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Biomedical subjects

S M Humphreys

Publications and source records attributed to S M Humphreys.

66 records · Page 4Linked to original sources
PubMed1991

Peripheral triacylglycerol extraction in the fasting and post-prandial states.

1. Triacylglycerol extraction by subcutaneous adipose tissue and forearm muscle was studied in nine normal subjects after an overnight fast and after the consumption of a mixed meal. 2. There was an inverse correlation between the total plasma fractional triacylglycerol extraction across the adipose tissue and the fasting arterial plasma triacylglycerol concentration. In contrast, there was no correlation between the lower fractional triacylglycerol extraction across the forearm muscle and the fasting plasma triacylglycerol concentration. 3. Chylomicron-triacylglycerol concentrations in arterial(ized) plasma increased post-prandially and peaked at 240-300 min. There was a comparable increase in the very-low-density lipoprotein-triacylglycerol concentration, peaking at 300 min. 4. Clearance of chylomicron-triacylglycerol by adipose tissue increased after the meal (P less than 0.05). In contrast, the clearance of very-low-density lipoprotein-triacylglycerol by adipose tissue decreased post-prandially (P less than 0.05). 5. Although there was significant uptake of chylomicron-triacylglycerol by the forearm muscle post-prandially, this was less than by the adipose tissue. Very-low-density lipoprotein-triacylglycerol was unaffected by passage through the forearm muscle at any time. 6. We conclude that the extraction of lipoprotein-triacylglycerol by human adipose tissue is important in determining the fasting plasma triacylglycerol concentration. Chylomicron-triacylglycerol, appearing in the plasma post-prandially, may compete with very-low-density lipoprotein-triacylglycerol for clearance by adipose tissue lipoprotein lipase, and this mechanism may explain, at least in part, the post-prandial rise in very-low-density lipoprotein-triacylglycerol. Forearm muscle, in contrast, appears to play a much smaller role in the extraction of plasma triacylglycerol, especially that in the very-low-density lipoprotein fraction.

Adipose Tissue↗
PubMed1990

Arteriovenous differences across human adipose and forearm tissues after overnight fast.

Measurements of arteriovenous differences across subcutaneous abdominal tissue (mainly adipose) and deep forearm tissue (mainly muscle) were made on 25 occasions in normal subjects after an overnight fast. Adipose tissue was shown to be strongly lipolytic (releasing nonesterified fatty acids and glycerol), to clear circulating triacylglycerol, glucose, ketone bodies and acetate, and to produce lactate. Uptake of circulating carbohydrate and ketones was sufficient to account for only 51% of the adipose tissue oxygen consumption, implying that adipose tissue utilizes fuel(s) stored within it. The mean fractional re-esterification rate of fatty acids in adipose tissue was 13% to 19%. Arteriovenous differences were converted to fluxes of carbon atoms to compare the movements of different fuels. (Amino acids were not included in these calculations.) Adipose tissue after an overnight fast was a net exporter of carbon, whereas in resting muscle the uptake of carbon atoms from circulating carbohydrate and lipid fuels approximately balanced the CO2 production. Fatty acids were the main form in which carbon left adipose tissue, and the main source of carbon atoms entering the resting forearm.

Acetates↗
PubMed1990

Metabolic responses of forearm and adipose tissues to acute ethanol ingestion.

Although excess ethanol consumption is often considered to lead to adiposity, the metabolic routes by which this might occur are not clear. We have investigated some metabolic consequences of acute ethanol ingestion by measuring arteriovenous differences across forearm muscle and subcutaneous adipose tissue for 6 hours after ingestion of 47.5 g ethanol, in seven normal subjects fasted overnight. The expected systemic effects of ethanol ingestion were observed: slight lowering of the plasma glucose concentration, depression of plasma nonesterified fatty acid (NEFA) concentrations, and elevation of the blood lactate/pyruvate and 3-hydroxybutyrate/acetoacetate ratios. There was a marked reduction in blood total ketone bodies in relation to plasma NEFA concentrations. However, the only major change observed in peripheral tissue metabolism was an increased uptake of acetate into forearm muscle, equivalent, in whole-body terms, to only 3% of the ethanol load. Adipose tissue appeared to show a reduced cytoplasmic state in that it exported an increased ratio of lactate to pyruvate after ethanol ingestion. However, this reduced state did not lead to increased fatty acid reesterification within adipose tissue. No mechanism was clearly identified whereby ethanol ingestion might lead to net deposition of triacylglycerol in adipose tissue.

Acetates↗
PubMed1990

Postprandial substrate deposition in human forearm and adipose tissues in vivo.

1. Substrate movements in forearm muscle and subcutaneous adipose tissue were studied, by measurement of arteriovenous differences and blood flow, in seven normal subjects after an overnight fast and then for 6 h after ingestion of a mixed meal. Overall substrate balances were examined in terms of the flux of gram-atoms of carbon. 2. As found previously, the forearm was approximately in carbon balance (import equal to export) after the overnight fast, whereas adipose tissue was a net exporter of carbon, mainly in the form of non-esterified fatty acids. 3. After the meal, arterialized plasma concentrations of glucose and lactate rose sharply (peak at 60 min), whereas those of non-esterified fatty acids and glycerol fell (nadir at 60-120 min). Plasma triacylglycerol concentrations rose slowly to peak at 240 min;much of this rise was accounted for by a rise in the chylomicron fraction. 4. Both tissues took up glucose at an increased rate after the meal. Release of non-esterified fatty acids and glycerol from adipose tissue was suppressed. Clearance of triacylglycerol by both tissues increased after the meal, but was more marked in adipose tissue, where the fractional extraction of chylomicron-triacylglycerol reached 44% at 240 min. 5. The forearm rapidly became a considerable net importer of carbon, and remained so until 6 h after the meal when it was again in approximate carbon balance. Glucose uptake dominated the forearm carbon balance. Adipose tissue was a net importer of carbon from 30 min until 5 h after the meal and then reverted to net export. Clearance of triacylglycerol carbon made the largest contribution to this positive balance, but towards the end of the study this was increasingly counterbalanced by simultaneous non-esterified fatty acid release.

