Calcium current inactivation in identified neurones of Helix aspersa.
1. A two-electrode voltage clamp method was used to study Ca inward currents in identified Helix aspersa neurones bathed in 25 mM-Ca, Na-free saline with TEA and 4-AP. 2. Inward currents were blocked by CdCl2. In Cd delayed outward currents appeared at +30 mV. When two identical depolarizations were separated by a gap inward current turned off to the same level during the two pulses up to +20 mV; above this potential the records cross over. 3. The turn-off of inward current at potentials up to +20 mV was not affected by 0.2 mM-quinine, which reduced outward currents at more depolarized potentials. Inward currents declined exponentially over the first 100 msec with a time constant around 60 msec at 0 mV. Double-pulse experiments gave the same time course of turn-off. 4. When Ca inward current was reduced by lowering [Ca]o or by partial block by Cd the rate and extent of turn-off was reduced. 5. The inactivation curve (obtained using a double pulse with gap method) was U-shaped. The curve was not significantly changed by addition of quinine (0.2 mM) or by changing test pulse size. 6. Recovery of inward currents after a depolarizing prepulse was a double-exponential process, with time constants of 120 msec and 9.4 sec at 10--11 degrees C. 7. Our results are discussed in terms of possible Ca-dependent Ca inactivation and in terms of the possibility of development of an outward Ca-dependent K current.
D015761:4-Aminopyridine; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D006372:Helix, Snails / Q000502:physiology*; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000502:physiology*; D011188:Potassium / Q000502:physiology; D011803:Quinine / Q000494:pharmacology; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D000631:Aminopyridines; D013757:Tetraethylammonium Compounds; D011803:Quinine; D015761:4-Aminopyridine; D011188:Potassium; D002118:Calcium