Chronic hypoxia reduces adenosine A2A receptor-mediated inhibition of calcium current in rat PC12 cells via downregulation of protein kinase A

1. Adenosine has been shown to decrease Ca2+ current (I-Ca) and attenuate the hypoxia-induced enhancement of intracellular free Ca2+ ([Ca2+](i)) in oxygen-sensitive rat phaeochromocytoma (PC12) cells. These effects are mediated via the adenosine A(A2) receptor and protein kinase A (PKA). The current study was undertaken to determine the effects of adenosine on Ca2+ current and hypoxia-induced change in [Ca2+](i) during chronic hypoxia. 2. Whole cell patch-clamp studies revealed that the effect of adenosine on I-Ca was significantly reduced when PC12 cells were exposed to hypoxia (10 % O-2) for 24 and 48 h. 3. Ca2+ imaging studies using fura-2 revealed that the anoxia-induced increase in [Ca2+](i) was significantly enhanced when PC12 cells were exposed to 10% O-2 for up to 48 h. In contrast, the inhibitory effects of adenosine on anoxia-induced elevation of [Ca2+](i) was significantly blunted in PC12 cells exposed to hypoxia for 48 h. 4. Northern blot analysis revealed that mRNA for the A(2A) receptor, which is the only adenosine receptor subtype expressed in PC12 cells, was significantly upregulated by hypoxia. Radioligand binding analysis with [H-3]CGS21680, a selective A(2A) receptor ligand, showed that the number of adenosine A(2A) receptor binding sites was similarly increased during exposure to 10% O-2 for 48 h. 5. PKA enzyme activity was significantly inhibited when PC12 cells were exposed to 10% O-2 for 24 and 48 h. However, we found that hypoxia failed to induce change in adenosine- and forskolin-stimulated adenylate cyclase enzyme activity Chronic hypoxia also did not alter the immunoreactivity level of the G protein G(s alpha), an effector of the A(2) signalling pathway. 6. Whole cell patch-clamp analysis showed that the effect of 8-bromo-cAMP, an activator of PKA, on I-Ca was significantly attenuated during 48 h exposure to 10% O-2. 7. We conclude therefore that the reduced effect of adenosine on I-Ca and [Ca2+](i) in PC12 cells exposed to chronic hypoxia is due to hypoxia-induced downregulation of PKA. This mechanism may serve to reduce the negative feedback on I-Ca and [Ca2+](i) by adenosine and therefore maintain enhanced membrane excitability of PC12 cells during long-term hypoxia.