Robert A. Gelfand

Role of counterregulatory hormones in the catabolic response to stress.

Patients with major injury or illness develop protein wasting, hypermetabolism, and hyperglycemia with increased glucose flux. To assess the role of elevated counterregulatory hormones in this response, we simultaneously infused cortisol (6 mg/m2 per h), glucagon (4 ng/kg per min), epinephrine (0.6 microgram/m2 per min), and norepinephrine (0.8 micrograms/m2 per min) for 72 h into five obese subjects receiving only intravenous glucose (150 g/d). Four obese subjects received cortisol alone under identical conditions. Combined infusion maintained plasma hormone elevations typical of severe stress for 3 d. This caused a sustained increase in plasma glucose (60-80%), glucose production (100%), and total glucose flux (40%), despite persistent hyperinsulinemia. In contrast, resting metabolic rate changed little (9% rise, P = NS). Urinary nitrogen excretion promptly doubled and remained increased by approximately 4 g/d, reflecting increased excretion of urea and ammonia. Virtually all plasma amino acids declined. The increment in nitrogen excretion was similar in three additional combined infusion studies performed in 3-d fasted subjects not receiving glucose. Cortisol alone produced a smaller glycemic response (20-25%), an initially smaller insulin response, and a delayed rise in nitrogen excretion. By day 3, however, daily nitrogen excretion was equal to the combined group as was the elevation in plasma insulin. Most plasma amino acids rose rather than fell. In both infusion protocols nitrogen wasting was accompanied by only modest increments in 3-methylhistidine excretion (approximately 20-30%) and no significant change in leucine flux. We conclude: (a) Prolonged elevations of multiple stress hormones cause persistent hyperglycemia, increased glucose turnover, and increased nitrogen loss; (b) The sustained nitrogen loss is no greater than that produced by cortisol alone; (c) Glucagon, epinephrine, and norepinephrine transiently augment cortisol-induced nitrogen loss and persistently accentuate hyperglycemia; (d) Counterregulatory hormones contribute to, but are probably not the sole mediators of the massive nitrogen loss, muscle proteolysis, and hypermetabolism seen in some clinical settings of severe stress.

Growth Hormone Stimulates Skeletal Muscle Protein Synthesis and Antagonizes Insulin's Antiproteolytic Action in Humans

We examined the effects of a combined, local intra-arterial infusion of growth hormone (GH) and insulin on forearm glucose and protein metabolism in seven normal adults. GH was infused into the brachial artery for 6 h with a dose that, in a previous study, stimulated muscle protein synthesis (phenylalanine Rd) without affecting systemic GH, insulin, or insulinlike growth factor I concentrations. For the last 3 h of the GH infusion, insulin was coinfused with a dose that, in the absence of infused GH, suppressed forearm muscle proteolysis by 30–40% without affecting systemic insulin levels. Measurements of forearm glucose, amino acid balance, and [3H]phenylalanine and [14C]leucine kinetics were made at 3 and 6 h of the infusion. Glucose uptake by forearm tissues in response to GH and insulin did not change significantly between 3 and 6 h. By 6 h, the combined infusion of GH and insulin promoted a significantly more positive net balance of phenylalanine, leucine, isoleucine, and valine (all P < 0.05). The change in net phenylalanine balance was due to a significant increase in phenylalanine Rd (51%, P < 0.05) with no observable change in phenylalanine Ra. For leucine, a stimulation of leucine Rd (50%, P < 0.05) also accounted for the change in leucine net balance, with no suppression of leucine Ra. The stimulation of Rd, in the absence of an observed effect on Ra, suggests that GH blunts the action of insulin to suppress proteolysis in addition to blunting insulins action on Rd.

Measurement of L-[1-14C]Leucine Kinetics in Splanchnic and Leg Tissues In Humans: Effect of Amino Acid Infusion

Although whole-body leucine flux is widely measured to study body protein turnover in humans, the contribution of specific tissues to the total-body measurement remains unknown. By combining the organ-balance technique with the systemic infusion of L-[1-14C]leucine, we quantitated leucine production and disposal by splanchnic and leg tissues and by the whole body, simultaneously, in six normal men before and during amino acid infusion. At steady state, disposal of arterial leucine by splanchnic and leg tissues was calculated from the percent extraction (E) of L-[1-14C]leucine counts: uptake = E × [Leu]a × flow. Tissue release of cold leucine (from protein turnover) into vein was calculated as the difference between leucine uptake and the net tissue leucine balance. In the postabsorptive state, despite substantial (P < .01) extraction of L-[1-14C]leucine by splanchnic (23 ± 1%) and leg (18 ± 2%) tissues, net leucine balance across both tissue beds was small, indicating active simultaneous disposal and production of leucine at nearly equivalent rates. Splanchnic tissues accounted for −50% of the measured total-body leucine flux. During amino acid infusion, the net leucine balance across splanchnic and leg tissues became positive, reflecting not only an increase in leucine uptake but also a marked suppression (by ∼50%, P < .02) of cold leucine release. This reduction in splanchnic and leg leucine release was indicated by a sharp decline in whole-body endogenous leucine flux. Conclusions: 1) combining the organ-balance method with systemic L-[1-14C]leucine infusion enables leucine kinetics to be measured simultaneously in the whole body and in specific tissues; 2) splanchnic tissues account for −50% of whole-body leucine flux in postabsorptive humans; and 3) amino acid infusion markedly suppresses leg and splanchnic tissue leucine release, which may indicate inhibition of proteolysis.

