Mitochondrial populations and succinic dehydrogenase in the heart of a hibernator

Thirteen-lined ground squirrels, Citellus tridecemlineatus, hibernating at 3–5 °C, awakened at 25–27 °C. Ten animals were killed for each experimental group as follows: (1) [Formula: see text] awakening to a body temperature of 9–13 °C, (2) 15 min awakening to 13–15 °C, and (3) 30 min awakening to 16–19 °C. At death a sample of the heart's apex was fixed in osmium tetroxide, dehydrated, and embedded in Vestopal-W for electron microscopy, and a 10% w/v water homogenate was made of the rest of the organ. Mitochondrial counts and measurements of dimensions were made on electron micrographs of at least 10 fields of each sample, and duplicate samples of homogenate were incubated at 37 °C and assayed spectrophotometrically for succinic dehydrogenase activity. When compared with data on control animals, during awakening the number of mitochondria per field decreased 25% (P < 0.01). Mitochondrial length increased 10% (P < 0.05) and width increased 28% (P < 0.01). In tissue obtained from hibernating animals, the succinic dehydrogenase activity was 60% of the control. This value decreased further to 42% of the control value during the first 7.5 min of awakening, thereafter returning to 94% of the control by 15 min.