ABSTRACT
At the cellular level, prolactin actions may be divisible into several hierarchical domains. These domains include general trophic actions that are shared with several other hormones, prolactin-growth hormone actions that are physiological responses which are shared by both prolactin and growth hormone, "conserved" prolactin effects, and "recent" prolactin effects which include several well-known effects in the tetrapods. The evolutionary history of the prolactin-growth hormone family and several physiological considerations, especially tissue and developmental specificities, suggest potentially important considerations about target receptor mechanisms. Target cell responsiveness to prolactin has been shown to be highly dependent upon the time of hormone administration during the daily cycle. Mechanisms responsible for the osmoregulatory influences of prolactin include reduced water influx through permeability control and increased conservation of ions through influences on transport mechanisms and permeability. Of all the multifarious effects ascribed to prolactin in nonmammalian vertebrates, none has more experimental and historical prominence than the stimulation of "crop milk" production in Columbid birds.
