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Biomedical subjects

M Reddington

Publications and source records attributed to M Reddington.

At least 55 records · Page 3Linked to original sources
PubMed1986

1,3-Dipropyl-8-cyclopentylxanthine (DPCPX) inhibition of [3H]N-ethylcarboxamidoadenosine (NECA) binding allows the visualization of putative non-A1 adenosine receptors.

The binding of the adenosine receptor agonists, [3H]N-ethylcarboxamidoadenosine (NECA) and [3H]cyclohexyladenosine (CHA) to membrane preparations and to cryostat sections of the rat brain was examined. The xanthine derivative, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) was ca. 500-fold more effective at A1 than at A2 sites. [3H]CHA binding to A1 adenosine receptors was virtually eliminated by the inclusion of DPCPX (50 nM), while [3H]NECA binding was only partially inhibited. The pattern of DPCPX-insensitive [3H]NECA binding sites was strikingly different from that of A1 receptors and is believed to represent an association with A2 type adenosine receptors and perhaps another or several, previously undescribed non-A1 sites.

Adenosine↗
PubMed1986

Properties of binding sites for [3H]cyclohexyladenosine in the hippocampus and other regions of rat brain: a quantitative autoradiographic study.

The properties of binding sites for the adenosine receptor ligand, [3H]cyclohexyladenosine ([3H]CHA), were investigated in rat brain using quantitative autoradiography. Scatchard analysis of the binding data showed that there were no significant differences between Kd values for [3H]CHA in any of the regions investigated. The highest concentrations of [3H]CHA binding sites were found in the cerebellum (molecular layer) and the stratum oriens and stratum radiatum of the hippocampus (CAI region). Displacement curves obtained using N-ethylcarboxamidoadenosine (NECA) and the R- and S-diastereoisomers of phenylisopropyl adenosine (PIA) showed the [3H]CHA binding sites to have the pharmacological properties of A1-adenosine receptors, i.e. the order of potency for these derivatives was R-PIA greater than NECA greater than S-PIA, in all regions tested. Further, [3H]CHA binding was in all cases attenuated by the guanosine triphosphate derivative, beta, gamma-imidoguanosine triphosphate. These results indicate that [3H]CHA binding sites throughout the central nervous system have the properties of A1-adenosine receptors and that these are in all regions associated with guanine nucleotide regulatory proteins.

Adenosine↗
PubMed1986

Autoradiographic evidence for multiple CNS binding sites for adenosine derivatives.

The binding of the adenosine analogues, 5'-N-ethylcarboxamido[3H]adenosine and N6-cyclohexyl[3H]adenosine, to the rat brain was examined utilizing light-microscopic autoradiographic techniques. While associated with many of the same structures, binding sites for these compounds showed distinct differences in both their patterns of distribution and their capacities to be inhibited by the adenosine analogue, R-phenylisopropyladenosine. Previous studies have shown that, at nanomolar concentrations, cyclohexyladenosine and R-phenylisopropyladenosine bind rather exclusively to the A1 type of adenosine receptors. In contrast, 5'-N-ethylcarboxamidoadenosine has almost equal affinity for A1 and A2 sites. Taking advantage of these characteristics, non-A1 binding sites were resolved by examining 5'-N-ethylcarboxamidoadenosine binding in the presence of micromolar concentrations of unlabeled R-phenylisopropyladenosine. The autoradiographically demonstrated distribution of R-phenylisopropyladenosine-insensitive 5'-N-ethylcarboxamidoadenosine binding sites differed significantly from that of the cyclohexyladenosine binding sites. Such non-A1 binding sites were concentrated in the striatum, nucleus accumbens, medial geniculate, olfactory tubercle, amygdala and certain thalamic nuclei. In contrast to the distribution of A1 adenosine receptor sites, R-phenylisopropyladenosine-insensitive 5'-N-ethylcarboxamidoadenosine binding was only low to moderate in the hippocampus, cerebellum and superior colliculus, regions which are strongly positive for cyclohexyladenosine binding. The present study provides the first autoradiographic evidence for multiple adenosine binding sites in the brain by demonstrating that the adenosine analogue 5'-N-ethylcarboxamidoadenosine can bind to a site or sites distinct from the A1 adenosine receptor site. The 5'-N-ethylcarboxamidoadenosine binding site which is not displaced by low concentrations of R-phenylisopropyladenosine may correspond to an A2 adenosine receptor site and/or an as yet uncharacterized type of adenosine receptor.

