Inhibition of Voltage‐Sensitive Calcium Channels by the A2A Adenosine Receptor in PC12 cells

Abstract

The role of the A_2A adenosine receptor in regulating voltage‐sensitive calcium channels (VSCCs) was investigated in PC12 cells. Ca²⁺ influx induced by membrane depolarization with 70 mM K⁺ could be inhibited with CGS21680, an A_2A receptor‐specific agonist. Both L‐ and N‐type VSCCs were inhibited by CGS21680 treatment. Effects of adenosine receptor agonists and antagonists indicate that the typical A_2A receptor mediates inhibition of VSCCs. Cholera toxin (CTX) treatment for 24 h completely eliminated the CGS21680 potency. Similar inhibitory effects on VSCCs were obtained by membrane‐permeable activators of protein kinase A (PKA). These effects were blocked by Rp‐adenosine‐3′,5′‐cyclic monophosphothioate, a PKA inhibitor. The data suggest that activation of the A_2A receptor leads to inhibition of VSCCs via a CTX‐sensitive G protein and PKA. ATP pretreatment caused a reduction in subsequent rise in cytosolic free Ca²⁺ concentration induced by 70 mM K⁺, presumably by inactivation of VSCCs. Simultaneous treatment with ATP and CGS21680 produced significantly greater inhibition of VSCCs than treatment with CGS21680 or ATP alone. Furthermore, the CGS21680‐induced inhibition of VSCCs was not affected by the presence of reactive blue 2. CGS21680 still significantly inhibited ATP‐evoked Ca²⁺ influx without VSCC activity after cobalt or 70 mM K⁺ pretreatment. These data suggest that the A_2A receptor‐sensitive VSCCs differ from those activated by ATP treatment. Although A_2A receptors induce inhibition of VSCCs as well as ATP‐induced Ca²⁺ influx, the two inhibitory effects are clearly distinct from each other.