Biomedical subjects

Z Q Shi

Publications and source records attributed to Z Q Shi.

At least 37 records · Page 2Linked to original sources

PubMed1999

Opposite effects of acute hypoglycemia and acute hyperglycemia on glucose transport and glucose transporters in perfused rat skeletal muscle.

This study was undertaken to characterize the effects of glycemia per se (glucose effectiveness) on muscle glucose transport. Isolated rat hindlimbs were perfused in situ for 2 h with perfusate containing either low (2 mmol/l, n = 7), normal (6.5 mmol/l, n = 6), or high (20 mmol/l, n = 6) concentrations of glucose, without insulin, to simulate hypo-, eu-, and hyperglycemic conditions. The effect of varying glucose concentrations on muscle glucose transport was assessed by an ensuing 30-min perfusion with 5.5 mmol/l glucose perfusate without insulin. The 2-h of low glucose perfusion induced significant increases in both muscle glucose clearance (approximately 2.3-fold, P < 0.01) and plasma membrane GLUT4 content (approximately 20%, P < 0.05) relative to normal. In contrast, high glucose perfusion decreased glucose clearance (approximately 1.7-fold, P < 0.01) and plasma membrane GLUT4 content (approximately 20%, P < 0.05). Glucose extraction during the following 30-min perfusion was 2.5-fold greater (P < 0.0001) in the low group and threefold less (P < 0.0001) in the high group, relative to normal. 2-[3H]deoxyglucose-6-phosphate content in both red (soleus) and white (extensor digitorum longus) muscles increased approximately twofold after 2 h of low glucose perfusion (P < 0.0001) and decreased > or =2-fold after high glucose perfusion (P < 0.0001), relative to normal. It is concluded that glycemia regulates glucose transport in skeletal muscle independently of insulin, achieved at least partially via changes in plasma membrane GLUT4. We propose that high glucose levels can acutely downregulate GLUT4 and glucose clearance, thus limiting excessive glucose uptake in muscle. Conversely, low glucose-induced upregulation of muscle glucose clearance and GLUT4 can compensate for reduced glucose availability in the circulation.

Acute Disease↗

PubMed1999

Leptin restores euglycemia and normalizes glucose turnover in insulin-deficient diabetes in the rat.

Leptin has been shown to improve insulin sensitivity and glucose metabolism in normoinsulinemic healthy or obese rodents. It has not been determined whether leptin may act independently of insulin in regulating energy metabolism in vivo. The present study was designed to examine the effects of leptin treatment alone on glucose metabolism in insulin-deficient streptozotocin (STZ)-induced diabetic rats. Four groups of STZ-induced diabetic rats were studied: 1) rats treated with recombinant methionine murine leptin subcutaneous infusion with osmotic pumps for 12-14 days (LEP; 4 mg x kg(-1) x day(-1), n = 10); 2) control rats infused with vehicle (phosphate-buffered saline) for 12-14 days (VEH; n = 10); 3) pair-fed control rats given a daily food ration matching that of LEP rats for 12-14 days (PF; n = 8); and 4) rats treated with subcutaneous phloridzin for 4 days (PLZ; 0.4 g/kg twice daily, n = 10). Phloridzin treatment normalizes blood glucose without insulin and was used as a control for the effect of leptin in correcting hyperglycemia. All animals were then studied with a hyperinsulinemic-euglycemic clamp (6 mU x kg(-1) x min(-1). Our study demonstrates that leptin treatment in the insulin-deficient diabetic rats restored euglycemia, minimized body weight loss due to food restriction, substantially improved glucose metabolic rates during the postabsorptive state, and restored insulin sensitivities at the levels of the liver and the peripheral tissues during the glucose clamp. The effects on glucose turnover are largely independent of food restriction and changes in blood glucose concentration, as evidenced by the minimal improvement of insulin action and glucose turnover parameters in the PF and PLZ groups. Our results suggest that the antidiabetic effects of leptin are achieved through both an insulin-independent and an insulin-sensitizing mechanism.

Animals↗

PubMed1998

Downregulation of platelet-derived growth factor receptor-beta in Shp-2 mutant fibroblast cell lines.

