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Hippocampal neurochemical profile and glucose transport kinetics in patients with type 1 diabetes
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Abstract
Hippocampal MRS at 3 Tesla revealed no differences in hippocampal neurochemical
composition and glucose transport between subjects with type 1 diabetes and healthy controls.
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To prospectively evaluate the frequency and severity of hypoglycemic episodes in IDDM subjects who declare themselves to have reduced awareness of hypoglycemia, to validate their self-designations in their natural environment, and to determine objectively the presence or absence of autonomic and neuroglycopenic symptoms associated with their low blood glucose (BG) levels.
A total of 78 insulin-dependent diabetes mellitus (IDDM) subjects (mean age 38.3 +/- 9.2 years; duration of diabetes 19.3 +/- 10.4 years) completed two sets of assessments separated by 6 months. The assessments included reports of frequency and severity of low BG, symptoms associated with low BG, and a BG symptom/estimation trial using a hand-held computer (HHC). Diaries of hypoglycemic episodes were kept for the intervening 6 months. HbA1 levels were determined at each assessment.
Of the subjects, 39 declared themselves as having reduced awareness of hypoglycemia (reduced-awareness subjects). There were no differences between these reduced-awareness subjects and aware subjects with regard to age, sex, disease duration, insulin dose, or HbA1. During the HHC trials, reduced-awareness subjects were significantly less accurate in detecting BG < 3.9 mmol/l (33.2 +/- 47 vs. 47.6 +/- 50% detection, P = 0.001) and had significantly fewer autonomic (0.41 +/- 0.82 vs. 1.08 +/- 1.22, P = 0.006, reduced-awareness vs. aware) and neuroglycopenic (0.44 +/- 0.85 vs. 1.18 +/- 1.32, P = 0.004, reduced-awareness vs. aware) symptoms per subject. Prospective diary records revealed that reduced-awareness subjects experienced more moderate (351 vs. 238, P = 0.026) and severe (50 vs. 17, P = 0.0062) hypoglycemic events. The second assessment results were similar to the first and verified the reliability of the data.
IDDM subjects who believe they have reduced awareness of hypoglycemia are generally correct. They have a history of more moderate and severe hypoglycemia, are less accurate at detecting BG < 3.9 mmol/l, and prospectively experience more moderate and severe hypoglycemia than do aware subjects. Neither disease duration nor level of glucose control explains their reduced awareness of hypoglycemia. Reduced-awareness individuals may benefit from interventions designed to teach them to recognize all of their potential early warning symptoms.
An in vivo model of chronic hypoglycemia was used to investigate changes in blood-brain barrier (BBB) glucose transport activity and changes in the expression of GLUT1 mRNA and protein in brain microvasculature occurring as an adaptive response to low circulating glucose levels. Chronic hypoglycemia was induced in rats by constant infusion of insulin via osmotic minipumps; control animals received infusions of saline. The criterion for chronic hypoglycemia was an average blood glucose concentration of < 2.3 mmol/l (42 mg/dl) after 5 days. The average blood glucose concentration at the end of the experimental period in the rats selected for study was 2.0 +/- 0.1 mmol/l (36 +/- 1 mg/dl) vs. 4.9 +/- 0.1 mmol/l (88 +/- 1 mg/dl) in the controls. Internal carotid artery perfusion studies demonstrated an increase in the BBB permeability-surface area (PS) product of 40% (P < 0.0005) in the chronically hypoglycemic animals as compared with controls. Western blotting of solubilized isolated brain capillaries demonstrated a 51% increase (P < 0.05) in immunoreactive BBB GLUT1 in the chronically hypoglycemic rats, and Northern blotting of whole-brain poly(A+) mRNA revealed a 50% increase in the GLUT1-to-actin ratio in the insulin-treated group (P < 0.05). Northern blotting analysis of microvessel-depleted total brain poly(A+) showed that the increase in GLUT1 mRNA in the chronically hypoglycemic rats was restricted to the BBB. The present study demonstrates increased expression of GLUT1 mRNA and protein at the BBB in chronic hypoglycemia and suggests that this increase is responsible for the compensatory increase in BBB glucose transport activity that occurs with chronically low circulating blood glucose levels.
Context
Upregulated brain glucose transport in response to recurrent hypoglycemia may contribute to the development of hypoglycemia associated autonomic failure (HAAF) and impaired awareness of hypoglycemia. Whether recurrent hypoglycemia alters glucose transport in the hypothalamus is not known.
Objective
To test the hypothesis that hypothalamic glucose transport will increase in healthy volunteers preconditioned with recurrent hypoglycemia to induce HAAF.
Setting
University medical center.
Design and participants
Thirteen healthy subjects underwent paired euglycemic and hypoglycemic preconditioning studies, which included three 2-hour long clamps over two days, separated by at least one month. Following preconditioning, hypothalamic glucose transport was measured by magnetic resonance spectroscopy (MRS) in the afternoon on day 2 of each preconditioning protocol.
Outcome Measure
The ratio of maximal transport rate to cerebral metabolic rate of glucose (Tmax/CMRglc), obtained from MRS-measured glucose in the hypothalamus as a function of plasma glucose.
