65 research outputs found
Spin Connection and Renormalization of Teleparallel Action
In general relativity, inertia and gravitation are both included in the
Levi-Civita connection. As a consequence, the gravitational action, as well as
the corresponding energy-momentum density, are in general contaminated by
spurious contributions coming from inertial effects. In teleparallel gravity,
on the other hand, because the spin connection represents inertial effects
only, it is possible to separate inertia from gravitation. Relying on this
property, it is shown that to each tetrad there is naturally associated a spin
connection that locally removes the inertial effects from the action. The use
of the appropriate spin connection can be viewed as a renormalization process
in the sense that the computation of energy and momentum naturally yields the
physically relevant values. A self-consistent method for solving field
equations and determining the appropriate spin connection is presented.Comment: v2: 13 pages, minor corrections, new section about solving the field
equations include
Flat connection for rotating vacuum spacetimes in extended teleparallel gravity theories
Teleparallel geometry utilizes Weitzenb\"ock connection which has nontrivial
torsion but no curvature and does not directly follow from the metric like
Levi-Civita connection. In extended teleparallel theories, for instance in
or scalar-torsion gravity, the connection must obey its antisymmetric
field equations. So far only a few analytic solutions were known. In this note
we solve the gravity antisymmetric vacuum field equations for a
generic rotating tetrad ansatz in Weyl canonical coordinates, and find the
corresponding spin connection coefficients. By a coordinate transformation we
present the solution also in Boyer-Lindquist coordinates, often used to study
rotating solutions in general relativity. The result hints for the existence of
another branch of rotating solutions besides the Kerr family in extended
teleparallel gravities.Comment: minor clarifications and references added; 14 pages, no figures;
contributed to the special issue "Selected Papers from Teleparallel Universes
in Salamanca 2018
On higher dimensional Kerr-Schild spacetimes
Institute of Theoretical PhysicsÚstav teoretické fyzikyFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult
Multinuclear Magnetic Resonance Spectroscopy of Human Skeletal Muscle Metabolism in Training and Disease
In this chapter, techniques and application of multinuclear (1H, 13C, and 31P) in vivo magnetic resonance spectroscopy (MRS) for the assessment of skeletal muscle metabolism in health and disease are described. Studies focusing on glucose transport and utilization, lipid storage and consumption, handling of energy rich phosphates, and measurements of newly emerging noninvasive biomarkers, i.e., acetylcarnitine and carnosine are summarized. Muscle metabolism connections to exercise physiology and the development as well as possible treatment of metabolic diseases, such as obesity and diabetes are also discussed. Taken together, multinuclear in vivo MRS on humans helped to uncover defects in skeletal muscle metabolic pathways in insulin-resistant conditions; and to discover links between defects in mitochondrial activity/capacity and lipid metabolism, as well as defects in whole-body and/or muscle glucose metabolism. There is also to mention that several of the MR-derived readouts are affected by both training status and metabolic disease in a specific way, and thus could serve as potential markers of training status and metabolic flexibility
Alterations in Postprandial Hepatic Glycogen Metabolism in Type 2 Diabetes
Decreased skeletal muscle glucose disposal and increased endogenous glucose production (EGP) contribute to postprandial hyperglycemia in type 2 diabetes, but the contribution of hepatic glycogen metabolism remains uncertain. Hepatic glycogen metabolism and EGP were monitored in type 2 diabetic patients and nondiabetic volunteer control subjects (CON) after mixed meal ingestion and during hyperglycemic-hyperinsulinemic-somatostatin clamps applying 13C nuclear magnetic resonance spectroscopy (NMRS) and variable infusion dual-tracer technique. Hepatocellular lipid (HCL) content was quantified by 1H NMRS. Before dinner, hepatic glycogen was lower in type 2 diabetic patients (227 ± 6 vs. CON: 275 ± 10 mmol/l liver, P < 0.001). After meal ingestion, net synthetic rates were 0.76 ± 0.16 (type 2 diabetic patients) and 1.36 ± 0.15 mg · kg−1 · min−1 (CON, P < 0.02), resulting in peak concentrations of 283 ± 15 and 360 ± 11 mmol/l liver. Postprandial rates of EGP were ∼0.3 mg · kg−1 · min−1 (30–170 min; P < 0.05 vs. CON) higher in type 2 diabetic patients. Under clamp conditions, type 2 diabetic patients featured ∼54% lower (P < 0.03) net hepatic glycogen synthesis and ∼0.5 mg · kg−1 · min−1 higher (P < 0.02) EGP. Hepatic glucose storage negatively correlated with HCL content (R = −0.602, P < 0.05). Type 2 diabetic patients exhibit 1) reduction of postprandial hepatic glycogen synthesis, 2) temporarily impaired suppression of EGP, and 3) no normalization of these defects by controlled hyperglycemic hyperinsulinemia. Thus, impaired insulin sensitivity and/or chronic glucolipotoxicity in addition to the effects of an altered insulin-to-glucagon ratio or increased free fatty acids accounts for defective hepatic glycogen metabolism in type 2 diabetic patients
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