The SH2-containing tyrosine phosphatase Shp-2 appears to function downstream of a variety of growth factor receptors and might play a positive role in cell proliferation. Here we report that expression of the beta subunit of platelet-derived growth factor receptor (PDGFR-beta) was specifically downregulated in mutant fibroblasts lacking a functional Shp-2, while the levels of PDGFR-alpha EGFR and IGFIR were not changed. PDGF-stimulated DNA synthesis and extracellular signal regulated kinase (Erk) activation was severely suppressed in mutant cells. RasGAP, that responds to activation of PDGFR-beta but not PDGFR-alpha, was not phosphorylated on tyrosine in mutant cells upon PDGF-treatment. Northern blot analysis failed to detect PDGFR-beta mRNA in mutant cells. The transcription initiation from the PDGFR-beta gene promoter was not significantly changed, but the half-life of its mRNA was shortened in Shp-2 mutant cells. These observations indicate that Shp-2 not only participates in transmission of signals from growth factor receptors but also plays a specific role in the control of the PDGFR-beta expression. We propose that this is an important mechanism for the positive control of cell proliferation by Shp-2.

Animals↗

PubMed1998

The Shp-2 tyrosine phosphatase has opposite effects in mediating the activation of extracellular signal-regulated and c-Jun NH2-terminal mitogen-activated protein kinases.

Shp-2 is a widely expressed cytoplasmic tyrosine phosphatase with two SH2 domains. A targeted mutant allele of the Shp-2 gene with a deletion of 65 amino acids in the NH2-terminal SH2 domain was created that leads to embryonic lethality at mid-gestation in homozygous mutant mice. To define the Shp-2 function in cell signaling, we have established mutant fibroblast cell lines, and have examined the effect of the Shp-2 mutation on extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) mitogen-activated protein (MAP) kinase pathways. Insulin-like growth factor (IGF)-I-induced ERK activation was completely abolished, while ERK activity upon platelet-derived growth factor and epidermal growth factor stimulation was significantly reduced and shortened in mutant cells. Stimulation of ERK by phorbol 12-myristate 13-acetate was not affected in mutant cells, but the phorbol 12-myristate 13-acetate-induced ERK activity decayed much faster compared with that in wild-type cells. In contrast, JNK activation upon heat shock was significantly enhanced in Shp-2 mutant cells. Based on these results, we conclude that Shp-2 plays differential positive regulatory roles in various mitogenic signaling pathways leading to ERK activation, and that Shp-2 is a negative effector in JNK activation by cellular stress. This is the first evidence that a tyrosine phosphatase has opposite effects in mediating the activation of ERK and JNK MAP kinases.

Animals↗

PubMed1998

Intracerebroventricular administration of leptin markedly enhances insulin sensitivity and systemic glucose utilization in conscious rats.

This study examines the acute, subacute (overnight), and chronic (7-day) effects of intracerebroventricular (i.c.v.) administration of r-metMuLeptin on insulin sensitivity and systemic glucose turnover in conscious unrestrained rats (body weight, 250 to 300 g). Under postabsorptive conditions, acute i.c.v. leptin ([AL] 10 microg bolus) did not affect tracer (3-(3)H-glucose)-determined glucose production (GP) and utilization (GU) rates during the 2-hour hyperinsulinemic (2 mU x kg(-1) x min(-1)) euglycemic clamp. Chronic i.c.v. leptin ([CL] 10 microg/d for 7 days) administered by osmotic pumps markedly reduced the daily food consumption (P < .05), body weight (P < .05), and postabsorptive basal plasma glucose level (P < .01). During the glucose clamp, GP was markedly suppressed (55%) with CL (P < .001 v vehicle and pair-fed control groups). The insulin-induced increment in GU was significantly greater with CL (23.3 +/- 1.8 mg(-1) x kg(-1) x min(-1)) than with vehicle (16.9 +/- 0.2) and pair-feeding (17.1 +/- 0.6, both P < .001). Subacute i.c.v. leptin ([SL] 10 microg bolus) moderately but insignificantly decreased overnight food consumption (-18%) and body weight (-2.5 +/- 1.5 g). The glucose infusion rate during the final 60 minutes of the glucose clamp was 43% greater than for the vehicle group (P < .0001). SL also significantly increased GU (P < .005) and suppressed GP (P < .05) during the glucose clamp. Thus, we conclude that i.c.v. administered leptin has strong actions on the central nervous system that result in significant increases in insulin sensitivity and systemic GU, and these effects are achieved as early as overnight after leptin administration.

