Taurine kinetics assessed using [1,2-13C2]taurine in healthy adult humans

To assess the dynamics of taurine metabolism in vivo, two sets of studies were carried out in healthy volunteers. First, pilot studies were carried in a single human subject to determine the time course of plasma and whole blood isotope enrichment over the course of an 8-h, unprimed continuous infusion of [1,2-(13)C(2)]taurine. Second, five healthy adult males received two tracer infusions on separate days and in randomized order: 1) a 6-h continuous infusion of [1,2-(13)C(2)]taurine (3.1 +/- 0.2 micromol x kg(-1) x h(-1)) and 2) a bolus injection of [(13)C(2)]taurine (3.0 +/- 0.1 micromol/kg). Isotope enrichments in plasma and whole blood taurine were determined by gas chromatography-mass spectrometry. The pilot experiments allowed us to establish that steady-state isotope enrichment was reached in plasma and whole blood by the 5th h of tracer infusion. The plateau enrichment reached in whole blood was lower than that obtained in plasma taurine (P < 0.02). In the second set of studies, the appearance rate (R(a)) of plasma taurine, determined from continuous infusion studies was 31.8 +/- 3.1 micromol x kg(-1) x h(-1). After a bolus injection of tracer, the enrichment decay over the subsequent 2 h was best fitted by a two-exponential curve. Taurine R(a) was approximately 85% higher when determined using the bolus injection technique compared with continuous infusion of tracer. We conclude that 1) taurine R(a) into plasma is very low in healthy postabsorptive humans, and, due to taurine compartmentation between the extra- and intracellular milieus, may represent only interorgan taurine transfer and merely a small fraction of whole body taurine turnover; and 2) the bolus injection technique may overestimate taurine appearance into plasma. Further studies are warranted to determine whether alterations in bile taurine dynamics affect taurine R(a).     

Influence of alpha or beta adrenergic antagonists on catecholamine dependent metabolic effects in pigs

In 4 Piétrain-pigs and 4 crossbred (Duroc X Landrace) pigs (32-47 kg body weight; b.w.) the effect of an intravenous injection of epinephrine (80 micrograms/kg b.w.) or isoprenaline (55 micrograms/kg b.w.) was investigated during a continuous infusion of 0.9% NaCl-solution (1 ml/min and pig), propranolol or phentolamine (priming dose 100 micrograms/kg b.w. and thereafter 2 micrograms/kg and min over 45 min) on the plasma concentration of glucose, lactate, free fatty acids (FFS) and free over 45 min) on the plasma concentration of glucose, lactate, free fatty acids (FFS) and free glycerol. Furthermore the effect of a continuous infusion of the blocking agents alone was examined in the 4 crossbred animals. Lipolysis was stimulated via beta-adrenergic receptors and was inhibited through an alpha-adrenergic mediated effect in pigs. The lean Piétrain-pigs showed a significant higher response than the crossbred pigs. The catecholamine induced increase in plasma glucose and lactate was equal in both breeds. The rise of glucose concentration resulted from an alpha- and beta-adrenergic component, with the alpha-adrenergic effect dominating. Compared to isoprenaline, the higher increase in plasma lactate after adrenaline injection is attributed to clinical reactions.     

Rates of production and utilization of lactate by microbial communities from the human colon

Lactate metabolism was studied in mixed bacterial communities using single-stage continuous flow fermentors inoculated with faecal slurries from four different volunteers and run for 6 days at pH 5.5 and 6.0, using carbohydrates, mainly starch, as substrates. A continuous infusion of [U-(13) C]starch and l-[3-(13) C]lactate was performed on day 5 and a bolus injection of l-[3-(13) C]lactate plus dl-lactate on day 6. Short-chain fatty acids and lactate concentrations plus enrichments and numbers of lactate-producing and -utilizing bacteria on day 5 were measured. Faecal samples were also collected weekly over a 3-month period to inoculate 24-h batch culture incubation at pH 5.9 and 6.5 with carbohydrates alone or with 35 mmol L(-1) lactate. In the fermentors, the potential lactate disposal rates were more than double the formation rates, and lactate concentrations usually remained below detection. Lactate formation was greater (P<0.05) at the lower pH, with a similar tendency for utilization. Up to 20% of butyrate production was derived from lactate. In batch cultures, lactate was also efficiently used at both pH values, especially at 6.5, although volunteer and temporal variability existed. Under healthy gut environmental conditions, bacterial lactate disposal seems to exceed production markedly.     

