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PMID: 2504882 Published · ppublish English Journal Article Research Support, U.S. Gov't, P.H.S.

"Caged calcium" in Aplysia pacemaker neurons. Characterization of calcium-activated potassium and nonspecific cation currents.

The Journal of general physiology ·Vol. 93 ·No. 6 ·1989-06-00 ·Pages 1017-60

Landò L, Zucker RS

Abstract

We have studied calcium-activated potassium current, IK(Ca), and calcium-activated nonspecific cation current, INS(Ca), in Aplysia bursting pacemaker neurons, using photolysis of a calcium chelator (nitr-5 or nitr-7) to release "caged calcium" intracellularly. A computer model of nitr photolysis, multiple buffer equilibration, and active calcium extrusion was developed to predict volume-average and front-surface calcium concentration transients. Changes in arsenazo III absorbance were used to measure calcium concentration changes caused by nitr photolysis in microcuvettes. Our model predicted the calcium increments caused by successive flashes, and their dependence on calcium loading, nitr concentration, and light intensity. Flashes also triggered the predicted calcium concentration jumps in neurons filled with nitr-arsenazo III mixtures. In physiological experiments, calcium-activated currents were recorded under voltage clamp in response to flashes of different intensity. Both IK(Ca) and INS(Ca) depended linearly without saturation upon calcium concentration jumps of 0.1-20 microM. Peak membrane currents in neurons exposed to repeated flashes first increased and then declined much like the arsenazo III absorbance changes in vitro, which also indicates a first-order calcium activation. Each flash-evoked current rose rapidly to a peak and decayed to half in 3-12 s. Our model mimicked this behavior when it included diffusion of calcium and nitr perpendicular to the surface of the neuron facing the flashlamp. Na/Ca exchange extruding about 1 pmol of calcium per square centimeter per second per micromolar free calcium appeared to speed the decline of calcium-activated membrane currents. Over a range of different membrane potentials, IK(Ca) and INS(Ca) decayed at similar rates, indicating similar calcium stoichiometries independent of voltage. IK(Ca), but not INS(Ca), relaxes exponentially to a different level when the voltage is suddenly changed. We have estimated voltage-dependent rate constants for a one-step first-order reaction scheme of the activation of IK(Ca) by calcium. After a depolarizing pulse, INS(Ca) decays at a rate that is well predicted by a model of diffusion of calcium away from the inner membrane surface after it has entered the cell, with active extrusion by surface pumps and uptake into organelles. IK(Ca) decays somewhat faster than INS(Ca) after a depolarization, because of its voltage-dependent relaxation combined with the decay of submembrane calcium. The interplay of these two currents accounts for the calcium-dependent outward-inward tail current sequence after a depolarization, and the corresponding afterpotentials after a burst

MeSH Terms
Animals Aplysia/physiology Calcium/metabolism,physiology Chelating Agents Egtazic Acid/analogs & derivatives Electrophysiology Models, Neurological Neurons/physiology Photolysis Potassium Channels/physiology
Chemicals
Chelating Agents Potassium Channels nitr 5 nitr 7 Egtazic Acid Calcium
Authors & Affiliations
2 authors, click to expand affiliations / ORCID
Landò L
Department of Physiology-Anatomy, University of California, Berkeley 94720.
Zucker R S
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Article Info
Journal
The Journal of general physiology
Abbr.
J Gen Physiol
ISSN
0022-1295
DOI
10.1085/jgp.93.6.1017
Published
1989-06-00
Pages
1017-60
Language
English
Region
United States
NLM ID
2985110R
PMCID
PMC2216242
Subset
IM
Grants
NINDS NIH HHS · NS-15114 · United States
External Links
PubMed source DOI: 10.1085/jgp.93.6.1017 PMC Full Text View journal details
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