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Calcium channel currents and their inhibition by (‐)‐baclofen in rat sensory neurones: modulation by guanine nucleotides.
Author(s) -
Dolphin A C,
Scott R H
Publication year - 1987
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1987.sp016518
Subject(s) - chemistry , gtp' , calcium channel , biophysics , calcium , t type calcium channel , guanosine , voltage dependent calcium channel , pertussis toxin , baclofen , g protein , endocrinology , medicine , biochemistry , agonist , biology , receptor , enzyme , organic chemistry
1. The effect of intracellular application of the hydrolysis‐resistant GTP and GDP analogues, guanosine 5'‐O‐3‐thiotriphosphate (GTP‐gamma‐S), and guanosine 5'‐O‐2‐thiodiphosphate (GDP‐beta‐S) has been examined on voltage‐activated calcium‐channel currents and the ability of the gamma‐aminobutyric acid B agonist baclofen to inhibit them, in cultured rat dorsal root ganglion (d.r.g.) neurones. 2. Under control conditions, the calcium‐channel current, recorded using the whole‐cell patch technique with Ba2+ rather than Ca2+ as the permeant divalent cation, consists of an inactivating and a sustained current. In the presence of 500 microM‐GTP‐gamma‐S included in the patch pipette, the calcium‐channel current was activated more slowly and was largely non‐inactivating during the 100 ms depolarization voltage step. The effects of GTP‐gamma‐S were abolished by pre‐treatment of cells with pertussis toxin. 3. The calcium‐channel current recorded in the presence of 500 microM‐GDP‐beta‐S had a more marked transient component than the control calcium‐channel current. The proportion of transient calcium‐channel current in the presence of GDP‐beta‐S was not reduced in Na+‐free medium. 4. No statistically significant effects of GTP‐gamma‐S and GDP‐beta‐S were observed on the calcium‐activated potassium current IK(Ca), the transient outward potassium current activated in Ca2+‐free medium, or on the inwardly rectifying current (Ih) activated by hyperpolarization. 5. GTP‐gamma‐S increased the ability of baclofen to inhibit calcium‐channel currents, whereas this was decreased by GDP‐beta‐S and by pre‐treatment of cells with pertussis toxin. The half‐maximal effective dose (EC50) for baclofen was 2 microM in the presence of GTP‐gamma‐S, 15 microM for control and 50 microM in the presence of GDP‐beta‐S. Comparable results were obtained using a single concentration of the adenosine agonist 2‐chloroadenosine (2‐CA, 0.05 microM) to inhibit calcium‐channel currents; its effect was significantly increased by GTP‐gamma‐S and reduced by GDP‐beta‐S. 6. The ability of baclofen to inhibit calcium‐channel currents was not affected by 1 microM‐forskolin or 50 microM‐intracellular cyclic AMP. 7. It is concluded that calcium channels in d.r.g.s are associated with a nucleotide binding protein, and that this mediates the effect of baclofen and 2‐CA on calcium‐channel currents. The ability of GTP‐gamma‐S to inhibit the transient component of calcium‐channel currents in the absence of agonist may represent a means of differentially regulating calcium‐channel activity.