Autophagic vacuoles in heart muscle and liver. A comparative morphometric study including circadian variations in meal-fed rats

Abstract

Large test areas of 36 000 to 38 000 μm 2 of scctioned cytoplasm/animal and organ were morphometrically evaluated for autophagic vacuoles (AVs) in heart muscle and in liver parenchymal cells. Rats were kept in a nutritional steady state by feeding a timely, i.e. shortly before the onset of darkness, single daily meal (10 g of a standard diet). Circadian variations in volume fraction as well as in numerical density of AVs were found. The values reached a maximum in the late light period, whereas minimal values were obtained in the early dark period. The variations were synchronous in heart muscle and in liver. The average cytoplasmic volume fraction of AVs in heart muscle was 0.79 × 10 −4, that is less than half the value found in liver 1.74 × 10 −4. The differences were less pronounced in the segregated fraction of mitochondria which amounted to 1.68 × 10 −4 in heart muscle and 2.85 × 10 −4 in the liver. These data suggest that in heart muscle cytoplasmic components are degraded by cellular autophagy to a similar, perhaps some lower rate than in liver. The absence of myofibrils in the AVs does not exclude a possible lysosomal degradation of myofibrillar proteins.