Prolactin and Growth Hormone: Specialists on One Hand and Mutual Mimics on the Other (1982)

PROLACTIN AND GROWTH HORMONE: SPECIALISTS ON ONE HAND AND MUTUAL MIMICS ON THE OTHER CHARLES S. NICOLL* Much of the early work on the functions of the adenohypophysis was characterized by controversies as to the number of pituitary hormones that existed and the number of functions that each of them regulated [I]. One of the more memorable of these disputes was that which raged between Oscar Riddle of the Carnegie Institution of Washington and Herbert M. Evans of the Institute of Experimental Biology of the University ofCalifornia at Berkeley. Their controversy centered around the question of whether prolactin (PRL) and growth hormone (GH) were separate and distinct entities. Riddle and his associates isolated prolactin in semipure form as early as 1932 [2] and found that the preparation stimulated body growth in dwarf mice and pigeons [3, 4]. Accordingly, they concluded that prolactin could account for the growth-promoting activity that had been detected in crude pituitary extracts. Evans and his associates were not convinced by these findings at the Carnegie Institution, and they were impressed by the poor somatotrophic activity of PRL preparations in the laboratory rat. Furthermore, they had prepared a crude fraction from bovine pituitary glands which had somatotrophic activity with little or no PRL activity [5]. The controversy was ultimately resolved when highly purified preparations of PRL and GH were isolated in Evans's laboratory [6, 7]. W. R. Lyons and C. H. Li figured prominently in these accomplishments, which established as part of the glandular gospel that PRL and GH are separate and distinct entities with separate and distinct functions. Growth hormone acquired the reputation of being the principle that regulates body growth, along with a variety of associated anabolic processes. Prolactin was viewed as a hormone of reproductive significance, controlling func- *Department of Physiology-Anatomy and the Cancer Research Laboratory, University of California, Berkeley, California 94720.© 1982 by The University of Chicago. All rights reserved. 003 1 -5982/82/2503-0280$01 .00 Perspectives in Biology and Medicine, 25, 3 ¦ Spring 1982 | 369 tions such as corpus luteum secretion, mammary growth and lactation, crop milk production in pigeons and doves, and parental behavior. Our view of the physiological role of GH has not changed much over the years, but our understanding of prolactin's functions has evolved considerably . Highly purified preparations of PRL and GH from sheep and cattle pituitaries became available to endocrinologists throughout the world for physiological studies after the Second World War, and endocrinologists and physiologists of every description tested these hormones for effects in a wide variety of species. A substantial body of information on the comparative physiology of both hormones was thus acquired. Although the import of this information escaped the attention of most endocrinologists, there was at least one notable exception. Early in the 1960s Oscar Riddle was moved to emerge from retirement to write an essay on the unappreciated significance of PRL in vertebrate "function and organization" [8]. He reemphasized that PRL was indeed a hormone of growth-regulating and metabolic significance among vertebrates, as he and his colleagues had stressed in earlier years. In addition, he pointed out that the hormone had a greater diversity of actions than was generally appreciated by endocrinologists. Riddle's review [8] had a considerable impact on the way that endocrinologists in general and comparative endocrinologists in particular viewed PRL. Consequently, studies on the comparative physiology ofthe hormone became increasingly popular during the 1960s, and this popularity has persisted. As a result of the growing interest in prolactin's functions, it was possible to catalogue about 85 different actions of the hormone [9] some 8 years after Riddle's essay [8]. A more up-to-date list would probably include over 100 actions of prolactin [see 10, 11], which substantiates the suggestion that prolactin has more distinct functions among the vertebrates than all other adenohypophysial hormones combined [9]. The diverse actions of prolactin fall into four major areas of physiological control: osmoregulation, growth and development (particularly in amphibian metamorphosis), metabolism, and reproduction. Bern [12] suggested that the osmoregulatory and the growth-developmental effects of the hormone may be its primary functions. In considering the possible evolution of prolactin's functions, Nicoli [13...