The postexercise state: altered effects of insulin on skeletal muscle and their physiologic relevance

Sustained postexercise increases in AS160 Thr642 and Ser588 phosphorylation in skeletal muscle without sustained increases in kinase phosphorylation

Prior exercise by rats can induce a sustained increase in muscle Akt substrate of 160 kDa (AS160) phosphorylation on Thr(642) (pAS160(Thr642)). Because phosphorylation of AS160 on both AS160(Thr642) and AS160(Ser588) is important for insulin-stimulated glucose transport (GT), we determined if exercise would also induce a sustained increase in pAS160(Ser588) concomitant with persistently elevated pAS160(Thr642) and GT. Given that the mechanisms for sustained postexercise (PEX) effects on pAS160 were uncertain, we also studied the four kinases known to phosphorylate AS160 (Akt, AMPK, RSK, and SGK1). In addition, because the serine/threonine phosphatase(s) that dephosphorylate muscle AS160 were previously unidentified, we assessed the ability of four serine/threonine phosphatases (PP1, PP2A, PP2B, and PP2C) to dephosphorylate AS160. We also evaluated exercise effects on posttranslational modifications (Tyr(307) and Leu(309)) that regulate PP2A. In isolated epitrochlearis muscles from rats, GT at 3hPEX with insulin significantly (P < 0.05) exceeded SED controls. Muscles from 0hPEX vs. 0hSED and 3hPEX vs. 3hSED rats had greater pAS160(Thr642) and pAS160(Ser588). AMPK was the only kinase with greater phosphorylation at 0hPEX vs. 0hSED, and none had greater phosphorylation at 3hPEX vs. 3hSED. Each phosphatase was able to dephosphorylate pAS160(Thr642) and pAS160(Ser588) in cell-free assays. Exercise did not alter posttranslational modifications of PP2A. Our results revealed: 1) pAMPK as a potential trigger for increased pAS160(Thr642) and pAS160(Ser588) at 0hPEX; 2) PP1, PP2A, PP2B, and PP2C were each able to dephosphorylate AS160; and 3) sustained PEX-induced elevations of pAS160(Thr642) and pAS160(Ser588) were attributable to mechanisms other than persistent phosphorylation of known AS160 kinases or altered posttranslational modifications of PP2A.

Postcontraction insulin sensitivity: relationship with contraction protocol, glycogen concentration, and 5' AMP-activated protein kinase phosphorylation

Exercise enhances insulin-stimulated glucose transport (GT) in skeletal muscle. Evidence suggests that 5' AMP-activated protein kinase (AMPK) and glycogen may be important for enhanced insulin sensitivity. Our goals were to investigate the effect of various in situ muscle contraction protocols on insulin-stimulated GT and assess the relationship of contraction-induced changes in AMPK and glycogen with postcontraction improvement in insulin-stimulated GT. Rats were anesthetized, both ulnar nerves were exposed, and one nerve was electrically stimulated to contract forelimb muscles. We performed a series of five experiments, sequentially varying only one contraction parameter (train duration, train rate, pulse frequency, number of 5-min bouts, or pulse duration) while holding the others constant. Both epitrochlearis muscles were dissected out and incubated for 3.5 h before measurement of GT. For each contraction parameter studied, we identified an apparent threshold value that did not induce a significant increase in insulin-stimulated GT and an apparent peak value, above which there was a plateau or decline in insulin-stimulated GT. Using other rats, we evaluated muscle AMPK phosphorylation and glycogen concentration immediately postcontraction. AMPK phosphorylation and reduction in glycogen were increased compared with resting controls in each protocol, which had previously been shown to increase insulin-stimulated GT, as well as in several protocols that did not significantly increase insulin-stimulated GT. These data suggest that contraction-induced AMPK phosphorylation and decrease in glycogen may be necessary but are not sufficient for the postcontraction increase in insulin-stimulated GT in rat skeletal muscle.

