Walter A. Müller

Effect of insulin on muscle glutamate uptake. Whole blood versus plasma glutamate analysis.

For decades, investigators concerned with protein metabolism in man have performed detailed amino acid analyses of human plasma obtained under a wide range of experimental situations. A large body of information has been used to calculated rates of protein synthesis and proteolysis. During the course of an investigation of the effect of intrabrachial artery infusion of insulin (70 muU/min per kg body weight) on glutamate uptake by human forearm muscle, it was discovered that plasma arterio-deep venous glutamate difference analysis failed to document any increase in the uptake of this amino acid, suggesting that insulin had little influence on glutamate uptake by muscle. However, whole blood glutamate analyses, performed on the same blood samples, revealed that (a) the resting muscle uptake of glutamate is smaller than previously reported and (b) insulin is capable of markedly increasing glutamate uptake by muscle from whole blood. Since the hematocrit was obtained on all samples, detailed analyses of the various compartments in which glutamate could be found were performed. It was determined that circulating blood cells have a dynamic role in glutamate transport. These data underscore the need for both whole blood and plasma amino acid analysis in investigations concerned with protein synthesis and/or amino acid flux, for analysis of plasma samples alone could be misleading as illustrated in the present study.

Blood Cell and Plasma Amino Acid Levels Across Forearm Muscle During a Protein Meal

To elucidate the role of blood cells in amino acid metabolism, substrate balance across the forearm was studied in a nitrogen-depleted subject fed 200 gm. of meat. After ingestion of the meal, there was the expected outpouring of amino acids from the splanchnic bed into the general circulation. Both cell and plasma levels of most amino acids in arterial blood increased rapidly. Whole blood arterio-deep venous amino acid differences frequently differed from that of plasma. In conclusion, it appears that both blood cells and plasma transport amino acids from the splanchnic bed to the periphery and that both participate actively in the deposition of amino acids in the forearm of the subject studied.

Blood amino acid levels in patients with insulin excess (functioning insulinoma) and insulin deficiency (diabetic ketosis)

Blood amino acid concentrations were determined in the postabsorptive state in nine patients with insulin excess (functioning insulinomas), nine juvenile-type diabetics with insulin deficiency (diabetic ketosis due to insulin withdrawal), six juvenile diabetics in moderate metabolic control, and five healthy control subjects. Blood branched-chain amino acid (BCAA) levels were elevated in diabetic ketosis and decreased in patients with insulinomas. Blood concentrations of BCAA were significantly correlated to blood glucose levels, and in diabetics they were also correlated to blood ketone bodies, serum free fatty acids, and glycerol levels. These data indicate an inverse relationship between circulating effective insulin levels and blood BCAA concentrations. It is suggested that blood levels of BCAA might represent an indicator of insulin-dependent alterations of protein metabolism.

Studies of Glucagon Secretion in Pancreatectomized Patients

Intravenous arginine infusions were performed in two totally pancreatectomized patients and two age/sex-matched normal subjects. Plasma glucagon concentrations did not increase in the pancreatectomized patients, whereas a four- to sixfold rise of the glucagon levels following arginine administration was seen in the control subjects. Measurements of plasma glucagon-like im-munoreactivity revealed no difference between normal and pancreatectomized subjects. The data suggest the absence of a significant number of normally functioning alpha cells in extrapancreatic sites.

Extrapancreatic Glucagon and Glucagonlike Immunoreactivity in Depancreatized Dogs: A QUANTITATIVE ASSESSMENT OF SECRETION RATES AND ANATOMICAL DELINEATION OF SOURCES

The anatomical sites and the rates of extrapancreatic secretion of glucagon and of glucagon-like immunoreactivity (GLI) were assessed in dogs 2 h after pancreatectomy by catheterization of the gastrosplenic and mesenteric veins. Glucagon release from the gastrosplenic area approximated one-fourth that of a normal pancreas and rose from 0.25 to 1.0 ng/kg per min during arginine stimulation. Intestinal glucagon secretion was small and did not respond to arginine, suggesting that the stomach is the only important extrapancreatic source of glucagon. Glucagon concentrations attained by gastrosplenic secretion were in close proportion to those obtained during the administration of exogenous glucagon, indicating similar clearance rates of extrapancreatic and pancreatic glucagon, approximating 10 ml/kg per min.GLI secretion (0.3 ng eq/kg per min) was limited to the intestinal area and was transiently stimulated by arginine and exogenous glucagon. Base-line GLI clearance approximated 1 ml/kg per min. No insulin secretion could be detected. Gastrointestinal glucose uptake rose from 0.56 to 2.2 mg/kg per min after glucagon administration suggesting that as much as 10% of total glucose production can be taken up by the gastrointestinal tract. In two dogs both the stomach and pancreas were removed. Intestinal glucagon release remained small and did not increase during arginine administration. By contrast, GLI release was stimulated by both arginine and exogenous glucagon.