Animals↗

PubMed1998

Insulin acutely suppresses glucose production by both peripheral and hepatic effects in normal dogs.

To determine whether the predominant effect of insulin in suppressing tracer-determined glucose production (Ra) is hepatic or peripheral, we infused insulin peripherally (PER) and portally (POR) at both low (0.75 pmol.kg-1.min-1) and high physiological rates (2.7 pmol.kg-1.min-1) during euglycemic clamps in normal dogs. We also infused insulin peripherally at one-half these rates (1/2 PER) to match the peripheral insulin levels in POR and thus obtain a selective POR vs. 1/2 PER difference in hepatic insulin levels. At the high-rate insulin infusion, peripheral insulin levels were greatest with PER (PER = 212 +/- 10 pM, n = 5; POR = 119 +/- 5 pM, n = 6; 1/2 PER = 122 +/- 5 pM, n = 6). Calculated hepatic insulin levels were greatest with POR (POR = 227 +/- 13 pM, PER = 206 +/- 19 pM, 1/2 PER = 123 +/- 8 pM). High-dose PER yielded a greater suppression of Ra than POR (79 +/- 18 vs. 56 +/- 6%, P < .001). Ra was only suppressed by 45 +/- 6% with 1/2 PER (P < 0.01 vs. POR on 6 paired experiments). Free fatty acid (FFA) was suppressed by 57 +/- 8% with PER and only by 33 +/- 5 and 37 +/- 2% with POR and 1/2 PER, respectively. The low-dose PER and POR yielded an equal Ra suppression (PER = 46 +/- 9%, POR = 43 +/- 4%). Only 1/2 PER was associated with a lower suppression of Ra (36 +/- 8, P < 0.05 vs. POR). FFA showed similar suppression in all three groups (approximately 25%). Using both insulin infusion rates, the percent Ra suppression per unit difference in peripheral insulin was approximately twofold greater than that per unit difference in hepatic insulin. These results suggest that, during euglycemic clamps without somatostatin in normal dogs, Ra suppression is mediated by both peripheral and hepatic effects of insulin and that peripheral insulin, at least at high physiological infusion rates, is more potent than hepatic insulin in suppressing Ra.

Animals↗

PubMed1997

Quantitative measurement of islet glucagon response to hypoglycemia by confocal fluorescence imaging in diabetic rats: effects of phlorizin treatment.

