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T K Narayanan

Publications and source records attributed to T K Narayanan.

At least 19 recordsLinked to original sources
PubMed2002

A comparative study on the uptake and incorporation of radiolabeled methionine, choline and fluorodeoxyglucose in human astrocytoma.

PURPOSE: The goal of this investigation was to evaluate uptake and incorporation of 2-deoxy-2-[18F]fluoro-D-glucose (FDG), 11C-methionine, and 11C-choline in 17 patients suspected of grade-II and grade-III tumors using positron emission tomography (PET) and use in vitro astrocytoma cell lines in order to support in vivo findings. METHODS: Seventeen patients with suspected astrocytomas (9 grade-II and 8 grade-III) were studied by PET with FDG and 11C-methionine; and one patient (grade-III) with FDG, 11C-methionine and 11C-choline. Uptake of PET molecular imaging probe was quantitative based on tumor to corresponding contralateral-region uptake ratio, tumor to mean-cortical-uptake ratio, and tumor to white matter uptake ratio. This was correlated with World Health Organization histology grading system and clinical follow-up. Uptake and incorporation of 3H-methionine, 3H-choline and FDG into lipid, RNA, DNA, and protein were investigated in a grade-III human tumor brain-14 astrocytoma cell line. RESULTS: A time-dependent increase in the total uptake of 3H-methionine, 3H-choline and FDG was observed in human tumor brain-14 astrocytoma-III cell line. 3H-methionine was incorporated predominantly into proteins (in excess of 40% at 1 h) while 3H-choline incorporated primarily into lipids (in excess of 60% at 1 hr). Total uptake of FDG was accounted for in the free-pool supernatant fraction. In all patients, PET images of 11C-methionine and FDG provided higher tumor to white matter ratios than tumor to corresponding contra-lateral region ratios and tumor to mean cortical uptake ratios. In grade II patients, FDG did not exhibit significant increase in tumor uptake, while 11C-methionine was a good predictor with ratios of approximately 1.50 +/- 0.48. In grade III patients, both FDG and 11C-methionine exhibited higher ratios than for grade II, with 11C-methionine being the greatest (ratios of 2.50 +/- 0.85), possibly suggesting enhanced protein synthesis. With respect to tumor delineating potential, 11C-choline may be equal to or slightly better than 11C-methionine in the subject evaluated with all three probes. CONCLUSIONS: Results suggest that a combination of FDG and 11C-methionine is useful in the prediction of histological grade of astrocytomas. In addition, 11C-methionine is better than FDG in delineating tumor boundary for low-grade gliomas. In vitro results suggest that 3H-methionine is significantly incorporated into proteins and provides the major driving force in the uptake of 11C-methionine observed in PET images.

Journal Article↗
PubMed2001

Evaluation of dopamine D-2 receptor occupancy by clozapine, risperidone, and haloperidol in vivo in the rodent and nonhuman primate brain using 18F-fallypride.

We have used the high-affinity dopamine D-2 receptor radioligand, 18F-fallypride for evaluating receptor occupancy by the antipsychotic drugs, clozapine, risperidone, and haloperidol in rodents and nonhuman primates. In rodents, clozapine (0.1 mg/kg to 100 mg/kg) competed with 18F-fallypride at all the doses administered. At doses over 40 mg/kg, clozapine was able to displace all the administered 18F-fallypride. A pseudobiphasic profile of receptor occupancy by clozapine was observed. This behavior was compared with such other neuroleptics as risperidone and haloperidol that exhibited over 90% receptor occupancy at doses over 0.1 mg/kg and did not exhibit a biphasic nature. Dopamine D-2 receptor occupancy in the monkeys was studied using positron emission tomography (PET) after acute subcutaneous doses of the various drugs. At therapeutically relevant doses, clozapine, risperidone, and haloperidol were able to compete significantly with the binding of 18F-fallypride in all brain regions in rhesus monkeys, and our analyses indicate that these drugs (clozapine, risperidone, and haloperidol) do not discriminate between the striatal (caudate and putamen) and the extrastriatal (thalamus and cortical regions) dopamine receptors. The following extent of D-2 receptor occupancies were measured in the monkey brain using PET: clozapine approximately 70% (dose of 9.7 mg/kg), risperidone approximately 75% (0.05 mg/kg), and haloperidol approximately 90% (0.05 mg/kg).

