Treating type 1 diabetes: from strategies for insulin delivery to dual hormonal control.

Type 1 diabetes is a disorder where slow destruction of pancreatic ß-cells occurs through autoimmune mechanisms. The result is a progressive and ultimately complete lack of endogenous insulin. Due to ß-cell lack, secondary abnormalities in glucagon and likely in incretins occur. These multiple hormonal abnormalities cause metabolic instability and extreme glycemic variability, which is the primary phenotype. As the disease progresses patients often develop hypoglycemia unawareness and defects in their counterregulatory defenses. Intensive insulin therapy may thus lead to 3-fold excess of severe hypoglycemia and severely hinder the effective and safe control of hyperglycemia. The main goal of the therapy for type 1 diabetes has long been physiological mimicry of normal insulin secretion based on monitoring which requires considerable effort and understanding of the underlying physiology. Attainment of this goal is challenged by the nature of the disease and our current lack of means to fully repair the abnormal endocrine pancreas interactive functions. As a result, various insulin preparations have been developed to partially compensate for the inability to deliver timely exogenous insulin directly to the portal/intrapancreatic circulation. It remains an ongoing task to identify the ideal routes and regimens of their delivery and potentially that of other hormones to restore the deficient and disordered hormonal environment of the pancreas to achieve a near normal metabolic state. Several recent technological advances help addressing these goals, including the rapid progress in insulin pumps, continuous glucose sensors, and ultimately the artificial pancreas closed-loop technology and the recent start of dual-hormone therapies.

Optimizing reduction in basal hyperglucagonaemia to repair defective glucagon counterregulation in insulin deficiency.

In health, the pancreatic islet cells work as a network with highly co-ordinated signals over time to balance glycaemia within a narrow range. In type 1 diabetes (T1DM), with autoimmune destruction of the ß-cells, lack of insulin is considered the primary abnormality and is the primary therapy target. However, replacing insulin alone does not achieve adequate glucose control and recent studies have focused on controlling the endogenous glucagon release as well. In T1DM, glucagon secretion is disordered but not absolutely deficient; it may be excessive postprandially yet it is characteristically insufficient and delayed in response to hypoglycaemia. We review our system-level analysis of the pancreatic endocrine network mechanisms of glucagon counterregulation (GCR) and their dysregulation in T1DM and focus on possible use of α-cell inhibitors (ACIs) to manipulate the glucagon axis to repair the defective GCR. Our results indicate that the GCR abnormalities are of 'network origin'. The lack of ß-cell signalling is the primary deficiency that contributes to two separate network abnormalities: (i) absence of a ß-cell switch-off trigger and (ii) increased intraislet basal glucagon. A strategy to repair these abnormalities with ACI is proposed, which could achieve better control of glycaemia with reduced hypoglycaemia risk.

Distribution of glucose transporter messenger RNA transcripts in tissues of rat and man.

We used the complementary DNA for the human hepatoma Hep G2 glucose transporter to determine the distribution of glucose transporter messenger RNA (mRNA) in rat and human tissues. Under stringent hybridization conditions, a single 2.8-kilobase (kb) transcript is seen in all rat and human tissues examined. The mRNA is most abundant in brain, and is especially enriched in the brain microvascular fraction. The mRNA abundance in rat muscle and fat is 5% that in brain. Rat liver (both adult and fetal) and human liver have very little 2.8-kb mRNA, but it is abundant in cultured human fibroblasts and EB virus-transformed lymphoblasts. The same size mRNA is present in leg muscle of two type II diabetic patients. A very homologous glucose transporter mRNA is expressed in both insulin-sensitive and -insensitive tissues of rat and man. Hepatocytes, which have abundant glucose transport, may express a homologous but nonidentical glucose transporter.

Factor(s) released by glucose-deprived astrocytes enhance glucose transporter expression and activity in rat brain endothelial cells.

