Upregulation of eNOS and unchanged energy metabolism in increased susceptibility of the aging type 2 diabetic GK rat heart to ischemic injury

Abstract

We investigated the tolerance of the insulin-resistant diabetic heart to ischemic injury in the male Goto-Kakizaki (GK) rat, a model of type 2 diabetes. Changes in energy metabolism, nitric oxide (NO) pathway, and cardiac function were assessed in the presence of physiological substrates. Age-matched control Wistar (n = 19) and GK (n = 18) isolated rat hearts were perfused with 0.4 mM palmitate, 3% albumin, 11 mM glucose, 3 U/l insulin, 0.2 mM pyruvate, and 0.8 mM lactate for 24 min before switching to 1.2 mM palmitate (11 rats/group) during 32 min low-flow (0.5 ml·min(-1)·g wet wt(-1)) ischemia. Next, flow was restored with 0.4 mM palmitate buffer for 32 min. A subset of hearts from each group (n = 8 for control and n = 7 for GK groups) were freeze-clamped for determining baseline values after the initial perfusion of 24 min. ATP, phosphocreatine (PCr), and intracellular pH (pH(i)) were followed using (31)P magnetic resonance spectroscopy with simultaneous measurement of contractile function. The NO pathway was determined by nitric oxide synthase (NOS) isoform expression and total nitrate concentration (NOx) in hearts. We found that coronary flow was 26% lower (P < 0.05) during baseline conditions and 61% lower (P < 0.05) during reperfusion in GK vs. control rat hearts. Rate pressure product was lower during reperfusion in GK vs. control rat hearts (P < 0.05). ATP, PCr, and pH(i) during ischemia-reperfusion were similar in both groups. Endothelial NOS expression was increased in GK rat hearts during baseline conditions (P < 0.05). NOx was increased during baseline conditions (P < 0.05) and after reperfusion (P < 0.05) in GK rat hearts. We report increased susceptibility of type 2 diabetic GK rat heart to ischemic injury that is not associated with impaired energy metabolism. Reduced coronary flow, upregulation of eNOS expression, and increased total NOx levels confirm NO pathway modifications in this model, presumably related to increased oxidative stress. Modifications in the NO pathway may play a major role in ischemia-reperfusion injury of the type 2 diabetic GK rat heart.

Fig. 1.

Rate pressure product during the experimental time course in control and Goto-Kakizaki (GK) rat hearts. *P < 0.05 vs. control. Results are expressed as means ± SE in mmHg/min.

Fig. 2.

Coronary flow rates during preischemic baseline conditions and during reperfusion in control and GK rat hearts. Results are expressed as means ± SE in ml·min⁻¹·g wet wt⁻¹. *P < 0.05 vs. control.

Fig. 3.

Kinetics of phosphocreatine (PCr) (A), ATP (B), and intracellular pH (pH_i) (C) during the experimental time course in control and GK rat hearts. Results are expressed in mM except pH_i and are means ± SE.

Fig. 4.

Total nitrate concentration (NOx) in freeze-clamped hearts during preischemic baseline conditions and after reperfusion in control and GK rat hearts. Results are expressed as means ± SE in nmol/mg protein. *P < 0.05 vs. control.

Fig. 5.

Endothelial nitric oxide synthase (eNOS) expression in freeze-clamped hearts during preischemic baseline conditions and after reperfusion in control and GK rat hearts. Equal protein loading was checked using Ponceau red staining. The bar graphs show results from six control and six GK rat hearts. P < 0.05 vs. control (*) and vs. after reperfusion (†). Results are expressed as means ± SE in arbitrary units.

Fig. 6.

Neuronal nitric oxide synthase (nNOS) expression in freeze-clamped hearts during preischemic baseline conditions and after reperfusion in control and GK rat hearts. Equal protein loading was checked using Ponceau red staining. The bar graphs show results from six control and six GK rat hearts. Results are expressed as means ± SE in arbitrary units.