Effect of aminophylline on metabolic and thermoregulatory responses during hypothermia associated with cold exposure in lambs

Zimmermann, B., Diebold, G., Galbraith, J., Whitmore, W., Okamoto, M., Robinson, J. B., Young, B. A., Murdoch, G., Mosenthin, R. and Christopherson, R. J. 2003. Effect of aminophylline on metabolic and thermoregulatory responses during hypothermia associated with cold exposure in lambs. Can. J. Anim. Sci. 83: 739-748. Three experiments were conducted with lambs to test the hypothesis that the phosphodiesterase inhibitor/adenosine blocker, aminophylline would enhance metabolic rate and delay the development of hypothermia. In exp. I, eight lambs were treated either with aminophylline or control (0.9% saline) injections. During hypothermia, metabolic rate was increased from a resting value of 4.8 W kg^-1 to maximum values of 15-16 W kg^-1 The rate of development of hypothermia and recovery therefrom were inversely related to age and body weight. Aminophylline (8 mg kg^-1) did not affect resting or maximum metabolic rate, but after a second injection, aminophylline increased metabolic rate after warming (P < 0.05) and tended to shorten the absolute period of time to restore normal rectal temperature (P < 0.10). In exp. 2, a dose-response trial with six lambs each receiving 0,2, 16 or 32 mg aminophylline indicated that dose rates of 16 and 32 mg kg^-1 increased (P < 0.05) metabolic rate of lambs. In exp. 3, 21 lambs received either 0.9% saline or aminophylline (24 mg kg^-1) injections. Mean heat production ranged from 13 to 15W kg^-1 and was increased approximately 7% (P < 0.01) by aminophylline. Aminophylline, therefore, has some potential as a treatment for hypothermia or to improve recovery from hypothermia in lambs by increasing metabolic rate during cold stress.