Abstract
Two Hereford steers were adjusted to an environmental chamber which was maintained at 20 ± 1 C, 60 ± 2% relative humidity and 12-hr daily photoperiod. After a 3-week adjustment period, steers were exposed on 2 consecutive days to either an increase (from 20 to 40 C) or a decrease (from 20 to 4 C) in temperature at a rate of approximately 9 C/hr. Subsequently, four other Hereford steers were preconditioned to 20, 10 and 30 C. At the end of each period, steers were injected with either 2 mg of bovine prolactin or .85% NaCl and immediately infused for 2 hr with either prolactin (3.2 mg/16 ml/hr) or saline (16 ml/hr), respectively. Prolactin concentration in serum of steers averaged 23.2 ng/ml at 20 C and increased (P<.01) to 106.2 ng/ml at 40 C. Conversely, serum prolactin decreased (P<.001) from 34 ng/ml at 20 C to 4.4 ng/ml at 4 C. Stable serum prolactin concentrations were achieved within 30 min after infusions of exogenous prolactin were started. During this stable period serum prolactin concentration averaged 67.3, 82.4 and 114.3 ng/ml for steers infused at 10, 20 and 30 C, respectively. As ambient temperature increased from 10 to 30 C there was a reduction (P<.01) in the metabolic clearance rate and an increase (P<.01) in the secretion and disappearance rates of serum prolactin concentrations. It was concluded that changes in the metabolic clearance, secretion and disappearance rates of prolactin in cattle might account for at least a part of the inverse relationship between serum prolactin concentration and environmental temperature in this species. Copyright © 1977. American Society of Animal Science . Copyright 1977 by American Society of Animal Science.