ca current. 5. This curre nt most likely provides Ca2+ necessary for contraction of the ARC musc le.