Lactate extraction during net lactate release in legs of humans during exercise

Lactate metabolism was studied in six normal males using a primed continuous infusion of lactate tracer during continuous graded supine cycle ergometer exercise. Subjects exercised at 49, 98, 147, and 196 W for 6 min at each work load. Blood was sampled from the brachial artery, the iliac vein, and the brachial vein. Arteriovenous differences were determined for chemical lactate concentration and L-[1-14C]-lactate. Tracer-measured lactate extraction was determined from the decrease in lactate radioactivity per volume of blood perfusing the tissue bed. Net lactate release was determined from the change in lactate concentration across the tissue bed. Total lactate release was taken as the sum of tracer-measured lactate extraction and net (chemical) release. At rest the arms and legs showed tracer-measured lactate extraction, as determined from the isotope extraction, despite net chemical release. Exercise elicited an increase in both net lactate release and tracer-measured lactate extraction by the legs. For the legs the total lactate release (net lactate release + tracer-measured lactate extraction) was roughly equal to twice the net lactate release under all conditions. The tracer-measured lactate extraction by the exercising legs was positively correlated to arterial lactate concentration (r = 0.81, P less than 0.001) at the lower two power outputs. The arms showed net lactate extraction during exercise, which was correlated to the arterial concentration (r = 0.86). The results demonstrate that exercising skeletal muscle extracts a significant amount of lactate during net lactate release and that the working skeletal muscle appears to be a major site of blood lactate removal during exercise.     

Acute anemia increases lactate production and decreases clearance during exercise

We evaluated whether elevated blood lactate concentration during exercise in anemia is the result of elevated production or reduced clearance. Female Sprague-Dawley rats were made acutely anemic by exchange transfusion of plasma for whole blood. Hemoglobin and hematocrit were reduced 33%, to 8.6 +/- 0.4 mg/dl and 26.5 +/- 1.1%, respectively. Blood lactate kinetics were studied by primed continuous infusion of [U-14C]lactate. Blood flow distribution during rest and exercise was determined from injection of 153Gd- and 113Sn-labeled microspheres. Resting blood glucose (5.1 +/- 0.2 mM) and lactate (1.9 +/- 0.02 mM) concentrations were not different in anemic animals. However, during exercise blood glucose was lower in anemic animals (4.0 +/- 0.2 vs. 4.6 +/- 0.1 mM) and lactate was higher (6.1 +/- 0.4 vs. 2.3 +/- 0.5 mM). Blood lactate disposal rates (turnover measured with recyclable tracer, Ri) were not different at rest and averaged 136 +/- 5.8 mumol.kg-1.min-1. Ri was significantly elevated in both control (260.9 +/- 7.1 mumol.kg-1.min-1) and anemic animals (372.6 +/- 8.6) during exercise. Metabolic clearance rate (MCR = Ri/[lactate]) did not differ during rest (151 +/- 8.2 ml.kg-1.min-1); MCR was reduced more by exercise in anemic animals (64.3 +/- 3.8) than in controls (129.2 +/- 4.1). Plasma catecholamine levels were not different in resting rats, with pooled mean values of 0.45 +/- 0.1 and 0.48 +/- 0.1 ng/ml for epinephrine (E) and norepinephrine (NE), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fever and lethargy induced by subcutaneous pyrogen infusion in unrestrained rats

