Liquid chromatography with electrochemical detection for plasma norepinephrine and epinephrine

Determination of Catecholamines in a Small Volume (25 μL) of Plasma from Conscious Mouse Tail Vein

The determination of plasma catecholamine levels is commonly used as a measure of the sympathetic nervous system's response to stress and is highly important for diagnosis, therapy, and prognosis of cardiovascular diseases, catecholamine-secreting tumors arising from the chromaffin cells of the sympathoadrenal system, and affective disorders. Diseases in which catecholamines are significantly elevated include pheochromocytoma, Parkinson's disease, Alzheimer's disease, neuroblastoma, ganglioneuroblastoma, von Hippel-Lindau disease, baroreflex failure, chemodectina (nonchromaffin paraganglioma), and multiple endocrine neoplasia. Plasma norepinephrine levels provide a guide to prognosis in patients with stable, chronic, and congestive heart diseases. The method described here for the determination of plasma catecholamines is based on the principle that plasma catecholamines are selectively adsorbed on acid-washed alumina at pH 8.7 and then eluted at a pH between 1.0 and 2.0. Upon injection, catecholamines in elutes were separated by a reversed phase C-18 column. After separation, the catecholamines present within the mobile phase enter the electrochemical detector. Electrochemical detection occurs because electroactive compounds oxidize at a certain potential and thereby liberate electrons that create measurable current. Catecholamines readily form quinones under these conditions, get oxidized, release two electrons, and create current. The electrochemical detector detects this electrical current that linearly correlates to the catecholamine concentration loaded into the ultra-performance liquid chromatography instrument. A 15-min mixing time during the adsorption and desorption steps was found to be optimal. If the washing step was omitted, the catecholamines could not be eluted from the acid-washed alumina. To prevent dilution, the alumina had to be centrifuged and not aspirated to dryness after the washing step. We report here that by changing the range in the electrochemical detector, plasma catecholamines were measured with only 12.5 μL of plasma and more reliably with 25 μL of plasma. The detection limit was 1 ng/mL. This assay method is very useful as blood can be collected from the tail vein in a conscious mouse and the same mouse can be used for time-dependent or age-dependent studies.

The Endocrine Stress Response after Oral Premedication with Low-Dose Midazolam for Intraocular Surgery in Retrobulbar Anaesthesia

Purpose

It was the purpose of this study to examine the endocrine stess response in patients undergoing elective intraocular surgery in retrobulbar anaesthesia (RBA), and the effect of an oral premedication with 3.75 mg midazolam on this reaction.

Methods

In a prospective double-blind study, 23 patients were randomly assigned to either group I (n=12), receiving 3.75 mg midazolam, or group II (n=11), receiving a matched placebo 60 min before RBA. Plasma concentrations of known mediators of the endocrinological stress response were analysed at five perioperative times: adrenocorticotropic hormone (ACTH), cortisole (CORT), somatotropic hormone (STH), epinephrine (EPI), norepinephrine (NOR), and dopamine (DOP).

Results

The observed endocrinological reaction was moderate. Perioperative anxiety and, above all, RBA were the main stressors. The endocrine stress response could best be seen within the fast-reacting sympathoadrenergic system. Midazolam significantly suppressed the epinephrine and the cortisole answer (p < 0.01). After RBA an ACTH plasma peak only was observed after placebo. Plasma-STH showed extreme variability. An STH-peak after midazolam is presumably due to a gonadotropic hormone-releasing hormone (GHRH) secretion directly stimulated by midazolam. Midazolam had no significant effect upon norepinephrine. A significant (p<0.01) increase for plasma-cortisole was seen 60 min after the operation when compared to preoperative values.

Conclusions

The results indicate that even at a low dose midazolam positively influences the endocrine stress response. The combination of this stress-reducing effect with the well-known hemodynamic stability, the distinct anxiolytic and sedative action, suggest low-dose midazolam as a favourable premedication for intraocular surgery.

Endoluminal pelvic perfusion with norepinephrine causes only minor systemic effects and diminishes the increase in pelvic pressure caused by perfusion

OBJECTIVE

To evaluate the effect of endoluminal norepinephrine (NE) on transport pressures of the normal upper urinary tract of the pig and on plasma levels of NE in relation to possible systemic effects.

MATERIAL AND METHODS

Six anaesthetized pigs weighing approximately 39 kg were studied. Transparenchymally, two 6-F catheters were introduced into the renal pelvis bilaterally to measure pressure and perfusion. Ultrasonic flow probes recorded renal arterial blood flow, and a transurethral 10-F catheter drained the bladder and monitored diuresis. In all six animals, the bilateral pelvic pressure response was examined at increasing perfusion rates (2, 4, 6, 8, 10 and 15 ml/min) and with increasing doses of NE (0, 5, 50 and 100 microg/ml). Arterial blood samples were analysed for NE, epinephrine and blood glucose. The systemic blood pressure, heart rate and electrocardiogram were registered.

RESULTS

At all the investigated concentrations, endoluminal NE significantly diminished the increase in pelvic pressure caused by pelvic perfusion at all flow rates. At the lowest concentration of NE, no significant increase in the plasma level of NE was observed and the blood pressure did not increase. During perfusion with 50 and 100 microg/ml NE, plasma levels of NE increased significantly from 487+/-398 to 1798+/-910 and 2961+/-2093 pg/ml, respectively. This was accompanied by significant rises in mean systolic blood pressure from a baseline value of 95+/-10 mmHg to 111+/-20 and 118+/-23 mmHg, respectively. Heart rate, renal arterial blood flow and plasma levels of epinephrine and glucose did not change.

CONCLUSIONS

Endoluminal NE diminished the increase in pelvic pressure caused by pelvic perfusion even at concentrations too low to cause significant changes in NE plasma levels or systemic effects. Very high NE concentrations in the perfusion fluid caused increased plasma levels and a modest but significant increase in blood pressure. Administration of endoluminal NE may be useful in upper urinary tract stone treatment and endoscopy.

Persistent Hypotension Associated with Hypermedullipinemia: A New Syndrome

A new syndrome is described in a patient with advanced renal insufficiency. This consists of severe and persistent hypotension causing weakness but associated with a clear mental status. Also present is evidence for decreased vascular reactivity. The hypotension was not orthostatic. The hypotension was associated with a circulating vasodepressor substance having the characteristics of medullipin 1. The medullipin appears to have been derived from the remaining right kidney. Hypotension existed despite the presence of major prohypertensive mechanisms, including an endstage kidney, hyperreninemia and hyperaldosteronemia. It is likely that hypotension due to hypermedullipinemia is an entity occurring in the human being.

