Differential effect of cold acclimation on blood composition in rats and hamsters

Male rats and hamsters were exposed to a progressively lower air temperatur e and shorter photoperiod to simulate the onset of winter. Normothermic ham sters had a higher haematological oxygen carrying capacity (OCC) and coagul ability (shorter prothrombin time and activated partial thromboplastin time ) than rats. Cold acclimation significantly increased the OCC of rats, whic h parallels an increased metabolic rate, while no differences were observed in hamsters. Red cell transit time through filters was faster in the accli mated rats but not in hamsters, reflecting the lower mean cell volume due t o a decreased rate of clearance from the circulation. Platelet counts were significantly lower in both cold-acclimated rats and hamsters, and there wa s a significant leucopenia in rats, which would reduce the degree of microv ascular blockade. Whole blood viscosity, plasma viscosity, and serum osmola rity showed little change in either species. However, whole blood viscosity was significantly lower in cold-acclimated hamsters than control hamsters at the lowest shear rate tested (0.95 s(-1)). Interestingly, plasma viscosi ty and serum osmolarity were significantly lower in hamsters exposed to low temperatures for a shorter period (4 weeks), and may reflect the developme nt of a reduced coagulability. These data suggest that blood composition in hamsters contributes to an innate tolerance of low temperatures, maintaini ng tissue perfusion under hypothermic conditions and aiding arousal from hi bernation.