Animals↗
PubMed2000

In vitro and in vivo evaluation of the binding of the dopamine D2 receptor agonist (11)C-(R,S)-5-hydroxy-2-(di-n-propylamino)tetralin in rodents and nonhuman primate.

The in vitro autoradiographic binding characteristics as well as in vivo imaging characteristics of a dopamine D2 receptor agonist, (R, S)-2-(N-propyl-N-1'-(11)C-propyl)amino-5-hydroxytetralin ((11)C-5-OH-DPAT), were studied. In (3)H-spiperone assays using rat striata, 5-OH-DPAT exhibited an affinity of IC(50) = 2.5 nM. In vitro autoradiographs in rat brain slices with (11)C-5-OH-DPAT revealed selective binding to the dopaminergic regions in the striata which was displaceable by sulpiride. Varying concentrations of dopamine displaced this selective binding of (11)C-5-OH-DPAT to the striata in rat brain slices. This selective binding to the striata was also removed in the presence of the GTP analog, 5'-guanylylimidodiphosphate, indicative of the binding of (11)C-5-OH-DPAT to the high-affinity state of the D2 receptor. Ex vivo autoradiographic study in rats exhibited selective binding of (11)C-5-OH-DPAT to the striata. A PET study in a rhesus monkey showed selective localization of (11)C-5-OH-DPAT in the striata and the ratio between striata and cerebellum approached approximately 2 at 40 min postinjection.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed2000

Quantitation of striatal and extrastriatal D-2 dopamine receptors using PET imaging of [(18)F]fallypride in nonhuman primates.

[(18)F]Fallypride is a highly selective, high-affinity dopamine D-2 receptor ligand. The high affinity, K(D) = 30 pM, makes it a suitable candidate for visualizing both striatal and extrastriatal binding in the brain. In this work, dynamic PET studies of two macaque monkeys were acquired along with arterial plasma samples. Compartmental analysis and Logan plots were used to analyze the striatum, thalamus, frontal, and temporal cortices and to validate a reference region of analysis which yields a distribution volume ratio (DVR). The cerebellum was used as the reference region. The results indicate that all methods of analysis are in close agreement over all the analyzed regions in the brain. The average DVRs for the two monkeys was found to be: caudate = 26, putamen = 29, thalamus = 3.8, frontal ctx = 1.7, and temporal ctx = 1.7 on a high-resolution PET scanner. It was found that a scan time of 2 h is needed to accurately estimate the DVR for all regions of the brain. The striatal regions require the longest to linearize and are the most sensitive to variations in the average tissue-to-plasma efflux constant, k(2). For the extrastriatal regions, the effect of the k(2) term on DVR calculation is negligible. Repeatability measurements for all regions were found to be within 10% using the DVR parameter.

Animals↗
PubMed1999

Radiosynthesis and in vitro evaluation of 2-(N-alkyl-N-1'-11C-propyl)amino-5-hydroxytetralin analogs as high affinity agonists for dopamine D-2 receptors.

We have developed radiotracers based on agonists that may potentially allow the in vivo assessment of the high affinity (HA) state of the dopamine D-2 receptors. The population of HA state, which is likely the functional state of the receptor, may be altered in certain diseases. We carried out radiosyntheses and evaluated the binding affinities, lipophilicity, and in vitro autoradiographic binding characteristics of three dopamine D-2 receptor agonists: (+/-)-2-(N,N-dipropyl)amino-5-hydroxytetralin (5-OH-DPAT), (+/-)-2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin (PPHT), and (+/-)-2-(N-cyclohexylethyl-N-propyl)amino-5-hydroxytetralin (ZYY-339). In 3H-spiperone assays using rat striata, ZYY-339 exhibited subnanomolar affinity for D-2 receptor sites (IC50 = 0.010 nM), PPHT was somewhat weaker (IC50 = 0.65 nM), and 5-OH-DPAT exhibited the weakest affinity (IC50 = 2.5 nM) of the three compounds. Radiosynthesis of these derivatives, 2-(N-propyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-5-OH-DPAT), 2-(N-phenethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-PPHT), and 2-(N-cyclohexylethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-ZYY-339) was achieved by first synthesizing 11C-1-propionyl chloride and subsequent coupling with the appropriate secondary amine precursor to form the respective amide, which was then reduced to provide the desired tertiary amine products. The final products were obtained by reverse-phase high performance liquid chromatography (HPLC) purification in radiochemical yields of 5-10% after 60-75 min from the end of 11CO2 trapping and with specific activities in the range of 250-1,000 Ci/mmol. In vitro autoradiographs in rat brain slices with 11C-5-OH-DPAT, 11C-PPHT, and 11C-ZYY-339 revealed selective binding of the three radiotracers to the dopamine D-2 receptors in the striata.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed1998

