Elena Burdett

Insulin induces the translocation of GLUT4 from a unique intracellular organelle to transverse tubules in rat skeletal muscle.

Skeletal muscle surface membrane is constituted by the PM domain and its specialized deep invaginations known as TTs. We have shown previously that insulin induces a rapid translocation of GLUT4s from an IM pool to the PM in rat skeletal muscle (6). In this study, we have investigated the possibility that insulin also stimulates the translocation of GLUT4 proteins to TTs, which constitute the largest area of the cell surface envelope. PM, TTs, and IM components of control and insulinized skeletal muscle were isolated by subcellular fractionation. The TTs then were purified further by removing vesicles of SR origin by using a Ca-loading procedure. Ca-loading resulted in a five- to sevenfold increase in the purification of TTs in the unloaded fraction relative to the loaded fraction, assessed by immunoblotting with an anti-OHP-receptor monoclonal antibody. In contrast, estimation of the content of Ca2+-ATPase protein (a marker of SR) with a specific polyclonal antibody revealed that most, if not all, SR vesicles were recovered in the Ca-loaded fraction. Western blotting with an anti-COOH-terminal GLUT4 protein polyclonal antibody revealed that acute insulin injection in vivo (30 min) increased the content of GLUT4 (by 90%) in isolated PMs and markedly enhanced (by 180%) GLUT4 content in purified TTs. Importantly, these insulin-dependent changes in GLUT4 content of PM and purified TTs were seen in the absence of changes in the α1-subunit of the Na+-K+-ATPase, a surface membrane marker. Isolated IM components such as LSR, HSR, and triads (terminal cisternae plus junctional TT) contained low or barely detectable amounts of GLUT4; furthermore, insulin treatment did not change the distribution of the transporter protein within these fractions. In contrast, a unique IM fraction that was not associated with either SR or triad markers, contained significant amounts of GLUT4 and showed an insulin-dependent decrease (40%) in GLUT4 protein content. These results show that acute insulin treatment induces the translocation of GLUT4s to both the PM and TTs from a unique intracellular organelle not associated with the SR.

Differential expression of the GLUT1 and GLUT4 glucose transporters during differentiation of L6 muscle cells.

Skeletal muscle is the main tissue responsible for glucose utilization in the fed state, and it expresses the ubiquitous GLUT1 glucose transporter and the muscle/fat specific GLUT4 glucose transporter. Here we investigated the expression of these transporters during muscle cell differentiation in vitro. Rat L6 muscle cells were grown to the stages of myoblasts, alignment and fused myotubes. Glucose (2-deoxy-D-glucose) transport was higher in myoblasts, decreasing with the progression of alignment and cell fusion. Conversely, insulin-stimulated glucose uptake was negligible in myoblasts, and increased with cell alignment and fusion. The cellular content of GLUT1 transporters decreased and that of GLUT4 transporters increased with cell fusion. Insulin rapidly stimulated glucose uptake in fused myotubes maintained in 2% serum but not in 10% serum. In 10% serum, basal glucose uptake increased as did the cellular content of GLUT1 transporters, while GLUT4 transporter content did not change. These results indicate that both transporters are regulated oppositely during muscle cell differentiation, and that high serum concentrations override the capacity of insulin to regulate transport by inducing overexpression of the GLUT1 transporter.

Insulin-induced decrease in 5′-nucleotidase activity in skeletal muscle membranes

Insulin releases inositol phosphoglycans from myocytes in culture [(1986) Science 233, 967–972], which display insulinomimetic activity. Because 5′‐nucleotidase is anchored to the membrane through inositol‐containing phospholipid glycans, we investigated whether insulin could release the enzyme from the membrane. Membranes prepared from hindquarter muscles of rats perfused with insulin showed a 23% decrease in 5′‐nucleotidase activity. Isolated membranes from muscle exposed to insulin in vitro also showed a small but reproducible decrease (9%) in 5′‐nucleotidase activity relative to unexposed controls. Phospholipase C from Staphylococcus aureus released 60% of the membrane‐bound 5′‐nucleotidase. We propose that insulin may activate an endogenous phospholipase C that cleaves phospholipid‐glycan‐anchored proteins.

Distribution of glucose transporters and insulin receptors in the plasma membrane and transverse tubules of skeletal muscle

The distribution of glucose transporters and of insulin receptors on the surface membranes of skeletal muscle was studied, using isolated plasma membranes and transverse tubule preparations. (i) Plasma membranes from rabbit skeletal muscle were prepared according to Seiler and Fleischer (1982, J. Biol. Chem. 257, 13862-13871), and transverse tubules from rabbit skeletal muscle were prepared according to Rosemblatt et al. (1981, J. Biol. Chem. 256, 8140-8148) as modified by Hidalgo et al. (1983, J. Biol. Chem. 258, 13937-13945). The membranes were identified by the abundance of nitrendipine receptors in the transverse tubules, and their relative absence from the plasma membranes. (ii) Plasma membranes and transverse tubules were also isolated from rat skeletal muscle, according to a novel procedure that isolates both fractions from the same common homogenate. (iii) Glucose transporters were detected by D-glucose protectable binding of the specific inhibitor [3H]cytochalasin B, and insulin receptors were detected by saturable binding of 125I-insulin. The concentration of glucose transporters was about threefold (rabbit) or fivefold (rat) higher in the transverse tubule membrane compared to the plasma membrane, whereas the insulin receptor concentration was about the same in both membranes. These results indicate that the glucose transporters on the surface of the muscle are preferentially segregated to the transverse tubules, and this poses interesting consequences on the functional response of glucose transport to insulin in skeletal muscle.

