Effect of triiodothyronine on mitochondrial energy coupling in human skeletal muscle.

The mechanism underlying the regulation of basal metabolic rate by thyroid hormone remains unclear. Although it has been suggested that thyroid hormone might uncouple substrate oxidation from ATP synthesis, there are no data from studies on humans to support this hypothesis. To examine this possibility, we used a novel combined (13)C/(31)P nuclear magnetic resonance (NMR) approach to assess mitochondrial energy coupling in skeletal muscle of seven healthy adults before and after three days of triiodothyronine (T(3)) treatment. Rates of ATP synthesis and tricarboxylic acid (TCA) cycle fluxes were measured by (31)P and (13)C NMR spectroscopy, respectively, and mitochondrial energy coupling was assessed as the ratio. Muscle TCA cycle flux increased by approximately 70% following T(3) treatment. In contrast, the rate of ATP synthesis remained unchanged. Given the disproportionate increase in TCA cycle flux compared with ATP synthesis, these data suggest that T(3) promotes increased thermogenesis in part by promoting mitochondrial energy uncoupling in skeletal muscle.

Pancreatic resection: effects on glucose metabolism.

Pancreatic resection results in hormonal abnormalities that are dependent on the extent and location (proximal versus distal) of the resected portion of the gland. The form of glucose intolerance which results from pancreatic resection is termed pancreatogenic diabetes. It is associated with features distinct from both type I (insulin-dependent) and type II (insulin-independent, or adult-onset) diabetes. Hepatic insulin resistance with persistent endogenous glucose production and enhanced peripheral insulin sensitivity result in a brittle form of diabetes which can be difficult to manage. In addition to insulin deficiency, the endocrine abnormalities that accompany pancreatic resection can include glucagon deficiency or pancreatic polypeptide (PP) deficiency if the resection is distal or proximal, respectively. Glucagon deficiency can contribute to iatrogenic hypoglycemia, and PP deficiency can contribute to persistent hyperglycemia due to impaired hepatic insulin action. Pancreatic resections that spare the duodenum, such as distal pancreatectomy, duodenum-preserving pancreatic head resection (Beger procedure), or extended lateral pancreaticojejunostomy with excavation of the pancreatic head (Frey procedure), are associated with a lower incidence of new or worsened diabetes than the standard or pylorus-preserving pancreaticoduodenectomy (Whipple procedure) or total pancreatectomy. Operative considerations for the treatment of pancreatic disease should include strategies to minimize the hormonal impairment of pancreatic resection.

Isolated hepatic cholinergic denervation impairs glucose and glycogen metabolism.

BACKGROUND: Hepatic innervation plays an essential role in insulin extraction and glucose production, but the specific role of hepatic cholinergic innervation remains unclear. We sought to establish a model of isolated hepatic cholinergic denervation (IHCD), and to assess whether glycogen storage or the control of net hepatic glucose production (HGP) was altered by IHCD. MATERIALS AND METHODS: Sprague-Dawley rats underwent either hepatic vagotomy or sham operation. Liver tissue was stained for vesicular acetylcholine transporter (VAChT) and (nonspecific neural) protein gene product 9. 5 (PGP) for verification of IHCD. Liver glycogen content was quantified in fed and fasted IHCD or sham-operated animals. HGP was determined after single-pass isolated liver perfusion, during which a 30-min 12 ng/ml glucagon infusion was begun after equilibration, and after 10 min, a 200 microU/ml insulin infusion was added. RESULTS: Uniform staining of PGP and absence of VAChT staining in hepatic vagotomized rats demonstrated the validity of our model. Glycogen content of sham-operated livers (n = 8) increased from 6.0 +/- 1.7 in the fasting state to 10.6 +/- 1.8 mg/g liver, after feeding (P < 0.05). IHCD livers (n = 8) showed no comparable increase (3.5 +/- 0.6 to 4.0 +/- 0.7 mg/g liver). Perfusion with glucagon alone resulted in less HGP in IHCD livers (n = 12) compared with sham-operated livers (n = 10) (integrated HGP 3.3 +/- 0.3 mg/g liver min(-1) vs 5.1 +/- 0.5 mg/g liver min(-1), P < 0.05). Insulin infusion revealed impaired responsiveness to insulin after IHCD; the ratio of HGP in the final 10 min of perfusion (glucagon and insulin) to HGP in the initial 10 min (glucagon alone) was 90.3 +/- 2.4% for IHCD livers versus 68.1 +/- 4.4% for sham-operated controls, respectively (P = 0.0002). CONCLUSIONS: Our study shows that IHCD results in significant impairment in liver glycogen storage and impaired hepatic sensitivity to glucagon and, possibly, to insulin. We conclude that hepatic cholinergic integrity is essential to normal hepatic glucose metabolism.

Binding forces of hepatic microsomal and plasma membrane proteins in normal and pancreatitic rats: an AFM force spectroscopic study.

The docking and fusion of membrane-bound vesicles at the cell plasma membrane are brought about by several participating vesicle membrane, plasma membrane, and soluble cytosolic proteins. An understanding of the interactions between these participating proteins will provide an estimate of the potency and efficacy of secretory vesicle docking and fusion at the plasma membrane in cells of a given tissue. Earlier studies suggest that in chronic pancreatitis, glucose intolerance may be associated with impaired exocytosis/endocytosis of hepatic insulin receptor and glucose transporter proteins. In this study, the binding force profiles between microsome membrane proteins and plasma membrane proteins in liver obtained from normal and pancreatitic rats have been examined using atomic force microscopy. The ability of a VAMP-specific antibody to alter binding between microsome- and plasma membrane-associated membrane proteins was examined. In pancreatitic livers, a significant loss in microsome-plasma membrane binding is observed. Furthermore, our study shows that, in contrast to control livers, the microsome-plasma membrane binding in pancreatitic livers is VAMP-independent, which suggests an absence of VAMP participation in membrane-microsome binding. In confirmation with our earlier findings, these studies suggest altered membrane recycling in liver of rats with chronic pancreatitis.

Effects of free fatty acids on glucose transport and IRS-1-associated phosphatidylinositol 3-kinase activity.

To examine the mechanism by which free fatty acids (FFA) induce insulin resistance in human skeletal muscle, glycogen, glucose-6-phosphate, and intracellular glucose concentrations were measured using carbon-13 and phosphorous-31 nuclear magnetic resonance spectroscopy in seven healthy subjects before and after a hyperinsulinemic-euglycemic clamp following a five-hour infusion of either lipid/heparin or glycerol/heparin. IRS-1-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity was also measured in muscle biopsy samples obtained from seven additional subjects before and after an identical protocol. Rates of insulin stimulated whole-body glucose uptake. Glucose oxidation and muscle glycogen synthesis were 50%-60% lower following the lipid infusion compared with the glycerol infusion and were associated with a approximately 90% decrease in the increment in intramuscular glucose-6-phosphate concentration, implying diminished glucose transport or phosphorylation activity. To distinguish between these two possibilities, intracellular glucose concentration was measured and found to be significantly lower in the lipid infusion studies, implying that glucose transport is the rate-controlling step. Insulin stimulation, during the glycerol infusion, resulted in a fourfold increase in PI 3-kinase activity over basal that was abolished during the lipid infusion. Taken together, these data suggest that increased concentrations of plasma FFA induce insulin resistance in humans through inhibition of glucose transport activity; this may be a consequence of decreased IRS-1-associated PI 3-kinase activity.