Acute effects of a bicyclophosphate neuroconvulsant on monoamine neurotransmitter and metabolite levels in the rat brain.

Naive male Sprague-Dawley rats were injected intraperitoneally (i.p.) with the bicyclophosphate convulsant trimethylolpropane phosphate (TMPP) at dose levels from 0.2 to 0.6 mg/kg. Rats were observed for convulsive activity, and were sacrificed 15 min posttreatment. Levels of the monoamine neurotransmitters norepinephrine (NE), epinephrine (EPI), dopamine (DA), and serotonin (5-HT) and the major metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were assayed in forebrain, midbrain, hindbrain, cerebellum and brainstem regions. Neurotransmitter and metabolite levels were compared between control rats and rats that did and did not experience seizures. TMPP administration induced significant decreases in levels of measured neurotransmitters that varied as a function of brain region, dose, and expression of the seizure activity. These results show that tonic or tonic-clonic seizures induced by TMPP administration (0.6 mg/kg) are reliably associated with regional decreases in serotonin, dopamine, and norepinephrine. Convulsive activity resulting from lower dose administrations (0.2-0.4 mg/kg) of TMPP result only in decreased regional levels of serotonin.

Animals↗
PubMed1990

Effects of adenosine and analogs on adenylate cyclase activity in cultured bovine aortic endothelial cells.

We studied the effects of adenosine and analogs on adenylate cyclase (AC) activity in membranes from long-term cultured bovine aortic endothelial cells, using [alpha-32]ATP as substrate and chromatographic separation of [32P]cAMP. Compared to our previous findings in cultured bovine pulmonary arterial endothelial cells (Legrand et al., Biochem Pharmacol 38: 423-430, 1989), the present results were qualitatively and quantitatively comparable between the two cell types. In aortic cells, AC activity was stimulated in a concentration-dependent manner by isoproterenol, forskolin and 5'-guanylylimidodiphosphate (Gpp(NH)p), by 2.6-, 5.2- and 4.8-fold respectively. The A2 adenosine agonist 5'-(N-ethyl)-carboxamidoadenosine induced a smaller (60%) increase of AC activity. Adenosine (10(-3) M) partially inhibited (30%) the Gpp(NH)p-stimulated AC activity. Similarly, adenosine partially reversed, but 2',5'-dideoxyadenosine (DDA) totally blocked (IC50: 540 microM), the forskolin-induced stimulation of AC activity. DDA and 2'-deoxyadenosine-3'-monophosphate (2'-deoxy-3'-AMP) also inhibited the isoproterenol-induced stimulation of AC activity (IC50: 350 and 23 microM respectively). Adenosine-induced inhibition of stimulated AC activity does not appear to involve adenosine A1 receptors since the specific A1 agonist cyclohexyladenosine did not reverse forskolin stimulation of AC activity. Instead, it suggests a direct action of adenosine on the catalytic subunit of the adenylate cyclase (P site). We conclude that membranes from long-term cultured bovine aortic endothelial cells, express beta-adrenergic and adenosine A2 receptors coupled to adenylate cyclase activation. The two P site agonists, DDA and 2'-deoxy-3'-AMP, and, with a weaker effect, adenosine itself, inhibited the activated cyclase at the P site. The natural nucleotide 2'-deoxy-3'-AMP was a strong inhibitor in aortic cell types (as in pulmonary arterial endothelial cells) and may possibly act as a modulator of adenylate cyclase in these cells.