AMPK as a metabolic switch in rat muscle, liver and adipose tissue after exercise

An increasing body of evidence has revealed that activation of adenosine monophosphate (AMP)-activated protein kinase (AMPK)-activated protein kinase increases fatty acid oxidation by lowering the concentration of malonyl coenzyme A (CoA), an inhibitor of carnitine palmitoyl transferase 1. Studies carried out primarily in skeletal muscle suggest that AMPK modulates the concentration of malonyl CoA by concurrently phosphorylating and inhibiting acetyl CoA carboxylase (ACC), the rate limiting enzyme in malonyl CoA synthesis, and phosphorylating and activating malonyl CoA decarboxylase (MCD), an enzyme involved in its degradation. We have recently observed that AMPK and MCD activities are increased and ACC activity diminished in skeletal muscle, liver and, surprisingly, in adipose tissue 30 min following exercise (treadmill run) in normal rats. In liver and adipose tissue these changes were associated with a decrease in the activity of glycerol-3-phosphate acyltransferase (GPAT), which catalyses the first committed reaction in glycerolipid synthesis and, which like ACC, is phosphorylated and inhibited by AMPK. Similar changes in ACC, MCD and GPAT were observed following the administration of 5-aminoimidazole 4-carboxamide-riboside (AICAR), further indicating that the exercise-induced alterations in these enzymes were AMPK-mediated.

CONCLUSIONS

(1) AMPK plays a major role in regulating lipid metabolism in multiple tissues following exercise. (2) The net effect of its activation is to increase fatty acid oxidation and diminish glycerolipid synthesis. (3) The relevance of these findings to the regulation of muscle glycogen repletion in the post-exercise state and to the demonstrated ability of AMPK activation to decrease adiposity and increase insulin sensitivity in rodents remains to be determined.

Role of kallikrein-kininogen system in insulin-stimulated glucose transport after muscle contractions

Serum proteins [molecular weight (MW) > 10,000] are essential for increased insulin-stimulated glucose transport after in vitro muscle contractions. We investigated the role of the kallikrein-kininogen system, including bradykinin, which is derived from kallikrein (MW > 10,000)-catalyzed degradation of serum protein kininogen (MW > 10,000), on this contraction effect. In vitro electrical stimulation of rat epitrochlearis muscles was performed in 1) rat serum +/- kallikrein inhibitors; 2) human plasma (normal or kallikrein-deficient); 3) rat serum +/- bradykinin receptor-2 inhibitors; or 4) serum-free buffer +/- bradykinin. 3-O-methylglucose transport (3-MGT) was measured 3.5 h later. Serum +/- kallikrein inhibitors tended (P = 0.08) to diminish postcontraction insulin-stimulated 3-MGT. Contractions in normal plasma enhanced insulin-stimulated 3-MGT vs. controls, but contractions in kallikrein-deficient plasma did not. Supplementing rat serum with bradykinin receptor antagonist HOE-140 during contraction did not alter insulin-stimulated 3-MGT. Muscles stimulated to contract in serum-free buffer plus bradykinin did not have enhanced insulin-stimulated 3-MGT. Bradykinin was insufficient for postcontraction-enhanced insulin sensitivity. However, results with kallikrein inhibitors and kallikrein-deficient plasma suggest kallikrein plays a role in this improved insulin action.

Searching for ways to upregulate GLUT4 glucose transporter expression in muscle

1. Skeletal muscle is a major glucose-utilizing tissue in the absorptive state and alterations in muscle insulin-stimulated glucose uptake lead to derangements in whole body glucose disposal. 2. Furthermore, muscle GLUT4 overexpression in transgenic animals ameliorates insulin resistance associated with obesity or diabetes, which suggests that increasing GLUT4 in muscle by pharmacological intervention may be an effective therapy in insulin-resistant states. 3. This highlights the importance of understanding the pathways that upregulate GLUT4 glucose transporter expression in muscle. 4. We review studies describing the regulation of GLUT4 and the information currently available on the mechanisms that control GLUT4 expression in muscle.

