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Biomedical subjects

B R Martin

Publications and source records attributed to B R Martin.

At least 343 records · Page 19Linked to original sources
PubMed1971

Metabolism of pyruvate and malate by isolated fat-cell mitochondria.

1. Metabolism of pyruvate and malate by isolated fat-cell mitochondria incubated in the presence of ADP and phosphate has been studied by measuring rates of pyruvate uptake, malate utilization or production, citrate production and oxygen consumption. From these measurements calculations of the flow rates through pyruvate carboxylase, pyruvate dehydrogenase and citrate cycle have been made under various conditions. 2. In the presence of bicarbonate, pyruvate was largely converted into citrate and malate and only about 10% was oxidized by the citrate cycle; citrate and malate outputs were linear after lag periods of 6-9min and 3min respectively, and no other end products of pyruvate metabolism were detected. On the further addition of malate or hydroxymalonate, the lag in the rate of citrate output was less marked but no net malate disappearance was detected. If, however, bicarbonate was omitted then net malate uptake was observed. Addition of butyl malonate was found to greatly inhibit the metabolism of pyruvate to citrate and malate in the presence of bicarbonate. 3. These results are in agreement with earlier conclusions that in adipose tissue acetyl units for fatty acid synthesis are transferred to the cytoplasm as citrate and that this transfer requires malate presumably for counter transport. They also support the view that oxaloacetate for citrate synthesis is preferentially formed from pyruvate through pyruvate carboxylase rather than malate through malate dehydrogenase and that the mitochondrial metabolism of citrate in fat-cells is restricted. The possible consequences of these conclusions are discussed. 4. Studies on the effects of additions of adenine nucleotides to pyruvate metabolism by isolated fat-cell mitochondria are consistent with inhibition of pyruvate carboxylase in the presence of ADP and pyruvate dehydrogenase in the presence of ATP.

Adenosine Diphosphate↗
PubMed1971

Regulation of adipose tissue pyruvate dehydrogenase by insulin and other hormones.

1. In epididymal adipose tissue synthesizing fatty acids from fructose in vitro, addition of insulin led to a moderate increase in fructose uptake, to a considerable increase in the flow of fructose carbon atoms to fatty acid, to a decrease in the steady-state concentration of lactate and pyruvate in the medium, and to net uptake of lactate and pyruvate from the medium. It is concluded that insulin accelerates a step in the span pyruvate-->fatty acid. 2. Mitochondria prepared from fat-cells exposed to insulin put out more citrate than non-insulin-treated controls under conditions where the oxaloacetate moiety of citrate was formed from pyruvate by pyruvate carboxylase and under conditions where it was formed from malate. This suggested that insulin treatment of fat-cells led to persistent activation of pyruvate dehydrogenase. 3. Insulin treatment of epididymal fat-pads in vitro increased the activity of pyruvate dehydrogenase measured in extracts of the tissue even in the absence of added substrate; the activities of pyruvate carboxylase, citrate synthase, glutamate dehydrogenase, acetyl-CoA carboxylase, NADP-malate dehydrogenase and NAD-malate dehydrogenase were not changed by insulin. 4. The effect of insulin on pyruvate dehydrogenase activity was inhibited by adrenaline, adrenocorticotrophic hormone and dibutyryl cyclic AMP (6-N,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate). The effect of insulin was not reproduced by prostaglandin E(1), which like insulin may lower the tissue concentration of cyclic AMP (adenosine 3':5'-cyclic monophosphate) and inhibit lipolysis. 5. Adipose tissue pyruvate dehydrogenase in extracts of mitochondria is almost totally inactivated by incubation with ATP and can then be reactivated by incubation with 10mm-Mg(2+). In this respect its properties are similar to that of pyruvate dehydrogenase from heart and kidney where evidence has been given that inactivation and activation are catalysed by an ATP-dependent kinase and a Mg(2+)-dependent phosphatase. Evidence is given that insulin may act by increasing the proportion of active (dephosphorylated) pyruvate dehydrogenase. 6. Cyclic AMP could not be shown to influence the activity of pyruvate dehydrogenase in mitochondria under various conditions of incubation. 7. These results are discussed in relation to the control of fatty acid synthesis in adipose tissue and the role of cyclic AMP in mediating the effects of insulin on pyruvate dehydrogenase.