Adenosine↗
PubMed1984

Subcellular localization of 5'-nucleotidase in rat brain.

The subcellular distribution of the ectoenzyme, 5'-nucleotidase, in cerebral cortex and cerebellum of the rat was studied both biochemically and cytochemically. The fractions were characterized biochemically by marker enzymes. The localization of 5'-nucleotidase activity was also investigated cytochemically in the myelin, synaptosomal, mitochondrial, and microsomal fractions. Biochemically 5'-nucleotidase was found to be enriched in the membrane-containing fractions, i.e., myelin, synaptosomal, and microsomal fractions. Cytochemistry showed the reaction product in the myelin fraction to be associated with myelin profiles. In the synaptosomal fraction reaction product could occasionally be seen at synaptosomal membranes, although it could not be attributed unequivocally to the synaptosome itself, since in positions with reaction product unidentifiable membrane structures could always be seen attached. Mitochondria were virtually without any reaction product. In the microsomal fraction 5'-nucleotidase activity was associated with unidentifiable membrane structures. It is concluded that 5'-nucleotidase is associated with myelin profiles and that the high activity found in the synaptosomal fraction is probably not associated with nerve ending plasma membranes.

5'-Nucleotidase↗
PubMed1984

Tubuloglomerular feedback-mediated decreases in glomerular pressure in Munich-Wistar rats.

These experiments were performed to evaluate directly measured glomerular capillary pressure and single nephron glomerular filtration rate (SNGFR) tubuloglomerular feedback responses in Munich-Wistar rats during increased distal flow rate achieved by infusing an isotonic electrolyte solution into unblocked late proximal tubules. Arterial pressure averaged 114 +/- 2 mmHg and proximal tubule pressure was 14 +/- 1 mmHg. In eight tubules, control SNGFR based on distal tubular fluid collections averaged 22 +/- 3 nl/min, decreased to 15 +/- 2.3 nl/min when 10-12 nl/min of perfusate was infused into the late proximal tubule, and further decreased to 9 +/- 1.7 nl/min at an infusion of 20-24 nl/min. In 22 tubules, control glomerular capillary pressure was 55 +/- 1.6 mmHg, decreased to 43 +/- 2.5 mmHg with addition of perfusate into a late proximal tubule at a rate of 24 nl/min, and returned to 53 +/- 3.1 mmHg when perfusion was stopped. In eight nephrons, glomerular capillary pressure was shown to be responsive to smaller increments in the late proximal infusion rate and was reduced by 4 +/- 0.5 and 7 +/- 1.1 mmHg at the intermediate rates of 10 and 15 nl/min, respectively. These results demonstrate that glomerular pressure decreases during increased distal delivery even when the tubule is not blocked. They are consistent with the hypothesis that increases in afferent arteriolar resistance are primarily responsible for feedback-mediated reductions in glomerular filtration rate.

Animals↗
PubMed1983

Adenosine receptor density and the depression of evoked neuronal activity in the rat hippocampus in vitro.

The relationship between the heterogeneous distribution of A-1 adenosine receptors and the capacity of adenosine to depress neuronal activity was examined in the rat hippocampus. Utilizing autoradiographic techniques, the distribution of A-1 adenosine receptors was assessed by the binding of [3H]cyclohexyladenosine ([3H]CHA) to cryostat sections of rat brain. The apical dendritic region of CA1 showed a differential distribution of adenosine receptors between the stratum radiatum and stratum lacunosum/moleculare. The physiological relevance of this binding difference was studied in the hippocampal slice by examining the capacity of adenosine to depress evoked potentials in these two strata. It was observed that the receptor differences correlated with differential sensitivities to adenosine modulation of the evoked potentials. These data suggest that receptor density, as shown by binding techniques, may provide not only a qualitative but also a quantitative map of the sites of adenosine action.

Adenosine↗
PubMed1983

Regulation of the strength of adenosine modulation in the hippocampus by a differential distribution of the density of A1 receptors.

Correlative studies examining: (1) the binding of [3H]cyclohexyladenosine to A1 adenosine receptor sites, and (2) the depressive action of adenosine on evoked activity, were performed in the hippocampal formation. A greater number of A1 sites was observed in the dorsal versus ventral aspect of the hippocampus, and a larger depression of evoked potentials by adenosine was obtained in the dorsal aspect. These observations suggest that a differential density of A1 receptors might control the magnitude of modulatory action by adenosine on hippocampal circuitry.

