Islet transplantation improves glucose oxidation and mechanical function in diabetic rat hearts

Diabetic cardiomyopathy revisited

Diabetes mellitus increases the risk of heart failure independently of underlying coronary artery disease, and many believe that diabetes leads to cardiomyopathy. The underlying pathogenesis is partially understood. Several factors may contribute to the development of cardiac dysfunction in the absence of coronary artery disease in diabetes mellitus. This review discusses the latest findings in diabetic humans and in animal models and reviews emerging new mechanisms that may be involved in the development and progression of cardiac dysfunction in diabetes.

Effects of streptozocin diabetes and diabetes treatment by islet transplantation on in vivo insulin signaling in rat heart

The insulin signaling cascade was investigated in rat myocardium in vivo in the presence of streptozocin (STZ)-induced diabetes and after diabetes treatment by islet transplantation under the kidney capsule. The levels of insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit, insulin receptor substrate (IRS)-2, and p52(Shc) were increased in diabetic compared with control heart, whereas tyrosine phosphorylation of IRS-1 was unchanged. The amount of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) and the level of PI 3-kinase activity associated with IRS-2 were also elevated in diabetes, whereas no changes in IRS-1-associated PI 3-kinase were observed. Insulin-induced phosphorylation of Akt on Thr-308 was increased fivefold in diabetic heart, whereas Akt phosphorylation on Ser-473 was normal. In contrast with Akt phosphorylation, insulin-induced phosphorylation of glycogen synthase kinase (GSK)-3, a major cellular substrate of Akt, was markedly reduced in diabetes. In islet-transplanted rats, the majority of the alterations in insulin-signaling proteins found in diabetic rats were normalized, but insulin stimulation of IRS-2 tyrosine phosphorylation and association with PI 3-kinase was blunted. In conclusion, in the diabetic heart, 1) IRS-1, IRS-2, and p52(Shc) are differently altered, 2) the levels of Akt phosphorylation on Ser-473 and Thr-308, respectively, are not coordinately regulated, and 3) the increased activity of proximal-signaling proteins (i.e., IRS-2 and PI 3-kinase) is not propagated distally to GSK-3. Islet transplantation under the kidney capsule is a potentially effective therapy to correct several diabetes-induced abnormalities of insulin signaling in cardiac muscle but does not restore the responsiveness of all signaling reactions to insulin.

Sustained improvements in cardiac geometry and function following kidney-pancreas transplantation

Kidney-pancreas (KP) transplantation has been shown to improve left ventricular (LV) geometry and function 6-24 months after the procedure, yet whether these improvements are sustained in long-term survivors has not been demonstrated. This study examined whether early improvements in LV geometry and function were sustained 3-5 years after KP transplantation. Left ventricular function and geometry were prospectively evaluated prior to, and at 1, 2, and 3-5 years posttransplant using two-dimensional, M-mode, echocardiography with Doppler interrogation in the parasternal and apical views. The sample included 21 KP and a comparison group of 12 diabetic kidney-alone (KA) recipients. Long-term (3-5 years) data were obtained for KP recipients only. Although KA recipients had a longer duration of dialysis and worse diastolic function pretransplant, the groups were similar on other baseline measures. KA recipients experienced minimal improvements while KP recipients had significant improvements in cardiac function and geometry, both in terms of mean values and the percentage of KP recipients who experienced normalization posttransplant (p < 0.05). KP recipient improvements were also sustained at 3-5 years posttransplant on three of five measures, with 75% of long-term KP recipients achieving normal LV mass posttransplant compared with 31% pretransplant. Data indicate that significant impairments in cardiac geometry and function occur in diabetic KA and KP recipients. Though both groups experienced early improvements posttransplant, KP recipients achieved more dramatic and clinically significant improvements at 1, 2, and 3-5 years posttransplant. Additional studies are needed to examine the relevance of these findings with regard to the cardiac morbidity and mortality of these patients.

