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Inhibition of the T-Type Ca²⁺ Current by the Dopamine D1 Receptor in Rat Adrenal Glomerulosa Cells: Requirement of the Combined Action of the Gß Protein Subunit and Cyclic Adenosine 3',5'-Monophosphate

Department of Physiology and Biophysics (P.D., L.B., A.C., M.D.P.) and Department of Medicine Endocrine Service (P.D., L.L., N.G-P.), Faculty of Medicine, University of Sherbrooke Sherbrooke, Quebec, Canada J1H 5N4

Modulation of ionic Ca²⁺ currents by dopamine (DA) could play a pivotal role in the control of steroid secretion by the rat adrenal glomerulosa cells. In the present study, we report that DA decreases the T-type Ca²⁺ current amplitude in these cells. The use of pharmacological agonists and antagonists reveals that this effect is mediated by activation of the D1-like receptors. Modulation by cAMP is complex inasmuch as preincubation of the cells with 8-Br-cAMP or the specific adenylyl cyclase inhibitor, 2',3'-dideoxyadenosine, have no effect per se, but prevent the DA-induced inhibition. The inhibitory effect of DA was abolished by addition of GDPßS to the pipette medium but not by pertussis toxin. If a cell is dialyzed with medium containing G_s-GDP, the inhibitory effect is reduced and cannot be recovered by the addition of GTPS, indicating that the _s is not involved, but rather the ß-subunit. Indeed, DA-induced inhibition was mimicked by Gß in the pipette and 8-Br-cAMP in the bath. Similarly, Gß release from the activation of the AT₁ receptor of angiotensin II did affect the current amplitude only in the presence of 8-Br-cAMP in the bath. The mitogen-activated protein kinase cascade, which can be activated by receptors coupled to G_s, was not involved as shown by the lack of activation of p42^mapk by DA and the absence of effect of the mitogen-activated protein kinase inhibitor, PD 098059, on the DA-induced inhibition. Because the binding of Gß-subunits to various effectors involves the motif QXXER, we therefore tested the effect of the QEHA peptide on the inhibition of the T-type Ca²⁺ current induced by DA. The peptide, added to the medium pipette (200 µM), abolished the effect of DA. We conclude that the presence of the Gß and an increase in cAMP concentration are both required to inhibit the T-type Ca²⁺ current in rat adrenal glomerulosa cells.

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