Adenosine A2B‐receptor‐mediated cyclic AMP accumulation in primary rat astrocytes

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Abstract
1 The effects of adenosine receptor agonists and antagonists on the accumulation of cyclic AMP have been investigated in primary cultures of rat astrocytes. 2 Adenosine A2-receptor stimulation caused a concentration-dependent increase in the accumulation of [3H]-cyclic AMP in cells prelabelled with [3H]-adenine. The rank order of agonist potencies was 5′-N-ethylcarboxamidoadenosine (NECA; EC50 = 1 μm) > adenosine (EC50 = 5 μm) > 2-chloroadenosine (EC50 = 20 μm) >> CGS 21680 (EC50 > 10 μm). The presence of 0.5 μm dipyridamole, an adenosine uptake blocker, had no effect on the potency of adenosine. 3 The response to 10 μm NECA was antagonized in a concentration-dependent manner by the non-selective adenosine receptor antagonists, xanthine amine congener (apparent KD = 12 nM), PD 115,199 (apparent KD = 134 nM) and 8-phenyltheophylline (apparent KD = 126 nM). However, the A1-receptor-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine, had no significant effect on the responses to NECA or 2-chloroadenosine at concentrations up to 1 μm. 4 Stimulation of A1-receptors with the selective agonist, N6-cyclopentyladenosine, did not alter the basal accumulation of [3H]-cyclic AMP but inhibited a forskolin-mediated elevation of [3H]-cyclic AMP accumulation by a maximal value of 42%. This inhibition was fully reversed in the presence of 0.1 μm, 8-cyclopentyl-1,3-dipropylxanthine. 5 The time course for NECA-mediated [3H]-cyclic AMP accumulation was investigated. The results suggest that there is a substantial efflux of cyclic AMP from the cells in addition to the rapid and sustained elevation of intracellular cyclic AMP (5 fold over basal) which was also observed. 6 These data indicate that rat astrocytes in primary culture express an A2B-adenosine receptor coupled positively to adenylyl cyclase. Furthermore, the presence of A1-receptors negatively coupled to adenylyl cyclase appears to have no significant effect on the A2B-receptor-mediated cyclic AMP responses to NECA and 2-chloroadenosine.