Plasma membrane Ca2+ pumping plays a prominent role in adenosine A1 receptor mediated changes in [Ca2+]i in DDT1 MF-2 cells

A novel approach for the determination of contractile and calcium responses of the basilar artery employing real-time confocal laser microscopy

INTRODUCTION

Intracellular calcium concentration ([Ca(2+)](i)) modifications in endothelial and smooth muscle cells represent a key element in the pathogenesis of cerebral artery vasospasm. Therefore, the present study documented potential application of confocal laser microscopy in the determination of contractile and [Ca(2+)](i) responses in basilar artery.

METHODS

Experiments were performed on the rat isolated basilar artery. Changes in [Ca(2+)](i) were determined by ratiometry involving Fluo-4/AM and Fura Red/AM. Contractile function was calculated from the change in fluorescent area by Fluo-4/AM.

RESULTS

KCl (50 mM) elicited an increase in [Ca(2+)](i) and contraction in basilar artery; moreover, nearly well maintained responses were evident for at least 120 min following the first application. 10 microM 5-hydroxytryptamine (5-HT), 10 microM alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-meATP) and 10 nM vasopressin (VP) also induced increases in [Ca(2+)](i) and contraction dose-dependently. Additionally, 10 microM acetylcholine elicited a transient [Ca(2+)](i) decrease and sustained relaxation. In individual cells, rhythmical changes in [Ca(2+)](i) were observed after 10 microM 5-HT. VP (10 nM) evoked modest Ca(2+) oscillation in individual cells; however, Ca(2+) oscillation was not detectable with 10 microM alpha,beta-meATP.

DISCUSSION

These results indicate that this method offers reproducibility and quantifiable effects. Imaging technology may therefore be applied to the estimation of [Ca(2+)](i) responses at the tissue level as well as at the level of the individual cell. Thus, confocal laser microscopy is a suitable tool for estimation of small artery function.

Mitochondria play an important role in adenosine-induced ATP release from Madin-Darby canine kidney cells

We previously found that adenosine stimulates ATP release from Madin-Darby canine kidney (MDCK) cells, by activating an Ins(1,4,5)P(3) sensitive-calcium (Ca(2+)) pathway through the stimulation of A(1) receptors. Thus, we investigated the intracellular pathway of ATP efflux after the rise in intracellular Ca(2+) in MDCK cells. Adenosine evoked an increase in mitochondrial Ca(2+) using Rhod-2/AM, a mitochondrial Ca(2+) indicator. Adenosine-induced ATP release was inhibited by mitochondrial modulators, such as oxidative phosphorylation modulators (carbonyl cyanide 3-chlorophenylhydrazone and oligomycin), mitochondrial ADP/ATP carrier inhibitors (N-ethylmaleimide, carboxyatractyloside and bongkrekic acid), a mitochondrial Na(+)-Ca(2+) exchange inhibitor (CGP-37157). In addition, mitochondrial modulators significantly reduced intracellular ATP content. On the other hand, 2-deoxy-glucose (2-DG) induced a greater decrease in intracellular ATP content than mitochondrial modulators. ATP release was still induced by adenosine in the presence of 5mM 2-DG. These results suggest that mitochondria play an important role in the signaling pathway of adenosine-triggered ATP release in MDCK cells.

Adenosine induces ATP release via an inositol 1,4,5-trisphosphate signaling pathway in MDCK cells

ATP is released into extracellular space as an autocrine/paracrine molecule by mechanical stress and pharmacological-receptor activation. Released ATP is partly metabolized by ectoenzymes to adenosine. In the present study, we found that adenosine causes ATP release in Madin-Darby canine kidney cells. This release was completely inhibited by CPT (an A1 receptor antagonist), U-73122 (a phospholipase C inhibitor), 2-APB (an inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) receptor blocker), thapsigargin (a Ca2+-ATPase inhibitor), and BAPTA/AM (an intracellular Ca2+ chelator), but not by DMPX (an A2 receptor antagonist). However, forskolin, epinephrine, and isoproterenol, inducers of cAMP accumulation, failed to release ATP. Adenosine increased intracellular Ca2+ concentrations that were strongly blocked by CPT, U-73122, 2-APB, and thapsigargin. Moreover, adenosine enhanced accumulations of Ins(1,4,5)P3 that were significantly reduced by U-73122 and CPT. These data suggest that adenosine induces the release of ATP by activating an Ins(1,4,5)P3 sensitive-Ca2+ pathway through the stimulation of A1 receptors.

Exposure to TARC alters beta2-adrenergic receptor signaling in human peripheral blood T lymphocytes

The beta(2)-adrenergic receptor (beta(2)-AR) negatively regulates T cell activity through the activation of the G(s)/adenylyl cyclase/cAMP pathway. beta(2)-AR desensitization, which can be induced by its phosphorylation, may have important consequences for the regulation of T cell function in asthma. In the present study we demonstrate that the C-C chemokine thymus and activation-regulated chemokine (TARC) impairs the ability of beta(2)-agonist fenoterol to activate the cAMP downstream effector cAMP-responsive element binding protein (CREB) in freshly isolated human T cells. The TARC-induced activation of Src kinases resulted in membrane translocation of both G protein-coupled receptor kinase (GRK) 2 and beta-arrestin. Moreover, TARC was able to induce Src-dependent serine phosphorylation of the beta(2)-AR as well as its association with GRK2 and beta-arrestin. Finally, in contrast to CREB, phosphorylation of Src and extracellular signal-regulated kinase was enhanced by fenoterol upon TARC pretreatment. In summary, we show for the first time that TARC exposure impairs beta(2)-AR function in T cells. Our data suggest that this is mediated by Src-dependent activation of GRK2, resulting in receptor phosphorylation, binding to beta-arrestin, and a switch from cAMP-dependent signaling to activation of the MAPK pathway. We propose that aberrant T cell control in the presence of endogenous beta-agonists promotes T cell-mediated inflammation in asthma.

Cloning, pharmacological characterization, and polymorphism screening of the guinea pig beta(2)-adrenoceptor

In asthma, beta(2)-adrenoceptor agonist responsiveness has been associated with Arg16Gly and Gln27Glu polymorphisms of the beta(2)-adrenoceptor. Since the guinea pig is extensively used as an animal model for asthma, we investigated the occurrence of possible polymorphism of the guinea pig beta(2)-adrenoceptor. The guinea pig beta(2)-adrenoceptor coding region was amplified by sequence homology-based cloning. Homology of the translated protein with the human beta(2)-adrenoceptor was 88% with Ala at position 16 and Glu at position 27. Radioligand binding and cAMP- accumulation experiments of Chinese hamster ovary (CHO) cells transfected with the guinea pig beta(2)-adrenoceptor revealed a homogeneous population of functional receptors. Five degenerate single nucleotide polymorphisms were found within the beta(2)-adrenoceptor coding region of outbred Dunkin Hartley guinea pigs, at residues 354, 453, 483, 534 and 642. In conclusion, we have cloned the guinea pig beta(2)-adrenoceptor, which shows to be functional upon expression in a recombinant system and contains five single nucleotide polymorphisms dissimilar to human polymorphisms.