Citations

To determine the effects of acute metabolic acidosis and alkalosis on leucine metabolism in vivo, mongrel dogs were infused with [1-14C] leucine for 8 h, along with NaCI, HCI, or NaHCO3 over the last 4 h. Arterial pH did not change from the basal value during NaCI infusion but decreased (P <.01) and increased (P <.01) during HCI and NaHCO3 infusions, respectively. Total leucine carbon entry did not change from the basal value during saline infusion but increased (P <.01) with acidosis and decreased (P <.05) with alkalosis. Compared with saline controls, acidosis increased (P <.01) leucine oxidation, whereas alkalosis decreased (P <.01) leucine oxidation. During acidosis, total plasma essential and nonessential amino acid concentrations increased (P <.05), whereas during alkalosis, total plasma essential and nonessential amino acid concentrations decreased (P <.05). These studies suggest that acute alterations in arterial pH may affect the regulation of protein metabolism in vivo and must be considered in the interpretation of results from experiments in which alterations of acid-base homeostasis may have occurred.

Basal insulin supplementation has been used as a therapy for patients with non-insulin-dependent diabetes mellitus (NIDDM) who require insulin. To determine whether basal insulin supplementation in addition to lowering postabsorptive plasma glucose concentration also improves the postprandial pattern of glucose disposition, glucose metabolism after ingestion of a solid mixed meal was assessed in obese patients with NIDDM before and after treatment with ultralente and compared with glucose metabolism observed in nondiabetic subjects. Splanchnic uptake of ingested glucose clearance was assessed by including [2-3H]glucose (a tracer that only minimally cycles through glycogen) in a solid mixed meal. Postprandial gluconeogenesis was estimated by measuring the rate of incorporation of carbon dioxide into glucose. Net glucose and lipid oxidation were measured by indirect calorimetry. Both splanchnic uptake of ingested glucose (27 ± 1 vs. 14 ± 2 g) and postprandial hepatic glucose release (51 ± 5 vs. 24 ± 3 g) were greater (P < .001) in diabetic than in nondiabetic subjects. Although the percentage of postprandial hepatic glucose release accounted for by glucose synthesis from bicarbonate was similar in the two groups (25 ± 2 vs. 35 ± 5%), the absolute rate was greater in the diabetic patients (13 ± 1 vs. 8 ± 1 g; P < .05). Postprandial glucose oxidation and glucose disposal (measured either isotopically or by the forearm-catheterization technique) were similar in both groups. However, total lipid oxidation was increased in the diabetic patients. (P < .05). Two weeks of basal insulin supplementation lowered fasting glucose concentrations (from 219 ± 22 to 144 ± 21 mg/dl; P < .01) and integrated postprandial glycemic response (from 814 ± 68 to 621 ± 72 min · mg· ml−1) but not to normal. Although circulating insulin concentrations were two- to threefold greater (P < .02) after 3 mo of basal insulin supplementation, the postprandial pattern of glucose metabolism remained essentially the same. Basal insulin supplementation decreased (P < .05) both splanchnic uptake of ingested glucose and hepatic glucose release. The addition of a preprandial injection of soluble insulin to basal insulin supplementation further suppressed (P < .05) postprandial hepatic glucose release, thereby further improving postprandial glucose tolerance. These studies indicate that initial splanchnic glucose clearance, hepatic glucose release, and new glucose synthesis, as well as extrahepatic substrate metabolism, are altered in NIDDM after ingestion of a mixed meal. Basal insulin supplementation improves but does not normalize postprandial glucose metabolism.

To determine whether insulin regulation of lipolysis is abnormal in subjects with poorly controlled insulin-dependent diabetes mellitus (IDDM), free-fatty acid flux ([1-14C]palmitate) was measured under conditions ranging from complete insulin withdrawal to hyperinsulinemia. Seven nondiabetic and seven IDDM subjects were studied with the pancreatic clamp technique to control plasma insulin, growth hormone, and glucagon concentrations at the desired levels. Preliminary studies were performed to validate the experimental design. The palmitate flux response to insulin withdrawal (2.5 ± 0.2 vs. 2.5 ± 0.2 μmol · kg−1 · min−1) and maximally antilipolytic insulin concentrations (0.17 ± 0.02 vs. 0.23 ± 0.03 μmol · kg−1 · min−1) were not different in nondiabetic and IDDM subjects, respectively. In contrast, IDDM subjects required significantly greater plasma free-insulin concentrations to result in equivalent suppression of palmitate flux compared with nondiabetic subjects. Lipolysis was found to be very sensitive to insulin in nondiabetic humans, with half-maximal suppression occurring at plasma free-insulin concentrations of ∼12 pM (< 2 μU/ml). We conclude that adipose tissue lipolysis is normally exquisitely sensitive to insulin and that sensitivity, but not responsiveness to insulin, is impaired in poorly controlled IDDM.

