Abstract
GLP-1 receptors (GLP-1R) have been found in the brain and it has been hypothesized that GLP-1R agonists could improve brain glucose metabolism. The aims of the study were to evaluate the effects of Exenatide (EX) on cerebral as well as hepatic/peripheral glucose metabolism.
We studied 15 male subjects with impaired glucose tolerance (n=12) or newly diagnosed with type 2 diabetes (n=3) (age=56±8 y, BMI=29±1 kg/m2, HbA1c=5.7±0.1%). Each subject underwent 2 oral glucose tests (OGTT 75 g) with double blind injection of EX (5 mcg) or placebo (PLC) 30min before OGTT; 6,6-2H- glucose was infused for 4h (2h before and 2h during OGTT) and U-13C-glucose was added to oral glucose to assess glucose absorption (RaO), production (EGP), total rate of glucose appearance (Ra) and disposal (Rd). Brain glucose uptake was measured by PET following injection of 18FDG (5mCi) at t=0 and acquiring brain images 1h into the OGTT.
EX delayed gastric emptying (RaO AUC0-120min=660±167 vs 1885±148 µmol/min•kg); glucose (12607±760 vs 18353±1038 mg/dl) and insulin AUC0-120min
(2704±562 vs 4872±650 mU/l) were lower in EX (all p<0.05). While glucose Rd AUC0-60min was comparable (127±9 vs 132±10 µmol/min•kg, EX vs PLC), cerebral glucose metabolic rate (CGMR) was increased with EX (0.18±0.01 vs 0.12±0.01 µmol/min•ml; p=0.02). Total CGMR was inversely correlated with RaO (r=-0.63;
p<0.0005).
The brain areas with the highest CGMR were: Thalamus 0.23±0.02 vs 0.14±0.01, Occipital 0.24±0.02 vs 0.16±0.02 and Frontal lobes 0.19±0.02 vs 0.12±0.01 µmol/min•ml (EX vs PLC, all p<0.004). However, in the Hypothalamus EX reduced the CGMR from 0.13±0.02 to 0.09±0.01 µmol/min•ml (p=0.003).
Conclusion: EX modulates CGMR by increasing glucose uptake in multiple areas of the brain but reduces glucose uptake in the hypothalamus. These results provide a previously unrecognized mechanism via which EX-mediated changes in glucose metabolism can influence the regulation of glucose absorption and hepatic/peripheral glucose metabolism.
Acknowledgements
There are a number of people without whom this thesis might not have been written, and to whom I am greatly grateful. I would like to sincerely thank Amalia for her guidance and persistent help in this interesting project and also for her generous support and constructive comments in writing this thesis. I would like to thank Patricia for her precious contribution in the analysis of the data and for her suggestions. In addition I would like to acknowledge the crucial role of the staff of Research Imaging Institute of University of Texas, Health Science Center at San Antonio, TX, USA, particularly Jack Lancaster and Peter Fox who gave me a precious guidance and technical support to realize this thesis. I would like to express my gratitude to Stefano Del Prato and Ralph DeFronzo for their encouragement and guidance. I would especially like to thank nurses at Health Science Center for their support in recruiting patients and collecting data for my Ph.D. thesis.
Special thanks to my family who daily encourage me increasing dedication and hope.
At the end I would like express appreciation to my beloved wife Graziana who spent sleepless nights with me and is always my real support in all the moments of my life.
CONTENTS
Abstract pag. i
Acknowledgements pag. ii
INTRODUCTION pag. 1
References pag. 5
CHAPTER 1: Metabolic abnormalities from prediabetes toward overt type 2 diabetes
1.1 Introduction pag. 9
1.2 The role of β-cell dysfunction and insulin resistance pag. 10
1.3 The role of Incretin System pag. 10
1.4 The role of endogenous glucose production in postabsorptive state pag. 11 1.5 The role of hepatic insulin resistance pag. 13 1.6 The role of Splanchnic (hepatic) glucose uptake and Glucagon pag. 15 1.7 The role of adipocytes and fat metabolism pag. 16
1.9 Conclusions pag. 17
References pag. 18
CHAPTER 2: The acute effects of Exenatide on postprandial glucose metabolism in prediabetic individuals
2.1 Introduction pag. 25
2.2 Protocol and Characteristics of the study subjects pag. 25 2.3 The effect of Exenatide on Postprandial glucose metabolism pag. 27 2.4 Effect of Exenatide on Insulin, C-peptide and Glucagon profiles pag. 31
2.5 Effect of Exenatide on lipid metabolism pag. 34
2.6 Discussion pag. 35
References pag. 39
CHAPTER 3: The role of the Brain in glucose homeostasis
3.1 Introduction pag. 44
3.2 Sites of gluco-detection pag. 46 3.3 Mechanisms of gluco-detection pag. 47 3.4 Central glucose sensing modulates food intake and energy
expenditure: the gut-to-brain GLP-1 dependent axis pag. 49 3.5 Function of GLP-1 and GLP-1r agonists on (diabetic) brain pag. 51
3.6 Conclusions pag. 52
References pag. 53
CHAPTER 4: Positron emission tomography (PET) to evaluate glucose organ metabolism
4.1 Introduction pag. 60
4.2 Fundamentals of Positron Emission Tomography Imaging pag. 60
4.3 Radiopharmaceuticals used in PET pag. 62
4.4 Radiopharmaceuticals used in neurochemical imaging pag. 63 4.5 Compartmental model of FGD uptake pag. 65
4.5.1 Compartmental model pag. 66
4.5.2 Graphical analysis pag. 68
4.6 FDG and glucose transporters pag. 71
4.7 Excretion of FDG pag. 73
4.8 FDG Radioactivity pag. 73
4.9 FDG Specific Activity pag. 74
4.10 Conclusions pag. 75
References pag. 76
CHAPTER 5: Effect of Exenatide on postprandial cerebral glucose metabolism
5.1 Rationale pag. 79
5.2 Intracellular effects of brain GLP-1R activation pag. 80 5.3 Exenatide crosses the blood brain barrier (BBB)
after peripheral administration pag. 81
5.4 Method pag. 81
5.5 Results pag. 82
5.5.1 Fasting Glucose Metabolism pag. 82
5.5.2 OGTT Glucose Metabolism pag. 83
5.5.3 The brain FDG uptake and cerebral glucose
metabolic rate pag. 85
5.6 Discussion pag. 90
5.6.1 Exenatide increases brain glucose metabolic rate pag. 91 5.6.2 The effect of Exenatide on the Hypothalamus pag. 93
References pag. 96
CHAPTER 6: The role of Exenatide in the hepatic glucose metabolism:
a cross-talk hypothesis between the liver and the brain
6.1 Hepatic glucose metabolism and its modulation by Exenatide pag. 101 6.2 Hormonal response to a glucose load and its modulation by Exenatide pag. 103
6.3 Discussion pag. 116
References pag. 120
Conclusions pag. 122
Perspectives pag. 124
APPENDIX
A.1 Study Population pag. 125
A.2 Study design and protocol pag. 125
A.3 PET study pag. 127
A.3.1 PET scanner characteristics pag. 127
A.3.2 Liver and Adipose Tissue pag. 127
A.3.2.1 PET Imaging pag. 127
A.3.2.2 Image Processing pag.128
A.3.2.3 Kinetic Analyses pag.129
A.3.3 Brain pag.129
A.3.3.1 PET Imaging pag.129
A.3.3.2 Image Processing pag.130
A.3.3.3 Kinetic Analyses pag.132
A.4 Metabolic Fluxes Calculation pag.132
A.5 Analitical Analysis pag.133
References pag.134