Catabolic effects of thyroid hormone excess: The contribution of adrenergic activity to hypermetabolism and protein breakdown

Although patients with thyrotoxicosis improve clinically after treatment with beta-adrenergic blocking drugs, it has never been established whether the hypermetabolism and body protein wasting caused by thyroid hormone excess are actually mediated by adrenergic mechanisms. To evaluate this issue, we measured basal energy expenditure, epinephrine-stimulated calorigenesis, and leucine kinetics (an index of body protein catabolism) in six normal volunteers before and after triiodothyronine (T3) administration (150 micrograms/d for 1 week). Serum T3 rose nearly threefold (P less than 0.001) during T3 administration, producing significant increases in basal metabolic rate (21%, P less than 0.001), nitrogen excretion (45%, P less than 0.001), and leucine flux (45%, P less than 0.01). In response to epinephrine infusion, the absolute rise in metabolic rate above basal was 57% greater in the thyrotoxic condition (P less than 0.02). Although beta-adrenergic blockade with intravenous propranolol totally abolished the calorigenic response to epinephrine, it had no detectable effect on either the accelerated basal metabolic rate or the augmented body protein catabolism caused by thyroid horomone excess. Our data suggest that in the basal, resting state, the increased metabolic rate and accelerated protein breakdown caused by thyroid hormone are not adrenergically mediated. However, under nonbasal conditions (when sympathetic activity is stimulated), enhanced responsiveness to catecholamine calorigenesis may exaggerate the hypermetabolic state and thereby contribute to weight loss and other clinical manifestations of thyrotoxicosis. This mechanism may explain the clinical efficacy of beta-adrenergic blocking agents in the treatment of thyrotoxicosis.

Skeletal Muscle Proteolysis in Rats With Acute Streptozocin-Induced Diabetes

Skeletal muscle proteolysis was studied in rats 1 day after induction of diabetes with 65 mg/kg streptozocin. An evisceration procedure, including functional hepatectomy-nephrectomy, was performed, and the rate of proteolysis in the remaining tissues, primarily skeletal muscles, was evaluated over 2 h. With cycloheximide to block protein synthesis, total protein breakdown was measured from the rate of rise in plasma tyrosine concentration. The rate of degradation of contractile (myofibrillar) protein was estimated from the rate of rise in plasma concentration of 3-methylhistidine released from the breakdown of actomyosin. Compared with nondiabetic control preparations, the total protein degradation rate was increased 30% by diabetes (P < .001), and myofibrillar catabolism was accelerated by 60% (P < .005). In diabetes, the increase in proteolysis was accompanied by reductions in circulating insulin to 25–50% of normal level, whereas food intake did not differ from control. Treatment of diabetic rats with exogenous insulin, including acute infusions postoperatively, completely reversed the proteolytic effects of diabetes. The findings demonstrate that the hypoinsulinemia of acute diabetes increases the catabolism of skeletal muscle protein and that the inhibitory effect of normal levels of insulin includes a specific action to restrain myofibrillar proteolysis.

Defective Free-Fatty Acid and Oxidative Glucose Metabolism in IDDM During Hypoglycemia: Influence of Glycemic Control

To examine the impact of diabetes and its treatment on plasma free-fatty acid (FFA) and oxidative fuel metabolism during hypoglycemia, we combined indirect calorimetry with [3-3H]glucose during a 4-h low-dose insulin infusion (plasma insulin ∼2-fold above basal) in six poorly controlled and nine wellcontrolled insulin-dependent diabetes mellitus (IDDM) patients and in six healthy subjects. Diabetic subjects received insulin overnight to maintain euglycemia before study. Although free-insulin levels and counterregulatory hormone responses were similar, the plasma glucose fall was more pronounced in wellcontrolled diabetic subjects. In well-controlled diabetic and healthy subjects, the small increment in insulin rapidly suppressed plasma FFA and fat oxidation by ∼50% and stimulated carbohydrate oxidation by ∼80%. In contrast, plasma FFA levels did not fall in poorly controlled diabetic subjects, and glucose oxidation was not stimulated. To determine whether this resistance to the antilipolytic effect of insulin occurs in the absence of hypoglycemic counterregulation, we used a sequential low-dose euglycemic insulin clamp (0.2, 0.3, and 0.5 mU · k g−1 · mirr1). In healthy subjects, plasma FFA was nearly maximally suppressed at the lowest insulin dose. In contrast, plasma FFA remained persistently elevated in poorly controlled diabetic subjects at each insulin dose. However, the insulin dose-response curve for suppression of plasma FFA was near normal in well-controlled subjects. We conclude that poorly controlled IDDM diabetic patients are resistant to the antilipolytic effects of insulin and show impaired stimulation of glucose oxidation during insulin-induced hypoglycemia. Amelioration of these defects in well-controlled patients may be another factor contributing to the higher risk of hypoglycemia during intensified insulin therapy.