Somatostatin Receptor Type 2 Antagonism Improves Glucagon and Corticosterone Counterregulatory Responses to Hypoglycemia in Streptozotocin-Induced Diabetic Rats

Diminished responsiveness to hypoglycemia contributes to defective counterregulation in diabetes. Pancreatic and/or circulating somatostatin are elevated in diabetes, which may inhibit counterregulatory hormone release during hypoglycemia. Thus, a selective somatostatin receptor type 2 antagonist (SSTR2a) should improve hormone counterregulation to hypoglycemia. Nondiabetic (N) and streptozotocin-induced diabetic (D) rats underwent 4-h infusion of saline or SSTR2a with insulin-induced hypoglycemia clamped at 2.5 ± 0.5 mmol/L. To evaluate the effect of the SSTR2a in the absence of hypoglycemia, rats underwent a 4-h infusion of saline (Ctrl:N, Ctrl:D) or SSTR2a (Ctrl:D+SSTR2a) only. The attenuated glucagon response to hypoglycemia in D (P < 0.0002) was fully restored by SSTR2a (P < 0.0001). Furthermore, the attenuated corticosterone response in D (P < 0.002) was also enhanced by SSTR2a (P < 0.05). In the absence of hypoglycemia, SSTR2a did not alter basal blood glucose levels. D exhibited 62% more pancreatic somatostatin than N after hypoglycemia. In N rats, SSTR2a did not augment the glucagon or corticosterone response to hypoglycemia. Thus, somatostatin may contribute to impaired glucagon responsiveness to hypoglycemia in diabetes. We demonstrate that SSTR2 antagonism enhances hypoglycemia-stimulated glucagon and corticosterone release in D but not in N rats. SSTR2 antagonism does not affect basal glycemia in D rats.

Amelioration of Hypoglycemia via Somatostatin Receptor Type 2 Antagonism in Recurrently Hypoglycemic Diabetic Rats

Selective antagonism of somatostatin receptor type 2 (SSTR2) normalizes glucagon and corticosterone responses to hypoglycemic clamp in diabetic rats. The purpose of this study was to determine whether SSTR2 antagonism (SSTR2a) ameliorates hypoglycemia in response to overinsulinization in diabetic rats previously exposed to recurrent hypoglycemia. Streptozotocin diabetic rats (n = 19), previously subjected to five hypoglycemia events over 3 days, received an insulin bolus (10 units/kg i.v.) plus insulin infusion (50 mU/kg/min i.v.) until hypoglycemia ensued (≤3.9 mmol/L) (experimental day 1 [Expt-D1]). The next day (Expt-D2), rats were allocated to receive either placebo treatment (n = 7) or SSTR2a infusion (3,000 nmol/kg/min i.v., n = 12) 60 min prior to the same insulin regimen. On Expt-D1, all rats developed hypoglycemia by ∼90 min, while on Expt-D2, hypoglycemia was attenuated with SSTR2a treatment (nadir = 3.7 ± 0.3 vs. 2.7 ± 0.3 mmol/L in SSTR2a and controls, P < 0.01). Glucagon response to hypoglycemia on Expt-D2 deteriorated by 20-fold in the placebo group (P < 0.001) but improved in the SSTR2a group (threefold increase in area under the curve [AUC], P < 0.001). Corticosterone response deteriorated in the placebo-treated rats on Expt-D2 but increased twofold in the SSTR2a group. Catecholamine responses were not affected by SSTR2a. Thus, SSTR2 antagonism after recurrent hypoglycemia improves the glucagon and corticosterone responses and largely ameliorates insulin-induced hypoglycemia in diabetic rats.

Elevated Medium Chain-Acylcarnitines are Associated with Gestational Diabetes, and Early Progression to Type-2 Diabetes, and Induce Pancreatic β-Cell Dysfunction

Specific circulating metabolites have emerged as important risk factors for the development of diabetes. The acylcarnitines (acylCs) are a family of metabolites known to be elevated in type 2 diabetes (T2D) and linked to peripheral insulin resistance. However, the effect of acylCs on pancreatic β-cell function is not well understood. Here, we profiled circulating acylCs in two diabetes cohorts: 1) women with gestational diabetes mellitus (GDM) and 2) women with recent GDM who later developed impaired glucose tolerance (IGT), new-onset T2D, or returned to normoglycemia within a 2-year follow-up period. We observed a specific elevation in serum medium-chain (M)-acylCs, particularly hexanoyl- and octanoylcarnitine, among women with GDM and individuals with T2D without alteration in long-chain acylCs. Mice treated with M-acylCs exhibited glucose intolerance, attributed to impaired insulin secretion. Murine and human islets exposed to elevated levels of M-acylCs developed defects in glucose-stimulated insulin secretion and this was directly linked to reduced mitochondrial respiratory capacity and subsequent ability to couple glucose metabolism to insulin secretion. In conclusion, our study reveals that an elevation in circulating M-acylCs is associated with GDM and early stages of T2D onset and that this elevation directly impairs β-cell function.