Adenosine↗
PubMed1989

Modulation of adenylate cyclase activity in cultured bovine pulmonary arterial endothelial cells. Effects of adenosine and derivatives.

We studied the effects of adenosine and adenosine derivatives on adenylate cyclase activity in cultured endothelial cells from bovine pulmonary artery. Basal and stimulated enzyme activities were measured in membrane preparations using [alpha-32P]ATP as the substrate and chromatographic isolation of formed [32P]cAMP. Basal cyclase activity was 11 +/- 1 (mean +/- SEM) pmol/mg protein/min. Forskolin, 5'-guanylylimidodiphosphate (Gpp(NH)p) and (-)isoproterenol stimulated adenylate cyclase in a concentration-dependent manner, producing maximal stimulations of three, seven and four times the basal activity respectively. In the presence of adenosine deaminase, cyclohexyladenosine, an A1 agonist, had no effect on basal and forskolin- or Gpp(NH)p-stimulated activities, whereas 5'-(N-ethyl)-carboxamidoadenosine (NECA), an A2 agonist, had a small stimulatory effect (52% increase over basal). In the presence of IBMX, adenosine and two P-site agonists, 2',5'-dideoxyadenosine (DDA) and 2'-deoxyadenosine-3'-monophosphate (2'-deoxy-3'-AMP), inhibited forskolin (30 microM)-stimulated adenylate cyclase activity with an order of potency of 2'-deoxy-3'-AMP greater than DDA greater than adenosine. DDA and 2'-deoxy-3'-AMP were also able to inhibit cyclase activity stimulated by Gpp(NH)p (10(-5)M) or isoproterenol (10(-6)M) with the same order of potency. Only 2'-deoxy-3'-AMP inhibited the stimulated adenylate cyclase activity by more than 50% (IC50 = 19-32 microM). These findings indicate that (1) long-term cultured endothelial cells from bovine pulmonary artery express A2 and beta-adrenergic receptors which stimulate adenylate cyclase activity through Gs transducer proteins, and (2) the natural compound and P-site agonist, 2'-deoxy-3'-AMP, is a potent inhibitor, and possibly a natural regulator, of adenylate cyclase activity in this tissue.

1-Methyl-3-isobutylxanthine↗
PubMed1988

Diethyl ether effects on muscarinic acetylcholine receptor complexes in rat brainstem.

The influence of diethyl ether on muscarinic acetylcholine receptor-G protein interactions was studied using membranes isolated from rat brainstem. Membranes were equilibrated with diethyl ether (0.5 to 10%) for 20 min before, and then during, the binding assay. The affinity, but not the number, of [3H]N-methylscopolamine [( 3H]MS) binding sites was increased in the presence of diethyl ether (KD in air = 0.41 nM, KD in 2% diethyl ether = 0.21 nM). This increase in affinity reflected a decrease in the rapid dissociation rate constant (air k-1 = 13 X 10(-3) min-1, 2% diethyl ether k-1 = 7 X 10(-4) min-1) rather than a change in the association rate constant. Diethyl ether had no effect on the binding affinity of the muscarinic agonist carbamylcholine. However, the binding of a radiolabeled muscarinic agonist, [3H]oxotremorine-M [( 3H]Oxo-M), to high affinity binding sites decreased about 25% in the presence of 2% diethyl ether. The ability of a guanine nucleotide to depress the high affinity binding of both carbamylcholine and [3H]Oxo-M was decreased or eliminated by diethyl ether. Diethyl ether appears to interfere with muscarinic receptor-G protein interactions, perhaps by stabilizing receptor-G protein complexes or inhibiting the binding of guanine nucleotides.

Animals↗
PubMed1988

Halothane attenuation of muscarinic inhibition of adenylate cyclase in rat heart.

