G. Dietze

The antioxidant α-lipoic acid enhances insulin-stimulated glucose metabolism in insulin-resistant rat skeletal muscle

Insulin resistance of muscle glucose metabolism is a hallmark of NIDDM. The obese Zucker (fa/fa) rat—an animal model of muscle insulin resistance—was used to test whether acute (100 mg/kg body wt for 1 h) and chronic (5–100 mg/kg for 10 days) parenteral treatments with a racemic mixture of the antioxidant α-lipoic acid (ALA) could improve glucose metabolism in insulin-resistant skeletal muscle. Glucose transport activity (assessed by net 2-deoxyglucose [2-DG] uptake), net glycogen synthesis, and glucose oxidation were determined in the isolated epitrochlearis muscles in the absence or presence of insulin (13.3 nmol/1). Severe insulin resistance of 2-DG uptake, glycogen synthesis, and glucose oxidation was observed in muscle from the vehicle-treated obese rats compared with muscle from vehicle-treated lean (Fa/−) rats. Acute and chronic treatments (30 mgkg−1 · day−1, a maximally effective dose) with ALA significantly (P < 0.05) improved insulin-mediated 2-DG uptake in epitrochlearis muscles from the obese rats by 62 and 64%, respectively. Chronic ALA treatment increased both insulin-stimulated glucose oxidation (33%) and glycogen synthesis (38%) and was associated with a significantly greater (21%) in vivo muscle glycogen concentration. These adaptive responses after chronic ALA administration were also associated with significantly lower (15–17%) plasma levels of insulin and free fatty acids. No significant effects on glucose transporter (GLUT4) protein level or on the activities of hexokinase and citrate synthase were observed. Collectively, these findings indicate that parenteral administration of the antioxidant ALA significantly enhances the capacity of the insulinstimulatable glucose transport system and of both oxidative and nonoxidative pathways of glucose metabolism in insulin-resistant rat skeletal muscle.

Captopril enhances insulin responsiveness of forearm muscle tissue in non-insulin-dependent diabetes mellitus

Abstract. Bradykinin infusion has been shown to improve glucose metabolism in non‐insulin‐dependent diabetic subjects (NIDD). Therefore, we tested the following hypothesis: inhibition of Kininase II, the bradykinin (BK) degrading enzyme, by captopril may also improve glucose metabolism in NIDD. Immediate effects of captopril on total body and peripheral glucose disposal were examined in five normotensive, normal weight NIDD and compared with five NIDD control subjects, well matched for age, weight and degree of fasting hyperglycaemia. The euglycaemic insulin clamp technique was employed in combination with the forearm catheter technique. After 90 min of insulin infusion a single dose of 25 mg captopril was administered orally, whereas in the control group a placebo was given. Captopril lead to a significant rise in total body glucose disposal and forearm glucose uptake, while in the control group no change was observed. Simultaneously, captopril lead to reduction in muscular release of lactate and pyruvate. We conclude that these results demonstrate the stimulatory effect of captopril on insulin‐induced glucose disposal of the whole body, which appears to be a result of increased glucose utilization by peripheral tissues. Because of the described insulin‐like activity of bradykinin, the concomitant accumulation of local kinins by captopril‐induced inhibition of kininase II may represent an attractive hypothesis to explain the generated data sufficiently.

Insulin-induced glucose transporter (GLUT1 and GLUT4) translocation in cardiac muscle tissue is mimicked by bradykinin.