The cardiovascular effects of naloxone administration after fentanyl anesthesia in hypercapnic patients

Hemodynamic changes and plasma catecholamine levels after naloxone administration were studied in seventeen postoperative patients who received nitrous oxide, oxygen, and fentanyl anesthesia combined with epidural block. Group I consisted of ten postoperative hypercapnic (PaCO2 = 55.2 +/- 2.4 torr) and group II seven postoperative normocapnic patients (PaCO2 = 38.4 +/- 2.1 torr), respectively. In group I, naloxone reversal resulted in significant increases in heart rate (13.5%), mean arterial pressure (46.6%), systemic vascular resistance (32.1%), and rate pressure product (68.8%), whereas mean pulmonary artery pressure and pulmonary vascular resistance were significantly decreased. No significant hemodynamic changes after naloxone administration were observed in group II. There were no significant differences in arterial norepinephrine and epinephrine levels either before or after naloxone administration in the both groups. This study indicates that the postoperative hypercapnia elicits the cardiovascular stimulation after fentanyl reversal by naloxone.

Effect of Restraint Stress on the Progression of Experimental Periodontitis in Rats

BACKGROUND

A hypothesis of an increased risk for destructive periodontal diseases due to psychological stress has long been promoted. However, the research on stress and periodontal disease is still in its infancy. One of the reasons is thought to be that there is no suitable animal model for investigating the relationship.

METHODS

One hundred male Wistar rats were included. A nylon ligature was placed around the second right maxillary molars. The animals were then divided into group S, exposed to a restraint stress for 12 hours/day for up to 10 days, and group N, controls. Ten animals were sacrificed on days 2, 4, 6, 8, and 10. Blood samples were taken, and the blood glucose level and the concentrations of adrenocorticotropic hormone, corticosterone, and adrenaline were measured as the markers of stress. The atrophies of the thymus and the spleen were measured. The furcation area of the second maxillary molars was examined histologically and histometrically.

RESULTS

In group S, all values of stress markers were increased, and the thymus and the spleen were atrophied. Whereas group N showed only slight alveolar bone resorption, a marked alveolar bone resorption occurred in group S between days 8 and 10. An increase in beaded nerve terminals occurred around the vessels in the furcation area of group S.

CONCLUSION

The results of the present study suggest that the restraint stress modulates the progression of periodontal inflammation and that this rat model is suitable for investigating the association between stress and periodontal disease.

The roles of orthostatic hypotension, orthostatic tachycardia, and subnormal erythrocyte volume in the pathogenesis of the chronic fatigue syndrome

BACKGROUND

Orthostatic hypotension during upright tilt is an important physical disorder in patients with chronic fatigue syndrome. We have tested its occurrence during prolonged standing, whether it is correctable, and whether reduced circulating erythrocyte volume is present.

METHODS

Fifteen patients were randomly selected from a large population of patients with chronic fatigue syndrome, studied, and observed for several years (by DSB). Blood pressure (BP) and heart rate (HR) measured with Dinamap every minute for 30 minutes supine and 60 minutes standing were compared with these findings in 15 healthy age- and gender-matched control subjects and later during lower body compression with military antishock trousers (MAST). Plasma catecholamines and circulating erythrocyte and plasma volumes were also measured by isotopic dilution methods.

RESULTS

Abnormal findings in the patients included excessive orthostatic reductions in systolic (P < 0.001) and diastolic BP (P < 0.001) and excessive orthostatic tachycardia (P < 0.01), together with presyncopal symptoms in 11 of the 15 patients and in none of the control subjects after standing for 60 min. Lower body compression with the MAST restored all orthostatic measurements to normal and overcame presyncopal symptoms within 10 min. Circulating erythrocyte but not plasma volumes were subnormal in the 12 women (P < 0.01) and plasma norepinephrine concentration rose excessively after standing for 10 min.

CONCLUSION

Delayed orthostatic hypotension and/or tachycardia caused by excessive gravitational venous pooling, which is correctable with external lower-body compression, together with subnormal circulating erythrocyte volume, are very frequent, although not invariably demonstrable, findings in moderate to severe chronic fatigue syndrome. When present, they may be involved in its pathogenesis.

Studies of the neuronal transdifferentiation process in cultured human pheochromocytoma cells: effects of steroids with differing functional groups on catecholamine content and cell morphology

The neuronal differentiation of adrenal pheochromocytoma cells from human subjects was studied in vitro for periods of up to 65 days. Changes with time in culture were observed in both intracellular catecholamine content (progressive decreases in epinephrine, norepinephrine, and dopamine, except for a possible transient early increase in the latter) and in morphology (increases in neurite outgrowth) of cells cultured in control medium; supplementation of cultures with nerve growth factor resulted in a substantial increase in neurite formation. The effects on these changes of the presence in the culture medium of various steroids were examined. The addition of 11-oxygenated steroids (aldosterone, corticosterone, cortisol, or dexamethasone) at 10(-5) M concentrations caused at least 2.5-fold increases in mean intracellular dopamine and norepinephrine levels; with dexamethasone, 9-10-fold increases were observed. Intracellular epinephrine content was also enhanced by 11,17-oxygenated steroids (dexamethasone and cortisol), but not by the other 11-oxygenated compounds studied. These two 11,17-oxygenated glucocorticoids also inhibited the morphologic changes seen with extended periods in culture, decreasing the outgrowth of neurite projections and causing cells to attain a vacuolated and granular appearance; the presence of dexamethasone strongly inhibited the morphologic changes induced by nerve growth factor. 11-Deoxy steroid intermediates (pregnenolone, 11-deoxycorticosterone, and 11-deoxycortisol) had little or no effect on catecholamine content or on morphology. Preliminary observations suggest that C-18 and C-19 sex steroid hormones (17 beta-estradiol and testosterone) may have morphologic effects opposite to those of the 11-oxygenated compounds, showing a slight stimulatory influence on the formation of neurite projections, but no significant effect on catecholamine content.