Halothane stimulated basal adenylate cyclase activity in rat cardiac membranes. Maximal stimulation (54%) was obtained after equilibrating the membranes with 2% halothane. Halothane did not affect the fractional stimulation of adenylate cyclase activity produced by either forskolin or isoproterenol. However, halothane decreased carbamylcholine inhibition of adenylate cyclase activity stimulated by both forskolin and isoproterenol. Maximal depression of carbamylcholine inhibition of stimulated cyclase activity was obtained after equilibration with 1% halothane. These results are consistent with evidence from ligand binding studies and indicate that halothane disrupts muscarinic receptor-G-protein interactions.

Adenylyl Cyclase Inhibitors↗
PubMed1988

Temperature effect on the detection of muscarinic receptor-G protein interactions in ligand binding assays.

The ability of guanine nucleotides to lower agonist binding affinity provides a convenient indication of receptor-G protein coupling: guanine nucleotides convert muscarinic receptors from high-affinity states for agonists to low-affinity states. We studied the influence of assay temperature on the demonstration of this coupling in rat brainstem and atrium. Agonist affinity of brainstem receptors increased as temperature was lowered, reflecting a greater proportion of receptors in high-affinity conformations. The influence of 5'-guanylylimidodiphosphate, a stable analog of GTP, on agonist binding, determined directly (using [3H]oxotremorine-M) or indirectly (in [3H]N-methylscopolamine/carbamylcholine competition studies), was greatest from 16 to 20 degrees. Guanine nucleotide sensitivity was much reduced at 0-4 degrees and 37 degrees. Brainstem and atrial muscarinic receptors were similarly affected by temperature. We suggest that high-affinity receptor-G protein complexes are unstable at high temperatures, thereby decreasing agonist affinity and masking the guanine nucleotide effect. At low temperatures, the receptor-G protein complex is stabilized and fails to dissociate in the presence of guanine nucleotides. The optimum temperature for monitoring receptor-G protein interactions in binding assays was 16-20 degrees.

Animals↗
PubMed1987

Effects of halothane on high affinity agonist binding and guanine nucleotide sensitivity of muscarinic acetylcholine receptors from brainstem of rat.

The influence of halothane on muscarinic receptors with a high affinity for agonists was studied using [3H]oxotremorine-M. [3H]Oxotremorine-M bound with high affinity (KD = 2.8 nM) to a subpopulation of muscarinic receptors in the brainstem of rat, representing 32% of the total receptor pool. Agonist affinity for binding sites for [3H]oxotremorine-M was not affected by a guanine nucleotide (5'-guanylylimidodidiphosphate; Gpp(NH)p), although the level of binding was decreased, presumably due to the conversion of receptors to lower affinity conformations. However, only 58% of 3 nM binding of [3H]oxotremorine-M was sensitive to Gpp(NH)p. Halothane had two effects on the binding of [3H]oxotremorine-M: halothane (1) decreased the level of binding of [3H]oxotremorine-M without affecting agonist affinity for the surviving sites, and (2) lowered the sensitivity of the binding of [3H]oxotremorine-M to Gpp(NH)p by a factor of 120. The decrease in binding of [3H]oxotremorine-M binding was nonselective with regard to the sensitivity of the receptors to the guanine nucleotide, insofar as Gpp(NH)p inhibited the binding of [3H]oxotremorine-M to the same extent in the presence and absence of halothane. These results suggest that halothane (1) converts both G protein-coupled and -uncoupled muscarinic receptors to states of lower agonist affinity and (2) lowers the affinity of receptor-G protein complexes for guanine nucleotides.

Animals↗
PubMed1986

Interaction of gephyrotoxin and indolizidine alkaloids with the nicotinic acetylcholine receptor-ion channel complex of Torpedo electroplax.

The interactions of eighteen natural and synthetic gephyrotoxin and indolizidine alkaloids with binding sites on nicotinic acetylcholine receptor channel (AChR) complex from Torpedo californica electric organ were investigated using two radiolabeled probes, [3H]perhydrohistrionicotoxin and [3H]phencyclidine. Both gephyrotoxins and indolizidines were moderately active inhibitors of the binding of these probes (Ki's = 0.1-20 microM), but did not interact with the acetylcholine binding site. Structure-activity relationships indicate an important contribution of hydrophobic interactions to both gephyrotoxin and indolizidine binding. The stereoconfiguration of the alkaloids had little effect on binding. Carbamylcholine enhanced the affinity of certain alkaloids up to 6 to 8-fold suggesting that interactions with open or desensitized conformations of the AChR complex are favored over interactions with resting conformations.

