Effects of Ca2+ on the transient outward current of single isolated Helix central neurones.
1. Single Helix neurones were studied under voltage-clamp conditions with internal perfusion in order to examine the contribution of internal and external Ca2+ and the effects of 4-aminopyridine (4-AP) on the transient outward current (IA). 2. In Na+ -free snail Ringer, replacement of external Ca2+ [( Ca2+]o) with equimolar Co2+ reduced the maximum amplitude of IA and induced a shift of the steady-state part of the IA inactivation curve (I-V curve), in a positive direction along the voltage axis when the neurone was internally perfused with K-aspartate. 3. In Ca2+ -free solutions, precipitation or chelation of internal Ca2+ [( Ca2+]i) by internal perfusion with KF or EGTA shifted the curve in a more negative direction without affecting the maximum amplitude of IA. Thus, the kinetics of IA are dependent not only upon [Ca2+]o, as previously suggested, but also upon [Ca2+]i. 4. In the presence of 4-AP the I-V curve for IA shifted in a hyperpolarizing direction and the maximal amplitude was reduced when the neurone was internally perfused with K-aspartate. However, 4-AP had little or no effect on the I-V relationship of IA when the neurone was internally perfused with the Ca2+ precipitating or chelating agent, KF or EGTA. These results suggest that the actions of 4-AP on IA are at least partly dependent upon [Ca2+]i.
Tsuda
Y
Y
Department of Physiology, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
Oyama
Y
Y
Carpenter
D O
DO
Akaike
N
N
D015761:4-Aminopyridine; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002800:Cholinesterase Inhibitors / Q000494:pharmacology; D006372:Helix, Snails / Q000502:physiology*; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism
D000631:Aminopyridines; D002800:Cholinesterase Inhibitors; D015761:4-Aminopyridine; D002118:Calcium