Glucose transporter (GLUT) expression and regulation were studied in rat brain endothelial cells in primary culture (RBEC) and in immortalised RBE4 cells. Immunoblotting analysis showed a low expression of the endothelium-specific GLUT1 in RBEC and RBE4 cells compared to isolated brain capillaries. RBEC and RBE4 cells also expressed the GLUT3 isoform, whereas it was not present in isolated brain capillaries. No change in GLUT expression was observed in endothelial cells treated with astrocyte-conditioned medium. However, treatment with conditioned medium obtained from glucose-deprived astrocytes increased endothelial GLUT1 expression and glucose uptake. These results suggest that astrocytes submitted to hypoglycaemic conditions may release factor(s) that increase glucose uptake through the blood-brain barrier.

The impact of diabetes on the CNS.

The brain is not usually thought to be a target of chronic diabetes complications. Nonetheless, substantial evidence, summarized herein, suggests that diabetes causes brain damage. Clinical syndromes of diabetes-related brain abnormalities are discussed along with possible causes. Various physiological effects of diabetes are reviewed, and questions are raised about gaps in our knowledge. Appropriate directions for future research are suggested.

The prevalence of metoclopramide-induced tardive dyskinesia and acute extrapyramidal movement disorders.

BACKGROUND: Metoclopramide hydrochloride, a neuroleptic dopamine receptor antagonist used to treat gastric ailments, is reported to cause extrapyramidal movement disorders. The goals of this study were (1) to determine the prevalence and severity of tardive dyskinesia and acute extrapyramidal movement syndromes including akathisia, acute dystonia, and drug-induced parkinsonism in metoclopramide-treated patients and (2) to compare the prevalence and severity of tardive dyskinesia in metoclopramide-treated diabetics and nondiabetics. METHODS: From a list of metoclopramide-treated patients received from the Portland (Ore) Veterans Affairs Medical Center pharmacy, 53 patients met inclusion criteria and 51 (96%) agreed to participate. Controls consisted of a convenience sample drawn from the Portland Veterans Affairs Medical Center Outpatient Clinic who were matched to subjects on age (+/- 10 years), gender, and presence or absence of diabetes. Of 61 potential controls contacted, 51 (84%) agreed to participate. Metoclopramide-treated subjects and controls were seen by a rater who was "blind" to all diagnoses and treatments. The rater performed a standardized examination used to elicit signs and symptoms of tardive dyskinesia and acute extrapyramidal movement syndromes. RESULTS: The relative risk for tardive dyskinesia was 1.67 (95% confidence interval, 0.93 to 2.97), and the relative risk for drug-induced parkinsonism was 4.0 (95% confidence interval, 1.5 to 10.5). Metoclopramide-treated patients had significantly greater severity of tardive dyskinesia, drug-induced parkinsonism, and subjective akathisia than controls. Use of metoclopramide was associated with impairment in ambulation and increased use of benzodiazepines. Metoclopramide-treated diabetics had significantly greater severity of tardive dyskinesia than metoclopramide-treated nondiabetics. CONCLUSIONS: Metoclopramide use is associated with a significantly increased prevalence and severity of several extrapyramidal movement disorders.

Chronic seizures increase glucose transporter abundance in rat brain.

Pentylenetetrazole and kainic acid, seizure-inducing agents that are known to increase glucose utilization in brain, were used to produce chronic seizures in mature rats. To test the hypothesis that increased brain glucose utilization associated with seizures may alter glucose transporter expression, polyclonal carboxyl-terminal antisera to glucose transporters (GLUT1 and GLUT3) were employed with a quantitative immunocytochemical method and immunoblots to detect changes in the regional abundances of these proteins. GLUT3 abundances in control rats were found to be correlated with published values for regional glucose utilization in normal brain. Following treatment with kainic acid and pentylenetetrazole, both GLUT3 and GLUT1 increased in abundance in a region and isoform-specific manner. GLUT3 was maximal at eight hours, whereas GLUT1 was maximal at three days. Immunoblots indicated that most of the GLUT3 increase was accounted for by the higher molecular weight component of the GLUT3 doublet. The rapid response time for GLUT3 relative to GLUT1 may be related to the rapid increase in neuronal metabolic energy demands during seizure. These observations support the hypothesis that glucose transporters may be upregulated in brain under conditions when brain glucose metabolism is elevated.

