A(2B) receptors mediate antimitogenesis in vascular smooth muscle cells

Effects of extracellular triphosphate nucleotides and nucleosides on airway smooth muscle cell proliferation

Extracellular ATP and uridine triphosphate (UTP) have a range of effects on a wide variety of cells through the activation of P(2) receptors. The aim of this work was to establish if stimulation with ATP and UTP enhances airway smooth muscle (ASM) cell proliferation and to determine the type of receptor mediating this effect. Proliferation of rat ASM cells was assessed through bromodeoxyuridine (BrdU) uptake and by cell counting. At concentrations of 10(-6) and 10(-5) M, ATP and UTP induced significant increases in BrdU incorporation. ATP analogs specific for the P(2X) and P(2Y1) receptor subtypes had no effect. UDP (a P(2Y6) receptor agonist) produced significant decreases in BrdU incorporation and cell counts. Adenosine, the metabolite of ATP, produced an increase in cell proliferation through stimulation of the A(1) receptor. A(2) and A(3) receptor stimulation had no effect. Reverse transcription and polymerase chain reaction analysis showed that mRNA transcripts for the P(2Y2), P(2Y4), P(2Y6), A(1), A(2), and A(3) receptor subtypes were present in cultured ASM cells. These data show that extracellular UTP, ATP, and their metabolites may affect airway remodeling by increasing or by reducing (P(2Y6) receptor) ASM cell proliferation.

Localization of adenylyl cyclase isoforms and G protein-coupled receptors in vascular smooth muscle cells: expression in caveolin-rich and noncaveolin domains

A number of different agonists activate G protein-coupled receptors to stimulate adenylyl cyclase (AC), increase cAMP formation, and promote relaxation in vascular smooth muscle. To more fully understand this stimulation of AC, we assessed the expression, regulation, and compartmentation of AC isoforms in rat aortic smooth muscle cells (RASMC). Reverse transcription-polymerase chain reaction detected expression of AC3, AC5, and AC6 mRNA, whereas immunoblot analysis indicated expression of AC3 and AC5/6 protein primarily in caveolin-rich membrane (cav) fractions relative to noncaveolin (noncav) fractions. Beta(1)-adrenergic receptors (AR), beta(2)AR, and G(s) were detected in both cav and noncav fractions, whereas the prostanoid receptors EP(2)R and EP(4)R were excluded from cav fractions. We used an adenoviral construct to increase AC6 expression. Overexpressed AC6 localized only in noncav fractions. Two-fold overexpression of AC6 caused enhancement of forskolin-, isoproterenol- and prostaglandin E(2)-stimulated cAMP formation but no changes in basal levels of cAMP. At higher levels of AC6 overexpression, basal and adenosine receptor-stimulated cAMP levels were increased. Stimulation of cAMP levels by agents that increase Ca(2+) in native cells was consistent with the expression of AC3, but overexpression of AC6, which is inhibited by Ca(2+), blunted the Ca(2+)-stimulable cAMP response. These data indicate that: 1) RASMC express multiple AC isoforms that localize in both caveolin-rich and noncaveolin domains, 2) expression of AC6 in non-caveolin-rich membranes can increase basal levels of cAMP and response to several stimulatory agonists, and 3) Ca(2+)-mediated regulation of cAMP formation depends upon expression of different AC isoforms in RASMC. Compartmentation of GPCRs and AC is different in cardiomyocytes than in RASMC, indicating that targeting of these components to caveolin-rich membranes can be cell-specific. Moreover, our results imply that the colocalization of GPCRs and the AC isoforms they activate need not occur in caveolin-rich fractions.

Expression of adenosine receptors in the preglomerular microcirculation

The purpose of this study was to systematically investigate the abundance of each of the adenosine receptor subtypes in the preglomerular microcirculation vs. other vascular segments and vs. the renal cortex and medulla. Rat preglomerular microvessels (PGMVs) were isolated by iron oxide loading followed by magnetic separation. For comparison, mesenteric microvessels, segments of the aorta (thoracic, middle abdominal, and lower abdominal), renal cortex, and renal medulla were obtained by dissection. Adenosine receptor protein and mRNA expression were examined by Western blotting, Northern blotting, and RT-PCR. Our results indicate that compared with other vascular segments and renal tissues, A1 and A2B receptor protein and mRNA are abundantly expressed in the preglomerular microcirculation, whereas A2A and A3 receptor protein and mRNA are barely detectable or undetectable in PGMVs. We conclude that, relative to other vascular and renal tissues, A1 and A2B receptors are well expressed in PGMVs, whereas A2A and A3 receptors are notably deficient. Thus A1 and A2B receptors, but not A2A or A3 receptors, may importantly regulate the preglomerular microcirculation.