We have investigated the effects of continuous subcutaneous infusion of lipopolysaccharide (LPS), muramyldipeptide (MDP), or saline on abdominal temperature and voluntary activity in unrestrained rats. Both pyrogens were infused via osmotic pumps at a rate of approximately 2 microg.kg-1.min-1 for 7 d. LPS infusion evoked a 3-d and MDP a 1-d elevation in body temperature. Night-time activity was suppressed on days 1 and 2 during LPS infusion and on day 1 of MDP infusion. Body mass was significantly decreased on infusion day 4 in rats receiving either LPS or MDP; however, the rate of weight gain had been restored by day 8 (1 d after cessation of pyrogen infusion). We further tested the body temperature response of the same experimental animals to a single subcutaneous bolus injection (250 microg/kg) of the same pyrogen that had been infused for 7 d, 2 d after cessation of pyrogen infusion (day 9). The fever response in rats receiving a bolus injection of either LPS or MDP was significantly attenuated in rats that had previously been infused with the same pyrogen. These data suggest that tolerance developed to continuous infusion of both Gram-negative and Gram-positive pyrogens, and that mechanisms of tolerance development set in early during the 7-d infusion period of both pyrogens and persisted for at least 2 d after the cessation of pyrogen infusion. We propose that cytokine intermediates were involved or required in inducing these responses to continuous infusion of both LPS and MDP.     

Variables of myocardial backdiffusion,determined with 17-IODO-131 heptadecanoic acid in the normal dog heart

Summary Under normal and ischemic conditions backdiffusion of radiolabeled non-esterified fatty acids (NEFA) has been demonstrated. In the fasted normal canine heart the extraction fraction (EF) during interventions with glucose or lactate loading, vasodilation, and metabolic level augmentation was determined, and compared with the control EF. Backdiffusion alterations were deduced from the EF changes. After iv injection of 17-iodo-131 heptadecanoic acid (IHDA), 11 blood samples were drawn from aorta and coronary sinus in a time period of 60 minutes. In the control and vasodilation group the EF slowly decreased from 40 to 10%. In contrast, the EF in the noradrenaline group was constant. During glucose and lactate infusion the EF became negative within 10 min and remained negative. These results suggest that during physiological circumstances backdiffusion is determined by the metabolic level of the heart and its substrate availability.     

Effect of recombinant human GH and GHRH on plasma metabolite levels in rainbow trout (Oncorhynchus mykiss)

The aim of the present work was to study metabolic changes in rainbow trout treated with preparations of human recombinants of growth hormone (rhGH) and GH-releasing hormone (rhGHRH), by analysing time course variations in plasma metabolite levels. Trouts were given the hormones by intraperitoneal injection in a single dose (rhGH: 0.5microg/g b.w.; rhGHRH: 0.06microg/g b.w.) or as a weekly injection over a period of 4 weeks. The concentrations of glucose, lactate, free fatty acids and glycerol were determined in plasma immediately before injection and 1, 3, 6, 12, 24, 48 and 168 hours post-injection. Results indicate that glucose and lactate levels increase, while slight variations in free fatty acids was observed after the injection of rhGH and rhGHRH.     

KINETICS OF NEUTROPHILIC GRANULOCYTES IN THE BLOOD OF RATS

The kinetics of neutrophilic granulocytes in the blood of rats were investigated using in vivo ³H-TdR labelling and autoradiography. The radioactive precursor was administered by single injection, repeated injections at 5 hr intervals and continuous infusion. The appearance of labelled granulocytes in the blood and in the sputum was recorded up to 120 hr after tracer application. In contrast to results after a single injection of ³H-TdR, complete labelling of the blood granulocyte pool was achieved when the DNA precursor was given by continuous infusion or four repeated injections at 5 hr intervals. In the latter experiments, an exponential replacement of unlabelled blood granulocytes by labelled granulocytes could be demonstrated, the mean intravascular half-life being 5·7 hr. This figure is in good agreement with values obtained by isotopic techniques in other mammalian species.     

Reutilization of DNA catabolites in granulocytopoiesis.

Different DNA labelling procedures for the proliferating granulocyte precursor compartments of rat bone marrow were evaluated by studying the appearance of labelled granulocytes in the blood stream by means of autoradiography. 3-H-thymidine was administered by single injection and continuous infusion. 125-I-Iododeoxyuridine, a DNA precursor showing a neglegible extent of reutilization, was studied in comparison. Labelling patterns of blood neurtrophils were identical after single injection and continuous infusion of 3-H-thymidine, while a different pattern was observed after use of a single injection of 125-I-Iododeoxyuridine. The results provide evidence that reutilization of label is an important event to be considered when kinetics of granulocytopoiesis are studied after application of 3-H-thymidine and indicate that reutilization occurs at the level of thymidinemonophosphate in this cell system.     