The role of delayed orthostatic hypotension in the pathogenesis of chronic fatigue

Past studies have shown that severe fatigue was the presenting symptom in six of seven patients with delayed orthostatic hypotension and that tilt table-induced hypotension was found in 22 of 23 patients with the chronic fatigue syndrome. We have determined the prevalence of fatigue, volunteered in response to a nonspecific pre-examination questionnaire used in 431 patients, each subsequently diagnosed as having one of eight neurological or endocrine disorders. The results show that fatigue is a very common symptom in patients with delayed orthostatic hypotension (n = 21), as well as both primary (n = 30) and secondary (n = 106) hypocortisolism: 70-83% in all groups. In contrast, fatigue was an uncommon complaint in patients with multiple system atrophy (MSA) (n = 30), pituitary disorders without hypocortisolism (n = 106) or idiopathic hirsutism (n = 96): 7-33% in all groups, and was intermediate in prevalence in patients with acute hyperadrenergic orthostatic hypotension (n = 32): 41%. It is concluded that fatigue commonly results from delayed orthostatic hypotension and all forms of hypocortisolism but is less common in patients with acute orthostatic hypotension, both idiopathic and due to MSA, which more commonly present with lightheadedness or syncope.

Predictors of natural killer cell-mediated cytotoxicity deficiency in advanced heart failure secondary to either ischemic or idiopathic dilated cardiomyopathy

The effect of psychologic variables (situational emotional state and psychiatric diagnosis) and physiologic variables (plasma norepinephrine, decreased cardiac exercise capacity, and elevated pulmonary capillary wedge pressure) on natural killer cell activity was evaluated in 19 patients with advanced heart failure of ischemic or idiopathic origin. Only peak exercise capacity was independently predictive of natural killer cell deficiency.

Tyrosine hydroxylase gene expression in varying forms of human pheochromocytoma

We performed a comparative study of catecholamine content, tyrosine hydroxylase (TH) activity, and TH mRNA levels in normal human adrenals and various clinical forms of human pheochromocytoma. We studied sporadic, benign intra-adrenal chromaffin tumors and other non-malignant intra-adrenal tumors associated with multiple endocrine neoplasia type 2B (MEN 2B) and von Hippel-Lindau disease along with one extra-adrenal malignant pheochromocytoma. Our findings suggest substantial differences in TH transcriptional rates or the stability of TH mRNA or both may contribute to altered TH expression in human chromaffin cells associated with "normal" adrenal tissues and various forms of pheochromocytoma and distinctive patterns of expression in the different settings in which these tumors arise.

Relationship between tissue ischemia and venous endothelin-1 during abdominal aortic aneurysm surgery

OBJECTIVES

Several substances may be released from ischemic tissues with the declamping shock that occurs during abdominal aortic aneurysm surgery. To clarify the relationship between tissue ischemia and venous endothelin-1 (ET-1) level in humans, plasma ET-1 and oxygen content in the iliac vein were measured before anesthesia, after the induction of anesthesia, after the release of the proximal and first distal clamps, after the release of the second distal clamp, and 1 hour after the second clamping.

DESIGN

Prospective study.

SETTING

A University hospital.

PARTICIPANTS

Seven patients who underwent abdominal aneurysmectomy and replacement with bifurcated graft.

INTERVENTIONS

A 20G catheter was inserted into the radial artery for the direct measurement of blood pressure and for collecting arterial blood. An 18G, 20-cm catheter was inserted into the femoral vein for collecting venous blood from the lower extremities.

MEASUREMENTS AND MAIN RESULTS

The arterial and venous ET-1 levels did not change after the induction of anesthesia. Immediately after the initial release of the proximal and distal clamps, venous oxygen content dramatically decreased from 11.3 to 3.6 mL/dL (vol%) with significant increases in venous ET-1 concentration from 2.3 to 4.9 pg/mL. Concomitant with the decrease in venous oxygen content, venous plasma pH and base excess decreased with increase in PCO2, suggesting that ischemic changes in tissues distal to the cross-clamp may occur during aortic clamping. Venous ET-1 levels were significantly correlated with venous oxygen content, pH, PO2, oxygen saturation, base excess, blood sodium concentration, and potassium concentration. One hour after the second declamping, the venous ET-1 level remained high in comparison with the preanesthetic level, whereas the venous oxygen content returned to the preanesthetic level. There was no correlation between venous plasma ET-1 and venous plasma norepinephrine or epinephrine concentration.

CONCLUSIONS

Tissue ischemia may increase venous ET-1 levels in humans. Factor(s) other than tissue ischemia may provoke the increase in venous ET-1 that occurs after the release of the second distal clamp.

Treatment of orthostatic hypotension with erythropoietin

BACKGROUND AND METHODS

Patients with orthostatic hypotension caused by autonomic neuropathy frequently have a decreased red-cell mass. This would be expected to compromise their effective circulating blood volume and aggravate the orthostatic hypotension. We studied the effect of increasing the red-cell mass with erythropoietin, given subcutaneously in a dose of 50 U per kilogram of body weight three times a week for 6 to 10 weeks to eight patients with orthostatic hypotension--four men, one teenage boy, and three women (age range, 17 to 68 years). Four patients had type I diabetes mellitus and autonomic neuropathy, three patients had pure autonomic failure, and one patient had sympathotonic orthostatic hypotension. Seven patients received fludrocortisone (0.1 or 0.2 mg per day) before, during, and after the trial of erythropoietin. The red-cell volume, plasma volume, and hemodynamic response to orthostatic stress were measured before and after therapy.

RESULTS

Erythropoietin increased the mean (+/- SD) hematocrit from 0.34 +/- 0.04 to 0.45 +/- 0.04 (P < 0.005) and increased the red-cell volume from 16.8 +/- 3.9 to 25.3 +/- 3.1 ml per kilogram (P < 0.005), but had no effect on plasma volume. The systolic blood pressure increased from 81 +/- 11 to 100 +/- 24 mm Hg (P < 0.01) and the diastolic blood pressure increased from 46 +/- 10 to 63 +/- 18 mm Hg (P < 0.01) while the patients were standing. The average systolic and diastolic blood pressure while the patients were supine did not increase significantly, although hypertension in the supine position developed in three patients. Orthostatic dizziness improved during treatment in six of the eight patients.

CONCLUSIONS

In patients with orthostatic hypotension, increasing the red-cell volume with erythropoietin elevates blood pressure while standing. Possible long-term adverse effects are not known.

