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

Insulin-induced hypoglycemia decreases single-unit activity of serotonergic medullary raphe neurons in freely moving cats: relationship to sympathetic and motor output

Serotonergic single-unit activity during glucoregulatory challenges was studied in the nuclei raphe obscurus (NRO) and raphe pallidus (NRP) of freely moving cats. Systemic insulin administration (2-4 IU/kg, i.v.) suppressed neuronal activity by approximately 40% in direct relationship to blood glucose levels and in inverse relationship to plasma catecholamine levels. NRO and NRP serotonergic neurons displayed a temporary recovery in unit activity in response to i.v. glucose administration (500 mg/kg), which temporarily reversed insulin-induced hypoglycemia. The neuronal responses to insulin and subsequent glucose administration were also directly related to changes in integrated nuchal electromyographic activity. Serotonergic unit activity remained unchanged after glucose loading (500 mg/kg, i.v.), which produced a four-fold increase in blood glucose. Thus, medullary serotonergic neurons appear to be sensitive to reductions, but not increases, in blood glucose. The observed inverse relationship between unit activity and plasma catecholamines does not support a postulated sympathoexcitatory role for these neurons. Instead, the parallel changes in single-unit activity and integrated muscle activity support the hypothesis that the activity of medullary serotonergic neurons is linked to motor output. These neurons may modulate autonomic outflow, but only in relationship to their primary role in motor control. Finally, medullary serotonergic neurons may play a protective role in maintaining glucose homeostasis by disfacilitating the output of the somatomotor system, and hence diminishing muscle energy demands, when peripheral glucose availability is low.

Hydroquinone-based derivatization reagents for the quantitation of amines using electrochemical detection

Two new reagents, NDTE (2,5-dihydroxyphenylacetic acid, 2,5-bis-tetrahydropyranyl ether p-nitrophenyl ester) and HLTE (homogentisic gamma-lactone tetrahydropyranyl ether), are described for the chemical derivatization of primary and/or secondary amines to form an electrochemically active product. These reagents undergo reaction with the aforementioned analytes to form a product possessing the hydroquinone moiety, thus allowing for reversible electrochemical detection at mild oxidation potentials. The reactivity of each reagent was demonstrated by using N-ethylbenzylamine (EBzA) and the dipeptide isoleucine leucine methyl ester as model analytes. The investigation included the isolation and identification of the intermediates and final products from derivatization of EBzA. These isolated standards were subsequently characterized with respect to electrochemical properties by means of cyclic voltammetry. In LC-EC experiments, the concentration limit of detection (CLOD) of the purified EBzA product was determined to be 5 nM (100 fmol) at a detection potential of +200 mV vs Ag/AgCl ([Cl-] = 3 M). The CLOD values obtained by LC-EC after derivatization of aqueous solutions of EBzA and Ile-Leu-OMe with NDTE were 25 nM (250 fmol) and 250 nM (2.5 pmol), respectively.

Physical activity, skeletal muscle beta-adrenoceptor changes and oxidative metabolism in experimental chronic heart failure

In chronic heart failure (CHF), changes in sympathetic nervous activity and skeletal muscle metabolism contribute to a limitation in the capacity for exercise. The aim of this study was to investigate the potential relationships between physical deconditioning, skeletal muscle beta-adrenoceptor (beta-AR) characteristics and muscle metabolic changes in rats with coronary ligation-induced experimental CHF. Muscle beta-AR and norepinephrine levels were assessed in rats with CHF that had been treated with propranolol at 28 mg/kg/day and compared with rats with CHF that had not been treated and those that had undergone sham operations. The soleus muscle was investigated because of its predominantly oxidative fibre-type composition. Measurements of spontaneous locomotion activity were carried out using telemetry. After 85 days, muscle energetic phosphate levels were assessed using 31P-magnetic resonance spectroscopy. The phosphocreatine resynthesis rate was decreased in the untreated CHF rats (15 +/- 3 vs 33 +/- 5 mmol L-1 min-1 in the sham-operated rats, p < 0.05), but this had been partially reversed in the rats given propranolol (22 +/- 3 mmol L-1 min-1, non-significant (NS) when compared with the sham-operated rats). Spontaneous activity did not differ among the three groups of animals. Soleus beta-adrenoceptor density was decreased in rats with CHF (8.8 +/- 3.0 fM/mg of protein vs 22.0 +/- 7.0 fM/mg of protein in the sham-operated rats, p < 0.05) and normalized in the propranolol-treated rats (31.9 +/- 7.0 fM/mg of protein, NS vs the sham-operated rats; p < 0.05 vs the untreated rats with CHF). Unchanged spontaneous activity in the rats with CHF suggests that physical deconditioning could not account for the muscle metabolic changes. Changes in skeletal muscle energy metabolism were accompanied by changes in beta-AR density, occurring in typically oxidative beta-AR-rich muscles, reversible after beta-blocker therapy and therefore suggestive of beta-AR downregulation.

Simultaneous measurement of serotonin, catecholamines and their metabolites in cat and human plasma by in vitro microdialysis-microbore high-performance liquid chromatography with amperometric detection

A method for the simultaneous measurement of norepinephrine, epinephrine, dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, serotonin and 5-hydroxyindole-3-acetic acid in cat and human plasma by in vitro microdialysis-microbore high-performance liquid chromatography with electrochemical detection is described. The detection limit (signal-to-noise ratio = 3) is about 0.05-0.1 pg per injection. The volume of plasma samples required is very small (< 200 microliters), hence there is minimal blood loss in repeated blood sampling, especially in experiments using small animals. Within 15 min, a fast isocratic separation of these analytes by using a microbore reversed-phase ODS column is achieved, hence over 90 analyses can be performed in a single working day. As microdialysis per se is not destructive to plasma samples, the remaining plasma sample and perfusate can be repeatedly analysed for other substances. This simple, efficient and sensitive method can therefore be used as a routine clinical and basic research technique in the investigation of blood biogenic amines and their metabolites.

Sensitive method for measuring hydrolysis of enkephalins in plasma, using high-performance liquid chromatography with electrochemical detection

This paper describes a simple and sensitive method for detection of [Leu]- and [Met]enkephalin and their N-terminal tyrosine-containing metabolic fragments (Tyr, Tyr-Gly, Tyr-Gly-Gly, and Tyr-Gly-Gly-Phe), using high-performance liquid chromatography with electrochemical detection. The method employs a carbon graphite working electrode with increased working electrode surface area (40 mm2). The procedures were applied to assay of the activities of enkephalin-degrading enzymes in whole plasma collected from rats, mice, and chicks.

Nerve impulse-induced release of endogenous noradrenaline and adrenaline from the perfused cod spleen

Release of endogenous catecholamines (CA) by electrical nerve stimulation (NS) was studied in the isolated perfused spleen of the Atlantic cod, Gadus morhua. An HPLC-system for the analysis of endogenously released CA is described. Cocaine (COC) was used to block neuronal re-uptake of endogenous CA released by NS. Splanchnic NS at frequencies of 1-40 Hz for 20 s resulted in release of noradrenaline (NA) and adrenaline (A) with a maximal total overflow at 20 Hz for both amines. The release of CA was frequency-dependent. COC (0.1 mmol.l-1) increased NS-evoked (40 Hz) overflow of NA and A by 4.8 and 2.2 times, respectively. and reduced the overflow of dihydroxyphenylglycol (DOPEG) to spontaneous efflux levels or less. It can be concluded that the HPLC-technique used was adequate for measurement of NS-evoked release of endogenous CA and DOPEG from the isolated perfused cod spleen, and the model presented can therefore be used when studying adrenergic mechanisms in fish spleen.