Myocardial ultrastructure and electrocardiograms of the hummingbird under normal and experimental conditions.

Abstract

AbstractVentricular myocardium from several adult specimens of hummingbirds (Eupetomena macroura macroura) were subjected to study by electrocardiography and by light and electron microscopy under normal and experimental conditions as provided by injection of 2,4‐dinitrophenol (DNP) and ether anesthesia. The birds were captured in Brazil, and were studied because of their high heart rates 428/460 minute on the average, seeking correlations of structure and function under normal conditions as well as after pharmacological stimuli. Under normal conditions, the hummingbird showed a highly developed sarcoplasmic reticulum, many gigantic mitochondrial with numerous tightly packed parallel mitochondrial cristae and tubules, and few small, dark bodies. The amount of sarcosomes is approximately equivalent to that of myofibrils. As seen in longitudinal sections of muscle fibers, often the junctions between successive mitochondria and both indentations of mitochondria and of the nuclear envelope occurred at the level of the Z bands. This gave the impression that contraction of the myofibrils shortened the nucleus and caused it to wrinkle. Most mitochondrial bulged at their middle as if they had been compressed between successive Z bands, suggesting a more resistant area at the level of these bands than in the rest of the myofibril. Almost no glycogen granules were found, probably because the high metabolic rate of the heart did not allow free storage of such carbohydrates.The administration of DNP was responsible for changes in the ECG (tachycardia and other alterations) and in the structure of the myocardium: large dilations in the sarcoplasmic reticulum and the appearance of small spaces in the mitochondria.