Cell Cycle Parameters and Ornithine Decarboxylase Activity in the Red Bone Marrow of Hibernating Ground Squirrels Urocitellus undulatus

During the hibernation season, the values for the parameters of the cell cycle of red bone marrow cells in the hibernating ground squirrels Urocitellus undulatus, when they return to an active-like state between periods of torpor and interbout arousal, do not differ from those observed in summer-active animals. In animals that enter a state of torpor, the cumulative percentage of cells in the resting phase (G₀ phase) and pre-synthesis phase (G₁ phase) increased from 71.8 to 76.0%, the percentage of cells in the synthesis phase (S phase) decreased from 19.3 to 15.3% compared to those animals that return to an active-like state between periods of torpor and interbout arousal. The cumulative percentage of cells in the post DNA synthesis phase (G₂ phase) and mitosis (M) does not change, but (G₂ + M)/S ratio increases. When animals enter a state of torpor, changes in parameter values are observed when the animal’s body temperature drops below 25°C; this effect refers to a system whose thermal relaxation time is a nonmonotonic function of the initial temperature. The activity of the key enzyme of polyamine synthesis ornithine decarboxylase, a marker of cell activation and proliferation during interbout arousal does not significantly differ from that observed in summer-active animals; the enzymatic activity decreases sharply, when animals decrease their body temperature below 25°C and enter the state of torpor, and this activity remains at a low level during hibernation and arousal until body temperature reaches 30°C. The role of changes in the parameter values associated with proliferative activity in adaptation of hematopoietic tissue during hibernation of the Yakutian ground squirrel is discussed.