Adrenal steroidogenesis in methylandrostenediol-induced hypertension.

Adrenal vein catheterizations were done in rats made hypertensive by administration of methylandrostenediol (MAD; 17alpha-methyl-5-androstene-3beta,-17beta-diol), and in control rats at intervals during treatment. All MAD-treated rats were hypertensive by 7 weeks. Secretion of corticosterone was consistently decreased at all times in MAD-treated rats. 18-Hydroxy-11-deoxycorticosterone secretion and 11-deoxycorticosterone (DOC) secretion decreased and increased, respectively, compared to controls at 2, 4, and 6 weeks. Aldosterone secretion was decreased at 2 and 4 weeks. This study shows an in vivo block of adrenal 11- and 18-hydroxylation. Transient DOC accumulation by treatment with MAD produced hypertension, though DOC oversecretion and other changes in steroidogenesis were waning by the time hypertension developed.

Expression of leptin receptor isoforms in rat brain microvessels.

Leptin acts on specific brain regions to affect body weight regulation. As leptin is made by white adipose tissue, it is thought that leptin must cross the blood-brain barrier or the blood-cerebrospinal fluid barrier to reach key sites of action within the brain. High expression of a short form leptin receptor has been reported in the choroid plexus. However, whether one or more of the known leptin receptor isoforms is expressed in brain capillaries is unknown. To identify and quantitate leptin receptor isoforms in rat brain microvessels, we applied quantitative RT-PCR to RNA from purified rat brain microvessels in parallel with in situ hybridization. The results show that the amount of short form leptin receptor messenger RNA (mRNA) in brain microvessels is extremely high, exceeding that in choroid plexus. In contrast, low levels of this mRNA were detected in the cerebellum, hypothalamus, and meninges. The long form leptin receptor mRNA is only present at low levels in the microvessels, but surprisingly, its level in cerebellum is 5 times higher than that in the hypothalamus. In situ hybridization experiments confirmed strong expression of short leptin receptors in microvessels, choroid plexus, and leptomeninges. The distribution and type of leptin receptor mRNA isoforms in brain microvessels are consistent with the possibility that receptor-mediated transport of leptin across the blood-brain barrier is mediated by the short leptin receptor isoform.

Forebrain ischemia increases GLUT1 protein in brain microvessels and parenchyma.

Glucose transport into nonneuronal brain cells uses differently glycosylated forms of the glucose transport protein, GLUT1. Microvascular GLUT1 is readily seen on immunocytochemistry, although its parenchymal localization has been difficult. Following ischemia, GLUT1 mRNA increases, but whether GLUT1 protein also changes is uncertain. Therefore, we examined the immunocytochemical distribution of GLUT1 in normal rat brain and after transient global forebrain ischemia. A novel immunocytochemical finding was peptide-inhibitable GLUT1 immunoreactive staining in parenchyma as well as in cerebral microvessels. In nonischemic rats, parenchymal GLUT1 staining co-localizes with glial fibrillary acidic protein (GFAP) in perivascular foot processes of astrocytes. By 24 h after ischemia, both microvascular and nonmicrovascular GLUT1 immunoreactivity increased widely, persisting at 4 days postischemia. Vascularity within sections of brain similarly increased after ischemia. Increased parenchymal GLUT1 expression was paralleled by staining for GFAP, suggesting that nonvascular GLUT1 overexpression may occur in reactive astrocytes. A final observation was a rapid expression of inducible heat shock protein (HSP)70 in hippocampus and cortex by 24 h after ischemia. We conclude that GLUT1 is normally immunocytochemically detectable in cerebral microvessels and parenchyma and that parenchymal expression occurs in some astroglia. After global cerebral ischemia, GLUT1 overexpression occurs rapidly and widely in microvessels and parenchyma; its overexpression may be related to an immediate early-gene form of response to cellular stress.