A model for measurement of lactate disappearance with isotopic tracers in the steady state.

1. The irreversible disappearance of lactate carbon from the body (RdL) is commonly calculated from data obtained with a continuous infusion of isotopically labelled lactate tracer. The tracer infusion rate divided by the steady-state lactate specific radioactivity in blood is taken to give the rate of lactate disappearance. 2. Measurement of lactate disappearance is complicated by the fact that it is reversibly converted into pyruvate as well as being irreversibly removed from the system. 3. We analysed a four-compartment model of lactate metabolism, representing blood lactate, tissue lactate and pyruvate carbon pools. 4. The standard method of calculating RdL from the lactate tracer infusion rate divided by the specific radioactivity of lactate was not validated. 5. We found that RdL can be calculated from the infusion rate and the pyruvate specific radioactivity, multiplied by the fraction of the total carbon flow out of pyruvate that goes to lactate. 6. Therefore, if almost all of the pyruvate carbon flows back to lactate, then RdL approaches the tracer infusion rate divided by the pyruvate specific radioactivity. On the other hand, if the rate of oxidation is large in relation to the rate of pyruvate conversion into lactate, than RdL is overestimated when calculated from the pyruvate specific radioactivity. 7. Calculation of RdL with the arterial lactate specific radioactivity results in an underestimate of the true RdL.     

Tracer mixing: sites of tracer infusion and sampling

Controversy exists in the literature concerning the correct infusion and sampling sites in studies measuring substrate turnover rates. To investigate this problem, we examined the results obtained with various infusion and sampling sites in 7 anesthetized dogs. [1-14C]lactate was infused by a primed continuous infusion method in three different sites (the left ventricle, ascending aorta, and the aortic arch) in a sequential fashion; samples were obtained simultaneously from five sites (femoral artery, carotid artery, pulmonary artery, superior vena cava and inferior vena cava) for each of the three different infusion sites. [U-13C]lactate was also infused in a femoral vein and simultaneous samples were obtained in the carotid artery and femoral artery for analysis of the stable isotope. [14C]lactate analysis demonstrated that infusion of the tracer into the left ventricular chamber resulted in a uniform distribution in the systemic circulation. Infusion into the ascending aorta near the aortic valve resulted in uniform distribution of tracer in four out of five experiments. Tracer infusion into the aortic arch resulted in nonuniform systemic distribution of tracer. The [U-13C]lactate results showed that infusion into the femoral vein gives uniform systemic distribution, similar to that observed with left ventricular infusion. The pulmonary artery lactate specific activities varied from those in the superior vena cava. Thus, this study shows that the tracer must be infused in the left ventricle or upstream from this chamber to obtain optimal systemic distribution. Vena caval sampling, especially superior vena caval sampling, will not give a consistent mixed venous concentration of the lactate tracer. Therefore, aortic tracer infusion with vena caval sampling may lead to errors in determining substrate turnover values.     

Spinal cord excitability is not influenced by elevated blood lactate levels

The aim of the present study was to examine the association of high blood lactate levels, induced with a maximal cycling or with an intravenous infusion, with spinal cord excitability. The study was carried out on 17 male athletes; all the subjects performed a maximal cycling test on a mechanically braked cycloergometer, while 6 of them were submitted to the intravenous infusion of a lactate solution (3?mg/kg in 1?min). Before the exercise or the injection, also at the end as well as 5 and 10?min after the conclusion, venous blood lactate was measured and excitability of the spinal α-motoneurons was evaluated by using the H reflex technique. In both experimental conditions, it has been observed that an exhaustive exercise is associated with a strong increase of blood lactate (but not of blood glucose) and with a significant reduction of spinal excitability. Since a similar augment of blood lactate induced by an intravenous infusion, in subjects not performing any exercise, is not associated with significant changes of spinal excitability, it can be concluded that the increase of blood lactate levels during a maximal exercise is not per se capable of modifying the excitability of spinal α-motoneurons.     

