Adenosine inhibits L- and N-type calcium channels in pituitary melanotrophs. Evidence for the involvement of a G protein in calcium channel gating.
It has been previously demonstrated that activation of A1 adenosine receptors in frog melanotrophs causes inhibition of spontaneous action potential discharges and alpha-melanocyte-stimulating hormone secretion. In the present study, we have investigated the effect of adenosine on high-voltage-activated (HVA) calcium currents in cultured melanotrophs, using the whole-cell variant of the patch-clamp technique with barium as a charge carrier. Adenosine and the specific A1 adenosine receptor agonist R-PIA (50 microM each) produced a decrease of the amplitude of the barium current, while the selective A2 adenosine receptor agonist CGS 21680 did not affect the current. The inhibitory effect of R-PIA was observed throughout the activation range of the current, with stronger responses at more positive potentials. R-PIA inhibited both the L- and N-type components of the current, the effect on the N-component being two-fold higher than on the L-component. The inhibitory effect of R-PIA was rendered irreversible by addition of GTP gamma S (100 microM) to the intracellular solution. Pre-treatment of the cells with pertussis toxin (1 microgram/ml; 12 h) totally abolished the effect of R-PIA on the HVA calcium channels. Conversely, addition of a high concentration of cAMP (100 microM) together with the phosphodiesterase inhibitor IBMX (100 microM) to the intracellular solution did not modify the effect of R-PIA on the current. It is concluded that, in frog melanotrophs, adenosine induces inhibition of L- and N-calcium currents and that this effect is mediated by a pertussis toxin-sensitive G protein. Our data also indicate that the inhibitory effect of adenosine on the calcium currents is not mediated by inhibition of adenylyl cyclase.
Mei
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Laboratory of Cellular and Molecular Neuroendocrinology, INSERM U 413, University of Rouen, Mont-Saint-Aignan, France.
Le Foll
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Vaudry
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Cazin
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D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000241:Adenosine / Q000494:pharmacology*; D037361:Adenylate Cyclase Toxin; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004594:Electrophysiology; D019204:GTP-Binding Proteins / Q000502:physiology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D009074:Melanocyte-Stimulating Hormones / Q000096:biosynthesis*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D037342:Pertussis Toxin; D010660:Phenylisopropyladenosine / Q000494:pharmacology; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D011895:Rana ridibunda; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D037361:Adenylate Cyclase Toxin; D002121:Calcium Channel Blockers; D010726:Phosphodiesterase Inhibitors; D010566:Virulence Factors, Bordetella; D010660:Phenylisopropyladenosine; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D009074:Melanocyte-Stimulating Hormones; D037342:Pertussis Toxin; D017868:Cyclic AMP-Dependent Protein Kinases; D019204:GTP-Binding Proteins; D000241:Adenosine; D015056:1-Methyl-3-isobutylxanthine