Adipose Tissue↗
PubMed1970

The intracellular localization of enzymes in white-adipose-tissue fat-cells and permeability properties of fat-cell mitochondria. Transfer of acetyl units and reducing power between mitochondria and cytoplasm.

1. A method is described for extracting separately mitochondrial and extramitochondrial enzymes from fat-cells prepared by collagenase digestion from rat epididymal fat-pads. The following distribution of enzymes has been observed (with the total activities of the enzymes as units/mg of fat-cell DNA at 25 degrees C given in parenthesis). Exclusively mitochondrial enzymes: glutamate dehydrogenase (1.8), NAD-isocitrate dehydrogenase (0.5), citrate synthase (5.2), pyruvate carboxylase (3.0); exclusively extramitochondrial enzymes: glucose 6-phosphate dehydrogenase (5.8), 6-phosphogluconate dehydrogenase (5.2), NADP-malate dehydrogenase (11.0), ATP-citrate lyase (5.1); enzymes present in both mitochondrial and extramitochondrial compartments: NADP-isocitrate dehydrogenase (3.7), NAD-malate dehydrogenase (330), aconitate hydratase (1.1), carnitine acetyltransferase (0.4), acetyl-CoA synthetase (1.0), aspartate aminotransferase (1.7), alanine aminotransferase (6.1). The mean DNA content of eight preparations of fat-cells was 109mug/g dry weight of cells. 2. Mitochondria showing respiratory control ratios of 3-6 with pyruvate, about 3 with succinate and P/O ratios of approaching 3 and 2 respectively have been isolated from fat-cells. From studies of rates of oxygen uptake and of swelling in iso-osmotic solutions of ammonium salts, it is concluded that fat-cell mitochondria are permeable to the monocarboxylic acids, pyruvate and acetate; that in the presence of phosphate they are permeable to malate and succinate and to a lesser extent oxaloacetate but not fumarate; and that in the presence of both malate and phosphate they are permeable to citrate, isocitrate and 2-oxoglutarate. In addition, isolated fat-cell mitochondria have been found to oxidize acetyl l-carnitine and, slowly, l-glycerol 3-phosphate. 3. It is concluded that the major means of transport of acetyl units into the cytoplasm for fatty acid synthesis is as citrate. Extensive transport as glutamate, 2-oxoglutarate and isocitrate, as acetate and as acetyl l-carnitine appears to be ruled out by the low activities of mitochondrial aconitate hydratase, mitochondrial acetyl-CoA hydrolyase and carnitine acetyltransferase respectively. Pathways whereby oxaloacetate generated in the cytoplasm during fatty acid synthesis by ATP-citrate lyase may be returned to mitochondria for further citrate synthesis are discussed. 4. It is also concluded that fat-cells contain pathways that will allow the excess of reducing power formed in the cytoplasm when adipose tissue is incubated in glucose and insulin to be transferred to mitochondria as l-glycerol 3-phosphate or malate. When adipose tissue is incubated in pyruvate alone, reducing power for fatty acid, l-glycerol 3-phosphate and lactate formation may be transferred to the cytoplasm as citrate and malate.

Acetates↗
PubMedDate not supplied

Functional and dispositional tolerance develops during continuous cocaine exposure.

Despite the fact that high doses of cocaine are abused chronically, relatively little is known regarding the development of tolerance and/or sensitization under these circumstances. Therefore, male Sprague-Dawley rats were infused continuously i.v. for 10 days, at a rate of 150 mg/kg/day (0.1 mg/kg/min) with cocaine hydrochloride. Body weight, food and water consumption, urine and fecal excretion, as well as blood pressure, heart rate and stereotypic behavior were monitored daily. Blood samples were also drawn daily so that plasma could be analyzed for cocaine and BEG by gas chromatography/mass spectrometry. Severe body weight loss on days 1 through 4 was followed by a gradual return to pre-drug levels. In addition, cocaine's effects on food and water consumption and urine and fecal excretion, which were maximal by day 2, were imperceptible by day 5. Complete tolerance developed rapidly to the remarkable rise in blood pressure noted on the first day. However, tolerance did not develop to the cocaine-induced increase in heart rate. A profound decrease in heart rate was noted in some animals which was interpreted to be cardiotoxicity since these animals subsequently died. On the other hand, sensitization or intensification of behavioral stereotypy occurred during the first 2 days followed by complete tolerance to this effect by day 5. No withdrawal phenomena were noted 24 h after cocaine was abruptly withdrawn. Plasma concentrations of cocaine rose rapidly during the first day and remained elevated at a constant level until day 5. Then, a sharp decline in plasma levels occurred at day 6 which remained depressed for the duration of the infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMedDate not supplied

