Nutritional status and physiological development of the preweanling rat

¹ Department of Physiology, University of Rochester School of Medicine and Dentistry, Rochester, New York

Heart rate and responses to cold stress and anoxia were studied in rapidly and slowly gaining preweanling rats. Relative availability of milk was varied so that some animals increased birth weight four times and others ten times during initial 21 days of extrauterine life. Core temperatures during cold exposure decreased more slowly in large as compared to smaller animals 5–20 days of age. Temperature regulatory capacity was nearly complete in 20-day-old animals weighing 40 g, but incomplete in equal aged animals weighing 20 g. Heart rate of the infant rat increased from 300 to 500 by 20 days. An initial increase to 400/min occurred by day 5 in both rapidly and slowly gaining rats. At day 10 and day 15 heart rates of heavier weight animals were significantly greater than those of lighter animals of the same age. Rapidly gaining animals achieved heart rates of 500/min by day 15; slowly gaining animals required 20 days. Tolerance to anoxia was not influenced by rate of weight gain up to 21 days of age.