Marie-Cécile Jacobs

Plasma metanephrines in the diagnosis of pheochromocytoma.

Pheochromocytoma is a tumor of chromaffin cells that usually presents as hypertension. The tumor has potentially life-threatening consequences if it is not promptly diagnosed, located, and removed. Evidence of excessive production of catecholamines is essential for diagnosis of the tumor. Traditional tests have relied on measurements of the 24-hour urinary excretion of catecholamines (norepinephrine and epinephrine) or of the products of catecholamine metabolism [1-4]. Because of the common problems of incompleteness and inconvenience associated with 24-hour urine collections, clinicians have long sought a diagnostic test based on sampling of antecubital venous blood. Measurements of plasma catecholamines are useful in this respect [4, 5]. However, patients with a pheochromocytoma can have plasma concentrations of catecholamines that fall within the range of those in patients with essential hypertension [4, 6] (that is, false-negative results). In addition, emotional distress or pathologic conditions other than pheochromocytoma (such as heart failure) can produce abnormally high catecholamine concentrations [7, 8] (that is, false-positive results). Glucagon stimulation and clonidine suppression testing can enhance the accuracy of plasma catecholamine determinations in the diagnosis of pheochromocytoma [9, 10]. These tests, however, can still yield false-negative or false-positive results [9-11]; they also require considerable time and effort. The search has continued for a single simple, highly sensitive and specific blood test with which to confirm the presence of the tumor in patients with pheochromocytoma. We studied the diagnostic accuracy of tests for specific catecholamine metabolites for this purpose, notably the metanephrinesnormetanephrine and metanephrine. An understanding of why plasma metanephrines may be particularly useful for diagnosis of pheochromocytoma requires an understanding of catecholamine metabolism. Norepinephrine and epinephrine are first metabolized intraneuronally by deamination to dihydroxyphenylglycol or extraneuronally by o-methylation to the metanephrines [12]. Because most dihydroxyphenylglycol is formed from norepinephrine leaking from neuronal stores and little is formed from circulating catecholamines [13, 14], plasma levels of this metabolite are relatively insensitive to the release of catecholamines into the circulation from a pheochromocytoma [6, 15]. The formation of most methoxyhydroxyphenylglycol from dihydroxyphenylglycol [14] and the formation of most vanillylmandelic acid from methoxyhydroxyphenylglycol within the liver [16] explains why a test for vanillylmandelic acid is also a poorer marker for pheochromocytoma than other tests [17]. In contrast, preferential metabolism of circulating catecholamines compared with neuronal catecholamines by extraneuronal pathways [14] suggests that the metanephrinesas extraneuronal metabolitesmay provide good markers for release of catecholamines from a pheochromocytoma. Furthermore, substantial production of metanephrines within adrenal tissue [18] suggests that metanephrines may be produced within the tumor itself. In humans, metanephrines are extensively sulfate-conjugated [18, 19]. Assays of metanephrines in urine depend on measurements after deconjugation to free metanephrines [19] so that measurements represent the sum of free and conjugated metabolites (total metanephrines). In contrast, good sensitivity of the assay for plasma metanephrines [20] enables measurements of both free and total metanephrines. We compared the sensitivity, specificity, and positive and negative predictive values of tests for plasma free and total metanephrines with those of tests for plasma catecholamines and urinary total metanephrines. Study participants included a relatively large sample of patients with pheochromocytoma, patients with essential hypertension or secondary hypertension from causes other than pheochromocytoma, and patients with either heart failure or angina pectoris in whom sympathetically mediated catecholamine release would be expected to be increased. Methods Patients Fifty-two patients with a histologically proven pheochromocytoma were studied. Thirty patients were studied retrospectively, and 22 were studied before the final diagnosis was made. The pheochromocytoma was benign in 39 patients and malignant in 13. Sixty-seven healthy, normotensive persons and 51 patients with essential hypertension served as a reference group. Blood samples were obtained from 23 patients with secondary hypertension (12 patients with renal artery stenosis, 2 with kidney disease, 1 with Cushing disease, 1 with primary hyperaldosteronism, and 7 with cyclosporine-induced hypertension) and from 50 patients with either heart failure or angina pectoris. The age, sex, and specialty center where the patients were studied for each of the five groups are shown in Table 1. Except for the few patients who were