Contribution of malonyl-CoA decarboxylase to the high fatty acid oxidation rates seen in the diabetic heart

Myocardial glucose oxidation is markedly reduced in the uncontrolled diabetic. We determined whether this was due to direct biochemical changes in the heart or whether this was due to altered circulating levels of insulin and substrates that can be seen in the diabetic. Isolated working hearts from control or diabetic rats (streptozotocin, 55 mg/kg iv administered 6 wk before study) were aerobically perfused with either 5 mM [(14)C]glucose and 0.4 mM [(3)H]palmitate (low-fat/low-glucose buffer) or 20 mM [(14)C]glucose and 1.2 mM [(3)H]palmitate (high-fat/high-glucose buffer) +/-100 microU/ml insulin. The presence of insulin increased glucose oxidation in control hearts perfused with low-fat/low-glucose buffer from 553 +/- 85 to 1,150 +/- 147 nmol x g dry wt(-1) x min(-1) (P < 0. 05). If control hearts were perfused with high-fat/high-glucose buffer, palmitate oxidation was significantly increased by 112% (P < 0.05), but glucose oxidation decreased to 55% of values seen in the low-fat/low-glucose group (P < 0.05). In diabetic hearts, glucose oxidation was very low in hearts perfused with low-fat/low-glucose buffer (9 +/- 1 nmol x g dry wt(-1) x min(-1)) and was not altered by insulin or high-fat/high-glucose buffer. These results suggest that neither circulating levels of substrates nor insulin was responsible for the reduced glucose oxidation in diabetic hearts. To determine if subcellular changes in the control of fatty acid oxidation contribute to these changes, we measured the activity of three enzymes involved in the control of fatty acid oxidation; AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase (ACC), and malonyl-CoA decarboxylase (MCD). Although AMPK and ACC activity in control and diabetic hearts was not different, MCD activity and expression in all diabetic rat heart perfusion groups were significantly higher than that seen in corresponding control hearts. These results suggest that an increased MCD activity contributes to the high fatty acid oxidation rates and reduced glucose oxidation rates seen in diabetic rat hearts.

Pancreatic islet transplantation improves body composition, decreases energy intake and normalizes energy efficiency in previously diabetic female rats

We investigated the weight gain, body composition, and feed efficiency of female Wistar Furth rats (170 +/- 1 g) made diabetic with streptozotocin (55 mg/kg intravenously), then infused intraportally with 3519 +/- 838 (150 mu equivalent units) syngeneic pancreatic islets of Langerhans. After islet transplants (5-6 wk), nutritional energetics were evaluated in transplanted rats (Transplant), and also in 3- and 9-wk diabetic (Diab-3, 9) and control rats treated with sham infusions and similar surgical manipulations (Sham-3, 9). Diabetic rats demonstrated marked hyperphagia, which was corrected by islet transplantation (577 +/- 53 vs. 266 +/- 19 kJ/d; P < 0.0001) and was not different than sham control rats (285 +/- 24 kJ/d; P > 0.05). Three weeks of diabetes resulted in a lower protein (Diab-3, 24.8 +/- 2.6 g vs. Sham-3, 30.9 +/- 1.0 g) and fat content (1.9 +/- 0.8 g vs. 11.6 +/- 1.7 g) in the rats' carcasses. However, 6 wk after islet transplantation, rats receiving islets (Transplant) were not different than control rats (Sham-9) (31.9 +/- 1.7 g vs. 33.3 +/- 1.9 g protein and 15.4 +/- 3.0 g vs. 15.1 +/- 3.2 g fat). Three weeks of diabetes resulted in a lesser energy efficiency compared with Sham rats (2.7 +/- 2.0 vs. 7.1 +/- 1.9 kJ gained/100 kJ ingested); islet-transplanted rats were not different than Sham-9 rats (4.9 +/- 2.3 vs. 4.7 +/- 1.4 kJ gained/100 kJ ingested). These data illustrate that islet transplantation in previously diabetic female rats improves growth with proportional gains in body protein and fat mass. This is modulated in part by a reduced food intake and an energy efficiency that is improved relative to controls. These studies offer an optimistic outlook for the continued development of more physiological insulin delivery strategies that preclude the nutritional complications associated with exogenous insulin administration.