Influence of Glucocorticoids on Skeletal Muscle Proteolysis in Normal and Diabetic-Adrenalectomized Eviscerated Rats

The effect on skeletal muscle proteolysis of acute (20-hour) glucocorticoid treatment (dexamethasone 1.5 mg/kg, subcutaneously [SC]) was tested using the eviscerated rat preparation. According to this method, the peripheral tissues (primarily the skeletal muscles) are isolated by functional hepatectomy-nephrectomy. Total proteolysis is estimated from the rate of rise of plasma tyrosine concentration in the presence of cycloheximide to block protein synthesis. Myofibrillar proteolysis is measured from the rate of release into the plasma of the nonreutilized, nonmetabolized amino acid 3-methylhistidine (3MH), in the absence of cycloheximide. In normal rats, dexamethasone increased total proteolysis by 20% and myofibrillar proteolysis by 75% (both P less than .025 v saline controls). In diabetic-adrenalectomized rats prepared 2 weeks earlier (65 mg/kg streptozocin [STZ] followed by adrenalectomy), dexamethasone caused much greater increments in rates of total proteolysis (94%) and myofibrillar proteolysis (240%) (both P less than .001 v saline controls). Because diabetic animals are extremely sensitive to glucocorticoid-induced proteolysis, we also examined whether the acute proteolytic effect of diabetes itself might be mediated by adrenal cortical hormones. Previously adrenalectomized rats studied 20 hours after STZ showed a 40% augmentation of total proteolysis (P less than .01), an effect similar to that produced by acute diabetes in rats with intact adrenals. We conclude that glucocortical hormones cause a catabolic effect on total and myofibrillar skeletal muscle protein which is exaggerated when the counteracting action of insulin is reduced, but that the excess proteolysis of acute insulin deficiency is independent of the endogenous glucocorticoids secretion.

Nitrogen conservation in starvation revisited: protein sparing with intravenous fructose

The provision of small amounts of glucose during fasting is known to spare body protein and to attenuate markedly the metabolic response to starvation. These actions, which are not shared by fat, are generally thought to depend on the ability of exogenous glucose to stimulate insulin secretion. To determine whether fructose, a very weak insulin secretagogue, will also conserve nitrogen and alter the response to fasting, we infused small amounts of fructose, 100 g/d (375 kcal), into 7 obese subjects during a 10-day fast: 4 received fructose days 7 to 10, and 3 received fructose days 1 to 7. Fructose virtually abolished (all P less than 0.05-0.01) the fasting induced: (a) fall in glucose and insulin and rise in glucagon, (b) fall in triiodothyronine, (c) ketosis and acidosis, (d) increased ammonia excretion, (e) hyperuricemia (and hypouricosuria), and (f) fall in plasma alanine and rise in branched chain amino acids. Fructose also significantly reduced urinary sodium loss. Moreover, fructose exerted a prominent protein-sparing action, even though plasma insulin concentrations never exceeded postabsorptive levels. Excretion of total nitrogen was reduced by 40% to 50% during periods of fructose infusion, reflecting significant suppression of both urea and ammonia generation (all P less than 0.05-0.01). Most plasma glucogenic amino acids rose significantly during fructose administration. We conclude that low-dose fructose infusion essentially abolishes the entire hormone-substrate response to fasting, and spares body protein without raising insulin above postabsorptive levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Is Exogenous Fructose Metabolism Truly Insulin Independent

Although fructose is widely regarded as an insulin-independent fuel source, its in vivo conversion to glucose represents a theoretical limitation to its clinical usefulness in diabetics, particularly if given in large doses. To determine whether small amounts of fructose can be well utilized in the setting of insulinopenia, we administered a low-dose fructose infusion (4.2 g/hr) to a fasting type 1 diabetic patient receiving continuous subcutaneous insulin at a dose that had previously maintained stable euglycemia for 72 hr (plasma glucose = 80-110 mg/dl). Despite the low infusion rate (less than 20% of calorie requirement), fructose caused an immediate and marked rise in plasma glucose (to 370 mg/dl after 27 hr). Glucose loss in the urine and accumulation in plasma could account for fully half of the administered hexose load. Thus, the utilization of even small quantities of exogenous fructose is profoundly impaired under hypoinsulinemic conditions. It is misleading to regard fructose as a truly insulin-independent fuel source.