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C Londos

Publications and source records attributed to C Londos.

At least 55 records · Page 3Linked to original sources
PubMed1981

The action of adenosine analogs on PC12 cells.

PC12 cells, a nerve growth factor-responsive clone of rat pheochromocytoma, contain a membrane-bound adenylate cyclase, which can be activated by adenosine analogs. The characteristics of the cyclase response indicate the presence of stimulatory adenosine receptors. Adenosine analogs also produce a marked increase in the ornithine decarboxylase levels of the cells, and the characteristics of this response suggest that it is linked to the adenylate cyclase-stimulatory adenosine receptors. The ornithine decarboxylase response elicited by 5'-N-ethylcarboxamideadenosine (NECA), a potent stimulatory adenosine analog, is synergistic with that produced by nerve growth factor. Differentiation of the cells with nerve growth factor, however, does not substantially alter either the response of cyclase to the adenosine analog or the magnitude of the adenosine-evoked ornithine decarboxylase response. Treatment of the cells with NECA produces an increase in the phosphorylation of a specific non-histone nuclear protein. While causing little or no morphological alteration by itself, NECA is synergistic with nerve growth factor in producing neurite outgrowth in PC12 cells. NECA does not cause an induction of acetylcholinesterase in the cells. NECA does not cause an induction of acetylcholinesterase in the cells, nor does it appear to affect the induction of this enzyme by nerve growth factor.

Acetylcholinesterase↗
PubMed1980

Subclasses of external adenosine receptors.

Cell surface adenosine receptors mediate either stimulation or inhibition of adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1], and the receptors that mediate these different responses can be discriminated with selected adenosine analogs. 5'-N-Ethylcarboxamide-adenosine is a more potent agonist at stimulatory receptors (Ra) than is N6-phenylisopropyladenosine, whereas the reverse potency order is seen with inhibitory receptors (Ri). The potency of adenosine is intermediate between the potencies of these two analogs. The relative potencies of adenosine receptor agonists are maintained in physiological responses in intact cells, such as steroidogenesis and inhibition of lipolysis. As with adrenergic receptors, subclasses of adenosine receptors differ functionally and pharmacologically.

Adenosine↗
PubMed1979

Evaluation of the effects of adenosine on hepatic and adipocyte adenylate cyclase under conditions where adenosine is not generated endogenously.

Direct effects of adenosine on adipocyte and hepatic adenylate cyclase have been demonstrated in an assay system where adenosine is not generated. The substrate used, 2'-deoxy ATP may, on metabolism, only give rise to 2'-deoxyadenosine, which does not act at adenosine receptors. With a slight modification of existing assay techniques this assay system has been used to detect a hitherto undiscovered adenosine receptor on liver plasma membranes, which is antagonised by methylxanthines and which stimulates adenylate cyclase activity in a GTP-dependent manner. The potent inhibitory effects of purine-modified adenosine analogs on fat cell adenylate cyclase are reproduced by adenosine in this assay system. An application of this approach to the study of adenylate cyclase not only simplifies detection of the role of adenosine, but also yields insights into the interaction between guanine nucleotides and hormones.

Adenosine↗
PubMed1978

Adenosine analogs inhibit adipocyte adenylate cyclase by a GTP-dependent process: basis for actions of adenosine and methylxanthines on cyclic AMP production and lipolysis.

Adenylate cyclase in purified membranes from rat adipocytes is inhibited by low concentrations of purine-modified adenosine analogs, particularly those modified in the N6 position. Such inhibition is antagonized competitively by methylxanthines, but not by other cyclic nucleotide phosphodiesterase inhibitors, and it is dependent on "inhibitory" concentrations of GTP in the assay medium. Ribose-modified adenosine analogs inhibit adenylate cyclase through a process that is neither dependent upon the GTP concentration nor antagonized by methylxanthines. These results explain the potent effects of adenosine and methylxanthines on fat cell metabolism and demonstrate the importance of GTP in mediating inhibition by agents that act at cell surface receptors.

Adenosine↗
PubMed1977

Regulation by glucagon and divalent cations of inhibition of hepatic adenylate cyclase by adenosine.

Adenosine inhibits the rat liver adenylate cyclase system at a regulatory site that is distinct from the glucagon receptor, the guanine nucleotide regulatory site, and the active site involved in catalysis of ATP to cyclic AMP. The effects of the nucleoside are also independent of the concentration of uncomplexed ATP (ATP4-) in the assay medium. Glucagon, but not guanine nucleotides, sensitizes the system to inhibition by adenosine. Depending on assay conditions, the hormone can shift the concentration of adenosine required for 50% inhibition by as much as 10-fold. Under optimal conditions, the apparent Ki for adenosine is 25 micron. Both Mg2+ and Mn2+ increase adenylate cyclase activity and, in order of relative potency, increase the sensitivity of the enzyme to adenosine inhibition; Mn2+ is 50- to 100-fold more potent than Mg2+. The adenosine inhibitory site exhibits stringent structural requirements for nucleoside action. Most alterations of the purine ring result in loss of activity, whereas alterations in the ribose ring are tolerated, and some deoxyadenosine analogs are even more effective than adenosine. Naturally occurring nucleosides and nucleotides, such as inosine, guanosine, and 5'-AMP, are inactive. Analog studies reveal also that inhibition of the hepatic system occurs at a site which is clearly different from the sites through which adenosine activates other adenylate cyclase systems, and that the liver enzyme appears to have no site for activation by the nucleoside.

