Abstract
Abstract
Changing daily exposure of prepubertal bulls from 8 hr of light: 16 hr of dark (8L: 16D) to 16L:8D or 6L:8D:2L:8D increased basal secretion of prolactin 418% 6 weeks later. When daily exposure was changed from 8L:16D (6 weeks) to 6L:14D:2L:2D (6 weeks), basal secretion of prolactin increased only 173%. Among photoperiod exposures, prolactin released into blood after injection of 33 μg/100 kg body weight of thyrotropin-releasing hormone paralleled the changes described for basal conditions. There was no repeatable diurnal secretory pattern for secretion of prolactin. The data support the hypothesis that cattle possess a photosensitive rhythm for secretion of prolactin.
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