To determine the role insulin resistance may play in the catabolic effect of high-dose prednisone therapy, healthy volunteers were studied on four occasions with the hormone-clamp technique at two insulin infusion rates. Subjects were studied after 5 days of prednisone (60 mg/day) or no steroid treatment and were infused with somatostatin, glucagon, growth hormone, [3H]glucose, [14C]leucine, and insulin (0.1 or 0.2 mil · kg1 · min1). At each rate of insulin infusion, the rate of leucine oxidation was increased (P < .001) after steroid treatment. Leucine flux, an indicator of whole-body proteolysis, was similar in the presence or absence of steroid treatment (2.26 ± 0.08 vs. 2.13 ± 0.04 μmol · kg1 · min1 respectively) at the lower rate of insulin infusion but was higher during steroid treatment (2.18 ± 0.06 vs. 1.84 ± 0.13 μmol · kg1 · min1) at the 0.2-mU · kg1 · min1 insulin infusion. Steroid pretreatment had no significant effect on the nonoxidative rates of leucine disappearance. These data provide strong evidence that the protein wasting associated with glucocorticosteroid therapy is in part the result of steroid-induced resistance to the antiproteolytic effect of insulin and an increase in the oxidation (and thus wasting) of one essential amino acid, leucine.

Recent studies indicate that hydrogen-labeled glucose tracers underestimate glucose turnover in humans under conditions of high flux. The cause of this underestimation is unknown. To determine whether the error is time-, pool-, model-, or insulin-dependent, glucose turnover was measured simultaneously with [6-3H]-, [6,6-2H2]-, and [6-14C]glucose during a 7-h infusion of either insulin (1 mU · kg−1 · min−1) or saline. During the insulin infusion, steady-state glucose turnover measured with both [6-3H]glucose (8.0 ± 0.5 mg · kg−1 · min−1) and [6,6-2H2]glucose (7.6 ± 0.5 mg kg−1 min−1) was lower (P < .01) than either the glucose infusion rate required to maintain euglycemia (9.8 ± 0.6 mg · kg−1 · min−1) or glucose turnover determined with [6-14 C]glucose and corrected for Cori cycle activity (9.8 ± 0.7 mg · kg−1 · min1). Consequently “negative” glucose production rates (P <.01) were obtained with either [6-3 H]- or [6,6-2 H2]-but not [6-14 C]glucose. The difference between turnover estimated with [6-3 H]glucose and actual glucose disposal (or 14C glucose flux) did not decrease with time and was not dependent on duration of isotope infusion. During saline infusion, estimates of glucose turnover were similar regardless of the glucose tracer used. High-performance liquid chromatography of the radioactive glucose tracer and plasma revealed the presence of a tritiated nonglucose contaminant. Although the contaminant represented only 1.5% of the radioactivity in the [6-3H]glucose infusate, its clearance was 10-fold less (P <.001) than that of [6-3H]glucose. This resulted in accumulation in plasma, with the contaminant accounting for 16.6 ± 2.09 and 10.8 ± 0.9% of what customarily is assumed to be plasma glucose radioactivity during the insulin or saline infusion, respectively (P < .01). When corrected for the presence of the contaminant, glucose turnover determined with [6-3H]glucose during insulin infusion (9.5 ± 0.6 mg · kg−1 · min−1) no longer differed from either the glucose infusion rate or that determined with [6-14C]glucose. Therefore, the underestimation of glucose turnover during insulin infusion and negative glucose production rates observed with traditional methods to analyze plasma radioactivity and commercially available tracers is the result of an artifactual increase in [6-3H]glucose specific activity. The etiology of the underestimation of glucose turnover with [6,6-2H2]glucose remains to be determined.