Coupled glucose transport and metabolism in cultured neuronal cells: determination of the rate-limiting step.

In brain and nerves the phosphorylation of glucose, rather than its transport, is generally considered the major rate-limiting step in metabolism. Since little is known regarding the kinetic coupling between these processes in neuronal tissues, we investigated the transport and phosphorylation of [2-3H]glucose in two neuronal cell models: a stable neuroblastoma cell line (NCB20), and a primary culture of isolated rat dorsal root ganglia cells. When transport and phosphorylation were measured in series, phosphorylation was the limiting step, because intracellular glucose concentrations were the same as those outside of cells, and because the apparent Km for glucose utilization was lower than expected for the transport step. However, the apparent Km was still severalfold higher than the Km of hexokinase I. When [2-3H]glucose efflux and phosphorylation were measured from the same intracellular glucose pool in a parallel assay, rates of glucose efflux were three- to-fivefold greater than rates of phosphorylation. With the parallel assay, we observed that activation of glucose utilization by the sodium channel blocker veratridine caused a selective increase in glucose phosphorylation and was without effect on glucose transport. In contrast to results with glucose, both cell types accumulated 2-deoxy-D-[14C]glucose to concentrations severalfold greater than extracellular concentrations. We conclude from these studies that glucose utilization in neuronal cells is phosphorylation-limited, and that the coupling between transport and phosphorylation depends on the type of hexose used.

Localization of glucose transporter GLUT 3 in brain: comparison of rodent and dog using species-specific carboxyl-terminal antisera.

The carboxyl-terminal amino acid sequences of the canine and gerbil glucose transporter GLUT3 were determined and compared to the published rat sequence. Eleven of 16 amino acids comprising the carboxyl terminus of GLUT3 were found to be identical in rat and dog. However, the canine sequence "ATV" substitutes for the rat sequence "PGNA" at the end of the molecule. The gerbil sequence has 12 of 16 amino acids identical to the rat, including the PGNA terminus. Based on these sequences, four peptides were synthesized, and two polyclonal antisera (one to the canine sequence and one to the rat sequence) were raised to examine the distribution of GLUT3 in canine and rodent brain. Immunoblots of brain membrane preparations showed that both antisera identified peptide-inhibitable protein bands of molecular weight 45,000-50,000. Immunocytochemical studies demonstrated that binding sites for these antisera were abundantly distributed in neuropil in all brain regions. Areas rich in synapses and areas surrounding microvessels exhibited especially high reactivity. GLUT3 reactivity was similarly distributed in canine and rodent brain, except at the blood-brain barrier. GLUT3 was not detected in the blood-brain barrier in gerbil and rat but was present in many canine cerebral endothelial cells, particularly in cerebellum and brain stem. The carboxyl-terminal antisera employed in this study exhibited high degrees of species specificity, indicating that the three or four terminal amino acids of the immunizing peptides (ATV and PGNA) are important epitopes for binding the polyclonal antibodies. These antisera exhibited only minimal binding to brain tissue of non-target species, yet yielded similar staining patterns in neuropil of rodent and canine brain. This finding provides strong evidence that the observed staining patterns accurately reflect the distribution of GLUT3 in brain. In addition, the presence of vascular GLUT3 in dog brain suggests that the canine blood-brain barrier may be preferable to that of the rat as a model for studies of glucose transport relevant to human brain.

Diabetes causes differential changes in CNS noradrenergic and dopaminergic neurons in the rat: a molecular study.