Rat luminal cell nuclear area changes correlated with uterine growth responses induced by a low dose infusion or injection of estradiol-17 beta

Rat uterine luminal epithelial cells (LEC) responded differently when exposed to an injection of 1.0 microgram estradiol-17 beta (E2) compared to a continuous infusion of E2 at the rate of 1.0 microgram/24 hours. After injection or beginning infusion, LEC mean nuclear area significantly decreased by 4 h, then increased thereafter. After injection, nuclear area distributions were determined at each time point. The percentage of large nuclei (greater than 40 mu 2) decreased by 4h postinjection and remained a relatively small proportion of the population, while the percentage of nuclei of 20-30 mu 2 areas increased throughout the experiment. During infusion, the percentage of large nuclei decreased by 4h after pump implantation, then increased. Only infusion induced sustained, increased uterine protein content, DNA synthesis and ornithine decarboxylase activity. This study suggests that E2 treatment modality induces differences in nuclear size in target cells as well as in biochemical parameters.     

Chemically coupled spectrophotometric assays based on flow injection analysis: Determination of nitrogenase by assays for creatine, ammonia, hydrazine, phosphate, and dithionite

Micromethods of direct chemical coupling have been developed for several different enzyme reactions, using the principles of flow injection analysis. Samples of 1–25 μl are injected into a flowing stream of color-forming reagents and the peak of color change is measured after about 1 min. Alternatively, continuous slow infusion of a reacting system (5–100 μl/min) gives a continuous change of color which can be monitored to derive enzyme reaction rates. These techniques are highly sensitive, requiring a few nanomoles of the substance being detected. Phosphate, ammonia, dithionite, creatine, and hydrazine have been measured. Consumption of reagents is less than 75 ml per hour; typical sample throughput is 30–40 samples per hour by the injection method, and 5 samples per hour by continuous infusion. The precedure has been applied to nitrogenase, continuously monitoring creatine produced from creatine phosphate by creatine kinase which is used to supply a constant level of ATP for nitrogenase. In this way nitrogenase activity can be determined over a wide range of enzyme concentrations. Production of inorganic phosphate directly from ATP, by injection of formaldehyde-quenched samples, was used when coupling to creatine kinase was not possible. Both injection of aliquots and continuous infusion were used for detection of hydrazine during nitrogenase reduction of azide, and the injection method has been used for ammonia assay during dinitrogen reduction. Dithionite oxidation was measured directly from decolorization of iodine, after trapping both dithionite and bisulfite with formaldehyde.     

Independent coding of movement duration in a repetitive non-visually guided movement

The aim of the present study was to examine the association of high blood lactate levels, induced with a maximal cycling or with an intravenous infusion, with spinal cord excitability. The study was carried out on 17 male athletes; all the subjects performed a maximal cycling test on a mechanically braked cycloergometer, while 6 of them were submitted to the intravenous infusion of a lactate solution (3?mg/kg in 1?min). Before the exercise or the injection, also at the end as well as 5 and 10?min after the conclusion, venous blood lactate was measured and excitability of the spinal α-motoneurons was evaluated by using the H reflex technique. In both experimental conditions, it has been observed that an exhaustive exercise is associated with a strong increase of blood lactate (but not of blood glucose) and with a significant reduction of spinal excitability. Since a similar augment of blood lactate induced by an intravenous infusion, in subjects not performing any exercise, is not associated with significant changes of spinal excitability, it can be concluded that the increase of blood lactate levels during a maximal exercise is not per se capable of modifying the excitability of spinal α-motoneurons.     

Relationship between turnover rate and blood concentration of lactate in exercising dogs.

Steady-state blood lactate concentrationss and lactate turnover, or entry, rates were determined by use of constant infusion of L(+)-[14C]lactate in seven anesthetized dogs before and during electrically induced exercise. Lactate entry rates increased during exercise in all dogs with or without the infusion of additional exogenous cold lactate. Blood lactate concentrations, on the other hand, rose to levels considerably below those predicted for these entry rates in a previous study of the relationship in normal nonexercising dogs. It is concluded that improved efficiency of lactate removal during exercise allows low blood concentrations despite large increases in entry rates.