Adenosine↗
PubMed1983

5'-Nucleotidase activity in human astrocytomas.

Human astrocytomas contain 5'-nucleotidase as shown using histochemical, cytochemical and biochemical techniques. 5'-Nucleotidase activity was revealed by electron microscopy to be associated with the outer surface of the plasma membrane of astrocytoma cells. The results confirm and extend previous reports of the localisation of 5'-nucleotidase in normal astrocytes in situ. Biochemical assay of 5'-nucleotidase activity in 35 tumours showed it to be clearly higher in malignant (anaplastic astrocytomas and glioblastomas) than in benign astrocytomas.

5'-Nucleotidase↗
PubMed1983

Adenosine metabolism in a rat hippocampal slice preparation: incorporation into S-adenosylhomocysteine.

The incorporation of [14C]adenosine into various metabolites was studied in a hippocampal slice preparation in order to assess the extent of adenosine metabolism via synthesis of S-adenosylhomocysteine, a potent inhibitor of transmethylation reactions. Highest incorporation of 14C occurred into nucleotides, with only a few percent being recovered in inosine + hypoxanthine, S-adenosylhomocysteine, and the free adenosine pool. Labeling of S-adenosylhomocysteine did not significantly increase with higher concentrations of added adenosine despite greater accumulation of free [14C]adenosine in the tissue. Addition of L-homocysteine significantly increased the labelling of S-adenosylhomocysteine. The results indicate that S-adenosylhomocysteine synthesis is a minor pathway of adenosine metabolism in brain tissue under steady-state conditions. Further, changes in adenosine concentration, without a concomitant change in L-homocysteine availability, are unlikely to lead to a significant accumulation of S-adenosylhomocysteine. S-Adenosylhomocysteine is therefore not likely to play a significant role in mediating the biological effects of adenosine in the CNS via inhibition of transmethylations.

Adenosine↗
PubMed1982

An A1-adenosine receptor, characterized by [3H] cyclohexyladenosine binding, mediates the depression of evoked potentials in a rat hippocampal slice preparation.

In slices of rat hippocampus, adenosine and several adenosine derivatives depressed evoked neuronal responses to afferent stimulation. The nanomolar potency of adenosine derivatives and their relative effectiveness indicate that the depression of evoked potentials is mediated via an A1-adenosine receptor. A remarkable similarity was found between the relative potencies of nucleoside derivatives with respect to their electrophysiological effects and to their inhibition of high affinity [3H] cyclohexyladenosine ([3H]CHA) binding to rat brain membranes. We conclude that the [3H] CHA binding site in rat brain membranes represents a physiological receptor of the A1-type.

2-Chloroadenosine↗
PubMed1979

Synaptic membrane proteins as substrates for cyclic AMP-stimulated protein phosphorylation in various regions of rat brain.

Synaptosomal plasma membranes from mammalian brain contain protein kinase activity which phosphorylates endogenous membrane proteins and is stimulated by cyclic AMP. Using polyacrylamide gel electrophoresis it was shown that at least ten proteins in the synaptosomal plasma membrane fraction could be phosphorylated by endogenous cyclic AMP-stimulated protein kinase activity. The number of proteins whose phosphorylation was stimulated by cyclic AMP was strongly influenced by the pH and Mg2+ concentration used in the phosphorylation reaction. A complex pattern of cyclic AMP-stimulated protein phosphorylation was obtained only with synaptosomal plasma membranes and a crude microsomal fraction. Mitochondrial and myelin fractions exhibited no cyclic AMP-stimulated protein kinase activity. Investigation of the distribution of substrates for cyclic AMP-stimulated phosphorylation among various brain regions failed to reveal any regional differences.

Animals↗
PubMed1979

Parallel investigations of the effects of adenosine on evoked potentials and cyclic AMP accumulation in hippocampus slices of the rat.

In slices of rat hippocampus, adenosine and its receptor agonist, 2-chloro-adenosine, were both found to (a) enhance the intracellular accumulation of cyclic adenosine monophosphate (cAMP) and (b) depress neuronal evoked responses to electrical stimulation. The differences in the dose/effect curves for the biochemical and electrophysiological changes cast doubt on a causal relationship between the two effects.

Adenosine↗