Calcium channel currents and their inhibition by (‐)‐baclofen in rat sensory neurones: modulation by guanine nucleotides.

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Abstract
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 acidB 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 .mu.M-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 depolarizing 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 .mu.M-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 .mu.M in the presence of GTP-.gamma.-S, 15 .mu.M for control and 50 .mu.M in the presence of GDP-.gamma.-S. Comparable results were obtained using a single concentration of the adenosine agonist 2-chloroadenosine (2-CA, 0.05 .mu.M) 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 .mu.M-forskolin or 50 .mu.M-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.