The effects of cocaine and metanephrine on the cardiac responses to sympathetic nerve stimulation in dogs.
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The effects of infusions of cocaine (COC) and metanephrine (MET) on the inotropic and chronotropic responses to cardiac sympathetic nerve stimulation were studied in open-chest, anesthetized dogs. COC blocks the neuronal uptake of norepinephrine (NE), whereas MET blocks the extraneuronal uptake. Both blocking agents slightly enhanced the inotropic but not the chronotropic responses. COC prolonged the cardiac responses significantly, particularly the chronotropic responses, whereas MET had no appreciable effect on the durations of these responses. Hence, it appears that the neuronal uptake mechanism plays a major role in the dissipation of neurally released NE in the heart, but that the extra-neuronal uptake mechanism plays only a minor role in its dissipation. In contrast to the results in certain other tissues, the combined effects of COC and MET on the cardiac responses were no greater or more prolonged than the sum of the effects produced by each agent acting alone. (+info)
Urinary metanephrine radioimmunoassay: comparison with the colorimetric assay.
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A radioimmunoassay involving an 125I ligand has been developed and applied to the measurement of urinary metanephrine. To validate the clinical usefulness of this assay, we compared measurement of metanephrine by radioimmunoassay and of total urinary metanephrines by the Pisano colorimetric method. The radioimmunoassay is specific for metanephrine, whereas the colorimetric method measures both metanephrine and normetanephrine. We used both methods to determine urinary metanephrine or total metanephrines in subjects with essential hypertension, pheochromocytoma, the syndrome of multiple endocrine adenomatosis type 2, and normotensive volunteers. The mean and upper limit of normal (3 SD) for metanephrine by radioimmunoassay in our normotensive volunteers was 94.2 microgram/24 h and 229 microgram/24 h, respectively, which compares well with reported values of 87.6 microgram/24 h and 319 microgram/24 h by non-radioimmunoassay methods. Both radioimmunoassay and colorimetry accurately identified five patients with known pheochromocytoma. Good correlation (r = 0.993) was demonstrated between the two assays in a comparison of patients with essential hypertension and pheochromocytoma. We conclude that the radioimmunoassay is at least equivalent to the colorimetric methods in distinguishing pathological and normal catecholamine secretion, and is faster, more precise, and 1000-fold more sensitive. (+info)
Errors in the measurement of agonist potency-ratios produced by uptake processes: a general model applied to beta-adrenoceptor agonists.
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1. The sensitization of guinea-pig atria and trachea to noradrenaline, isoprenaline, and salbutamol, produced by an inhibitor of neuronal (cocaine) and extraneuronal (metanephrine) uptake, was studied quantitatively. The data were compared to a theoretical model. 2. Cocaine produced near maximal sensitization to noradrenaline in guinea-pig atria (5 fold) at concentrations which produced only partial sensitization in guinea-pig trachea (4.7 fold sensitization of a maximum 11 fold). These results agreed with the model which predicts that there is a direct relationship between the amount of uptake inhibitor required to produce full sensitization and the magnitude of maximal sensitization demonstrable in the tissue. This makes extrapolation of uptake inhibition concentrations from tissue to tissue a potentially erroneous practice. 3. In normal trachea, salbutamol is 20 times more potent than noradrenaline but this difference is abolished (to 0.9 times) by cocaine (100 microM). This reduction of potency-ratio is due to the selective cocaine-induced sensitization of trachea to noradrenaline and raises a serious objection to the classification of salbutamol as a beta 2 selective agonist. 4. Metanephrine produced very little sensitization of trachea to isoprenaline. Experiments with salbutamol showed metanephrine to be a simple competitive antagonist of beta-adrenoceptors (pKb = 4.3) and that this receptor antagonism masked sensitization to isoprenaline. 5. A theoretical model indicates that an inhibitor of agonist uptake requires a remarkable degree of selectivity for the uptake mechanism (i.e. Kb for receptors 10(4) x KI for uptake sites) to demonstrate tissue sensitization to the agonist. This analysis and the data with metanephrine indicate that a sinistral shift of the concentration-response curve is a poor indicator of the importance of uptake mechanisms in an isolated tissue. 6. An alternate method to determine the importance of agonist-uptake effects on concentration-response curves is described which utilizes agonist potency ratios. Agonist potency ratios in guinea-pig atria and trachea showed that the bronchoselectivity demonstrated by salbutamol (with respect to isoprenaline) is reduced from 54 to 7.8 by metanephrine reflecting the importance of extraneuronal uptake in trachea. (+info)
Determination of urinary normetanephrine and metanephrine by radial-compression liquid chromatography and electrochemical detection.