We have previously reported that chronic elevation of insulin in the CNS of rats results in opposing changes of the mRNA expression for the norepinephrine transporter (NET; decreased) and the dopamine transporter (DAT; increased). In the present study we tested the hypothesis that a chronic depletion of insulin would result in opposite changes of NET and DAT mRNA expression, from those observed with chronic elevation of insulin. Rats were treated with streptozotocin to produce hypoinsulinemic diabetes. One week later, steady state levels of mRNA were measured by in situ hybridization for NET in the locus coeruleus (LC) and for DAT in the ventral tegmental area/substantia nigra compacta (VTA/SNc). The mRNA for tyrosine hydroxylase (TH), the rate-limiting enzyme for NE and DA synthesis, was measured in these same brain regions. In the diabetic animals, NET mRNA was significantly elevated (159 +/- 22% of average control level) while DAT mRNA was non-significantly decreased (78 +/- 9% of average control level). Additionally, TH mRNA was significantly altered in both the LC (131 +/- 11% of average control level) and VTA/SNc (79 +/- 5% of average control level). We conclude that endogenous insulin is one physiological regulator of the synthesis and re-uptake of NE and DA in the CNS.

Immunohistochemical localization of the neuron-specific glucose transporter (GLUT3) to neuropil in adult rat brain.

The precise histologic localization of GLUT3, a glucose transporter thought to be restricted to neurons, is unknown. Using a high-affinity, specific antiserum against rodent GLUT3 for immunocytochemistry, light microscopic staining concentrates heterogeneously in the neuropil in a region- and lamina-specific manner; intense staining characterizes areas with high rates of glucose utilization such as inferior colliculus and pyriform cortex. Neuropil localization with little perikaryal staining suggests that GLUT3 may provide the energy needed locally for synaptic transmission.

Forebrain endothelium expresses GLUT4, the insulin-responsive glucose transporter.

The presence of GLUT4, the insulin-responsive glucose transporter, in microvascular endothelium and the responsiveness of glucose transport at the blood-brain barrier to insulin have been matters of controversy. To address these issues, we examined GLUT4 mRNA and protein expression in isolated brain microvessels and in cultured calf vascular cells derived from brain microvessels and aorta. We report here that GLUT4 mRNA can be detected in rat forebrain and its microvasculature using high stringency hybridization of poly(A)+ RNA isolated from these sources. This mRNA is identical to that found in adipose cells from rat. Immunoblot analysis of isolate brain microvessels reveals that GLUT4 protein is also present. Peptide preadsorption studies and absence of our antibody reaction to human red cells suggest these findings are specific. Immunohistochemical staining of cultured calf vascular cells reveals that GLUT4 is expressed in brain endothelial cells but not pericytes, nor in aortic endothelium or smooth muscle cells. The sensitivity of the methods required to detect GLUT4 in brain and comparison to its abundance in low density microsomes from rat adipose cells indicate that GLUT4 is expressed in relatively low abundance in brain microvascular endothelium. No significant differences are observed in steady state levels of GLUT4 mRNA in brain from streptozotocin diabetic compared to control rats. This last finding supports the concept of tissue-specific regulation of GLUT4. We conclude that brain microvascular endothelium specifically expressed GLUT4 while other vascular cells do not.

Progressive hippocampal loss of immunoreactive GLUT3, the neuron-specific glucose transporter, after global forebrain ischemia in the rat.

Brain damage after global forebrain ischemia is worsened by prior hyperglycemia and ameliorated by antecedent hypoglycemia. To assess whether GLUT3, the neuron specific glucose transporter and its mRNA, are affected by cerebral ischemia, we investigated the hippocampal pattern of GLUT3 immunoreactivity and GLUT3 gene expression 1, 4 and 7 days after global forebrain ischemia in a rat 2-vessel occlusion model. We used a newly generated, specific, C-terminally directed polyclonal antiserum against GLUT3 to stain coronal frozen sections. Thionin staining and the microglial marker, OX42, indicated the extent of ischemic damage in hippocampus and correlated with GLUT3 loss. One day after ischemia, no significant change in hippocampal GLUT3 immunoreactivity was observed; by 4 days however, there was consistent and pronounced loss; and at 7 days the loss of GLUT3 staining was maximal. The greatest loss of GLUT3 staining was in the CA1 region, especially the strata oriens and radiatum of Ammon's horn. By contrast, GLUT3 staining was undiminished in the stratum lacunosum moleculare, in the mossy fibers of the lateral aspect of CA3 and in all but the inner-most portion of the molecular layer of the dentate gyrus, immediately adjacent to the granule cells. GLUT3 mRNA levels were not significantly altered at 24 hours and significantly declined at 4 and 7 days after ischemia in the CA1 pyramidal layer. These data are consistent with the pattern of neuronal loss and microglial activation in hippocampus. Loss of GLUT3 may affect the availability of glucose, and possibly the viability of ischemically damaged neurons.

