Biomedical subjects

J F Markuns

Publications and source records attributed to J F Markuns.

6 recordsLinked to original sources

PubMed2000

Insulin signaling and insulin sensitivity after exercise in human skeletal muscle.

Muscle glucose uptake, glycogen synthase activity, and insulin signaling were investigated in response to a physiological hyperinsulinemic (600 pmol/l)-euglycemic clamp in young healthy subjects. Four hours before the clamp, the subjects performed one-legged exercise for 1 h. In the exercised leg, insulin more rapidly activated glucose uptake (half activation time [t1/2] = 11 vs. 34 min) and glycogen synthase activity (t1/2 = 8 vs. 17 min), and the magnitude of increase was two- to fourfold higher compared with the rested leg. However, prior exercise did not result in a greater or more rapid increase in insulin-induced receptor tyrosine kinase (IRTK) activity (t1/2 = 50 min), serine phosphorylation of Akt (t1/2 = 1-2 min), or serine phosphorylation of glycogen synthase kinase-3 (GSK-3) (t1/2 = 1-2 min) or in a larger or more rapid decrease in GSK-3 activity (t1/2 = 3-8 min). Thirty minutes after cessation of insulin infusion, glucose uptake, glycogen synthase activity, and signaling events were partially reversed in both the rested and the exercised leg. We conclude the following: 1) physiological hyperinsulinemia induces sustained activation of insulin-signaling molecules in human skeletal muscle; 2) the more distal insulin-signaling components (Akt, GSK-3) are activated much more rapidly than the proximal signaling molecules (IRTK as well as insulin receptor substrate 1 and phosphatidylinositol 3-kinase [Wojtaszewski et al., Diabetes 46:1775-1781, 1997]); and 3) prior exercise increases insulin stimulation of both glucose uptake and glycogen synthase activity in the absence of an upregulation of signaling events in human skeletal muscle.

Adult↗

PubMed1999

Insulin and exercise decrease glycogen synthase kinase-3 activity by different mechanisms in rat skeletal muscle.

Glycogen synthase activity is increased in response to insulin and exercise in skeletal muscle. Part of the mechanism by which insulin stimulates glycogen synthesis may involve phosphorylation and activation of Akt, serine phosphorylation and deactivation of glycogen synthase kinase-3 (GSK-3), leading to dephosphorylation and activation of glycogen synthase. To study Akt and GSK-3 regulation in muscle, time course experiments on the effects of insulin injection and treadmill running exercise were performed in hindlimb skeletal muscle from male rats. Both insulin and exercise increased glycogen synthase activity (%I-form) by 2-3-fold over basal. Insulin stimulation significantly increased Akt phosphorylation and activity, whereas exercise had no effect. The time course of the insulin-stimulated increase in Akt was closely matched by GSK-3alpha Ser(21) phosphorylation and a 40-60% decrease in GSK-3alpha and GSK-3beta activity. Exercise also deactivated GSK-3alpha and beta activity by 40-60%. However, in contrast to the effects of insulin, there was no change in Ser(21) phosphorylation in response to exercise. Tyrosine dephosphorylation of GSK-3, another putative mechanism for GSK-3 deactivation, did not occur with insulin or exercise. These data suggest the following: 1) GSK-3 is constitutively active and tyrosine phosphorylated under basal conditions in skeletal muscle, 2) both exercise and insulin are effective regulators of GSK-3 activity in vivo, 3) the insulin-induced deactivation of GSK-3 occurs in response to increased Akt activity and GSK-3 serine phosphorylation, and 4) there is an Akt-independent mechanism for deactivation of GSK-3 in skeletal muscle.

Animals↗

PubMed1999

Differential regulation of MAP kinase, p70(S6K), and Akt by contraction and insulin in rat skeletal muscle.

To study the effects of contractile activity on mitogen-activated protein kinase (MAP kinase), p70 S6 kinase (p70(S6K)), and Akt kinase signaling in rat skeletal muscle, hindlimb muscles were contracted by electrical stimulation of the sciatic nerve for periods of 15 s to 60 min. Contraction resulted in a rapid and transient activation of Raf-1 and MAP kinase kinase 1, a rapid and more sustained activation of MAP kinase and the 90-kDa ribosomal S6 kinase 2, and a dramatic increase in c-fos mRNA expression. Contraction also resulted in an apparent increase in the association of Raf-1 with p21Ras, although stimulation of MAP kinase signaling occurred independent of Shc, IRS1, and IRS2 tyrosine phosphorylation or the formation of Shc/Grb2 or IRS1/Grb2 complexes. Insulin was considerably less effective than contraction in stimulating the MAP kinase pathway. However, insulin, but not contraction, increased p70(S6K) and Akt activities in the muscle. These results demonstrate that contraction-induced activation of the MAP kinase pathway is independent of proximal steps in insulin and/or growth factor-mediated signaling, and that contraction and insulin have discordant effects with respect to the activation of the MAP kinase pathway vs. p70(S6K) and Akt. Of the numerous stimulators of MAP kinase in skeletal muscle, contractile activity emerges as a potent and physiologically relevant activator of MAP kinase signaling, and thus activation of this pathway is likely to be an important molecular mechanism by which skeletal muscle cells transduce mechanical and/or biochemical signals into downstream biological responses.

Animals↗

PubMed1999

Effects of streptozocin-induced diabetes and islet cell transplantation on insulin signaling in rat skeletal muscle.