being treated with phenoxybenzamine, no patients with pheochromocytoma had been receiving medication for at least 2 weeks at the time of blood sampling. No patients with essential hypertension had been receiving medication for at least 2 weeks at the time of blood sampling. Medications taken by the other patient groups included digoxin, calcium channel blockers, diuretics, acetylsalicylic acid, dipyridamole, and cyclosporine. Procedures used in our study were approved by the hospital ethics committee or intramural research board of each of the three centers where patients were studied. Table 1. Patient Characteristics* Blood and Urine Samples All patients refrained from ingesting methylxanthine-containing food products and from smoking after midnight on the day before blood sampling. Blood was collected from an indwelling catheter in an antecubital vein after the patients had rested supine for 20 minutes. In 39 patients with heart failure and 15 with secondary hypertension, arterial blood was obtained through an indwelling arm arterial catheter. Blood samples were collected into precooled tubes containing heparin or EGTA and glutathione and were centrifuged within 30 minutes to separate the plasma, which was stored frozen until assayed. All plasma catecholamine and urinary metanephrine assays were done within 2 weeks of sample collection. Seven of the 52 pheochromocytoma samples were assayed for plasma metanephrines after being stored at 80C for more than 2 years (range, 2 to 8 years), whereas the remaining 45 samples were assayed within 2 years of collection (22 samples within 4 weeks). In 46 of the 52 patients with pheochromocytoma, a 24-hour urine collection was obtained, with 30 mL of 6-M hydrochloric acid used as a preservative. Analytic Methods Plasma metanephrines were assayed at the National Institutes of Health (NIH) using liquid chromatography with electrochemical detection [20]. Concentrations of total metanephrines (the sum of concentrations of free and sulfoconjugated metanephrines) were measured after incubation of 0.25 mL of plasma with 0.1 units of sulfatase (Sigma Chemical Company, St. Louis, Missouri) at 37 C for 30 minutes. The detection limits were 0.013 nmol/L for normetanephrine and 0.019 nmol/L for metanephrine. At a plasma normetanephrine concentration of 0.31 nmol/L and a metanephrine concentration of 0.21 nmol/L, the interassay coefficients of variation were 12.2% for normetanephrine and 11.2% for metanephrine. As previously reported [20], the presence of acetaminophen in samples of plasma can substantially interfere with measurements of plasma normetanephrine concentrations. Therefore, this analgesic must not be used by patients for several days before blood samples are collected. No analytic interference of various other drugs with this assay has been shown [20]. Plasma catecholamines were assayed using liquid chromatography. Electrochemical detection was used for quantification at the NIH [21], and fluorometric detection was used at St. Radboud University Hospital, Nijmegen, the Netherlands [22]. At the NIH, the detection limits were 0.006 nmol/L for norepinephrine and 0.010 nmol/L for epinephrine. At a plasma norepinephrine concentration of 2.4 nmol/L and an epinephrine concentration of 0.39 nmol/L, the interassay coefficients of variation were 6.5% for norepinephrine and 11.4% for epinephrine. At St. Radboud University Hospital, the detection limits for norepinephrine and epinephrine were 0.002 nmol/L and 0.003 nmol/L, respectively. At plasma concentrations of 1.02 nmol/L for norepinephrine and 0.15 nmol/L for epinephrine, interassay coefficients of variation were 8.5% for norepinephrine and 7.2% for epinephrine. Urinary concentrations of metanephrines were measured according to a previously described method [23]; the upper reference limit of the normal range for the 24-hour urinary output of metanephrines was 6.8 mol/d. Data Analysis Because plasma concentrations of catecholamines and metanephrines were not normally distributed, only medians and ranges are presented for these concentrations. Differences in plasma concentrations of metanephrines and catecholamines among patients with pheochromocytoma and other groups were tested using the Kruskal-Wallis test. We assessed relations among variables using the Spearman rank correlation coefficient. Normal distributions of plasma concentrations of catecholamines and metanephrines were obtained after logarithmic transformation of the data. Thus, upper reference limits, defined as the 97.5th percentile, were determined after logarithmic transformation of individual values for the combined data from normotensive persons and those with essential hypertension (118 persons). The 97.5th percentiles were calculated from the antilogarithm of the mean plus 2 standard deviations of the transformed data. A false-negative result of a test for plasma metanephrines in a patient with pheochromocytoma was defined as plasma concentrations of both normetanephrines and metanephrines that were

