Lipid and Glycogen Metabolism in Patients With Impaired Glucose Tolerance and Calcium Sensing Receptor Mutations

Background: 1. Type 2 diabetes mellitus is a main risk factor for cardiovascular disease and heart failure, in part due to diabetic cardiomyopathy. Ectopic intracellular lipid accumulation and impaired glycogen metabolism in skeletal muscle and liver and are closely associated with metabolic impairment in insulin resistant subjects and patients with diabetes mellitus. Recent evidence suggests that increased myocardial lipid accumulation might contribute to the development of myocardial dysfunction by direct toxic effects (lipotoxicity). However, the association between intracellular lipid accumulation and (myocardial) functional impairment is likely more complex than originally imagined. Recent studies suggest that not fat per se, but the content of saturated or unsaturated fatty acids might predict the development of cardiac steatosis and myocardial dysfunction. In addition carbohydrates stored as glycogen in muscle cells serve as readily available energy supply for contracting muscle. Skeletal muscle and hepatic glycogen metabolism is impaired in patients with diabetes mellitus. Data from animal experiments suggest a relevant role of myocardial glycogen stores in ischemic preconditioning. Due to methodological limitations so far data on myocardial glycogen stores and myocardial lipid composition in humans are missing. 2. Heterozygote inherited inactivating mutations in Calcium Sensing Receptor (CaSR)-gene leads to familiar hypocalciuric hypercalcemia (FHH), specified by mildly elevated plasma Ca and parathyroid hormone concentrations, whereas urine Ca excretion is inadequately low. However, in addition to the parathyroid gland CaSR is expressed in various tissues including the endocrine pancreas and the heart. So far it is unknown whether the endocrine function of the pancreas or myocardial morphology and/or function is altered in patients with FHH. 3. Altered hepatic energy metabolism might play an important role in the development of type 2 diabetes. Additionally, the lack of insulin delivery to the liver via the portal vein in type 1 diabetes might alter liver ATP synthesis. Therefore we aim to investigate hepatic energy metabolism non invasively with MRS. Hypothesis: In addition to total ectopic lipid deposition in the myocardium, myocardial lipid composition, i.e. the relative abundance of saturated and unsaturated fatty acids, and impaired myocardial glycogen metabolism may play an important role in the development cardiac lipotoxicity leading to diabetic cardiomyopathy. Pancreatic endocrine function and myocardial morphology and function is altered in patients with heterozygote inactivating mutations of the CaSR-gene / FHH. Hepatic and cardiac lipid and energy metabolism is altered in T1DM. Aims: * Metabolic virtual biopsy of the myocardium for identification of specific patterns of intracellular lipid composition and myocardial glycogen metabolism as possible critical determinants of metabolic cardiomyopathy * Characterization of the metabolic interplay between the myocardium, skeletal muscle, liver and adipose tissues in different stages of development of type 2 diabetes compared to patients with calcium sensing receptor mutation Methods: * 1H/13C and 31P magnetic resonance spectroscopy (MRS) and imaging (MRI) for measurements of myocardial, skeletal and liver lipid and glycogen content, abdominal adipose tissue distribution and composition, ATP synthesis and myocardial functional parameters * Mixed meal tolerance tests to trace the postprandial partitioning of substrates between insulin sensitive tissues (myocardium, skeletal muscle, liver, adipose tissue). * Hyperinsulinemic-hyperglycemic glucose clamp (HHC) with enrichment of the infused glucose with the stable isotope \[1-13C\]glucose to trace the incorporation of circulating glucose into myocardial glycogen in healthy insulin sensitive volunteers, prediabetic insulin resistant volunteers with impaired glucose tolerance, healthy subjects, patients suffering from type 2 diabetes mellitus, type 1 diabetes and patients with heterozygote mutation in calcium sensing receptor. Relevance: Despite intensive treatment of cardiovascular risk factors, heart diseases are still the main cause of death in diabetic patients. Thus, elucidation of mechanisms that link impaired lipid and/or glycogen metabolism and energy homeostasis to the development of heart failure appears to be crucial for the development of novel treatment strategies. Additionally, hepatic steatosis plays a challenging, emerging role in the treatment of liver disease, wherefore further insight in hepatic energy metabolism in various endocrine disease is urgently needed.