The appearance of endogenous opiates in cerebrospinal fluid following opiate injection in various animal species.

Phenylquinone writhing, tail-flick, stimulated guinea pig ileum and mouse vas deferens were used to quantitate opiate-like activity in CSF taken prior to and at various times after the injection of morphine. The injection of various doses of morphine caused a dose responsive increase in opiate-like material in rabbit CSF taken at the peak of activity (60 min later). Morphine also caused an increase in opiate activity in dog CSF. Quantitation of radioactivity in CSF after the injection of radiolabelled morphine, stability studies and other assays were used to rule out the possibility that the increased activity in CSF was due to morphine itself. CSF taken after morphine injection produced a peak on HPLC with a 15 minute retention time which was not present in CSF taken prior to the injection. This peak was not caused by morphine, met-enkephalin, leu-enkephalin, dynorphin 1-13 or beta-endorphin which have retention times of 5, 7, 9, 22 and 30 minutes, respectively. Preinjection of the rabbit with naloxone blocked the increased opiate-like effects of CSF taken after morphine and the opiate effects of the CSF were antagonized by naloxone injected into mice or the isolated organ bath.

Animals↗
PubMedDate not supplied

Marihuana-like activity of new synthetic tetrahydrocannabinols.

11-Methy-delta8-, 9-nor-delta8, and 9-nor-delta9-tetrahydrocannabinol (THC), newly synthesized cannabinoids which are not 11-hydroxyated in vivo, were tested for cannabinoid activity. Delta8-, delta9-THC and each synthetic analog produced static ataxia in unanesthetized dogs, hypotension and bradycardia in anesthetized dogs, and decreased spontaneous activity in mice. All synthetic analogs tested produced a greater degree of tolerance to the behavioral effect in dogs than did delta8-THC. 11-Methyl-delta8-THC was more effective than delta8-THC in decreasing spontaneous activity in mice, but was less active in producing the behavioral and cardiovascular effects in dogs. 9-nor-delta9-THC was less active than delta9-TCH, but 9-nor-delta8-THC was as active as delta8-THC in all observations. These results suggest that the 11-hydroxy metabolites of delta8- and delta 9-THC are not solely responsible for the biological activity of tetrahydrocannabinol.

Analgesics↗
PubMedDate not supplied

Does chronic prenatal methadone exposure affect beta-receptor subtypes in placental, fetal and maternal brain homogenates?

Computer competition analysis of 3H-DHA (3H-dihydroalprenolol, a nonselective beta-adrenergic radioligand) binding in the presence of unlabeled metoprolol (a beta 1-selective antagonist) indicates the existence of both beta 1- and beta 2-adrenergic receptor subtypes in the rat placenta and confirms previous reports that both beta-adrenoceptors are present in adult rat cortex. In the fetal brain (20th day of gestation), however, only beta 1-receptors were detected. Pregnant rats were chronically exposed to methadone from day 7 to day 20 of gestation via implanted osmotic minipumps (6.3-9.0 mg/kg/day). This treatment schedule did not induce a change in the affinity and density of either beta-receptor subtype in the placental, fetal and maternal brain homogenates. The results are discussed in terms of the reported monoaminergic and opiate receptor functional interactions.

Animals↗
PubMedDate not supplied

Demonstration of physical dependence following chronic continuous methadone delivery via osmotic minipumps in pregnant rats.

