Rates of rewarming, heart and respiratory rates and their significance for oxygen transport during arousal from torpor in the smallest mammal, the Etruscan shrew Suncus etruscus

We investigated the process of rewarming from torpor with respect to respiratory and circulatory oxygen transport properties in the smallest mammal, the Etruscan shrew Suncus etruscus. In seven adult Etruscan shrews with a mean body mass of 2.4g, torpor was induced by deprivation of food and a cold environment. During arousal from torpor at an ambient temperature of 22 degrees C, the shrews actively rewarmed from the lowest mean (+/- S.D.) body temperature (Tb) of 12.1 +/- 1.2 degrees C to 20 degrees C at a rate of 0.43 +/- 0.14 degree C min-1, from 20 to 24 degrees C at a rate of 0.8 degree C min-1, and from 24 to 36 degrees C at a rate of 1.1 +/- 0.1 degrees C min-1. The mean rate from 12 degrees C to normothermia amounted to 0.83 degree C min-1, which is among the highest values recorded in mammals. During rewarming, the heart rate increased exponentially (Q10 = 2.2) from 100 to 800-1200 min-1, whereas the respiratory rate increased linearly from 50 to 600-800 min-1. These rates are higher than the heart and respiratory rates reported for other small mammals at the same Tb. The fraction of brown adipose tissue (BAT) was 9.2 +/- 1.6% of body mass, which is higher than in any other mammal. Up to a body temperature of approximately 17 degrees C, the heat for rewarming was mainly produced in the BAT; above this value, considerable activity of the skeletal muscles enhanced thermogenesis. Estimation of the mixed venous oxygen partial pressure showed that, at the tissue level, the rewarming process corresponds to heavy work conditions. The ventilatory system is adapted such that during rewarming, in addition to the appropriate oxygen transport capacity, there is also a capacity for hyperventilation.