Glucagon Immunoreactivities and Amino Acid Profile in Plasma of Duodenopancreatectomized Patients

Glucogon immunoreactivity (IRG) was measured in plasma of duodenopancreatectomized subjects with a nonspecific (K-4023) and a specific (30-K) glucagon antiserum. After an overnight fast, plasma IRG (K-4023) was significantly (P < 0.05) higher in the subjects without pancreas, averaging 782+/-79 (SEM) pgeq/ml, than in the controls (482+/-80 pgeq/ml). IRG (30-K) of 162+/-68 pg/ml did not change during an infusion of arginine (450 mg/kg per 40 min). Insulin deprivation during 3 d in one patient did not restore the IRG response to arginine as reported in depancreatized dogs.Bio-Gel P-30 column chromatography revealed that virtually all IRG (30-K) measured in whole plasma was of different molecular weight than glucagon, and primarily of a mol wt >/= 40,000. Intravenous arginine did not significantly alter the chromatographic pattern of these plasmas. Thus, as postulated by others, duodeno-pancreatectomized humans have virtually no circulating 3,500-dalton glucagon. Hence, the presence of 3,500-dalton glucagon in plasma is not a condition for the diabetic state. It might, nevertheless, when present in normal or excessive amounts, worsen the metabolic state of diabetic patients. Among 14 amino acids measured in plasma of these patients, the concentrations of alanine, serine, ornithine, and arginine were significantly (P < 0.05) elevated to approximately twice that of normal: alanine and serine are both substrates for gluconeogenesis, whereas ornithine and arginine are involved in the formation of urea, the second product of hepatic gluconeogenesis. As the concentrations of branched chain amino acids were not grossly altered, it is hypothesized that this amino acid pattern is a consequence of glucagon deficiency rather than secondary to the diabetic state of these patients.

Effect of glucagon on amino acid and nitrogen metabolism in fasting man

Abstract The infusion of small amounts of glucagon into fasting subjects has been reported to paradoxically decrease urinary urea nitrogen excretion. The mechanism and tissues by which this apparent protein sparing was accomplished was examined in the current study by infusing glucagon ( 0.1 mg 24 hr × 4 days ) into subjects who had fasted for 5–6 wk. It was found that circulating levels of alanine and glutamine declined while urinary urea nitrogen excretion decreased and ammonia nitrogen excretion increased. These findings suggested that hepatic gluconeogenesis had been diminished while renal ammoniagenesis and gluconeogenesis had increased. The further finding of a significant prolongation of the t , 1 2 of 14C-lalanine (U) 24 hr after the start of the infusion appeared to substantiate this diminution in hepatic gluconeogenesis. In addition, while serum insulin levels had declined significantly by the end of the infusion, circulating levels of the branched-chain amino acids had increased. It was concluded that the infusion of small amounts of glucagon may have resulted in a diminution of portal glucagon levels, which in turn resulted in a decrease in hepatic gluconeogenesis and, directly or indirectly, a compensatory increase in renal ammoniagenesis and gluconeogenesis. The coincidental decline in alamine and the increase in levels of the branched-chain amino acids suggest that the infused glucagon had affected peripheral amino acid metabolism as well.

Amino acid levels across normal forearm muscle: Whole blood vs. plasma☆

Abstract The role of human blood cells in interorgan amino acid flux has come under scrutiny. The cells have previously been thought to be inert in amino acid transport; however, evidence has recently become available attributing to the cells an active role in amino acid metabolism. In order to assess the implications of these findings with respect to basal calculations of protein mobilization and utilization, whole blood and plasma arterial and deep venous amino acid levels and arterio-deep venous differences across the forearm of 8 normal subjects were determined. Four of these subjects were then fed 200 g of broiled ground sirloin. Paired arterio-deep venous blood samples were obtained for determination of whole blood and plasma glutamine levels at 1, 2, 3 and 4 hr. It was found that in the basal state, plasma arterio-deep venous differences were considerably greater than whole blood arterio-deep venous differences suggesting a minimal role played by the cells. Following the ingestion of the meal, whole blood arterial and deep venous glutamine levels did not change significantly. However, compartmental analysis revealed a decrease in arterial blood cell glutamine and an increase in arterial and deep venous plasma glutamine. It was concluded that both plasma and whole blood amino acid determinations should be performed in studies concerned with interorgan amino acid transport.

Effects of β-hydroxybutyrate, glycerol, and free fatty acid infusions on glucagon and epinephrine secretion in dogs during acute hypoglycemia

The importance of glucagon in the regulation of carbohydrate metabolism is clearly established. However, the role played by this hormone in the regulation of the overall fuel economy is less certain, particularly with respect to such nonglucose fuels as free fatty acids, glycerol, and ketoacids. In order to elucidate glucagons role with respect to the latter substrates, dogs were infused with solutions of these three fuels, and their A-cell responses to concomitant insulin-induced hypoglycemia were studied. In addition, epinephrine levels were also monitored. It was found that while these infusions failed to suppress glucagon release, the ketoacid infusion did significantly reduce epinephrine secretion during the insulin-induced hypoglycemic period. It was therefore concluded that glucagon secretion under these experimental conditions is not responsive to prevailing non-glucose fuel levels. Indeed, these data suggest that the sympathetic nervous system may play an important role in the regulation of the over-all fuel economy.

Hormonal regulation of glutamine metabolism in fasting man

Abstract Regulation of glutamine metabolism has previously been thought to be a function of the acid-base state. However, following the intravenous administration of small amounts of glucagon and insulin to subjects undergoing therapeutic fasting, circulating plasma glutamine levels were markedly lowered. Coincident with these infusions, urinary NH 3 nitrogen excretion increased (glucagon) and urea nitrogen excretion decreased (glucagon, insulin). These data suggest that glutamine handling by kidney, liver, and possibly muscle is subject to hormonal regulation.