Energy transfer in acute diabetic rat hearts: adaptation to increased energy demands due to augmented calcium transients.

OBJECTIVES

Hearts of rats with diabetes mellitus (DM) are characterized by energy demands exceeding their energy production, but they might also exhibit decreased vulnerability to ischemia and calcium overload. This indicates adaptation in cardiac energetics (CE), where energy transport is not rate-limiting. Aim-This study was designed to elucidate the functional significance of the DM-induced adaptation in CE by investigating the formation of mitochondrial contact sites (MiCS), facilitating the Ca-dependent/high-capacity energy transfer from mitochondria, in conjunction with testing the ischemic tolerance (IT) of hearts.

METHODS

After 1 week of streptozotocin-induced DM (45 mg/kg iv), the hearts of male diabetic and age-matched control rats (C) were isolated and Langendorff-perfused with either 1.6 or 2.2 mmol/L of CaCl(2). MiCS formation was assessed by cytochemical detection of mCPK octamers and was quantified stereologically as MiCS to mitochondrial surface ratio (S(S)). IT was evaluated in anesthetized open-chest animals subjected to 30-min occlusion of the LAD coronary artery followed by 4-h reperfusion, by monitoring ischemic arrhythmias and by measuring the size of infarction (tetrazolium double staining).

RESULTS

In C hearts, increasing Ca2+ induced both positive inotropic response (dP/dt increase from 2270 +/- 220 to 2955 +/- 229, p < 0.01) and elevated MiCS formation (S(S) increase from 0.070 +/- 0.011 to 0.123 +/- 0.012, p < 0.01). In DM hearts, basic MiCS formation was already comparable with that induced by elevated Ca2+ in C hearts and could not be further stimulated by Ca2+. In C, ventricular tachycardia represented 55.4% of the total arrhythmias and occurred in 90% of the animals. In DM rats, the arrhythmia profile was similar to that in C, and the incidence of tachyarrhythmias and their severity were not enhanced (arrhythmia score: 3.18 +/- 0.4 vs. 3.30 +/- 0.3 in C). The infarct size normalized to the size of area at risk was smaller in the DM than in C hearts (52.3 +/- 5.8% vs. 69.2 +/- 2.2%, respectively; p < 0.05).

CONCLUSIONS

Ca-signaling represents the link between energy delivery from mitochondria (via MiCS) and energy requirements of the heart. In DM hearts, energy transport via MiCS is elevated to the maximum value. This contributes to increased resistance of DM hearts to irreversible cell damage.