To determine the effects of insulin on dietary and endogenous leucine metabolism, five normal subjects, seven insulin-insufficient insulin-dependent (IDDM) diabetic patients, and five diabetic patients controlled with continuous subcutaneous insulin infusion (CSII) were studied before and for 8 h after ingestion of a chemically defined elemental test meal (10 cal/kg) containing crystalline amino acids. L-[1-14C]leucine was included in the meal to trace the entry and oxidation of the dietary leucine. Total (meal + endogenous) entry of leucine into the circulation was estimated with a constant infusion of [2H3]leucine. Postabsorptive and meal-related increases in the plasma leucine concentration were greater (P < .05) in the insulin-insufficient IDDM than in the normal subjects but returned to near-normal values with CSII. Baseline leucine flux was ∼40% greater in the insulin-insufficient IDDM than in normal subjects (2.17 ± 0.17 vs. 1.55 ± 0.15 μmol · kg−1 · min−1, respectively; .05 < P < .01) but were near normal during CSII treatment (1.85 ± 0.25 μmol · kg−1 · min−1). Furthermore, total leucine entry during meal absorption was greater in the insulin-insufficient IDDM (1.41 ± 0.10 mmol · kg−1.8 h−1) than in either normal (0.96 ± 0.08 mmol · kg−1 · 8 h−1, P < .01) or IDDM subjects during CSII treatment (1.09 ± 0.11 mmol · kg−1 · 8 h−1, P < .05). Fractional oxidation (∼40–50%) and entry of dietary leucine were similar in all three groups. The calculated rate of leucine entry from endogenous proteins over the 8 h of study in insulin-insufficient IDDM (1.04 ± 0.12 mmol · kg−1 · 8 h−1) was significantly greater than in normal subjects (0.64 ± 0.07 mmol · kg−1 · 8 h−, P < .01) or in IDDM subjects during CSII treatment (0.76 ± 0.10 mmol · kg−1 · 8 h−1, P < .05). However, during meal absorption, the calculated rate of leucine entry from endogenous protein was significantly suppressed below baseline only in the normal subjects. In summary, during absorption of a mixed meal, 1) one-third of the leucine entering the peripheral plasma space is derived from the diet in normal humans; 2) the rate of absorption and oxidation of dietary leucine in IDDM is normal regardless of adequacy of insulin therapy; and 3) postprandial hyperleucinemia in poorly controlled diabetic patients is at least partly the result of inadequate suppression of the entry rate of leucine from endogenous protein, suggesting persistence of excessive rates of proteolysis, which is at least partly corrected by intensified insulin treatment. Thus, insulin-mediated suppression of endogenous proteolysis is an integral part of protein anabolism during meal absorption.

Clinical risk factors for nephropathy were assessed in a population-based study of Rochester, Minnesota, residents with diabetes mellitus initially diagnosed between 1945 and 1969 (incidence cohort). The 1031 Rochester residents with non-insulin-dependent diabetes mellitus (NIDDM) were followed through their complete medical records in the community to 1 January 1982. The prevalence of persistent proteinuria was 8.2% at the diagnosis of NIDDM. Among those initially free of persistent proteinuria, the subsequent incidence was 15.3/1000 person-yr. Twenty years after the diagnosis of diabetes, the cumulative incidence of persistent proteinuria was 24.6%. A proportional hazards model identified the following risk factors for persistent proteinuria in NIDDM: elevated initial fasting blood glucose (P < .01); older age at onset of diabetes (P < .01); male gender (P = .05); and presence of macrovascular disease (P = .05), diabetic retinopathy (P = .05), or glycosuria (P = .07) at the diagnosis of diabetes. Separate analyses controlling for attained age indicated no association between duration of NIDDM and the incidence of persistent proteinuria. Stratified analysis of the two most significant risk factors (fasting blood glucose and age) indicated that hyperglycemia was a stronger risk factor for proteinuria in younger diabetic subjects, perhaps because of a competing risk of death in the elderly diabetic patient. In contrast to a recently described decreasing secular trend of proteinuria in Danish insulin-dependent diabetes mellitus patients, there was no decrease over the past 40 yr in proteinuria risk in this NIDDM incidence cohort.

To assess the possible influence of moderate titer insulin antibodies on diabetic glycemic control, we examined insulin-antibody equilibrium binding characteristics, postprandial glucose tolerance, and plasma free-insulin profiles after subcutaneous injection of both porcine and human insulin (0.15 U/kg) in 12 patients with insulin-dependent diabetes mellitus under conditions simulating intensive insulin therapy. The patients' antibodies bound porcine and human insulin indistinguishably, and their plasma glucose and freeinsulin profiles after ingestion of a standard meal were similar with both insulins. Initial increases in plasma free-insulin levels after injection of both insulins were negatively correlated with both insulin-antibody binding (r = −.55, P < .006) and postprandial hyperglycemia (peak level r = −.56, P < .006); the latter was positively correlated with insulin-antibody binding (r = .48, P < .02). The effects of insulin antibodies on postprandial plasma free-insulin and glucose levels could be accounted for substantially by the association constant of the high-affinity insulin-antibody binding sites (K1); patients in the highest quartile for K1 had significantly slower initial increments in plasma free insulin (0.31 ± 0.04 vs. 0.46 ± 0.06 μU/min, P < .05) and greater postprandial hyperglycemia (peak value 237 ± 10 vs. 166 ± 12 mg/dl, P < .001) than patients in the lowest quartile. We conclude that moderate insulin-antibody titers commonly found in insulin-treated patients can slow the early increase in plasma free insulin after subcutaneous injection and that this impairs postprandial glucose tolerance; such an effect may Received for publication 22 April 1986 and accepted in revised form 18 September 1986. limit the effectiveness of intensive insulin therapy.