The effect of bradykinin on glucose transporter translocation in isolated rat heart was compared with the effect of insulin. Hearts from male obese (fa/fa) Zucker rats were perfused under normoxic conditions and constant pressure in a classic Langendorff preparation with 12 mmol/l glucose as substrate, and a set of functional parameters was measured simultaneously. Bradykinin was administered at a concentration (10−11 mmol/l) that did not increase coronary flow. Insulin was used at a concentration (8 × 10−8 mmol/1) known to maximally stimulate glucose transport in this model. After 15 min of perfusion with insulin or bradykinin, subcellular membrane fractions of the heart were prepared, and distribution of glucose transporter protein (GLUT1 and GLUT4) in fractions enriched with surface membranes (transverse tubules [TTs] and sarcolemmal membranes [PMs]) and with low-density microsomal membranes (LDMs) were determined by immunoblotting with the respective antibodies. Both glucose transporter isoforms were translocated after stimulation with insulin (increased transporter protein content in the PM+TT-enriched fraction with a concomitant decrease in the LDM-enriched fraction) and, to a smaller extent, also with bradykinin. These data suggest that in hearts of insulin-resistant obese (fa/fa) Zucker rats, bradykinin interacts with or facilitates the translocation process of both GLUT1 and GLUT4.

Potential Role of Bradykinin in Forearm Muscle Metabolism in Humans

Using the euglycemic-hyperinsulinemic glucose clamp and the human forearm technique, we have demonstrated that the improved glucose disposal rate observed after the administration of an angiotensin-converting enzyme (ACE) inhibitor such as captopril may be primarily due to increased muscle glucose uptake (MGU). These results are not surprising because ACE, which is identical to the bradykinin (BK)-degrading kininase II, is abundantly present in muscle tissue, and its inhibition has been observed to elicit the observed metabolic actions via elevated tissue concentrations of BK and through a BK B2 receptor site in muscle and/or endothelial tissue. These findings are supported by several previous studies. Exogenous BK applied into the brachial artery of the human forearm not only augmented muscle blood flow (MBF) but also enhanced the rate of MGU. In another investigation, during rhythmic voluntary contraction, both MBF and MGU increased in response to the higher energy expenditure, and the release of BK rose in the blood vessel, draining the working muscle tissue. Inhibition of the activity of the BK-generating protease in muscle tissue (kallikrein) with aprotinin significantly diminished these functional responses during contraction. Applying the same kallikrein inhibitor during the infusion of insulin into the brachial artery significantly reduced the effect of insulin on glucose uptake into forearm muscle. This is of interest, because in recent studies insulin has been suggested to elicit its actions on MBF and MGU via the accelerated release of endothelium-derived nitric oxide, the generation of which is also stimulated by BK in a concentration-dependent manner. This new evidence obtained from in vitro and in vivo studies sheds new light on the discussion of whether BK may play a role in energy metabolism of skeletal muscle tissue.

Immunolocalization of Bradykinin B2 Receptors on Skeletal Muscle Cells

The kallikrein-kinin system has been implicated in the inflammatory process, blood pressure regulation, renal homeostasis, and glucose utilization. The effects of kallikrein and kinin on glucose uptake by the skeletal muscle are well established; however, the occurrence and the cellular distribution of the kinin receptor(s) mediating these effects in the striated muscle are unknown. Using anti-peptide antibodies raised against the predicted intra- and extracellular domains of the B2 receptor and the peroxidase/antiperoxidase system, we have been able to detect the B2 receptor on the plasma membrane of striated skeletal muscle cells of the rat hindlimb. A strong immunostaining appeared as a rim of immunoreactive material located on the periphery of striated muscle cells. Cross-sectioned and longitudinally sectioned cells revealed a similar staining pattern. Alternatively, the immunostaining with specific antibodies to tissue kallikrein and to T-kininogen did not yield a significant staining of the striated muscle cells. Localization of the B2 receptor on the surface of striated muscle cells provides a structural basis for the hypothesized physiological functions of the kinin system in the skeletal muscle.