Effects of dexmedetomidine on systemic and coronary hemodynamics in the anesthetized dog

In addition to central effects, which are the basis of their use in anesthesiology, alpha 2-adrenergic agonists have direct peripheral cardiovascular effects. Dexmedetomidine (DM) has been found to depress cardiac function in dogs, even after autonomic denervation. The present experiments evaluated the effects of DM on coronary flow, myocardial oxygen extraction, and cardiac function in intact, open chest dogs under enflurane anesthesia. Heart rate (HR), mean arterial pressure (MAP), left ventricular end-diastolic pressure (LVEDP), the first derivative of systolic left ventricular pressure (dP/dtmax), and flow in the left anterior descending coronary artery (CBF) were measured and continuously recorded. Cardiac output (CO), plasma catecholamines (CA), hemoglobin and oxygen saturation in arterial, mixed venous, and coronary sinus blood were measured at intervals. Cardiac index (CI), systemic vascular resistance index (SVRI), regional coronary vascular resistance (CVR), and oxygen concentration differences across the systemic [C(a-v)O2], and coronary [C(a-cs)O2] circulations were calculated. DM doses of 0.25, 0.5, 1.0, 2.0, and 4.0 micrograms/kg were given IV at 20-minute intervals. Measurements and samples were taken at peak drug effects and just prior to the next dose. The alpha 2-antagonist atipamezole, 0.5 mg/kg, was given after the last dose of DM. DM caused immediate dose-dependent increases in SVRI, CVR, LVEDP, C(a-v)O2, and C(a-cs)O2, and decreases in HR, and CI, with recovery between doses. DP/dtmax declined after the first two doses and stabilized thereafter, as plasma CA fell to minimal levels. Atipamezole completely reversed all changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dexmedetomidine, an alpha 2-adrenergic agonist, in the isolated heart

Dexmedetomidine (DM) was studied in the isolated dog heart in the form of a Starling heart-lung preparation, (HLP). Hearts were subjected to increased loading by (a) increasing cardiac output, and (b) increasing systemic resistance. Results are depicted by cardiac function curves, prepared by plotting left atrial pressure against either systemic cardiac output or mean arterial pressure. DM, given in divided doses up to 44 micrograms, had no effect on heart rate or cardiac function, nor did injection of 0.5 mg of atipamezole, a selective alpha 2-antagonist. Additional injections of very large doses of DM, up to 4,444 micrograms, caused an increase in heart rate and a leftward shift of the function curves, ie, positive chronotropic and inotropic effects. Plasma catecholamine levels increased markedly between the 444 micrograms and the 4,444 micrograms cumulative doses of DM. Administration of 1 mg of prazosin had no effect, but 1 mg of propranolol returned the rate to baseline and markedly shifted function curves to the right and depressed their slopes. Thus, whereas low doses (corresponding to between 1 and 30 micrograms/kg in intact animals) of DM, given acutely IV, have been shown to depress cardiac function in intact and denervated dogs, this effect is not due to a direct effect on the myocardium. High doses, far beyond doses maximally effective in intact animals and man, release catecholamines from cardiac stores. Plasma DM levels after low doses in the HLP were between 1 to 10 times those seen in intact animals and human volunteers after the usual doses given clinically for their central effects. Because DM caused no myocardial depressant effect in the isolated, blood-perfused canine HLP, decreases in cardiac function seen after this drug is given to intact and autonomically denervated dogs must be due to factor(s) other than a direct action on the myocardium.

Effects of dorsal root entry zone lesions on CSF and plasma neuropeptides and catecholamines

Effects of dorsal root entry zone lesions (DREZLs) on cerebrospinal fluid (CSF) and plasma concentrations of neuropeptides, catecholamines, and cyclic nucleotides were studied in 9 patients with intractable chronic pain. Contents of beta-endorphin-like-material in CSF decreased in all patients 12-17 days following DREZLs during which complete to good pain relief was achieved. Contents of beta-endorphin-like-material in CSF increased again about one month after DREZLs in two and remained unchanged in one of three patients tested, who complained of partial reappearance of pain. Contents of beta-endorphin-like-materials in plasma showed no significant changes after DREZLs. Substance P, noradrenaline, adrenaline, and cyclic nucleotide levels in both CSF and plasma were variable among the subjects and did not change significantly following the operations. Thus, the results suggest that production of beta-endorphin-like-material in the central nervous system is decreased by DREZL, though the increase in its turn-over might not be neglected. The mechanisms of the decrease in contents of beta-endorphin-like-material in CSF after DREZLs were discussed in terms of our current knowledge of pain and pain inhibitory systems.

Measurement of plasma catecholamines by high-performance liquid chromatography with electrochemical detection in intensive care patients after dobutamine infusion

A procedure for the determination of plasma catecholamine concentrations in critical care patients after dobutamine infusion is presented. A modified chromatographic system is required with an additional washing procedure to achieve maximum sensitivity and stable chromatographic conditions. The influence of storage time on the catecholamine concentrations of plasma samples is reported in detail. A time-dependent decrease in catecholamine concentrations of up to 12 and 39% was found within two and ten months, respectively.

Evaluation of a simple plasma catecholamine extraction procedure prior to high-performance liquid chromatography and electrochemical detection

The modified extraction method for catecholamines described in this study is reproducible, simple, rapid, economical and relatively hazard-free. This method is based on the principle that plasma catecholamines are selectively adsorbed on acid-washed alumina at pH 8.6 and then eluted at a pH between 1.0 and 2.0. No statistically significant differences were obtained by using either 0.5 or 1.0 ml of plasma with 0.5 or 1.0 ml of Tris buffer. A 15-min mixing time during the adsorption and desorption steps was found to be practical, but any standardized time up to 1 h can be used. If the washing step was omitted, the catecholamines could not be eluted from the acid-washed alumina. To prevent dilution, the alumina had to be centrifuged and not aspirated to dryness after the washing step. An amount of 50 mg of WA-4 alumina was found to be the most practical in this study. Extracted or unextracted plasma as well as catecholamine standards were stable for four months at -20 degrees C.

