Adenosine-A2a Receptor Down-Regulates Cerebral Smooth Muscle L-Type Ca²⁺Channel Activity via Protein Tyrosine Phosphatase, Not cAMP-Dependent Protein Kinase

Abstract

Adenosine acting via A2a receptors (A2aR) is a potent cerebral vasodilator that relaxes vascular smooth muscle cells (VSMCs) by a mechanism attributed to activation of cAMP-dependent protein kinase (cAK). We examined effects of adenosine and its mechanism of action on L-type Ca 2ϩ channels in native VSMCs from rat basilar artery. Reverse transcription-polymerase chain reaction and immunofluorescence imaging confirmed transcription and expression of A2aR, and in situ hybridization confirmed presence of mRNA for L-type Ca v1.2b channels. In patch-clamp experiments, adenosine down-regulated Ca . Western blots of lysate or of immunoisolated Ca 2ϩ channels from arterial segments incubated with CGS-21680 showed 1) increased phosphorylation of vasodilator-stimulated phosphoprotein that was blocked by inhibiting cAK (KT-5720), consistent with activation of cAK by A2aR; and 2) decreased tyrosine phosphorylation of immunoisolated ␣1c subunit of the Ca 2ϩ channel. Our data show that cAK, although activated, was not germane to down-regulation of Ca 2ϩ channel activity by A2aR, and they delineate a novel signaling mechanism involving reduced tyrosine phosphorylation of Ca 2ϩ channels by A2aR probably caused by PTP activation.Adenosine is an important vasodilator that regulates cerebrovascular tone in both physiological and pathological conditions such as hypoxia and ischemia (Winn et al., 1981;Rudolphi et al., 1992). Two vasodilatory mechanisms of adenosine have been broadly defined in various circulatory beds. One involves endothelial receptors, whose occupation results in activation of nitric-oxide synthase (NOS), release of NO from endothelium, and activation of cGMP-dependent protein kinase (cGK) in vascular smooth muscle cells (VSMCs), a mechanism that has been well delineated in coronary vessels (Hein and Kuo, 1999) but that may not be active in the cerebral circulation (West et al., 2003). The second mecha-

Cilostazol, an Inhibitor of Type 3 Phosphodiesterase, Produces Endothelium-Independent Vasodilation in Pressurized Rabbit Cerebral Penetrating Arterioles

“…It is reported that the function of endothelium, NO, activation of ATP-dependent K + and L-type Ca 2+ channels, and cAMP-mediated intracellular signaling contribute to the adenosine-mediated vasodilation in cerebral arteries [22, 23]. However, the interaction between the PDE3 inhibitors and ADO-mediated vasodilation of the cerebral arterioles has been controversial.…”

Role of store-operated Ca2+ entry in adenosine-induced vasodilatation of rat small mesenteric artery

“…Cytoplasmic Ca 2ϩ is a primary determinant of vascular smooth muscle contraction. There is now considerable evidence supporting the ability of adenosine to interfere with Ca 2ϩ mobilization in several kinds of cells, including VSMCs (12,26). However, there is a paucity of information that relates the intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i )-regulating capacity of adenosine to its vasodilatory effect.…”

Bidirectional Control of Blood Flow by Astrocytes: A Role for Tissue Oxygen and Other Metabolic Factors

“…Adenosine acted in the reverse way. Instead of facilitating vasodilation, adenosine prevents the vessel's ability to constrict by activating A2A receptors directly on smooth muscle cells which results in a reduced Ca 2+ permeability of these cells [ 44 ]. Both the elevation in external lactate and adenosine were required simultaneously to switch the vessel's response from vasoconstriction to vasodilation in high O 2 and low O 2 respectively.…”