UNLABELLED: We have shown that the glucagon irresponsiveness to hypoglycemia in diabetic rats is markedly improved by correction of hyperglycemia independent of insulin. In contrast, normalization of glycemia by insulin did not improve this response. To find out whether these glucagon responses reflect changes in islet glucagon, we directly quantified glucagon area and content in each pancreatic islet by using fluorescent immunostaining and computerized image analysis with confocal laser scanning microscopy (CLSM). The pancreases were analyzed in four groups of rats. 1. Normal controls (NC, n = 4), streptozotocin (65 mg/kg) diabetic rats. 2. Diabetic untreated (DU, n = 4). 3. Diabetic Phlorizin-treated, (0.4 g/kg), twice daily for 4 d (DP, n = 4). 4. Diabetic insulin-treated, using sustained release (2-3 U/d) insulin implant for 5 d (DI, n = 4). Basal plasma glucose was 7.4 +/- 0.3 mM in NC, increased to 14.5 +/- 2.2 mM in DU, which was normalized in DP (5.5 +/- 0.5) and DI (6.7 +/- 0.8). Acute hypoglycemia (H) was induced by i.v. insulin injection. The rats were sacrificed 2 h after insulin injection and the pancreas was removed. By imaging with CLSM, we quantified: 1. Percent of glucagon containing A-cell area/islet area, 2. Fluorescence intensity per islet area, which indicated glucagon content in the islet. 3. Fluorescence intensity per glucagon area indicating glucagon concentration in A-cells. In NC, glucagon containing A cell area was 21 +/- 2% of the islet area, and glucagon intensity and concentration was 11 +/- 1 U and 36 +/- 3.0 U, respectively, in basal (O) state and did not change in (H). In DU, glucagon area increased 183% (O) and 166% (H), and islet glucagon intensity increased by 235% (O) (p < 0.05), but decreased to 135% in H. Glucagon area in DP and DI did not differ significantly from DU. However, hypoglycemia in DP increased glucagon intensity in islet further to 306% of normal control (p < 0.05), suggesting marked increase in glucagon content indicating increased synthesis. In contrast, DI compared to DP showed a decrease in glucagon intensity in islet (46 +/- 3, DP to 22 +/- 2 DI; p < 0.05) in (H) state. Glucagon concentration followed the same pattern as its intensity. CONCLUSION: 1. Increase in islet glucagon content in diabetic rats was associated with increase in glucagon containing A-cell area per islet. 2. Phlorizin-induced insulin independent correction of hyperglycemia increased glucagon content per islet in hypoglycemic state. This, in part, probably contributed to improved glucagon response to hypoglycemia observed earlier 3. Normalization of glycemia with insulin reduced glucagon content of each islet during hypoglycemia. This may explain, in part, unresponsiveness of glucagon to hypoglycemia often observed in insulin-dependent diabetes mellitus (IDDM) with intensive insulin therapy.

Animals↗

PubMed1997

Nerve-induced disruption and reformation of beta1-integrin aggregates during development of the neuromuscular junction.

The earliest biochemical change detected during synaptogenesis is a local elimination of muscle basal lamina proteins. To explore whether this provides signal(s) that regulate postsynaptic differentiation, we examined the effects of innervation on the distribution of beta1-integrins, which were initially present in scattered aggregates complexed with basal lamina ligands. These beta1-integrin aggregates disappear along paths of nerve contact as their basal lamina ligands are eliminated. New accumulations of these proteins then form during assembly of the postsynaptic apparatus. The new beta1-integrin aggregates at developing synapses form partly via a redistribution of mobile molecules on muscle surface. We thus consider whether (a) the removal of integrins' basal lamina ligands alters their cytoplasmic ligand-interactions, causing the dissociation of integrin clusters, and (b) this receptor modulation helps to transduce local changes in pericellular protease activity into cytoplasmic signals that control postsynaptic differentiation.

Animals↗

PubMed1997

A deletion mutation in the SH2-N domain of Shp-2 severely suppresses hematopoietic cell development.

Shp-1 and Shp-2 are cytoplasmic protein tyrosine phosphatases that contain two Src homology 2 (SH2) domains. A negative regulatory role of Shp-1 in hematopoiesis has been strongly implicated by the phenotype of motheaten mice with a mutation in the Shp-1 locus, which is characterized by leukocyte hypersensitivity, deregulated mast cell function, and excessive erythropoiesis. A targeted deletion of 65 amino acids in the N-terminal SH2 (SH2-N) domain of Shp-2 leads to an embryonic lethality at midgestation in homozygous mutant mice. To further dissect the Shp-2 function in hematopoietic development, we have isolated homozygous Shp-2 mutant embryonic stem (ES) cells. Significantly reduced hematopoietic activity was observed when the mutant ES cells were allowed to differentiate into embryoid bodies (EBs), compared to the wild-type and heterozygous ES cells. Further analysis of ES cell differentiation in vitro showed that mutation in the Shp-2 locus severely suppressed the development of primitive and definitive erythroid progenitors and completely blocked the production of progenitor cells for granulocytes-macrophages and mast cells. Reverse transcriptase PCR analysis of the mutant EBs revealed reduced expression of several specific marker genes that are induced during blood cell differentiation. Stem cell factor induction of mitogen-activated protein kinase activity was also blocked in Shp-2 mutant cells. Taken together, these results indicate that Shp-2 is an essential component and primarily plays a positive role in signaling pathways that mediate hematopoiesis in mammals. Furthermore, stimulation of its catalytic activity is not sufficient, while interaction via the SH2 domains with the targets or regulators is necessary for its biological functions in cells. The in vitro ES cell differentiation assay can be used as a biological tool in dissecting cytoplasmic signaling pathways.

