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Hepatic mitochondrial function was studied in lean and obese Zucker rats in the fed state and at 3 and 6 days of starvation. No significant differences in state 3 mitochondrial oxidative rates were found due to obesity or starvation. Palmitoylcarnitine utilization rates in mitochondria were unaffected by obesity or starvation; however, when expressed per gram liver weight, they were lower in the obese rats due to the decreased amount of mitochondrial protein per gram liver. For palmitoylcarnitine oxidation and acetoacetate and citrate production, the patterns were the same: per milligram mitochondrial protein, both lean and obese rates were equivalent; per total liver, the obese rates were higher; per gram liver, the obese rates were lower. Mitochondrial carnitine palmitoyltransferase specific activity was higher in fed obese than in lean rats and remained higher during starvation. The results indicate that mitochondrial capacity to oxidize fatty acids and to produce keto acids is not affected by genetic obesity or starvation. The differences in fatty acid oxidation and keto acid production that have been observed in hepatocytes and perfused liver might be explained by decreased mitochondrial protein per unit weight of liver or hepatocytes in obese rats.

The urinary excretion of acetylcarnitine was studied in patients with diabetic ketosis before and during insulin therapy and in normal-weight and obese subjects during fasting. In the diabetic ketotic patients, acetylcarnitine represented 61% of the total acylcarnitine excretion. During the first 24 h of insulin treatment, acetylcarnitine excretion decreased and on the 5th day of treatment was 18% of the acylcarnitines excreted. The urinary excretion of the other acylcarnitines fell slowly. In normal-weight subjects fasted for 3 days, the urinary excretion of acetylcarnitine increased on the 2nd day of fasting, and on the 3rd day acetylcarnitine accounted for 78% of the excreted acylcarnitine. In obese subjects there was a progressive increase in urinary acetylcarnitine excretion, but on day 6 it represented only 55% of the total acylcarnitine excreted. The urinary excretion of acetylcarnitine correlated with blood beta-hydroxybutyrate concentration in the normal-weight subjects during fasting and in the diabetic ketotic patients. Acetylcarnitine accounts for a major fraction of the acylcarnitines excreted in the three ketotic conditions studied. The contribution of acetylcarnitine to the change in acylcarnitines as ketosis appears or disappears is significantly less in the obese subjects than in the normal-weight subjects or in the diabetic patients. This difference may reflect an alteration in the production or disposition of acetyl-CoA and acetylcarnitine in obesity.

he Ca2+ -uptake capacity of two distinct populations of rat heart mitochondria was characterized by monitoring Ca2+ movements directly (using both 45Ca and dual-wavelength spectroscopy) and indirectly (monitoring effects of Ca2+ on respiration, enzyme release, and morphology). Interfibrillar mitochondria accumulated up to 930 nmol/mg protein, whereas the capacity of the subsarcolemmal mitochondria for Ca2+ uptake was limited to 620 nmol/mg protein. In both mitochondrial populations, uptake of the maximal amount of Ca2+ was accompanied by increased proton permeability and subsequent release of the accumulated Ca2+. Even when calcium was taken up and apparently retained by the subsarcolemmal mitochondria, resting respiratory rates increased. Morphological examination of these mitochondria revealed that, although the majority of mitochondria were in the aggregated, energized conformation, a considerable number were disrupted. These damaged mitochondria were probably responsible for the increased resting respiratory rates. No net release of calcium was observed, however, since the intact mitochondria could take up any calcium released by the damaged mitochondria. Morphological disruption of this subset of subsarcolemmal mitochondria was accompanied by release of not only cytochrome c but also of mitochondrial marker enzymes into the extramitochondrial milieu. In contrast, the interfibrillar mitochondria were relatively unaffected in terms of morphology and marker enzyme release, even when their capacity to retain calcium was exceeded.