The incidence of impaired glucose tolerance and diabetes (called CFRD) is high in cystic fibrosis (CF), and is expected to increase due to increased life-span of the patients. CFRD is an important cause of worsened morbidity and mortality, thus understanding the pathophysiology underlying its development is imperative. Insulin deficiency has been recognized as one cause of CFRD; however it is clear the etiology is more complex. The applicant's long-range goal is to understand the pathophysiologic changes which cause CFRD. Studies proposed in this application will allow us to characterize alterations in normal metabolism of glucose and protein which contribute to CFRD. Our global hypothesis is that increased rates of gluconeogenesis (GNG) result in hyperglycemia. This excess is not caused simply by portal hypoinsulinemia alone, but rather is driven by excessive amino acid substrate availability secondary to cytokine-mediated protein catabolism. We will recruit 48 adult CF subjects (12 per glucose tolerance category: normal, impaired, CFRD with fasting hyperglycemia and CFRD without fasting hyperglycemia) and 20 normal volunteers matched for age and gender. Subjects will recruited from the CF centers at The University and the South Central CF Consortium (all CF centers in Texas, Oklahoma and Arkansas). Subjects will be categorized according to clinical status (pulmonary function and modified NIH score), and we will measure cytokines TNF-a, IL- 6, IL-10 and TNF receptor antibodies. We will also measure thyroid function, estrogen and testosterone and IGF-1 levels. GNG and glycogenolysis will be quantified by measuring the incorporation 2H into the 2nd, 5th and 6th carbons of glucose, and reported as percentage of total hepatic glucose production (measured using [6,6-2H2]glucose). Whole body protein turnover (WBPT) will be measured using [15N]urea and [1-13C]leucine, and we will conduct a sub-study to evaluate the effect of intravenous protein hyperalimentation on GNG and WBPT. Insulin secretion will be quantified using a hyperglycemic clamp (target glucose levels 160 mg/dl and 350 mg/dl). Insulin effect on GNG and WBPT, as well as peripheral insulin sensitivity will be determined using a step-wise hyperinsulinemic euglycemic clamp (insulin doses of 10,20 and 120 mU/m2/min). We will measure resting energy expenditure using indirect calorimetry and intake of carbohydrate, protein and fat will be quantified with a three day food journal. Important support for this proposal includes the excellent scientific community at UT-Southwestern, the consultant role of Dr. Satish Kalhan, a noted expert in substrate metabolism, ready access to a GC mass spectrometer, institutional support, and an established track record of performing complex metabolic studies in CF subjects. It is therefore expected that these studies will provide new information regarding potential causes of CF related diabetes and protein catabolism.
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