The effects of a 14-day (gestation days 7-20) chronic methadone (6.3-9.0 mg/kg/day) infusion via osmotic minipumps were studied on the induction of physical dependence in both pregnant and nonpregnant female rats. Following continued methadone exposure, an acute injection of naloxone (2.0 mg/kg, SC) produced the following symptoms of withdrawal in both pregnant and nonpregnant methadone-exposed rats: increased frequency of head shakes, teeth-chattering and face-rubbing episodes, as well as the induction of burrowing, diarrhea, facial tremor, squeaking and vaginal sniffing. Increased fetal movement in the maternal abdomen was also observed in the pregnant rats. In the saline-exposed pregnant controls, naloxone failed to induce a significant effect. In addition, brain and plasma methadone levels during the various stages of pregnancy (gestation days 8-20) were determined. The methadone levels in plasma were initially variable (gestation days 8-12) but became more constant (approximately 50 ng/ml) from gestation day 14 to 20. Methadone brain levels also followed a similar pattern, except that the brain methadone content was at least 20-fold greater than plasma concentrations at any given time. Thus, relative to the high brain levels, the present data suggest that acute changes in methadone plasma concentration may not be a good index of pharmacological effect.

Animals↗
PubMedDate not supplied

Chronic prenatal methadone exposure alters central opioid mu-receptor affinity in both fetal and maternal brain.

The effects of chronic prenatal methadone exposure (6.3-9.0 mg/kg/day) via osmotic minipumps to pregnant dams on fetal and maternal brain opioid mu-receptors were assessed on gestation day 20 and day 7 postnatally. By using the 3H-DAMGO binding assay, it was shown that chronic methadone treatment (gestation days 7-20) did not affect mu-receptor capacity in both fetal and maternal brains during gestation day 20, nor when tested 7 days after delivery. However, this chronic exposure decreased mu-receptor affinity in both fetal and maternal brain homogenates when determined on day 20 of pregnancy. Scatchard analysis of binding data in both tissues indicated that the methadone-induced increase in KD returned to control values when tested 7 days after delivery. The change in mu-receptor affinity was not due to competition between 3H-DAMGO and residual methadone. Extensive washing of the brain homogenates failed to alter the affinity of the receptor but decreased the concentration of the residual methadone. This decrease in receptor affinity was also observed in extensively washed brain tissue from female adult rats treated acutely with methadone (9.0 mg/kg, IP) or when brain homogenates were exposed to methadone (50 ng/ml) in vitro. Thus, these data suggest that methadone alters mu-receptor affinity by some unknown mechanism.

Animals↗
PubMedDate not supplied

Precipitated cannabinoid withdrawal is reversed by Delta(9)-tetrahydrocannabinol or clonidine.

The availability of the cannabinoid antagonist, SR 141716A, to precipitate withdrawal following repeated cannabinoid administration provides a model to investigate the mechanisms underlying cannabinoid dependence as well as potential treatments to alleviate withdrawal symptoms. The goal of the present study was to determine whether SR 141716A-precipitated withdrawal symptoms in Delta(9)-tetrahydrocannabinol (Delta(9)-THC)-tolerant mice could be alleviated by either readministration of Delta(9)-THC or clonidine, an alpha(2)-receptor agonist. SR 141716A elicited paw tremors in Delta(9)-THC-tolerant mice, but produced a significant increase in head shakes independently of repeated Delta(9)-THC treatment. Readministration of Delta(9)-THC, following SR 141716A-precipitated withdrawal, reversed paw tremors (ED(50)=9.9 mg/kg), but failed to reduce head shaking behavior. Clonidine reversed SR 141716A-precipitated paw tremors (ED(50)=0.18 mg/kg) and blocked head shakes at all doses tested. The reversal effects did not appear to be the result of motor impairment because neither decreases in spontaneous locomotor activity nor motor incoordination, as assessed in the inverted screen test, could account for the effects. These findings suggest that SR 141716A precipitates paw tremors in mice by competing with Delta(9)-THC at the CB(1) receptor, though it also produced head shaking in nondependent animals. Finally, the observation that clonidine alleviated SR 141716A-precipitated paw tremors suggests its potential as a treatment for cannabinoid dependence.

Adrenergic alpha-Agonists↗