Pathogenesis of hyperadrenergic orthostatic hypotension. Evidence of disordered venous innervation exclusively in the lower limbs

The pathogenesis of hyperadrenergic orthostatic hypotension was studied in eight patients. Correction of the abnormal orthostatic changes by an inflated pressure suit (MAST) confirmed previous evidence of excessive gravitational pooling of blood in the leg veins. Intravenous L-norepinephrine infusion raised diastolic blood pressure in the same relationship to the infusion-induced increments in plasma norepinephrine concentrations as in normal subjects, indicating normal arteriolar responses. Contractile responses of the veins to infused L-norepinephrine were measured with a linear variable differential transformer (LVDT). The venous responses of hand veins in the patients fell within the 95% confidence limits of the responses of normal hand veins, as did the responses of foot veins in the seven normal subjects. However, foot veins of the patients with hyperadrenergic orthostatic hypotension, and both hand and foot veins of patients with "diffuse" autonomic failure, were supersensitive to norepinephrine, as reflected by a steeper slope of the regression of log (norepinephrine infusion rate) on percentage reduction in venous distensibility, and a significantly lower ED50 (i.e., norepinephrine infusion rate that induced 50% reduction in venous distensibility). The findings suggest anatomical or functional postganglionic denervation of lower limb veins causing excessive gravitational blood pooling with consequent orthostatic hypotension in these patients.

Effect of blockade of noradrenaline re-uptake on evoked tritium overflow from mouse vasa deferentia and rat cortex slices

1. In tissues previously incubated with [3H]-noradrenaline exposure to cocaine (0.1 to 10 microM) or desmethylimipramine (0.01 to 1 microM) produced a concentration-dependent increase (up to 2 fold) in electrically evoked (3 Hz, 2 ms, 20 mA, 120s every 20 min) fractional overflow of tritium from rat brain cortex slices but not from mouse vas deferens (2.5 Hz, 2 ms, 400 mA, for 90s every 14 min). 2. Yohimbine and idazoxan (0.01 to 1 microM) increased fractional evoked overflow of tritium by up to 2 fold; in the presence of these drugs, cocaine (10 microM) produced an increase in both tissues (up to 3.5 fold over control). 3. In brain slice an increase in stimulation frequency (0.1, 0.5, 1, 3 and 6 Hz) decreased fractional evoked overflow of tritium per pulse but cocaine (10 microM) produced a significant enhancement at each frequency except 6 Hz. In vas deferens fractional tritium overflow per pulse changed little with increasing frequency and cocaine produced no effect. 4. In both tissues fractional evoked overflow of tritium was dependent on the stimulation current; cocaine (10 microM) increased fractional evoked overflow from brain slice at every current tested but was without effect in vas deferens. 5. Chromatographic separation of the released tritium showed there was little difference in the proportions of [3H]-noradrenaline and 3H-metabolites overflowing from the tissues. Cocaine increased the proportion of [3H]-noradrenaline and decreased the proportion of [3H]-DOPEG overflowing both at rest and during stimulation. 6. In brain slice an increase in electrically evoked overflow was produced by cocaine (10 microM) whether total tritium overflow (1.8 fold), overflow of [3H]-noradrenaline (1.8 fold) or overflow of unlabelled noradrenaline (1.8 fold) was measured. Evoked overflow from vas deferens was unaffected when assessed by any of these three methods. 7. The mechanism responsible for this differential effect of cocaine is unclear but may involve differences in the physical relationship between release sites, reuptake sites and presynaptic autoreceptors.

Narcotic reversal in hypercapnic dogs: comparison of naloxone and nalbuphine

Reversal of opioid effects by naloxone (NX) can lead to significant cardiovascular problems. We have reported previously that hypercapnic dogs develop greater increases in blood pressure and plasma catecholamine (CA) levels than hypocapnic ones when reversed with naloxone. We have also demonstrated differences between NX and nalbuphine (NBPH) in producing excitatory adrenergic responses when administered during normocapnia. The present study was designed to investigate possible dissimilarities in cardiovascular and sympathetic events after administration of either NX or NBPH in dogs made hypercapnic following fentanyl administration. After induction of anaesthesia with thiopentone and intubation, two groups of dogs were maintained with controlled ventilation on enflurane in oxygen anaesthesia and given 50 micrograms.kg-1 fentanyl IV. This caused a significant decrease in heart rate (HR) (P less than 0.001), mean arterial blood pressure (MAP) (P less than 0.001), and plasma concentrations of norepinephrine (NE) (P less than 0.002). Then, ventilation was decreased to produce a PaCO2 of 60 mmHg; this was accompanied by a significant elevation in plasma level of both epinephrine (EPI) (P less than 0.02) and NE (P less than 0.001). Administration of 20 micrograms.kg-1 NX to six dogs resulted in immediate increases in HR (P less than 0.01) and MAP (P less than 0.01), and a further rise in CA levels to greater than pre-fentanyl baseline values. In six other dogs, NBPH (0.3 mg.kg-1) caused increases in HR (P less than 0.001) and MAP (P less than 0.001) only, and the MAP rise was significantly less than that seen in the NX group (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of training on the metabolic and respiratory profile of high-intensity cycle ergometer exercise

The tolerable work duration (t) for high-intensity cycling is well described as a hyperbolic function of power (W): W = (W'.t-1) + Wa, where Wa is the upper limit for sustainable power (lying between maximum W and the threshold for sustained blood [lactate] increase, theta lac), and W' is a constant which defines the amount of work which can be performed greater than Wa. As training increases the tolerable duration of high-intensity cycling, we explored whether this reflected an alteration of Wa, W' or both. Before and after a 7-week regimen of intense interval cycle-training by healthy males, we estimated ( ) theta lac and determined maximum O2 uptake (mu VO2); Wa; W'; and the temporal profiles of pulmonary gas exchange, blood gas, acid-base and metabolic response to constant-load cycling at and above Wa. Although training increased theta lac (24%), mu VO2 (15%) and Wa (15%), W' was unaffected. For exercise at Wa, a steady state was attained for VO2, [lactate] and pH both pre- and post-training, despite blood [norepinephrine] and [epinephrine] ([NE], [E]) and rectal temperature continuing to rise. For exercise greater than Wa, there was a progressive increase in VO2 (resulting in mu VO2 at fatigue), [lactate], [NE], [E] and rectal temperature, and a progressive decrease for pH. We conclude that the increased endurance capacity for high-intensity exercise following training reflects an increased W asymptote of the W-t relationship with no effect on its curvature; consequently, there is no appreciable change in the amount of work which can be performed above Wa.(ABSTRACT TRUNCATED AT 250 WORDS)

Routine measurement of plasma catecholamines in clinical pharmacology by high-performance liquid chromatography with electrochemical detection

A method for the simultaneous determination of epinephrine, norepinephrine and dopamine in human plasma is described, which combines the advantages of liquid-liquid extraction sample preparation, high-performance liquid chromatography on weak cation-exchange stationary phases and dual-electrode coulometric detection. The limits of quantification are less than 5 pg/ml (at a signal-to-noise ratio greater than 5) for each analyte. The influence of various experimental parameters (e.g., composition of the mobile phase, pretreatment of the assay buffer, components of the re-extraction system) on the performance of the assay is reported in detail. A number of applications are presented, which demonstrate the quality of the data obtained in terms of sensitivity, reproducibility and significance.

