Inhibition of voltage-sensitive calcium channels by the A(2A) adenosine receptor in PC12 cells

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.