Oxidative stress and HNE conjugation of GLUT3 are increased in the hippocampus of diabetic rats subjected to stress.

Recent studies demonstrate that cellular, molecular and morphological changes induced by stress in rats are accelerated when there is a pre-existing strain upon their already compromised adaptive responses to internal or external stimuli, such as may occur with uncontrolled diabetes mellitus. The deleterious actions of diabetes and stress may increase oxidative stress in the brain, leading to increases in neuronal vulnerability. In an attempt to determine if stress, diabetes or stress+diabetes increases oxidative stress in the hippocampus, radioimmunocytochemistry was performed using polyclonal antisera that recognize proteins conjugated by the lipid peroxidation product 4-hydroxy-2-nonenal (HNE). Radioimmunocytochemistry revealed that HNE protein conjugation is increased in all subregions of the hippocampus of streptozotocin (STZ) diabetic rats, rats subjected to restraint stress and STZ diabetic rats subjected to stress. Such increases were not significant in the cortex. Because increases in oxidative stress may contribute to stress- and diabetes-mediated decreases in hippocampal neuronal glucose utilization, we examined the stress/diabetes mediated HNE protein conjugation of the neuron specific glucose transporter, GLUT3. GLUT3 immunoprecipitated from hippocampal membranes of diabetic rats subjected to stress exhibited significant increases in HNE immunolabeling compared to control rats, suggesting that HNE protein conjugation of GLUT3 contributes to decreases in neuronal glucose utilization observed during diabetes and exposure to stress. Collectively, these results demonstrate that the hippocampus is vulnerable to increases in oxidative stress produced by diabetes and stress. In addition, increases in HNE protein conjugation of GLUT3 provide a potential mechanism for stress- and diabetes-mediated decreases in hippocampal neuronal glucose utilization.

Home blood glucose monitoring: keystone for modern diabetes care.

Home monitoring of capillary blood glucose concentrations has changed diabetes care, giving physicians and patients a way to adjust their therapy and achieve better diabetic control. The practical strategies and equipment for home diabetic monitoring are discussed, including the changing role of urine testing and how inexpensive machines may enhance the value of blood glucose monitoring.

Blood-brain barrier transport of caffeine: dose-related restriction of adenine transport.

We studied the transport of 14C-caffeine across the blood-brain barrier (BBB) by measuring brain 14C:3H ratios five seconds after rats received the caffeine, with 3H2O, by intracarotid injection. Caffeine was found to enter the brain by both simple diffusion and saturable, carrier-mediated transport. This latter observation suggested to us that caffeine's transport might involve macromolecules that are structurally similar to caffeine. Hence, we examined caffeine's ability to inhibit the BBB transports of 14C-adenosine and 14C-adenine. Caffeine caused a dose-dependent inhibition of 14C-adenine transport but no clear change in that of 14C-adenosine. At very high blood levels (Ki = 9.8 mM), caffeine may restrict the availability of circulating purines to the brain. This effect may be important neonatally, when carrier-mediated adenine transport apparently is maximal.

Home monitoring of diabetes mellitus--a quiet revolution.

Home monitoring of capillary blood glucose concentrations has changed diabetes care, giving physicians and patients a way to adjust their therapy and achieve better diabetic control. The practical strategies and equipment for home diabetic monitoring are discussed, including the changing role of urine testing and how inexpensive machines may enhance the value of blood glucose monitoring.