The effect of local anaesthetics on epinephrine absorption following rectal mucosal infiltration

This study was undertaken to investigate the effects of lidocaine and bupivacaine on epinephrine absorption following rectal mucosal infiltration, to assess the cardiovascular and metabolic effects of the absorbed epinephrine and to compare the systemic absorption of the local anaesthetics employed. Three groups of five greyhounds received 1.5 micrograms.kg-1 of epinephrine 1:200,000 in lidocaine 0.5 per cent, bupivacaine 0.5 per cent or 0.9 per cent saline. Plasma epinephrine, lidocaine, bupivacaine, lactate, glucose and potassium concentrations were measured at 1, 2, 5, 10, 15 and 30 minutes following infiltration. Plasma epinephrine concentrations were significantly higher in the lidocaine group at one and two minutes following infiltration. Plasma bupivacaine concentrations were significantly higher than plasma lidocaine concentrations throughout the study period. There were no significant differences in metabolic or biochemical indices within or between the three groups. A local vasodilatory action of lidocaine may enhance epinephrine absorption. Differences in hepatic uptake and rate of metabolism may explain the increased plasma bupivacaine measured. Lidocaine may be the local anaesthetic of choice for ano-rectal procedures, especially when large volumes of local anaesthetic are being infiltrated.

Plasma catecholamines: laboratory aspects

In this review the methods used for analysis of plasma catecholamines in clinical chemical laboratories are discussed. The physiology of catecholamines as well as their measuring indications are discussed, together with concise evaluation of the methods most commonly used, namely indirect radioenzymatic assays or direct determinations by high-performance liquid chromatography combined with either electrochemical or fluorometric detection. The main advantage of radioenzymatic assay is its sensitivity and thus the need for only a small sample. Liquid chromatographic methods in general are less tedious, relatively rapid, and cheap, and omit the use of radionuclides. Both of these methods, however, are subject to a number of analytical errors, which can only be avoided by proper development of methods and skilled use of these methods. Little routine work is done using either radioimmunoassay or gas-chromatography.

Catecholamines and their metabolites

The research on biosynthesis, physiology, pharmacology, regulation and degradation of catecholamines has continuously increased for more than 50 years. This is not unexpected because of the fact that catecholamines are involved in so many life processes such as nerve conduction, blood circulation and hormone regulations in health and disease. This demands that methods for their determination should be improved, and in fact during the years a number of analytical methods have been published. About 20 years ago radioenzyme techniques with thin-layer chromatographic (TLC) separation of radiolabelled catecholamine derivatives were developed which greatly contributed to our knowledge of physiological concentrations of catecholamines in biological media, particularly in plasma and brain. Radioimmune methods were successful for analysis of a number of analytes, but for catecholamines radioimmunoassays developed slowly. We believe that the greatest potential for radioimmunochemical methods lies in their ability to localize catecholamines and metabolites at the cellular and subcellular levels. With the advent of gas chromatographic-mass spectrometric (GC-MS) and high-performance liquid chromatographic (HPLC) procedures analysis of catecholamines improved greatly., The equipment for GC-MS is expensive and requires technical skillfulness, but in experienced hands a lot of new biological data have emerged. An outstanding quality with GC-MS is that the method offers the ability to identify unknown compounds and is relatively free from interferences from extraneous compounds. In comparison with GC-MS, HPLC is versatile and has gained a widespread use. Applications for research in the catecholamine field are numerous. In general, the sensitivity and specificity are satisfactory with HPLC, but it should be borne in mind that a number of pitfalls can obscure the results. This involves both sample handling, clean-up and chromatographic procedures. At present, HPLC is the most expanding field in chromatographic determination of catecholamines and their metabolites. This is particularly the case for HPLC with electrochemical detection which has revolutionized our analytical potential in this field. These chromatographic procedures continue to develop. The prerequisites for further improved methods such as capillary zone electrophoresis and combined HPLC-MS are at hand and hopefully will soon come into more general use for analysis of catecholamines in biological samples.

Determinations of catechols in small volumes of plasma using ion-pair reversed phase liquid chromatography/electrochemistry

A method with improved sensitivity for detection of catechols (CA) in small volumes of plasma using an ion-pair reversed phase HPLC system with electromechemical detection is presented. Fast isocratic separations were obtained by using 7.5 cm x 4.6 mm (i.d.) reversed phase columns with 3C18 3 micron silica particles. The CA:s L-DOPA, Noradrenaline (NA), Adrenaline (A), Dihydroxybenzylamine (DHBA, i.s.), DOPAC and Dopamine (DA) were separated in less than 4 min. The performance of three different electrochemical cells was compared with respect to hydrodynamic voltammogram, band broadening effect, linearity and detection limit. The sample preparation procedure using alumina extraction of CA:s, was modified to improve recoveries and decrease dilution factors. A modified carbon paste cell (CP-O) gave a response 4-8 times higher than what is previously reported for GC cells. Detection limits were: L-DOPA 80, NA 1.25, A 1.25, DHBA 0.4, DOPAC 1.25 and DA 0.6 pg/injection. Application to plasma from rat and fish (cod) under rest, exercise and stress is reported. The method allows determination of CA:s in small volumes of plasma (less than 500 microliter) obtained several times a day from the same animal even if it is small (less than 1/2 kg), is under rest and parts of the plasma sample are to be used for analysis of other parameters than CA:s.

Estimation of plasma catecholamines by high-performance liquid chromatography with electrochemical detection in patients with subarachnoid haemorrhage

Plasma catecholamine levels were determined by high-performance liquid chromatography with electrochemical detection following alumina extraction. Mean individual recoveries of 50.5, 49.9 and 48% were obtained for norepinephrine, epinephrine and dopamine, respectively, and the limits of detection for each catecholamine were 0.15, 0.34 and 0.6 pmol/ml. Total analysis time for each plasma sample was approximately 1 h. Catecholamine levels were measured in plasma from control subjects and the ranges obtained were: norepinephrine, 0.33-5.98 pmol/ml; epinephrine, 0-4.77 pmol/ml; dopamine, 0-0.8 pmol/ml. When patients with subarachnoid haemorrhage were investigated, the ranges were found to be: norepinephrine, 0.23-7.27 pmol/ml; epinephrine, 0-4.91 pmol/ml; dopamine, 0-0.23 pmol/ml.