Streptozocin-induced diabetes is associated with alterations in insulin signaling in rat skeletal muscle, including increased insulin receptor substrate-1 phosphorylation and phosphotidylinositol 3-kinase activity. In the current study, we determined the effects of streptozocin-induced diabetes and treatment of diabetes by islet cell transplantation on several proximal insulin-activated signaling proteins. Three groups of male Lewis rats (untreated streptozocin-diabetic animals, islet cell-transplanted diabetic rats, and nondiabetic control rats) were studied in the basal state or 30 min after i.p. insulin injection (20 U/rat). Mixed hindlimb skeletal muscle lysates were used to determine the expression and enzymatic activities of the extracellular regulated kinase 2 (ERK2), p90 ribosomal S6 kinase (RSK2), Akt, and p70 S6 kinase (p70S6k). In all three groups of rats, insulin significantly increased ERK2, RSK2, Akt, and p70S6k activities. There was no effect of diabetes on insulin-stimulated ERK2 activity or ERK2 protein levels. RSK2 expression and insulin-stimulated RSK2 activity were significantly elevated in diabetic rats compared with those in the control animals. Insulin-stimulated Akt activity was also significantly greater in the diabetic animals, but there was no change in protein expression. In contrast, there was a decrease in insulin-stimulated p70S6k activity with no change in protein expression in the diabetic rats. Islet transplantation partially (RSK2) or fully (Akt, p70S6k) normalized these diabetes-induced changes in insulin signaling proteins. We conclude that streptozocin diabetes results in the dysregulation of several critical insulin-activated proteins in rat skeletal muscle, but islet cell transplantation is an effective therapy to partially correct these alterations in insulin signaling.

Animals↗

PubMed1999

Effects of suprachiasmatic transplants on circadian rhythms of neuroendocrine function in golden hamsters.

Grafts of fetal tissue including the suprachiasmatic nucleus (SCN) of the hypothalamus restore locomotor rhythmicity to behaviorally arrhythmic, SCN-lesioned Syrian hamsters. We sought to determine whether such transplants also reinstate endocrine rhythms in SCN-lesioned hamsters. In Exp 1, SCN lesions interrupted estrous cycles in a 14 h light, 10 h dark photoperiod and locomotor rhythms in constant dim red light (DD). SCN grafts that reinstated behavioral circadian rhythms consistently failed to reestablish estrous cycles. After ovariectomy, estradiol implants triggered LH surges at approximately circadian time 8 in 10 of 12 brain-intact control females and 0 of 9 SCN-lesioned, grafted females. Daily rhythms of the principal urinary melatonin metabolite, 6alpha-sulfatoxymelatonin, were not reestablished by behaviorally functional grafts. In Exp 2, SCN lesions eliminated locomotor rhythmicity in adult male hamsters maintained in DD. Seven to 12 weeks after restoration of locomotor activity rhythms by fetal grafts, hosts and sham-lesioned controls were decapitated at circadian times 4, 8, 12, 16, 20, or 24. Clear circadian rhythms of both serum corticosterone and cortisol were seen in sham-lesioned males, with peaks in late subjective day. No circadian rhythms in either adrenal hormone were evident in serum from lesioned-grafted males. Testicular regression, observed in intact and sham-lesioned males maintained in DD, was absent not only in arrhythmic SCN-lesioned hamsters given grafts of cerebral cortex, but also in animals in which hypothalamic grafts had reinstated locomotor rhythmicity. The pineal melatonin concentration rose sharply during the late subjective night in control hamsters, but not in SCN-lesioned animals bearing behaviorally effective transplants. Even though circadian rhythms of locomotor activity are restored by SCN transplants, circadian endocrine rhythms are not reestablished. Endocrine rhythms may require qualitatively different or more extensive SCN outputs than those established by fetal grafts.

Animals↗

PubMed1996

Glucose ingestion causes GLUT4 translocation in human skeletal muscle.

In humans, ingestion of carbohydrates causes an increase in blood glucose concentration, pancreatic insulin release, and increased glucose disposal into skeletal muscle. The underlying molecular mechanism for the increase in glucose disposal in human skeletal muscle after carbohydrate ingestion is not known. We determined whether glucose ingestion increases glucose uptake in human skeletal muscle by increasing the number of glucose transporter proteins at the cell surface and/or by increasing the activity of the glucose transporter proteins in the plasma membrane. Under local anesthesia, approximately 1 g of vastus lateralis muscle was obtained from six healthy subjects before and 60 min after ingestion of a 75-g glucose load. Plasma membranes were isolated from the skeletal muscle and used to measure GLUT4 and GLUT1 content and glucose transport in plasma membrane vesicles. Glucose ingestion increased the plasma membrane content of GLUT4 per gram muscle (3,524 +/- 729 vs. 4,473 +/- 952 arbitrary units for basal and 60 min, respectively; P < 0.005). Transporter-mediated glucose transport into plasma membrane vesicles was also significantly increased (130 +/- 11 vs. 224 +/- 38 pmol.mg-1.s-1; P < 0.017), whereas the calculated ratio of glucose transport to GLUT4, an indication of transporter functional activity, was not significantly increased 60 min after glucose ingestion (2.3 +/- 0.4 vs. 3.0 +/- 0.5 pmol.GLUT4 arbitrary units-1.s-1; P < 0.17). These results demonstrate that oral ingestion of glucose increases the rate of glucose transport across the plasma membrane and causes GLUT4 translocation in human skeletal muscle. These findings suggest that under physiological conditions the translocation of GLUT4 is an important mechanism for the stimulation of glucose uptake in human skeletal muscle.

Biological Transport↗