A(2B) adenosine receptors stimulate growth of porcine and rat arterial endothelial cells

The goal of this study was to determine which adenosine receptor subtype mediates growth stimulation by adenosine in arterial endothelial cells. In porcine coronary artery and rat aortic endothelial cells, 2-chloroadenosine (Cl-Ad), a metabolically stable analog of adenosine, stimulated DNA synthesis ((3)H-thymidine incorporation), cellular proliferation (cell number), collagen synthesis ((3)H-proline incorporation), and cell migration. The growth effects of adenosine and Cl-Ad were mimicked by the adenosine receptor agonist 5'-N-methylcarboxamidoadenosine but not by the adenosine receptor agonists N(6)-cyclopentyladenosine, 4-aminobenzyl-5'-N-methylcarboxamidoadenosine or CGS21680, an agonist profile consistent with an A(2B) receptor-mediated effect. The adenosine receptor antagonists KF17837 and 1,3-dipropyl-8-p-sulfophenylxanthine but not 8-cyclopentyl-1,3-dipropylxanthine blocked the growth-stimulatory effects of Cl-Ad and 5'-N-methylcarboxamidoadenosine, an antagonist profile consistent with an A(2) receptor-mediated action. Treatment of endothelial cells with erythro-9-(2-hydroxy-3-nonyl) adenine plus iodotubericidin (inhibitors of adenosine deaminase and adenosine kinase, respectively) induced endothelial cell growth, and these effects were blocked by 1,3-dipropyl-8-p-sulfophenylxanthine and KF17837 but not 8-cyclopentyl-1,3-dipropylxanthine, suggesting that endothelial cell-derived adenosine induces growth via A(2) receptors. The growth-stimulatory effects of Cl-Ad, 5'-N-methylcarboxamidoadenosine, and erythro-9-(2-hydroxy-3-nonyl) adenine plus iodotubericidin were abolished by antisense but not scrambled or sense oligonucleotides to the A(2B) receptor. Our findings strongly support the hypothesis that adenosine induces endothelial cell growth by activating A(2B) receptors. Thus, A(2B) receptors may play a critical role in regulating vascular remodeling associated with endothelial cell proliferation in angiogenesis, collateral vessel development, and recovery after vascular injury. Pharmacological or molecular biological activation of A(2B) receptors may be useful in modulating vascular remodeling.

Role of the extracellular cAMP-adenosine pathway in renal physiology

Adenosine exerts physiologically significant receptor-mediated effects on renal function. For example, adenosine participates in the regulation of preglomerular and postglomerular vascular resistances, glomerular filtration rate, renin release, epithelial transport, intrarenal inflammation, and growth of mesangial and vascular smooth muscle cells. It is important, therefore, to understand the mechanisms that generate extracellular adenosine within the kidney. In addition to three "classic" pathways of adenosine biosynthesis, contemporary studies are revealing a novel mechanism for renal adenosine production termed the "extracellular cAMP-adenosine pathway." The extracellular cAMP-adenosine pathway is defined as the egress of cAMP from cells during activation of adenylyl cyclase, followed by the extracellular conversion of cAMP to adenosine by the serial actions of ecto-phosphodiesterase and ecto-5'-nucleotidase. This mechanism of extracellular adenosine production may provide hormonal control of adenosine levels in the cell-surface biophase in which adenosine receptors reside. Tight coupling of the site of adenosine production to the site of adenosine receptors would permit a low-capacity mechanism of adenosine biosynthesis to have a large impact on adenosine receptor activation. The purposes of this review are to summarize the physiological roles of adenosine in the kidney; to describe the classic pathways of renal adenosine biosynthesis; to review the evidence for the existence of the extracellular cAMP-adenosine pathway; and to describe possible physiological roles of the extracellular cAMP-adenosine pathway, with particular emphasis on the kidney.

