Hormonal effects on partitioning of nutrients for tissue growth: role of growth hormone and prolactin.

The growth process exemplifies changes in the priorities of different tissues for available nutrients according to a developmental program. We propose that a higher order of endocrine regulation over and above that provided by homeostatic mechanisms directs the flow of nutrients to support the physiological or developmental process of highest prevailing priority. The term homeorhesis is applied to this regulatory phenomenon and is distinguished from the more familiar concept of homeostasis. The documented actions of growth hormone and prolactin as somatotrophic agents are discussed and their candidacy as likely homeorhetic vectors is proposed. Certain shortcomings in the quality of available hormone preparations and inconsistencies between potencies in various tests performed in vitro and in vivo are noted. We question the appropriateness of the use of experimental routines suited to exploring acute metabolic phenomena in acquiring a deeper understanding of the long-term process of growth. Despite the varied nature of supportive data, growth hormone, prolactin, and the closely related placental somatomammotropin exhibit the desired properties of homeorhetic hormones--they direct the flow of nutrients to the process of highest priority, partly by coordinating nutrient utilization by competing tissues.