Adipose Tissue↗
PubMed1990

Stability of plasma glucose during storage.

Plasma glucose concentrations were found to decrease during storage. This phenomenon was not method-dependent; it was demonstrated using both a Beckman Glucose Analyser 2 (based on glucose oxidase) and a hexokinase-based technique on a centrifugal analyser. The loss of glucose was typically of the order of 10% in plasma frozen for periods from 24 h to 14 months. Repeated freezing and thawing (weekly for 1 month) did not produce additional loss of glucose compared with aliquots thawed for the first time at 1 month. Fluoride did not reduce the loss of glucose. Glucose was also lost from frozen neutralized perchloric acid extracts of blood (mean +/- SD: 7 +/- 2 per cent over 1 year). We conclude that this variable loss of glucose on storage may complicate the interpretation of metabolic research procedures.

Blood Glucose↗
PubMed1989

Metabolic characteristics of human adipose tissue in vivo.

1. A method was developed for sampling the venous drainage from the subcutaneous adipose tissue of the anterior abdominal wall. This is a large depot in many subjects, and seems well suited to such studies as it is completely separated from the venous drainage of the underlying muscle by the aponeurosis of the external oblique muscle. 2. Eight normal subjects were studied after an overnight fast, and for 120 min after ingestion of 75 g of glucose. Concentrations of substrates in the abdominal wall drainage were compared with those in arterialized blood and in forearm muscle drainage. 3. Non-esterified fatty acid and glycerol concentrations in the abdominal wall drainage were high (three to four times the arterial level) after overnight fast. After glucose ingestion, arterial and abdominal venous levels fell and the arteriovenous differences narrowed. The forearm showed uptake of non-esterified fatty acids when fasting but not after glucose ingestion, with no significant arteriovenous difference for glycerol at any time. 4. The abdominal wall tissues showed a small arteriovenous difference for glucose uptake during fasting, which increased after glucose ingestion. Although lactate was produced throughout, its molar ratio to glucose uptake was less than that reported for other superficial sites, suggesting only a minor contribution of skin metabolism. Forearm muscle showed a larger and more prolonged increase in arteriovenous difference for glucose uptake after the glucose load, but no consistent release or uptake of lactate. 5. We conclude that the tissue studied by this technique is predominantly adipose. This technique may have wide application in studies of the metabolic basis for body weight regulation in man.

Adipose Tissue↗
PubMed1989

Effects of insulin on human adipose tissue metabolism in vivo.

1. The metabolic effects of insulin on human adipose tissue were studied by combining the euglycaemic clamp technique with measurement of arteriovenous differences across the subcutaneous adipose tissue of the anterior abdominal wall. 2. Eight normal subjects were studied after an overnight fast, and for 120 min during infusion of insulin (mean arterialized plasma insulin 50-55 m-units/l). 3. During the insulin infusion, the arterialized and abdominal venous levels of both non-esterified fatty acids and glycerol fell, and the arteriovenous differences for the release of these substances narrowed. The fractional rate of re-esterification of fatty acids was around 20% in the fasting state and increased to almost 100% during hyperinsulinaemia. 4. In the fasting state the uptake of glucose and 3-hydroxybutyrate by adipose tissue could account for only 20% of the oxygen uptake. During insulin infusion, adipose tissue glucose uptake increased and could account for more than 100% of oxygen uptake, implying storage of glucose. 5. Net balances of different substrates across adipose tissue were examined by calculating fluxes in terms of microgram-atoms of carbon. In the fasting state adipose tissue was in marked negative carbon balance (because of the export of non-esterified fatty acids); during insulin infusion it just reached 'carbon balance'. These results were in contrast to those from a previous study of glucose ingestion, in which the adipose tissue showed marked positive carbon balance (net substrate deposition).

3-Hydroxybutyric Acid↗
PubMed1989

Plasma triacylglycerol extraction in human adipose tissue in vivo: effects of glucose ingestion and insulin infusion.

Extraction of endogenous triacylglycerol (TAG) was measured across the subcutaneous adipose tissue of the abdominal wall and across forearm muscle in 16 studies on 13 normal subjects. After overnight fast there was significant TAG extraction across both adipose tissue (4 per cent) and muscle (1 per cent). After 75 g oral glucose (8 subjects), there was a rise in the arterial TAG concentration to a peak at 60 min, with a concomitant increase in extraction across adipose tissue (to 9 per cent). During insulin infusion (8 subjects; plateau insulin concentration 50-55 mU/l) with the plasma glucose 'clamped' at the fasting level, arterial TAG levels fell steadily, and extraction across adipose tissue and muscle decreased. The ability to measure triacylglycerol extraction by adipose tissue in vivo opens up many possibilities for metabolic and nutritional studies.

Adipose Tissue↗