Regulation of G protein-coupled receptor-adenylyl cyclase responsiveness in human airway smooth muscle by exogenous and autocrine adenosine

Adenosine is a mediator of bronchoconstriction in asthmatics and is believed to mediate its effects through adenosine receptor activation in inflammatory cells. In this study, we identify human airway smooth muscle (ASM) as a direct target of adenosine. Acute exposure of human ASM cultures to adenosine receptor (AR) agonists resulted in rapid accumulation of cyclic adenosine monophosphate (cAMP) with a pharmacologic profile consistent with A(2b)AR activation. Little or no evidence of A1AR or A3AR expression was suggested on acute addition of various AR ligands, although a low level of A1ARs was identified in radioligand binding studies. Treatment with adenosine deaminase suggested that human ASM cultures secrete adenosine that feeds back on A(2b)ARs and regulates basal cAMP levels as well as a small degree of A(2b)AR, beta(2)AR, and prostaglandin E(2) receptor desensitization. When subjected to chronic treatment with AR agonists or agents that enhance accumulation of endogenous, extracellular adenosine, a dual effect of A(2b)AR desensitization and adenylyl cyclase (AC) sensitization was observed. This AC sensitization was eliminated by pertussis toxin and partially reversed by the A1AR antagonist 8-cyclopentyl-1,3-dipropylxanthine, suggesting a contributory role for the A1AR. Overexpression of A1ARs and A(2b)ARs in human ASM cultures resulted in differential effects on basal, agonist-, and AC-mediated cAMP production. These data demonstrate that human ASM is a direct target of exogenous and autocrine adenosine, with effects determined by differential contributions of A(2b) and A1 adenosine receptors that are time-dependent. Accordingly, the relative distribution and activation of AR subtypes in ASM in vivo may influence airway function in diseases such as asthma and warrant consideration in therapeutic strategies that target ARs or alter nucleotide/ nucleoside levels in the airway.

A(2b) receptors mediate the antimitogenic effects of adenosine in cardiac fibroblasts

Adenosine inhibits growth of cardiac fibroblasts; however, the adenosine receptor subtype that mediates this antimitogenic effect remains undefined. Therefore, the goals of this study were to determine which adenosine receptor subtype mediates the antimitogenic effects of adenosine and to investigate the signal transduction mechanisms involved. In rat left ventricular cardiac fibroblasts, PDGF-BB (25 ng/mL) stimulated DNA synthesis ((3)H-thymidine incorporation), cellular proliferation (cell number), collagen synthesis ((3)H-proline incorporation), and MAP kinase activity. The adenosine receptor agonists 2-chloroadenosine and 5'-N-methylcarboxamidoadenosine, but not N(6)-cyclopentyladenosine, 4-aminobenzyl-5'-N-methylcarboxamidoadenosine, or CGS21680, inhibited the growth effects of PDGF-BB, an agonist profile consistent with an A(2B) receptor-mediated effect. The adenosine receptor antagonists KF17837 and 1,3-dipropyl-8-p-sulfophenylxanthine, but not 8-cyclopentyl-1,3-dipropylxanthine, blocked the growth-inhibitory effects of 2-chloroadenosine and 5'-N-methylcarboxamidoadenosine, an antagonist profile consistent with an A(2) receptor-mediated effect. Antisense, but not sense or scrambled, oligonucleotides to the A(2B) receptor stimulated basal and PDGF-induced DNA synthesis, cell proliferation, and collagen synthesis. Moreover, the growth-inhibitory effects of 2-chloroadenosine, 5'-N-methylcarboxamidoadenosine, and erythro-9-(2-hydroxy-3-nonyl) adenine plus iodotubericidin (inhibitors of adenosine deaminase and adenosine kinase, respectively) were abolished by antisense, but not scrambled or sense, oligonucleotides to the A(2B) receptor. Our findings strongly support the hypothesis that adenosine causes inhibition of CF growth by activating A(2B) receptors coupled to inhibition of MAP kinase activity. Thus, A(2B) receptors may play a critical role in regulating cardiac remodeling associated with CF proliferation. Pharmacologic or molecular biological activation of A(2B) receptors may prevent cardiac remodeling associated with hypertension, myocardial infarction, and myocardial reperfusion injury after ischemia.