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A procedure has been developed for determining the O-methylated catecholamine metabolites, normetanephrine and metanephrine, in urine by use of radial-compression liquid chromatography followed by electrochemical detection. Normetanephrine and metanephrine are isolated from hydrolyzed urine by ion-exchange on small, commercially available, disposable columns and preconcentrated by solvent extraction. They are then separated by reversed-phase ion-pair chromatography, with use of a radial compression cartridge and radial compression module, and quantified with 3-methoxy-4-hydroxybenzylamine as internal standard. Normetanephrine, metanephrine, and the internal standard are separated from interfering peaks in about 15 min. The method is applicable to the relatively low amounts of normetanephrine (100-600 micrograms/24 h) and metanephrine (50-400 micrograms/24 h) found in normal subjects and patients with depressive disorders or hypertension. Within-day CVs ranged from 1.1 to 2.2% for normetanephrine and 1.2 to 6.9% for metanephrine; the corresponding between-day CVs were 4.9 and 5.7% over these ranges. (+info)
Quantitation of urinary normetanephrine and metanephrine by reversed-phase extraction and mass-fragmentographic analysis.
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Hydrolyzed urine with added ring-trideuterated normetanephrine and metanephrine is applied to wet C18-reversed-phase minicolumns. The "metanephrines" are eluted, dried, derivatized with pentafluoropropionic anhydride, and analyzed with the gas chromatograph-mass spectrometer. Ions for the nondeuterated and trideuterated compounds are monitored at ml z 458 and 461, respectively. For both normetanephrine and metanephrine, the standard curve is linear over the range 10-2000 micrograms/L and the procedure has adequate precision both within-run (CV less than 3%) and between-day (CV less than 7%). Alkaline pH in the extraction is important for optimal analytical recovery. We have examined the potential value of untimed urine specimens for screening purposes and compared 24-h urine concentrations of these analyses in normotensive and hypertensive persons. (+info)
Simplified fluorometry of total metanephrines in urine.
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We describe a simplified fluorometric method for quantitatively determining metanephrines in urine. The method is based on the elution of metanephrines from an ion-exchange column followed by production of a fluorescent derivative that has excitation and emission maxima of 405 nm and 505 nm, respectively. Metanephrine concentrations as great as 7.0 mg/L were within the linear range of the assay. Analytical recovery of metanephrines was 82-87%. Cross contamination by catecholamines was insignificant. The upper range of normal was 520 micrograms/24 h. Within-run and between-run CV's were 5% and about 9%, respectively. Twenty-four-hour urine specimens from five patients with pheochromocytoma showed above-normal amounts of the metanephrines by this method. (+info)
Urinary metanephrines as measured by liquid chromatography with an on-line post-column reaction detector.
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In this method for measuring metanephrine and normetanephrine in urine, they are freed from their conjugates by hydrolysis in acid, neutralized, and isolated on a cation-exchange resin. Both metanephrines are co-eluted with ammoniacal methanol and the eluate is concentrated 10-fold with respect to the original urine volume by evaporation and reconstitution. The recovered metanephrines ae separated by reversed-phase (C18) "high-pressure" liquid chromatography with use of a mobile phase consisting of 10 mmol/L trichloroacetate, pH 2.7, containing 30 mL of acetonitrile per liter. The metanephrines are detected at 365 nm after being converted to vanillin in a post-column reaction with alkaline periodate. Total metanephrine values for most patients were appreciably lower than values obtained by the comparison procedure, that of Pisano (Clin. Chim. Acta 5: 406, 1970); however, results obtained by the two methods for two patients with pheochromocytomas agreed well. (+info)
Interference by the 4-hydroxylated metabolite of propranolol with determination of metanephrines by the Pisano method.
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Measurements of urinary metanephrines by the Pisano procedure (Clin. Chim. Acta 5: 406, 1960) are unreliable in patients who are taking propranolol owing to the presence of 4-hydroxypropranolol in the urine. Three properties of this propranolol metabolite lead to interference: (a) it is absorbed and eluted from ion-exchange resins under the conditions Pisano used for metanephrine isolation, (b) at high pH it absorbs at 350 to 360 nm, and (c) it is oxidized by periodate to a substance with negligible absorption in this region. Because 350 to 360 nm is the wavelength used to quantitate vanillin, the product formed from periodate oxidation of metanephrines, and because the unoxidized eluate is used as a specimen blank, the presence of 4-hydroxypropranolol spuriously decreases the measured urinary metanephrines; this has special significance because patients being tested for increased metanephrines are also likely to be receiving propranolol for hypertension. (+info)