Increased dietary leucine changes the rhythm of prolactin secretion

As we have recently shown that prolactin secretion in vitro is regulated by extracellular leucine, we wanted to determine whether leucine had a similar regulatory action on prolactin secretion in the intact animal. Leucine was administered to cycling female rats by way of their drinking water (0.5%) for a period of 24 days. At daily intervals during this time, six control and six leucine-treated rats were killed and their trunk blood subsequently assayed for prolactin content. On days 7, 14, and 21 during this period, pituitaries from the sacrificed rats were fixed and processed for electron microscopy. Leucine treatment resulted in an alteration in the normal four-day cyclicity in serum prolactin levels and a stimulation of prolactin synthesis as evidenced by ultrastructural changes in the pituitary mammotrophs. In a parallel set of experiments, daily vaginal smears were taken from nonsynchronized animals for 24 days before and 24 days during leucine administration. Leucine treatment resulted in variable alterations in the estrus cycle depending upon the stage of the cycle when the leucine was first administered. The changes in serum prolactin and the disturbances of the estrus cycle in the leucine-treated animals persisted for approximately 20 of the 24 days. In a shorter control experiment (seven days), alanine treatment (0.5%) was found to have no effect on prolactin levels. It is concluded that a mild elevation in dietary leucine can affect prolactin synthesis and release and the normal progression of the estrus cycle.