Haemodynamic effects of verapamil administration after large doses of fentanyl in man

Thirteen patients with good left ventricular function undergoing coronary artery revascularization were studied to determine the cardiovascular effects of verapamil, 75-150 micrograms X kg-1, after a large dose (100 micrograms X kg-1) of fentanyl, with pancuronium for muscle relaxation. The patients were continued on their usual cardiovascular medications until the time of surgery, which included nitrates, beta adrenergic blockers, and nifedipine. Anaesthesia with fentanyl was associated with decreases in mean arterial blood pressure, systemic vascular resistance, left ventricular stroke work index, and circulating catecholamine levels. Mean values were not further changed by verapamil, but individual patients had additional modest decreases in blood pressure and systemic vascular resistance. Cardiac index, however, was well maintained. Plasma catecholamines remained depressed after verapamil under the study condition. Thus, in patients with good left ventricular function, clinically relevant doses of verapamil were well tolerated even in the presence of an anaesthetic that included large doses of fentanyl, with suppression of circulating catecholamine levels.

Plasma catecholamine measurements in resting and stressed conscious rats, using high performance liquid chromatography with electrochemical detection

Plasma epinephrine (EPI) and norepinephrine (NE) were measured in conscious, unrestrained rats at rest, and following exercise, cold stress and hemorrhage. Heart rate and mean arterial pressure were monitored at all points. Minor modifications of a standard, commercially available methodology achieved good chromatographic separation of both EPI and NE. Sensitivity was sufficient for all NE measurements and for EPI measurements in the stress conditions. Estimates of EPI by this procedure were somewhat higher than those in other reports. Absolute values of NE, as well as the selective, qualitative changes found in both EPI and NE under each stress condition, were consistent with those found in other studies which used the radioenzymatic method. Blood withdrawal of two 1.5 ml samples, 3 hours apart, had no detectable effect on EPI, NE, mean arterial pressure or heart rate in otherwise non-stressed rats.

Intraventricular 5,7-dihydroxytryptamine increases thyrotropin-releasing hormone content in regions of rat brain

Rats received intraventricular (i.v.t.) injections of 5,7-dihydroxytryptamine (5,7-DHT) (100-600 micrograms). Some animals also received intraperitoneal injections of the 5-hydroxytryptamine uptake blocker fluoxetine (FX) (20 mg/kg) or the norepinephrine uptake blocker desmethylimipramine (DMI) (48 mg/kg) 30-90 min prior to i.v.t. 5,7-DHT. Rats were killed between 2 and 35 days following i.v.t. 5,7-DHT, brains were dissected, and regions were assayed for thyrotropin-releasing hormone (TRH) by radioimmunoassay. Dose-dependent increases in TRH content following i.v.t. 5,7-DHT were noted in the brainstem and hippocampus. DMI pretreatment blocked the increase in hippocampal TRH, but not in brainstem TRH. FX pretreatment was ineffective in blocking any increases in TRH content. These results suggest differential regulation of regional TRH content by interactions with specific neurotransmitter systems.

Intraventricular 6-hydroxydopamine increases thyrotropin-releasing hormone (TRH) content in regions of rat brain

Rats were given intraventricular (ivt) injections of various doses (50-400 micrograms, hydrobromide salt) of 6-hydroxydopamine (6-OHDA) and killed 1, 3 or 6 days later. Brains were removed, dissected into 11 regions, and the thyrotropin-releasing hormone (TRH) content of each region was measured by radioimmunoassay. 6-OHDA (400 micrograms) caused significant elevations in the TRH content of 6 regions: olfactory bulb, anterior cortex, brainstem, posterior cortex, hippocampus, and amygdala-piriform cortex. The magnitude of these increases ranged from 59% in olfactory bulb to 497% in hippocampus and was, in all cases, greatest at 3 days. These results suggest that the TRH content of certain brain regions may be regulated by catecholamine neurotransmitters.

A method for very rapid determinations of catechols using ion-pairing reverse phase HPLC with electrochemical detection: effects of L-dopa treatment on the catechol content in various rat brain structures

A simple and rapid method for determination of 12 catechols (9 endogenous and 3 internal standards, i.s.) using ion-pairing reverse phase HPLC with electrochemical detection is presented. This study basically concentrates on the importance of optimizing the mobile phase composition in isocratic systems where ordinary 25 cm X 4.6 mm i.d. columns are used. Mobile phase compositions for three different purposes are reported: 1) separation of 9 endogenous catechols, possibly occurring in the samples, and 3 i.s. in a moderately short retention time (tR) (L-DOPA, DOPEG, alpha-Methyldopa (alpha-MeDOPA, i.s.), Noradrenaline (NA), DOPAC, Adrenaline (A), Dihydroxybensylamine (DHBA, i.s.), Norsalsolinol (NS), Dopamine (DA), Epinine (EPI), Salsolinol (S) and Isoprenaline (ISO, i.s.) within 11 min), 2) ultra rapid separation of detectable endogenous catechols except L-DOPA (NA, DOPAC, A, DHBA (i.s.), NS and DA within 5.2 min and with S within 5.7 min) and 3) moderately fast separation of detectable endogenous catechols (L-DOPA, NA, A, DHBA (i.s.), NS, DOPAC and DA within 7.6 min and with S within 10 min). By balancing the pH, concentration of organic modifier (2-propanol) and pairing ion (1-heptanesulphonic acid) as well as preconditioning new columns with more packing material (Nucleosil 5 micron C18) and to high pressures (5000 psi) for 7 days, very fast separations with good baseline resolution between the peaks are possible. The method was applied on L-DOPA treated rats (100 mg/kg), where the catechol content was analysed in 7 different brain structures during the time course of synthesis and degradation (4 hours) of catechols from L-DOPA.