Adenosine↗
PubMed1977

Reversible activation of hepatic adenylate cyclase by guanyl-5'-yl-(alpha,beta-methylene)diphosphonate and guanyl-5'-yl imidodiphosphate.

Guanyl-5'-yl-(alpha,beta-methylene)di[gamma-32P]phosphonate (Gp-(CH2)pp) is not hydrolyzed by rat liver membranes under conditions in which GTP and guanyl-5'-yl imidodiphosphate (Gpp(NH)p) are hydrolyzed. Gp(CH2)pp activates adenylate cyclase in hepatic membranes with characteristics similar to those of Gpp(NH)p activation but with lower potency and effectiveness. The analogs, although with lower potency than GTP, also share the ability to change the glucagon receptor from a high to a low affinity state. Both Gp(CH2)pp and Gpp(NH)p stimulate adenylate cyclase activity following a lag period of about 1 min addition of GTP after steady state rates are achieved results in reduction in the rate following a lag period of 6 min from the time of addition of GTP. Pretreatment of the enzyme with Gpp(NH)p or Gp(CH2)pp, followed by washing the membranes, leads to a high activity state of the enzyme which slowly decays in rate unless the analogs are continuously present in the medium. These data suggest that the guanosyl nucleotide analogs act on the enzyme system by a slowly reversible process that possibly reflects slow binding and dissociation from different transition states of the enzyme system and suggest that activation of adenylate cyclase by GTP, Gpp(NH)p, and Gp(CH2)pp does not involve covalent modification of the enzyme.

Adenylyl Cyclases↗
PubMed1977

Two distinct adenosine-sensitive sites on adenylate cyclase.

The effects of adenosine and adenosine analogs on adenylate cyclases from several tissues have been examined. Two adenosine-reactive sites have been identified: (i) the "R" site, occupancy of which usually leads to activation of cyclase and which requires integrity of the ribose ring for activity, and (ii) the "P" site, which mediates inhibition and requires integrity of the purine ring for activity. Biphasic effects of adenosine are explained by the presence of both sites on a single adenylate cyclase. Comparison of these data with those in the literature indicates that adenosine-reactive "P" and "R" sites are present generally.

Adenosine↗
PubMed1975

The hepatic adenylate cyclase system. I. Evidence for transition states and structural requirements for guanine nucloetide activiation.

Previous studies have shown that guanine nucleotides, acting at a site termed nucleotide regulatory site, are required for activation of hepatic adenylate cyclase and that glucagon facilitates this process. This study shows that only guanine nucleotides containing triphosphate groups at the 5' position of ribose (or 3'-deoxyribose) are capable of activating the enzyme. The terminal phosphate is not utilized in the activation process since 5'-guanylylimidodiphosphate (Gpp(NH)p and 5'-guanylyl methylenediphosphonate, analogues of GTP that are not utilized in transferase or hydrolase reactions, stimulate enzyme activity. The nucleotides bind in their free form at the regulatory site; chelation by magnesium ion shifts the apparent concentration dependence for activation by Gpp(nh)p. GDP inhibits competitively Gpp(NH)p-stimulated activity and inhibits basal activity and activities stimulated by glucagon. Activation of the enzyme by Gpp(NH)p is a slow process; the length of the lag time increases as an inverse function of nucleotide concentration and is as long as 4 min before onset of increased enzyme activity. Following pretreatment with Gpp(NH)p and extensive washing of hepatic membranes, the enzyme displays immediate increases in activity with rates that are a function of the nucleotide concentration during pretreatment; the rates remain constant for at least 6 min despite the absence of Gpp(NH)p in the medium. Studies with labeled Gpp(NH)p show that the intact nucleotide remains firmly bound to the membranes after extensive washing, suggesting that the persistence of adenylate cyclase activity may be related to slow dissociation of the nucleotide from the regulatory site. Addition of 1 nM glucagon, a submaximal concentration, does not abolish the lag phase of Gpp(NH)p activation even at saturating concentration of the nucleotide (1 muM or higher). The maximal steady state rate is achieved under these conditions. Addition of 2 muM glucagon, a saturating hormone concentration, does not alter the steady state rate but abolishes the lag phase of Gpp(NH)p activation. The transient kinetics of Gpp(NH)p activation and the effects of glucagon thereon are discussed in terms of a three state model in which the guanine nucleotide induces the formation of an intermediate transition state that displays no increase in enzyme activity over the basal state and which slowly isomerizes to a high activity state of the adenylate cyclase system; glucagon acts by accelerating the rate of isomerization.

Adenylyl Cyclases↗