Altered myocardial substrate metabolism is associated with myocardial dysfunction in early diabetic cardiomyopathy in rats: studies using positron emission tomography

Abstract

Background: In vitro data suggest that changes in myocardial substrate metabolism may contribute to impaired myocardial function in diabetic cardiomyopathy (DCM). The purpose of the present study was to study in a rat model of early DCM, in vivo changes in myocardial substrate metabolism and their association with myocardial function.

Methods: Zucker diabetic fatty (ZDF) and Zucker lean (ZL) rats underwent echocardiography followed by [11C]palmitate positron emission tomography (PET) under fasting, and [18F]-2-fluoro-2-deoxy-D-glucose PET under hyperinsulinaemic euglycaemic clamp conditions. Isolated cardiomyocytes were used to determine isometric force development.

Results: PET data showed a 66% decrease in insulin-mediated myocardial glucose utilisation and a 41% increase in fatty acid (FA) oxidation in ZDF vs. ZL rats (both p < 0.05). Echocardiography showed diastolic and systolic dysfunction in ZDF vs. ZL rats, which was paralleled by a significantly decreased maximal force (68%) and maximal rate of force redevelopment (69%) of single cardiomyocytes. Myocardial functional changes were significantly associated with whole-body insulin sensitivity and decreased myocardial glucose utilisation. ZDF hearts showed a 68% decrease in glucose transporter-4 mRNA expression (p < 0.05), a 22% decrease in glucose transporter-4 protein expression (p = 0.10), unchanged levels of pyruvate dehydrogenase kinase-4 protein expression, a 57% decreased phosphorylation of AMP activated protein kinase alpha1/2 (p < 0.05) and a 2.4-fold increased abundance of the FA transporter CD36 to the sarcolemma (p < 0.01) vs. ZL hearts, which are compatible with changes in substrate metabolism. In ZDF vs. ZL hearts a 2.4-fold reduced insulin-mediated phosphorylation of Akt was found (p < 0.05).

Conclusion: Using PET and echocardiography, we found increases in myocardial FA oxidation with a concomitant decrease of insulin-mediated myocardial glucose utilisation in early DCM. In addition, the latter was associated with impaired myocardial function. These in vivo data expand previous in vitro findings showing that early alterations in myocardial substrate metabolism contribute to myocardial dysfunction.

Figure 1

In vivo alterations in myocardial substrate metabolism. Myocardial [¹¹C]palmitate time-activity curve, where the rapid decline reflects clearance of palmitate from the myocardium (A). Two-tissue compartment model used for determining myocardial metabolic rate of glucose utilisation (MMRglu) (B). Typical example of a myocardial [¹¹C]palmitate and ¹⁸FDG image in a ZL and ZDF rat normalised for standard uptake value (summed images from 30 to 60 min) (C). Myocardial fatty acid (FA) oxidation rate constant measured under fasting conditions (D), and MMRglu measured under hyperinsulinaemic euglycaemic clamp conditions (E) for ZL rats (open bars) and ZDF rats (filled bars). Data are expressed as mean ± SEM, n = 6–11, * p < 0.05 vs. ZL rats.

Figure 2

In vitro function of single cardiomyocytes. Single cardiomyocyte mounted between a force transducer and piezoelectric motor (A). Maximal (F_max) and passive (F_passive) force (B), maximal rate of force redevelopment (K_tr max) (C) and calcium sensitivity (D) in ZL rats (open bars) and ZDF rats (filled bars). Data are expressed as mean ± SEM, n = 4, * p < 0.005.

Figure 3

Molecular alterations in ZDF hearts. Quantification of glucose transporter-4 (GLUT4) mRNA levels (A) and protein levels (B), pyruvate dehydrogenase 4 (PDK4) protein levels (C) and phosphorylation/total protein levels of AMP kinase a (AMPKα1/2-Thr172) (D) in ZL rats (open bars) and ZDF rats (filled bars). Protein expression and a typical example of subcellular localisation of the fatty acid transporter (FAT)/CD36 (E) in ZL rats and ZDF rats. Data are expressed as mean ± SEM, n = 4–8, * p < 0.05, ^# p = 0.10.

Figure 4

Molecular alterations in myocardial insulin signaling. Quantification of immunoblots showing relative phosphorylation/total protein levels of Akt-Ser473/Akt2 after saline (-; open bars) or insulin (+; filled bars) injection. Data are expressed as mean ± SEM, n = 4–8, * p < 0.05 different from basal, # p < 0.05 different from ZL.

Figure 5

Association between metabolism and function. Relationship of whole-body insulin sensitivity (M-value) with fractional shortening (A; r = 0.83, p < 0.001, n = 12) and E deceleration time (B; r = -0.62, p < 0.05, n = 12). Relationship of myocardial metabolic rate of glucose utilisation (MMRglu) with fractional shortening (C; r = 0.91, p < 0.005, n = 7) and E deceleration time (D; r = -0.70, p = 0.08, n = 7).