Animals↗

PubMed1997

Direct and indirect effects of insulin in suppressing glucose production in depancreatized dogs: role of glucagon.

We have previously shown that during glucose clamps in moderately hyperglycemic depancreatized dogs: 1) peripheral insulin infusion, resulting in greater systemic insulinemia and greater suppression of glucagon than equidose portal infusion, inhibited glucose production (GP) to a greater extent; and 2) portal and half-dose peripheral infusions, resulting in matched peripheral insulinemia and similar suppression of glucagon, inhibited GP equally. These findings are consistent with an indirect effect of insulin in suppressing GP in diabetic dogs, which might be partly mediated by the differential suppression of glucagon. To address this question, we performed the experimental protocols of the previous study under conditions of constant glucagon levels (approximately 550 ng/liter), achieved by a high rate portal glucagon infusion (5 ng/kg.min). As in the previous study (basal glucagon levels, approximately 170 ng/liter), we used depancreatized dogs and assessed GP with HPLC-purified [6(-3)H]glucose. After obtaining constant basal hyperglycemia (approximately 10 mM) with portal infusions of insulin (4.8 +/- 0.5 pmol/kg.min) and glucagon, an additional infusion of insulin was administered for 180 min, either portally (portal; n = 7) or peripherally (peripheral; n = 8) at the same rate (5.4 pmol/kg.min) or at half that rate peripherally (1/2 periph; n = 5). Plasma glucose and glucose specific activities were clamped at basal levels. Systemic insulin levels increased by 215 +/- 16,310 +/- 26, and 184 +/- 15 pM, and estimated hepatic insulin levels increased by 398 +/- 20, 310 +/- 26, and 184 +/- 15 pM with portal, peripheral, and 1/2 periph, respectively. GP was suppressed to the same extent with portal and peripheral (53 +/- 6% and 50 +/- 6%), but less with 1/2 periph (35 +/- 5%). FFA levels were suppressed to a greater extent with peripheral than portal or 1/2 periph, whereas the responses of lactate alanine and glycerol to insulin infusion were similar in the three groups. Thus, in the present report, unlike in our previous study, 1) suppression of GP was proportional to the hepatic insulin levels; and 2) systemic insulin levels did not dominate suppression of GP. We, therefore, conclude that in hyperglycemic depancreatized dogs 1) glucagon, at concentrations seen in poorly controlled diabetes, can unmask a direct effect of hepatic insulin levels on GP; and 2) the suppression of glucagon may play a role in the peripheral effect of exogenously delivered insulin on GP. This is the first in vivo study to show that the main direct effect of insulin on the liver is to counteract the effect of glucagon.

Animals↗

PubMed1997

Insulin-independent acute restoration of euglycemia normalizes the impaired glucose clearance during exercise in diabetic dogs.

At rest and during exercise, chronic hyperglycemia, high free fatty acid (FFA) oxidation, and insulin deficiency in diabetes are well known to impair glucose clearance (metabolic clearance rate [MCR]). The effect of acute restoration of glycemia per se on MCR has been less well characterized. We therefore studied normal and alloxan-diabetic dogs both at rest and during exercise, as diabetic hyperglycemic or after acutely induced euglycemia (<160 min) generated by infusion of either insulin or phlorizin. Glucose uptake was similar under hyperglycemic and normoglycemic conditions both at rest and during exercise, indicating a precise balance between the mass effect of glucose and decreased MCR. Rest and exercise MCR was fourfold lower under conditions of hyperglycemia, but insulin-independent restoration of euglycemia improved basal MCR threefold and normalized MCR during exercise. High FFA turnover did not affect glucose uptake but was correlated with plasma lactate concentrations (r = 0.72, P < 0.001), suggesting that muscle fuel requirements are controlled by glucose oxidation and not uptake. We conclude that in alloxan-diabetic dogs, the impaired MCR may be an adaptive phenomenon because correction of hyperglycemia corrects MCR despite partial insulin deficiency and high FFA turnover. We speculate that constant glucose uptake despite hyperglycemia in diabetes may protect the muscle from excessive exposure to glucose.