Adrenomedullary Secretion of Epinephrine Is Increased in Mild Essential Hypertension

To assess whether patients with mild essential hypertension have excessive activities of the sympathoneuronal and adrenomedullary systems, we examined total body and forearm spillovers and norepinephrine and epinephrine clearances in 47 subjects with mild essential hypertension (25 men, 22 women, aged 38.1 +/- 6.7 years) and 43 normotensive subjects (19 men, 24 women, aged 36.5 +/- 5.9 years). The isotope dilution method with infusions of tritiated norepinephrine and epinephrine was used at rest and during sympathetic stimulation by lower body negative pressure at -15 and -40 mm Hg. Hypertensive subjects had a higher arterial plasma epinephrine concentration (0.20 +/- 0.01 nmol.L-1: mean +/- SE) than normotensive subjects (0.15 +/- 0.01) (P < .01). The increased arterial plasma epinephrine levels appeared to be due to a higher total body epinephrine spillover rate in the hypertensive subjects (0.23 +/- 0.02 nmol.min-1.m-2) than the normotensive subjects (0.18 +/- 0.01) (P < .05) and not to a decreased plasma clearance of epinephrine. The arterial plasma norepinephrine level, total body and forearm norepinephrine spillover rates, and plasma norepinephrine clearance were not altered in the hypertensive subjects. The responses of the catecholamine kinetic variables to lower body negative pressure were not consistently different between normotensive and hypertensive individuals. These data indicate that individuals with mild essential hypertension (1) have elevated arterial plasma epinephrine concentrations that are due to an increased total body epinephrine spillover rate, indicating an increased adrenomedullary secretion of epinephrine; (2) have no increased generalized sympathoneuronal activity and no increased forearm norepinephrine spillover; and (3) have similar responses of both the sympathoneuronal and adrenomedullary systems to sympathetic stimulation by lower body negative pressure.

Intravenous instrumentation alters the autonomic state in humans

Intravascular instrumentation may induce syncope or presyncope. It is not known whether asymptomatic subjects also have autonomic reactions, albeit concealed. We addressed this issue by studying 44 healthy young male subjects of various levels of fitness, ranging from inactivity to athletic [mean maximal oxygen uptake was 49.1 (SD 10.7) ml·kg−1·min−1, range 28.7–71.9 ml·kg−1·min−1]. The autonomic response to venous cannulation was quantified by measuring heart rate before cannulation (HR1), after cannulation (HR2), and after complete pharmacological autonomic blockade (HR0 = the intrinsic heart rate). The sympathovagal balance before and after cannulation was computed as HR1/HR0 and HR2/HR0, respectively. The group means of heart rate and sympathovagal balance decreased significantly (paired Students t-test P <0.01) from 62.5 to 59.9 beats·min−, and from 0.71 to 0.68, respectively. The maximal decrease in heart rate was 8.8 beats·min−1, and in the sympathovagal balance was 0.11. Our study demonstrated that the asymptomatic subjects responded to intravenous instrumentation with a concealed autonomic reaction. Thus, from our findings it would seem that intravenous instrumentation interferes with measurements relating to autonomic nervous system activity.

Long term beta-1 adrenergic blockade restores adrenomedullary activity in primary hypertension

In this study we examined the effects of long-term treatment of 19 patients with primary hypertension with the beta 1-adrenoceptor antagonist atenolol on norepinephrine and epinephrine kinetics, at rest and during sympathoadrenal stimulation by lower body negative pressure. Norepinephrine and epinephrine kinetics were measured by using the radioisotope-dilution technique by steady-state infusion of tritiated norepinephrine and epinephrine. The patients were studied before and at the end of 3 months of treatment with atenolol (50 or 100 mg daily). A control group of four normotensive subjects was studied before and after 3 months without any drug treatment. In this group, only arterial blood samples were collected without infusion of the tritiated catecholamines. Atenolol decreased blood pressure and heart rate, but forearm vascular resistance was not affected by atenolol. During atenolol, baseline arterial plasma epinephrine decreased from 0.23 +/- 0.02 to 0.17 +/- 0.01 nM (p < 0.05), and this was accompanied by a decrease in total body epinephrine spillover from 0.50 +/- 0.05 to 0.35 +/- 0.04 nmol/min (p < 0.05). In the control group, arterial plasma epinephrine had not decreased after 3 months. In addition, the increment of arterial plasma epinephrine during lower body negative pressure at -40 mm Hg was attenuated during atenolol. Atenolol had no effect on total body and forearm norepinephrine spillover rates, either at rest or during lower body negative pressure. Clearance rates of epinephrine and norepinephrine were not significantly affected by atenolol. These results suggest that treatment of patients with primary hypertension with the beta 1-adrenoceptor blocker atenolol inhibits the adrenomedullary secretion of epinephrine, but it does not affect the biochemical indices of sympathoneural activity. It remains speculative whether this selective effect of atenolol on epinephrine secretion contributes to its hypotensive action and to its cardioprotective effects in the long term.

Heart rate and heart rate variability during 10- and 30-minute episodes of lower body negative pressure

With the purpose of investigating joint heart rate and heart rate variability changes, the authors studied two groups of young normal subjects with a 10-min (group 1) and 30 min (group 2) lower body negative pressure (LBNP) protocol. In both studies LBNP was applied two times, the first episode at a -15 mmHg and the second episode at a -40 mmHg level. All subjects in the first study completed the protocol. However, the protocol of the second study had to be aborted several times because of presyncopal symptoms during -40 mmHg LBNP. The question of what might have caused these symptoms is addressed.<<ETX>>