Exercise regulation of glucose transport in skeletal muscle

Exercise increases the rate of glucose uptake into the contracting skeletal muscles. This effect of exercise is similar to the action of insulin on glucose uptake, and the mechanism through which both stimuli increase skeletal muscle glucose uptake involves the translocation of GLUT-4 glucose transporters to the plasma membrane and transverse tubules. Most studies suggest that exercise and insulin recruit distinct GLUT-4-containing vesicles and/or mobilize different "pools" of GLUT-4 proteins originating from unique intracellular locations. There are different intracellular signaling pathways that lead to insulin- and exercise-stimulated GLUT-4 translocation. Insulin utilizes a phosphatidylinositol 3-kinase-dependent mechanism, whereas the exercise signal may be initiated by calcium release from the sarcoplasmic reticulum leading to the activation of other signaling intermediaries, and there is also evidence for autocrine- or paracrine-mediated activation of transport. The period after exercise is characterized by increased sensitivity of muscle glucose uptake to insulin, which can be substantially prolonged in the face of carbohydrate deprivation. The ability of exercise to utilize insulin-independent mechanisms to increase glucose uptake in skeletal muscle has important clinical implications, especially for patients with diseases that are associated with peripheral insulin resistance, such as non-insulin-dependent diabetes mellitus.

Intravenous glucose tolerance test-derived glucose effectiveness in physically trained humans

Glucose effectiveness (SG) and insulin sensitivity of sedentary and physically trained males were estimated by the minimal model approach. Trained subjects, who ran 86 +/- 10 km/wk and had 37% higher maximal oxygen consumption than that of sedentary subjects (56.2 +/- 1.2 vs. 40.9 +/- 1.4 ml.kg-1 x min-1, P < 0.01), were studied 16 h and 1 wk after their last training session. After overnight fasting, glucose was administered intravenously (300 mg/kg body wt) within 2 min, and insulin was infused (approximately 13-20 mU/kg given over 5 min) from 20 to 25 min after administration of glucose. Glucose disappearance constant values as an estimate of glucose tolerance were significantly higher in trained subjects after 16 h and 1 wk of their training session (3.29 +/- 0.48 and 3.60 +/- 0.64%/min) than in sedentary subjects (1.92 +/- 0.30%/min, P < 0.05). Insulin sensitivity in trained subjects measured after 16 h and 1 wk of their last training session (26.2 +/- 4.4 and 24.3 +/- 6.0 x 10(-5) min-1 x pM-1) was also higher than that of sedentary subjects (10.3 +/- 1.2 x 10(-5) min-1 x pM-1, P < 0.05). SG, the ability of glucose itself to increase peripheral glucose uptake and suppress hepatic glucose output, was significantly higher in trained subjects after 16 h and 1 wk of their last training session (0.028 +/- 0.003 and 0.030 +/- 0.004/min) than in sedentary subjects (0.017 +/- 0.002/min, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in glucose disposal and cellular insulin binding in obese black Southern African patients with type 2 diabetes mellitus before and after sulphonylurea therapy

Peripheral insulin action and cellular insulin binding were studied in 10 newly detected, obese, black, Southern African women with Type 2 diabetes mellitus before and after mid-term oral sulphonylurea therapy and in five obese, non-diabetic controls. Glucose disposal (assessed by the euglycaemic insulin clamp technique) was significantly reduced in diabetic patients compared to control subjects (4.4 +/- 0.5 vs 6.4 +/- 0.5 mg kg-1 min-1, p < 0.05), and increased after 1 and 3 months of sulphonylurea therapy to 6.8 +/- 0.6 mg kg-1 min-1 (p = 0.01) and 6.3 +/- 0.7 mg kg-1 min-1 (p = 0.04), respectively. The major change in the binding kinetics of insulin to peripheral monocytes was an increase in the mean receptor concentration in the diabetic patients which was significant after 3 months of therapy (0.2 +/- 0.08 to 0.6 +/- 0.01 nM, p = 0.05). The basal plasma C-peptide concentration was significantly lower in the diabetic patients than in the controls and remained so following sulphonylurea therapy, despite significant reductions in fasting glucose and HbA1 concentrations. We conclude that newly diagnosed, obese, black Southern Africans with Type 2 diabetes showed diminished peripheral glucose disposal which increased following sulphonylurea therapy. This was accompanied by an increase in insulin receptor concentration but not with changes in basal insulin secretion.