Animals↗

PubMed1997

The effect of pioglitazone on hepatic glucose uptake measured with indirect and direct methods in alloxan-induced diabetic dogs.

Pioglitazone, a thiazolidinedione derivative, ameliorates hyperglycemia by augmenting peripheral glucose disposal and suppressing hepatic glucose production in diabetic animals. However, the effect of this agent on hepatic glucose uptake has not been explored. To determine this, experiments were conducted in alloxan-induced diabetic dogs with (pioglitazone group, n = 7) or without (control group, n = 5) a 10-day oral treatment with pioglitazone (1 mg x kg(-1) x day(-1)). A euglycemic-hyperinsulinemic (insulin infusion rate 25.2 pmol x kg(-1) x min(-1)) clamp was maintained by adjusting the peripheral glucose infusion rate (GIR). After a 60-min basal period (period I), portal glucose infusion (Pinf, 33.3 micromol x kg(-1) x min(-1)) was administered for 120 min (period II). This was followed by a 60-min recovery period (period III). Arterial insulin levels were kept stable in the supraphysiological range throughout the experiment (1,623 +/- 52, pioglitazone group; 1,712 +/- 52 pmol/l, C group). There was no significant difference in whole-body glucose utilization determined by [3-3H]glucose between the pioglitazone and C groups in period I (68.4 +/- 2.8 vs. 70.1 +/- 2.8 micromol x kg(-1) x min(-1), respectively) and period III (81.2 +/- 5.0 vs. 74.5 +/- 3.3 micromol x kg(-1) x min(-1), respectively). Net hepatic glucose uptake (NHGU) determined by arteriovenous difference method was approximately zero in the basal period (-0.7 +/- 1.1, pioglitazone group; 0.1 +/- 1.2 micromol x kg(-1) x min(-1), C group). In period II, hepatic glucose uptake, determined by the changes in GIR, was significantly higher in the pioglitazone group (6.5 +/- 0.6 micromol x kg(-1) x min(-1)) than in the C group (-0.4 +/- 0.6 micromol x kg(-1) x min(-1), P < 0.001). This observation was also confirmed by NHGU during portal glucose infusion (6.9 +/- 1.4 vs. 2.1 +/- 1.8 micromol x kg(-1) x min(-1), pioglitazone vs. C, respectively; P < 0.025). We conclude that pioglitazone treatment enhances hepatic glucose uptake during portal glucose loading in alloxan-induced diabetic dogs. However, in hyperinsulinemic conditions, pioglitazone does not enhance the already high peripheral glucose uptake.

Animals↗

PubMed1996

Grap is a novel SH3-SH2-SH3 adaptor protein that couples tyrosine kinases to the Ras pathway.

A human cytoplasmic signaling protein has been cloned that possesses the same structural arrangement of SH3-SH2-SH3 domains as Grb2. This protein is designated Grap for Grb2-related adaptor protein. The single 2.3-kilobase (kb) grap transcript was expressed predominantly in thymus and spleen, while the ubiquitously expressed grb2 gene produced two mRNA species of 3.8 and 1.5 kb. Grap and Grb2 consist of 217 amino acids and share 59% amino acid sequence identity, with highest homology in the N-terminal SH3 domain. The GrapSH2 domain interacts with ligand-activated receptors for stem cell factor (c-kit) and erythropoietin (EpoR). Grap also forms a stable complex with the Bcr-Abl oncoprotein via its SH2 domain in K562 cells. Furthermore, Grap is associated with a Ras guanine nucleotide exchange factor mSos1, primarily through its N-terminal SH3 domain. These results show that a family of Grb2-like proteins exist and couple signals from receptor and cytoplasmic tyrosine kinases to the Ras signaling pathway.