The Kallikrein-Kinin system and muscle metabolism: Biochemical aspects

Infusion of bradykinin (BK) into the brachial artery in front of skeletal muscle of the human forearm yielding arterial concentrations of about 10−12 mol/l caused not only acceleration of blood flow but also of glucose and branchedchain amino acid uptake into the muscle in healthy volunteers and maturity-onset diabetics. These effects were almost entirely abolished after inhibition of prostaglandin biosynthesis.Papaverine, although causing identical acceleration of capillary blood flow, induced no metabolic action. Apart from causing enlargement of the capillary bed, bradykinin has another metabolic effect which was underlined by results obtained in the isolated perfused rat heart, indicating increased glucose uptake at constant rates of coronary blood flow.In order to clarify whether kinins play a physiological role in muscle carbohydrate metabolism, the well-known work-induced acceleration of muscle glucose uptake was studied during the inhibition of kinin liberation from kininogen by application of a protease inhibitor (Trasylol®) and during additional substitution with synthetic BK. The glucose uptake under a defined work load was almost completely abolished by the protease inhibitor; application of BK restored the normal effect. Almost identical responses have been observed concerning the well-known hypoxia-induced acceleration of muscle glucose uptake. Furthermore, insulin-induced acceleration of glucose uptake into the resting forearm was reduced by half when kinin liberation from kininogen was inhibited by Trasylol®; additional application of synthetic BK restored the normal response.From the data presented, one may suggest that kinins are involved in carbohydrate and amino acid metabolism of skeletal muscle, most probably by improving the action of insulin.

IMPROVEMENT OF GLUCOSE ASSIMILATION AND PROTEIN DEGRADATION BY BRADYKININ IN MATURITY ONSET DIABETICS AND IN SURGICAL PATIENTS

Impaired glucose assimilation and accelerated protein degradation are concomitant symptoms of insulin deficiency as in diabetes and also of reduced insulin sensitivity as during postoperative stress. Since it is known that kininogen levels are reduced in traumatic stress (1) and that kinin liberation is involved in the peripheral action of insulin (2,3), the influence of kinin infusion on carbohydrate tolerance and urinary nitrogen excretion was investigated in normal and diabetic subjects and in surgical patients.

On the mechanism of glucose release from the muscle of juvenile diabetics in acute insulin deficiency

Abstract. In fifty‐three healthy subjects and twenty‐three juvenile diabetics the measurement of arterial and deep venous glucose concentrations showed that the substrate was taken up by the tissues of the forearm of all the healthy subjects and released from it in all the diabetic ones. In six of the diabetics glucose output was accelerated almost five‐fold during the intrabrachial arterial administration of metaproterenol (1.62 nmol/min), indicating that basal glucose release from muscle may result from enhanced glycogenolysis during acute insulin deficiency. In line with this view a reduction of glucose uptake by muscle was observed in six healthy subjects receiving metaproterenol infusion. However, since the production of lactate by the forearm appeared to be smaller in the diabetics, the basal glucose output could also partly be due to impaired glycolysis. These data suggest that the glucose released from muscle during acute insulin deficiency may be of clinical importance, especially when the rate of glycogenolysis is further stimulated by, for example, enhanced catechola‐mine drive.

Possible Involvement of Kinins in Muscle Energy Metabolism

It is a well-known phenomenon since the end of the last century that muscle shortening as it is initiated in the forearm by moving one finger is accompanied by accelerated blood flow through the contracting forearm-muscle1 and by increased glucose uptake into the working tissue2, although cardiac output and the arterial concentrations of insulin and glucose do not change. That means that these phenomenons are controlled locally.

Inhibition of muscular glucose uptake by lipid infusion in man

During a high-dose intravenous infusion of a mixed MCT/LCT-lipid emulsion and a conventional LCT-emulsion respectively, muscle substrate metabolism was investigated using the human forearm technique. With both lipid emulsions, a decrease in fractional muscular glucose extraction was seen, leading to significantly reduced muscular glucose uptake rates. An inverse linear relation between arterial tree fatty acids supply and fractional glucose extraction was seen suggesting that a mechanism according to Randles glucose fatty acid concept is operating in skeletal muscle in man.