Intra and extracellular surface charges near Ca2+ channels in neurons and neuroblastoma cells.
The properties of low (LVA) and high (HVA) voltage-activated calcium currents were investigated in rat sensory neurons and a murine neuroblastoma cell line exposed to various concentrations of intra- or extracellular monovalent ([c+]i/o) and trivalent ([c3+]i/o) cations. In neurons, when [c+]i was changed from 150 to 20 mM, positive shifts of 18-28 mV were observed in activation curves of both LVA and HVA currents, as well as in LVA inactivation curves. Extracellularly, in divalent-free solutions, [c+]o of 20-50 mM produced medium (12-22 mV) negative shifts of the LVA channel properties. These data were used to estimate, by a "screening" model, a negative surface charge density around neuron's calcium channels of 1/1,000 and 1/1,325 eA-2 at the outside or inside face, respectively. In the presence of physiological concentrations of divalent cations, [c+]o of 20-60 mM caused smaller (4-11 mV) negative shifts of the activation and inactivation curves, which can be explained by assuming a partial neutralization of negative charges by divalent cations. By applying the above procedure to LVA channels of neuroblastoma cells, the ratio of extra- to intracellular surface charge density turned out to be more than tenfold higher than in neurons. Effects produced by [c3+]i/o were not in agreement with expectations based on screening or binding models.
Becchetti
A
A
Department of General Physiology and Biochemistry, University of Milan, Italy.
Arcangeli
A
A
Del Bene
M R
MR
Olivotto
M
M
Wanke
E
E
D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002413:Cations, Divalent; D002478:Cells, Cultured; D002614:Chelating Agents / Q000494:pharmacology; D004594:Electrophysiology / Q000379:methods; D005727:Ganglia, Spinal / Q000502:physiology*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008433:Mathematics; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D009447:Neuroblastoma / Q000503:physiopathology*; D009474:Neurons / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D014407:Tumor Cells, Cultured
D015220:Calcium Channels; D002413:Cations, Divalent; D002614:Chelating Agents; D008274:Magnesium; D002118:Calcium