Adaptor Proteins, Signal Transducing↗

PubMed1996

A moderate decline in specific activity does not lead to an underestimation of hepatic glucose production during a glucose clamp.

We have previously shown that modeling errors lead to underestimation of hepatic glucose production (HGP) during glucose clamps when specific activity (SA) declines markedly. We wished to assess whether the failure to keep SA constant substantially affects calculation of HGP during insulin infusion when glucose requirements to maintain the glucose clamp are moderate. Therefore, 150-minute hyperinsulinemic (5.4 pmol - kg (-1) - min (-1) clamps were performed in depancreatized dogs that were maintained hyperglycemic (approximately 10 mmol/L with either (l) unlabeled glucose infusate (COLD Ginf, n = 5) or (2) labeled glucose infusate (HOT Ginf, n = 6) containing high-performance liquid chromatography (HPLC purified [6-3H]glucose. Insulinemia and glucagonemia were similar between the two groups. Additionally, glucose infusion rates were equivalent with COLD and HOT Ginf, indicating comparable insulin effects on overall glucose metabolism. The SA decreased a maximum of 32% with COLD Ginf, but remained constant with HOT Ginf. HGP was suppressed equally with COLD or HOT Ginf treatments at each time point during the clamp (mean suppression during last hour of clamp, 69% +/- 4% and 69% +/- 5%, P = NS, COLD and HOT Ginf, respectively). We conclude that when glucose requirements are moderate and SA changes slowly, as in the diabetic dog, it is not necessary to keep SA perfectly constant to avoid significant modeling errors when calculating HPG during hyperinsulinemic clamps.

Animals↗

PubMed1996

Proteolytic disruption of laminin-integrin complexes on muscle cells during synapse formation.

To explore whether a neural modulation of muscle integrins' extracellular ligand interactions contributes to synapse induction, we compared the distributions of beta1-integrins and basal lamina proteins on Xenopus myotomal myocytes developing in culture. beta1-Integrins formed numerous organized aggregates scattered over the entire muscle surface, with particularly dense accumulations at specialized sites resembling myotendinous and neuromuscular junctions. Integrin aggregates on muscle cells differed from those on surrounding fibroblasts and epithelial cells, both in their lack of response to cross-linking by multivalent ligands and in their consistent association with the cells' own extracellular matrices. Muscle integrin clusters were usually associated with congruent basal lamina accumulations containing laminin and a heparan sulfate proteoglycan (HSPG), sometimes including fibronectin and vitronectin acquired from the surrounding medium. Immediately prior to synaptic differentiation, any existing laminin and HSPG accumulations along the path of cell contact were eliminated, disrupting otherwise stable laminin-integrin complexes. This apparently proteolytic modulation of integrins' extracellular ligand interactions was soon followed by the accumulation of new congruent accumulations of laminin and HSPG in the developing synaptic basal lamina. Combining these results with earlier findings, we consider the possibility that postsynaptic differentiation is induced, at least in part, by the proteolytic disruption of integrin-ligand complexes at sites of nerve-muscle contact.

Animals↗

PubMed1996

Glucagon response to hypoglycemia is improved by insulin-independent restoration of normoglycemia in diabetic rats.