Electrochemical detection of catecholamines in urine and plasma after separation with HPLC

The catecholamines in plasma and urine were determined by electrochemical detection after separation on a HPLC column RCM 100 C 18. Linear calibration plots of epinephrine, norepinephrine and dopamine have been obtained in the range expected to appear in urine or plasma. Coefficients of variation (0.5 to 10%), recovery rates (60 to 80%) and the standard deviation were satisfactory, whereas plasma dopamine showed a greater variation (10-15%). The values ascertained by this technique were compared with those determined by the radioenzymatic assay of Da Prada et al. They showed a good correlation for norepinephrine r = 0.92, p less than 0.001 and epinephrine r = 0.80, p less than 0.01, but less for dopamine r = 0.1. Experimental details of isolation from plasma with Al2O3 adsorption and desorption with HClO4, and from urine with ion exchange chromatography on Bio-Rex 70 and elution with boric acid are described. Technical modifications which improve the method are reported. The optimized assay could reliably be employed in investigations of more than 3000 urine and plasma samples obtained from patients and athletes before and after exercise.

Catecholamine measurements by high-performance liquid chromatography

The development of sensitive detectors has allowed the use of high-performance liquid chromatography (HPLC) for measurements of catecholamines in extracts of plasma, urine, and tissue samples. Separation of the catecholamines may be effected by reversed phase chromatography or cation-exchange chromatography and quantitation by electrochemical detection (EC) or by fluorometry coupled with postcolumn derivatization according to the trihydroxyindole (THI) method. EC has a somewhat lower sensitivity than the THI method for norepinephrine (NE) and epinephrine (E). The THI method is insensitive to dopamine (DA). Basal plasma E levels of 0.1 nM (20 pg/ml) or less may be measured in sample volumes of 1-2 ml with EC. Sensitivity and reproducibility of an assay is not necessarily a guarantee of accuracy. It is argued that new methods and modifications of old methods should be validated against accepted methodology. This is rarely the case. Cation exchange HPLC with EC has been adequately validated, but only one of the reversed phase methods has been compared with radioenzymatic methodology. HPLC has the advantages of economy, speed, and more stimulating laboratory work, as compared with radioenzymatic methodology.

Sensitive high-performance liquid chromatographic determination of catecholamines in rat brain using a laser fluorimetric detection system

A sensitive high-performance liquid chromatographic method for the determination of catecholamines in rat brains has been developed using a fluorescence detector equipped with a continuous wave laser as an excitation light source. A new pre-purification and derivatization method was established and confirmed to be useful for the determination of catecholamines in biological samples. This pre-treatment method was simple, reproducible and specific. About 1 mg of the rat brain tissue was enough to determine catecholamines levels. The levels of dopamine and norepinephrine in rat brain were 0.40 and 0.87 ng, respectively, which agree with the findings of other workers.

Catecholamine measurements in phaeochromocytoma: a review

The early diagnosis of phaeochromocytoma by measurement of catecholamines or their metabolites is important, since it brings the prospect of permanent cure to a small but significant proportion of the hypertensive population. The development of sensitive and specific methods for the analysis of catecholamines has led to the increased use of such measurements in the biochemical diagnosis and localisation of this rare tumour.

Routine determination of plasma catecholamines using reversed-phase, ion-pair high-performance liquid chromatography with electrochemical detection

A procedure is described for the determination of plasma catecholamines using reversed-phase, ion-pair high-performance liquid chromatography coupled with electrochemical detection. Optimisation of chromatographic conditions with respect to detector performance and adherence to procedures and precautions described, render the method applicable to both neurochemical research and routine clinical analysis. The limit of quantitative detection of the method was found to be approximately 30 pg per injection for individual catecholamines. A single chromatographic run, providing adequate resolution of each component, could be completed in approximately 12 min.

Measurement of catecholamines in biological fluids by high-performance liquid chromatography: a comparison of fluorimetric with electrochemical detection

An improved method for the determination of catecholamines in biological fluids, by reversed-phase high-performance liquid chromatography (HPLC) with fluorimetric detection is presented. The pH titration previously employed in the alumina extraction was abandoned in favour of the use of a molar excess of PH 8.5 Tris--HCl buffer. A novel lyophilisation step serves to concentrate the catechols and by reconstituting in mobile phase, chromatography disturbances are minimised. The addition of 2 mM octanesulphonic acid to citrate--phosphate mobile phase at pH 6.0 gave optimal resolution and sensitivity. That HPLC separation can improve the specificity of the trihydroxyindole reaction, to the extent of providing a reliable analytical method, has been demonstrated and validated by the technique of HPLC with electrochemical detection. A correlation coefficient of 0.98 was obtained between the two techniques as applied to the measurement of urinary catecholamines. The HPLC--fluorimetric method was sensitive enough to measure 0.1 ng/ml of noradrenaline or adrenaline at a signal-to-noise ratio of 2.0. Application of the method to the quantitative determination of catecholamines in human urine, plasma and rat brain homogenates is demonstrated.

Investigations of catecholamine metabolism using high-performance liquid chromatography: analytical methodology and clinical applications

High-performance liquid chromatography, particularly in its reversed-phase mode, coupled with electrochemical or fluorometric detection, is becoming increasingly popular as an analytical tool for metabolic profiling of substances of neurochemical interest, such as catecholamines and their metabolites. During the last decade, a continued effort has been made to improve and simplify the analytical methodology for routine use in clinical laboratories where this technique is tremendously needed. New developments in column technology, reliable detectors, simplified sample cleanup procedures, and particularly better understanding of the complex physicochemical phenomena underlying the operation of electrochemical detection, have resulted in a steady and encouraging progress. The purpose of this review was to describe the current analytical methodology and recent applications of HPLC in the field of catecholamine metabolism. Although this discussion is by no means detailed and complete, it, at least, hints at the impact of this technique on biochemical investigations and its future potential in clinical laboratories.

Determination of catecholamines in rat heart tissue and plasma samples by liquid chromatography with electrochemical detection

Liquid chromatography with electrochemical detection is used for the determination of adrenaline, noradrenaline and dopamine in rat heart tissue, and the method has also been applied to the determination of basic levels of these compounds in blood plasma. The catecholamines are isolated from the biological sample by adsorption onto alumina and are then desorbed by elution with perchloric acid. The stability of the compounds during the different stages in the work-up process has been studied. A greatly simplified procedure for the preparation of alumina is presented. Both ion-pair reversed-phase and ion-exchange liquid chromatography have been used for the separation of the catecholamines. For plasma samples the method has been validated against radioenzymatic assay and the choice of method is discussed.