Alkaloids↗
PubMed1986

Allosteric effect of gallamine on muscarinic cholinergic receptor binding: influence of guanine nucleotides and conformational state.

Gallamine interacts with an allosteric site on muscarinic acetylcholine receptor complexes in rat brain membranes, thereby slowing the dissociation of a radiolabelled ligand ([3H]N-methylscopolamine) from the receptor complex. This effect involves the elimination of the fast component of the biphasic dissociation curve. The allosteric effect of gallamine is equally prominent in membranes containing predominantly M1 (cerebral cortex) and M2 (brainstem) subtypes of muscarinic receptor. Gallamine's action is not affected by a variety of treatments which influence the conformational state of the receptor as reflected by agonist binding affinity, including treatments with heat, N-ethylmaleimide and trypsin. A guanine nucleotide (5'-guanylylimidodiphosphate), however, moderates the effects of gallamine on muscarinic receptors in brainstem, but not in cortical, membranes.

Allosteric Regulation↗
PubMed1984

Detection and quantitation of fluorescamine-labeled bradykinin, its analogues and metabolites using high-performance liquid chromatography.

A sensitive technique is described for detecting and quantitating fluorescamine-labeled kinins and their usual metabolic products using reversed-phase high-performance liquid chromatography (HPLC) linked with a fluorescence detector. Kinins and their enzymatic products were labeled with fluorescamine, subjected to HPLC, and scanned for the fluorescence signal with excitation at 390 nm and emission at 476 nm. The fluorescence signal was linear with bradykinin, Lys-bradykinin and Met-Lys-bradykinin in amounts upward from 2.5 ng. Separation of the fluorescamine-labeled kinins using HPLC was carried out with a solvent system of methanol-triethylammonium formate buffer. Labeled kinins were eluted in the following order: bradykinin, Lys-bradykinin, and Met-Lys-bradykinin. When native (unlabeled) kinins were subjected to HPLC using a solvent system of acetonitrile-triethylammonium formate buffer, the minimum amount of native kinin detected at 210 nm was 1 microgram. All three kinins showed linearity at 210 nm in amounts upward from 1 microgram. Kinins were eluted in the following order: Lys-bradykinin, bradykinin and Met-Lys-bradykinin. The different elution patterns of kinins by means of these two separation techniques provide a useful method for identification of purified kinins. The fluorescamine label provides a 400-fold more sensitive detection technique than ultraviolet absorbance of the native kinins and may be used to identify the metabolic products of kinins.

Aminopeptidases↗
PubMed1980

Depressed growth of Morris hepatomas in altitude- and heat-stressed but not in cold-stressed buffalo rats.

Female inbred BUF rats bearing Morris hepatomas 5123C, 5123D, 7795, and 7800 bilaterally in the femoral musculature were exposed for 3 weeks to ether 4,500-m simulated altitude or sea level or to an ambient temperature of either 7, 23, or 33 degrees C. Rats were given inoculations 12 days before these exposures. Tumor size, body weight, food consumption, and body temperatures were measured weekly in these treated rats and in normal rats. At time of killing, tumor mass, DNA synthesis (by [3H]thymidine incorporation), and respiration (by conversion of [1,4-14C]succinic acid to 14CO2) were measured in each of the 4 hepatoma lines, in the livers of normal and host rats, and in regenerated livers 10 days post 70% hepatectomy. Growths of all 4 tumors and regenerated livers were significantly impaired in rats stressed by exposure to altitude and heat but not to cold. Neither DNA synthesis nor respiration was altered in the hepatomas and livers by any environmental stress. The environmentally stressed rats gained weight at a slower rate and consumed less food than did their controls, but no differences were found in these variables for tumor-bearing and non-tumor-bearing rats. However, whereas the ratio of body weight gain to food consumed was reduced under the three stressful environments, that of tumor weight gain to food consumed was not altered by any environment. Host survivorship was not influenced by any of these effects.

Altitude↗