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Dopamine D 2 ‐Receptor Isoforms Expressed in AtT20 Cells InhibitQ‐Type High‐Voltage‐Activated Ca 2+ Channels via a Membrane‐Delimited Pathway
Author(s) -
Wolfe Shannyn E.,
Howard Daniel E.,
Schetz John A.,
Cheng Caroline J.,
Webber Robert,
Beatty Diane M.,
Chronwall Bibie M.,
Morris Stephen J.
Publication year - 1999
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1999.0720479.x
Subject(s) - pertussis toxin , biophysics , receptor , depolarization , biology , agonist , microbiology and biotechnology , medicine , endocrinology , chemistry , g protein , biochemistry
: Dopamine D 2 receptors both acutely andchronically inhibit high‐voltage‐activated Ca 2+ channels (HVA‐CCs).Two alternatively spliced isoforms, D 2L (long) and D 2S (short), are expressed at high levels in rat pituitary intermediate lobemelanotropes but are lacking in anterior lobe corticotropes. We stablytransfected D 2L and D 2S into corticotrope‐derived AtT20cells. Both isoforms coupled to inhibition of Q‐type calcium channels throughpertussis toxin‐sensitive G proteins. Thus, we have created a model system inwhich to study the kinetics of D 2 ‐receptor regulation ofCa 2+ channels. Rapid inhibition of HVA‐CCs was characterized usinga novel fluorescence video imaging technique for the measurement ofmillisecond kinetic events. We measured the time elapsed (lag time) betweenthe arrival of depolarizing isotonic 66 m M K + , sensed byfluorescence from included carboxy‐X‐rhodamine (CXR), and the beginning ofincreased intracellular Ca 2+ levels (sensed by changes in indo 1fluorescence ratio). The lag time averaged 350‐550 ms, with no significantdifferences among cell types. Addition of the D 2 ‐agonist quinpirole(250 μ M ) to the K + /CXR solution significantly increasedthe lag times for D 2 ‐expressing cells but did not alter the lagtime for AtT20 controls. The increased lag times for D 2L ‐ andD 2S ‐transfected cells suggest that at least a fraction of theCa 2+ channels was inhibited within the initial 350‐550 ms. As this inhibition time is too fast for a multistep second messenger pathway, we conclude that inhibition occurs via a membrane‐delimited diffusion mechanism.