Adenosine inhibits a non-inactivating K+ current in bovine adrenal cortical cells by activation of multiple P1 receptors

Abstract

1. Bovine adrenal zona fasciculata (AZF) cells express a non-inactivating K+ current (I(AC)) that sets the resting potential while it is activated by intracellular ATP. In whole-cell patch clamp recordings from bovine AZF cells, we found that adenosine selectively inhibited I(AC) by a maximum of 78.4 ± 4.6% (n = 8) with an IC50 of 71 nM. The non-selective adenosine receptor agonist NECA effectively inhibited I(AC), by 79.3 ± 2.9% (n = 24) at a concentration of 100 nM. 2. Inhibition of I(AC) was mediated through multiple P1 adenosine receptor subtypes. The A1-selective agonist CCPA (10 nM), the A(2A)-selective agonist CGS 21680 (100 nM) and the A3-selective agonist IB-MECA (10 nM) inhibited I(AC) by 64.8 ± 8.4, 78.4 ± 4.6 and 69.3 ± 6.9%, respectively. 3. Specific adenosine receptor subtype antiagonists including DPCPX (A1), ZM 241385 (A(2A)) and MRS 1191 (A3) effectively blocked inhibition of I(AC) by adenosine receptor-selective agonists. 4. A mixture of the three adenosine receptor antagonists completely suppressed inhibition of I(AC) by adenosine, but failed to alter inhibition by external ATP which acts through a separate P2 nucleotide receptor. 5. Inhibition of I(AC) by adenosine or NECA was eliminated by substituting GDP-β-S for GTP in the pipette, or by replacing ATP with AMP-PNP or UTP. 6. In addition to inhibiting I(AC), adenosine (10 μM) depolarized AZF cells by 46.2 ± 5.8 mV (n = 6). 7. These results show that bovine AZF cells express at least three adenosine receptor subtypes (A1, A(2A), A3), each of which is coupled to the inhibition of I(AC) K+ channels through a G-protein-dependent mechanism requiring ATP hydrolysis. Adenosine-mediated inhibition of I(AC) is associated with membrane depolarization. Adenosine and other purines may co-ordinate the stress-induced secretion of corticosteroids and catecholamines from the adrenal gland.