Competition between glucose and free fatty acids as metabolic fuels is supported by both in vitro and in vivo data, but whether amino acids can also compete with glucose as a source of energy in vivo remains to be established. To determine the effect of increased availability of an amino acid on whole-body glucose flux and glucose carbon uptake by the human forearm, five groups of overnight-fasted normal subjects were infused with either saline, leucine (at 0.5 or 1.0μmol · kg−1 · min−1), isoleucine (0.5 μmol · kg−1 · min−1), or threonine (0.5 μmol · kg−1 · min−1). Plasma glucose concentrations and glucose flux decreased similarly in all groups. No significant changes in forearm output of leucine carbon, isoleucine carbon, or threonine were seen during saline infusion. In contrast, during leucine infusion there was a dose-dependent increase (r = .86, P < .001) in leucine carbon uptake with increased arterial leucine and a-ketoisocaproate concentrations. During infusions of isoleucine and threonine, increases (P < .05) in isoleucine carbon uptake and threonine uptake, respectively, were observed. Glucose uptake by forearm tissues did not change during the saline infusion, but it decreased (P < .05) in all four groups receiving an amino acid infusion. Changes in leucine carbon uptake were strongly correlated (r = −.76, P < .001) with changes in glucose uptake. Therefore, amino acids affect glucose uptake in human forearm tissue and presumably compete as oxidative fuels.

To determine whether therapy with exogenous insulin or sulfonylureas results in a postprandial pattern of carbohydrate metabolism in patients with non-insulin-dependent diabetes mellitus (NIDDM) that resembles that in nondiabetic individuals, we employed a dual-isotope technique combined with forearm catheterization to examine meal disposition in NIDDM patients, before and after 3 mo of therapy with tolazamide and after 3 mo of therapy with exogenous insulin, with a randomized crossover design. Results were compared with those observed in nondiabetic subjects. Although both forms of therapy improved chronic glycemic control (glycosylated hemoglobin concentration went from 9.6 ± 0.7 to 7.6 ± 0.5 and 7.1 ± 0.2%, respectively, P <.01), exogenous insulin resulted in a lower postprandial glycemic response than tolazamide (P <.001). Both agents comparably increased (P <.01) fasting and integrated postprandial insulin concentrations. However, the initial rate of postprandial increase was greater with exogenous insulin (P <.05). Tolazamide (P <.05) but not exogenous insulin increased postprandial C-peptide concentrations. However, tolazamide did not improve the deficient early insulin release. Both agents (P <.05) lowered postabsorptive hepatic glucose release (from 2.8 ± 0.3 to 2.3 ± 0.2 mg · kg−1 · min−1), but not to normal rates (1.8 ± 0.1 mg · kg−1 · min−1). Endogenous glucose release after carbohydrate ingestion was lowered (P <.001) by insulin (from 633 ± 52 to 460 ± 41 mg kg−1 7 h−1) but not by tolazamide (625 ± 62 mg · kg−1 · 7 h−1) to rates observed in nondiabetic subjects (470 ± 32 mg · kg−1 · 7 h−1). Unmetabolized meal glucose reaching the systemic circulation did not differ before or after therapy with either insulin or tolazamide. Total postprandial glucose uptake was not changed by tolazamide (1308 ± 85 mg kg−1 · 7 h−1) but was lowered (P <.01) by exogenous insulin (from 1269 ± 57 to 1088 ± 67 mg · kg−1 · 7 h−1) to normal rates (1023 ± 64 mg kg−1 7 h−1). Forearm glucose uptakedid not differ among any group. We conclude that, although tolazamide and exogenous insulin, as administered in this study, both improve carbohydrate tolerance, therapy with exogenous insulin results in a pattern of postprandial carbohydrate metabolism that more closely approximates that observed in nondiabetic subjects.