Short-term stimulation of vascular smooth muscle cells with platelet-derived growth factor (PDGF)-BB and angiotensin II induces

Growth factors such as the platelet-derived growth factor (PDGF)-BB and angiotensin II (Ang II) have been shown to induce vascular smooth muscle cell (VSMC) proliferation after long stimulation periods. Little is known though, about the effects of PDGF-BB and Ang II on VSMC proliferation after short stimulation periods. The purpose of our study was to examine whether a short term (3-60 min) stimulation of VSMC with PDGF-BB or Ang II is sufficient to induce cell proliferation. Incubation of VSMC with Ang II (100 nM) or PDGF-BB (50 ng/ml) caused a significant increase in [3H]thymidine incorporation starting after a 3-min stimulation, while the cell counts required 32 and 8 h of stimulation, respectively. Mitogen-activated protein kinase activation reached a maximum at 5-10 min of PDGF-BB or Ang II stimulation. This study demonstrates that the growth-promoting effects of PDGF-BB and Ang II are strongly dependent on the length of the stimulation period and that while prolonged stimulation periods (>8-32 h) result in VSMC proliferation, short ones (3-60 min) result only in [3H]thymidine incorporation without an increase in cell count, a fact of considerable pathophysiological significance, considering that the time kinetics of growth factors in the VSMC microenvironment have not as yet been clarified.

The pharmacology of nucleotide receptors on primary rat brain endothelial cells grown on a biological extracellular matrix: effects on intracellular calcium concentration

1. Brain capillary endothelial cells express a variety of nucleotide receptors, but differences have been reported between culture models. This study reports examination of nucleotide receptors on primary cultured rat brain capillary endothelial cells (RBCEC) grown on a biological extracellular matrix (ECM) to produce a more differentiated phenotype. 2. Fura-2 fluorescence ratio imaging was used to monitor intracellular free calcium concentration [Ca(2+)](i). ATP, UTP, and 2-methylthioATP (2-MeSATP) increased [Ca(2+)](i) to similar levels, while 2-MeSADP, ADP and adenosine gave smaller responses. 3. Removal of extracellular calcium caused no significant change in the [Ca(2+)](i) response to 2-MeSATP, evidence that the response was mediated by a metabotropic (P2Y) receptor. 4. All cells tested responded to ATP, UTP, 2-MeSATP and ADP, while 63% responded to adenosine and 50% to 2-MeSADP. No cells responded to alpha, beta-methyleneATP. Cells grown on rat tail collagen instead of ECM gave smaller and less uniform [Ca(2+)](i) responses, suggesting that the differentiating effect of the ECM contributed to a more uniform receptor profile. 5. The [Ca(2+)](i) response to the P2Y(1)-selective agonist 2-MeSADP was abolished in the presence of the subtype-selective antagonist adenosine 3'-phosphate 5'-phosphosulphate (PAPS). 6. The P2Y(2) antagonist suramin completely blocked the response to ATP and inhibited the response to UTP by 66%. 7. The A(1) subtype-selective adenosine receptor agonist N(6)-Cyclopentyladenosine (CPA) gave a small but characteristic [Ca(2+)](i) response, while A(2A) and A(2B) subtype-selective agonists failed to generate [Ca(2+)](i) changes. 8. The results are consistent with the presence on RBCEC of a P2Y(2)-like receptor coupled to phospholipase C, and a P2Y(1)-like receptor mobilizing intracellular Ca(2+). The role of multiple nucleotide receptors in the function of the brain endothelium is discussed.

Selective transport of adenosine into porcine coronary smooth muscle

Adenosine (ADO), an endogenous regulator of coronary vascular tone, enhances vasorelaxation in the presence of nucleoside transport inhibitors such as dipyridamole. We tested the hypothesis that coronary smooth muscle (CSM) contains a high-affinity transporter for ADO. ADO-mediated relaxation of isolated large and small porcine coronary artery rings was enhanced 12-fold and 3.4-fold, respectively, by the transport inhibitor, S-(4-nitrobenzyl)-6-thioinosine (NBTI). Enhanced relaxation was independent of endothelium and was selective for ADO over synthetic analogs. Uptake of [(3)H]ADO into freshly dissociated CSM cells or endothelium-denuded rings was linear and concentration dependent. Kinetic analysis yielded a maximum uptake (V(max)) of 67 +/- 7.0 pmol. mg protein(-1). min(-1) and a Michaelis constant (K(m)) of 10. 5 +/- 5.8 microM in isolated cells and a V(max) of 5.1 +/- 0.5 pmol. min(-1). mg wet wt(-1) and a K(m) of 17.6 +/- 2.6 microM in intact rings. NBTI inhibited transport into small arteries (IC(50) = 42 nM) and cells. Analyses of extracellular space and diffusion kinetics using [(3)H]sucrose indicate the V(max) and K(m) for ADO transport are sufficient to clear a significant amount of extracellular adenosine. These data indicate CSM possess a high-affinity nucleoside transporter and that the activity of this transporter is sufficient to modulate ADO sensitivity of large and small coronary arteries.