Results
HAAF was successfully induced based on lower epinephrine, glucagon, and cortisol during the third vs. first hypoglycemic preconditioning clamp (p≤0.01). Hypothalamic glucose transport was not different following recurrent euglycemia vs. hypoglycemia (Tmax/CMRglc = 1.62±0.09 after euglycemia preconditioning and 1.75±0.14 after hypoglycemia preconditioning, p=ns). In addition, hypothalamic glucose concentrations measured by MRS were not different following the two preconditioning protocols.
Conclusions
Glucose transport kinetics in the hypothalamus of healthy humans with experimentally induced HAAF were not different from those measured without HAAF. Future studies of patients with diabetes and impaired awareness of hypoglycemia will be necessary to determine if the existence of the diabetes state is required for this adaptation to hypoglycemia to occur.
Recurrent hypoglycemia (RH), the most common side-effect of intensive insulin therapy for diabetes, is well established to diminish counter-regulatory responses to further hypoglycemia. However, despite significant patient concern, the impact of RH on cognitive and neural function remains controversial. Here we review the data from both human studies and recent animal studies regarding the impact of RH on cognitive, metabolic, and neural processes. Overall, RH appears to cause brain adaptations which may enhance cognitive performance and fuel supply when euglycemic but which pose significant threats during future hypoglycemic episodes.
Near-normalization of glycemia reduces the risks of chronic diabetic complications but increases the risk of serious hypoglycemia. Hypoglycemia can impair neuronal function in the brain and diminish awareness of subsequent hypoglycemic episodes, yet little is known about how neurons adapt to hypoglycemia. This study tests the hypothesis that isoform-specific alterations in brain glucose transport proteins occur in response to chronic hypoglycemia. To study this, groups of rats were injected with approximately 25 U/kg ultralente insulin daily at 1700 for 8 days to maintain hypoglycemia. Vascular-free and microvessel membrane fractions from brain were prepared for immunoblot analysis of GLUT-1 and GLUT-3 by use of isoform-specific antisera. Insulin treatment reduced blood glucose levels from 4.0 +/- 0.1 (vehicle-injected controls) to 1.7 +/- 0.1 mmol/l on day 8 (P < 0.001) and increased GLUT-3 protein expression (175.6% of control; P < 0.05). Microvascular GLUT-1 (55 kDa) tended to increase (195.6% of controls; P = 0.08) variably, whereas nonvascular GLUT-1 (45 kDa) was unchanged. We conclude that neuronal glucose transport protein (GLUT-3) expression adapts to chronic hypoglycemia. This adaptation may spare neuronal energy metabolism but could dampen neuronal signaling of glucose deprivation.
Understanding the mechanism of brain glucose transport across the blood-brain barrier is of importance to understanding brain energy metabolism. The specific kinetics of glucose transport have been generally described using standard Michaelis-Menten kinetics. These models predict that the steady-state glucose concentration approaches an upper limit in the human brain when the plasma glucose level is well above the Michaelis-Menten constant for half-maximal transport, Kt. In experiments where steady-state plasma glucose content was varied from 4 to 30 mM, the brain glucose level was a linear function of plasma glucose concentration. At plasma concentrations nearing 30 mM, the brain glucose level approached 9 mM, which was significantly higher than predicted from the previously reported Kt of approximately 4 mM (p < 0.05). The high brain glucose concentration measured in the human brain suggests that ablumenal brain glucose may compete with lumenal glucose for transport. We developed a model based on a reversible Michaelis-Menten kinetic formulation of unidirectional transport rates. Fitting this model to brain glucose level as a function of plasma glucose level gave a substantially lower Kt of 0.6 +/- 2.0 mM, which was consistent with the previously reported millimolar Km of GLUT-1 in erythrocyte model systems. Previously reported and reanalyzed quantification provided consistent kinetic parameters. We conclude that cerebral glucose transport is most consistently described when using reversible Michaelis-Menten kinetics.
Although it is well established that recurrent hypoglycemia leads to hypoglycemia unawareness, the mechanisms responsible for this are unknown. One hypothesis is that recurrent hypoglycemia alters brain glucose transport or metabolism. We measured steady-state brain glucose concentrations during a glucose clamp to determine whether subjects with type 1 diabetes and hypoglycemia unawareness may have altered cerebral glucose transport or metabolism after exposure to recurrent hypoglycemia. We compared 14 subjects with diabetes and hypoglycemia unawareness to 27 healthy control subjects. Brain glucose concentrations were measured under similar metabolic conditions using in vivo (1)H nuclear magnetic resonance (NMR) spectroscopy at 4 Tesla during a hyperglycemic clamp (plasma glucose = 16.7 mmol/l) with somatostatin and insulin. Subjects with type 1 diabetes and hypoglycemia unawareness had significantly higher brain glucose concentrations compared to that in controls under the same conditions (5.5 +/- 0.3 vs. 4.7 +/- 0.1 micromol/g wet weight, P = 0.016). These data suggest that changes in brain glucose transport or metabolism may occur as a result of recurrent hypoglycemia.
Feasibility and reproducibility of neurochemical profile quantification in the human hippocampus at 3 T
G. Feasibility and reproducibility of neurochemical profile quantification in the human
hippocampus at 3 T. NMR Biomed 2015; 28:685-693