The aim of this study was to determine whether the impaired glucagon response to insulin-induced hypoglycemia in the diabetic rat can be improved by correction of hyperglycemia independent of insulin. Four groups of age-matched male Sprague-Dawley rats (246 +/- 13 g BW) were studied: 1) normal controls (NC; n = 7); 2) diabetic, untreated (DU; n = 6); 3) diabetic, treated for 5-7 days using sustained release (2-3 U/day) insulin implants (DI; n = 6); and 4) diabetic, treated for 3-4 days with phlorizin (0.4 g/kg), given sc twice daily (DP; n = 7). Diabetes was induced by a single injection of streptozotocin (65 mg/kg). Basal plasma glucose was 7.4 +/- 0.3 mM in NC, but rose to 14.5 +/- 2.2 mM in DU. Basal hyperglycemia was corrected with phlorizin and insulin treatments (5.5 +/- 0.5 and 6.7 +/- 0.8 mM, respectively). NC rats responded to insulin-induced hypoglycemia with a rapid and marked increase in glucagon (peak, 2059 +/- 311 pg/ml). The glucagon response was blunted in DU (635 +/- 180 pg/ml) and was partially improved by prolonged normalization of glycemia in DP (1335 +/- 295 pg/ml; P < 0.05). Plasma somatostatin levels in all diabetic groups were 2- to 3-fold higher in the basal state, but were not different during hypoglycemia, than those in NC rats. Compared to levels in NC rats, diabetes resulted in decreased insulin, but elevated glucagon and somatostatin concentrations in the pancreatic tissue. Treatment with both insulin and phlorizin reversed the changes in the pancreatic content of both glucagon and somatostatin. Pancreatic proglucagon messenger RNA did not show significant differences among the four groups in either state. Insulin treatment in the DI group resulted in a delayed and much smaller increase in the glucagon response (740 +/- 138 pg/ml) to hypoglycemia despite normalization of glycemia. We, therefore, conclude that in streptozotocin-diabetic rats, the impaired glucagon responsiveness to hypoglycemia is significantly improved by insulin-independent correction of hyperglycemia, suggesting the importance of normoglycemia per se in maintaining, at least in part, the glucose sensitivity of pancreatic alpha-cells.

Animals↗

PubMed1996

Determinants of glucose turnover in the pathophysiology of diabetes: an in vivo analysis in diabetic dogs.

Hyperglycaemia in diabetes results from a combination of increased hepatic glucose production and decreased metabolic clearance of glucose. Our report summarizes recent work conducted in our laboratory to investigate the regulatory factors involved in the control of glucose turnover in diabetes. The action of insulin, both directly and indirectly, in regulating glucose turnover in diabetic dogs is considered. 1) In the depancreatized diabetic dog, peripheral rather than portal insulin levels determine the suppression of hepatic glucose production via indirect mechanisms such as limiting, precursors for gluconeogenesis and/or inhibiting glucagon secretion. 2) The differential effects of insulin and insulin-like growth factor I on glucose turnover may be dependent on a decline in glycaemia since previously observed differential effects on glucose turnover were masked under conditions of clamped hyperglycaemia in the depancreatized dog. 3) In a paradoxical dichotomous fashion, hyperglycaemia both contributes to, and compensates for, defective glucose clearance in diabetes. Acute restoration of euglycaemia significantly improves glucose clearance at rest and normalizes the exercise-induced increment in clearance in alloxan-diabetic dogs. 4) Our model of centrally-induced stress also shows that an increase in glucose utilization and clearance is largely independent of changes in insulin and that the combined effects of catecholamines and glucagon are responsible for increasing glucose production.

Animals↗

PubMed1995

Erratic deposition of agrin during the formation of Xenopus neuromuscular junctions in culture.

In order to disclose the mechanism that regulate synapse development we compared the distributions of agrin, acetylcholine receptors (AChR) and a basal lamina heparan sulfate proteoglycan (HSPG) in sections and cultures prepared from Xenopus laevis and Ambystoma mexicanum embryos. While agrin, AChR and HSPG may accumulate almost synchronously at synapses in vivo, agrin deposition usually lagged well behind the other synaptic markers during development in culture, and was not detectable at many differentiated junctions. Agrin deposition at nerve-muscle contacts in culture also appeared to require the presence of other synaptic components. A similarly variable deposition occurred on noninnervated myocytes, where agrin again collected near sites of HSPG and AChR accumulation on some cells. Profuse agrin accretion occurred consistently, however, within the extracellular matrices of surrounding epithelial cells derived from both myotomes and neural tubes. In cocultures of Ambystoma neurons and Xenopus myocytes Ambystoma agrin collected at some chimeric neuromuscular junctions, but also accumulated on noninnervated myocytes and in the extracellular matrices of salamander neuroendothelial cells. Based upon these observations we conclude that (a) focal agrin deposition is not required for synaptic differentiation on Xenopus myocytes and (b) agrin may be one of several muscle basal lamina components that stem mainly from the secreted products of nearby epithelial cells.

Agrin↗