412 - Multicellular Animals, Especially Man 



quately studied side effects, medical usage 

 must be conducted with great caution and 

 under rigorous supervision. 



Hormones of the Posterior Lobe. Damage 

 to the posterior lobe, or to its controlling 

 center in the brain (see below), gives rise to 

 diabetes insipidus, a drastic condition in 

 which the kidney fails to concentrate the 

 urine (p. 376). The water reabsorption of the 

 nephric tubules is greatly impaired; and such 

 diabetics void as much as 3 to 10 gallons of 

 urine daily and must drink corresponding 

 quantities of water to avoid insufferable 

 thirst. Under this condition the urine is 

 quite devoid of sugar and low in salts (or, in 

 other words, insipid). 



For some years it has been known that 

 crude extracts of the posterior pituitary could 

 be separated into two potent fractions, called 

 oxytocin and vasopressin. Oxytocin was 

 found to augment the rhythmic contractions 

 of the uterus anil hence proved useful in 

 some obstetrical cases. Vasopressin, on the 

 other hand, produced a marked rise in blood 

 pressure — by constricting the smaller arteri- 

 oles throughout the body. Moreover, vaso- 

 pressin was found to be very effective in re- 

 lieving the symptoms of diabetes insipidus. 



No purified posterior pituitary hormones 

 were isolated, however, until 1954. Then 

 Vincent du Vigneaud and co-workers at 

 Cornell University Medical College suc- 

 ceeded not only in obtaining pure prepara- 

 tions of oxytocin and vasopressin, but also 

 in artificially synthesizing both compounds. 

 Both compounds proved to be relatively sim- 

 ple polypeptides, with eight amino acids rep- 

 resented in the chainlike molecules, although 

 the amino acids are slightly different in each 

 case. The action of pure oxytocin and vaso- 

 pressin is essentially similar to that of the ex- 

 tracts of the same name. However, the pure 

 compounds display a much greater potency. 

 Extremely small doses of oxytocin, injected 

 intravenously, produce strong contractions of 

 the uterine wall. Hence oxytocin is proving 

 to be useful in initiating childbirth when 

 abnormal delays are encountered; and vaso- 



pressin has proved to be very effective in con- 

 trolling diabetes insipidus. 



Another pituitary hormone, the melano- 

 cyte-stimulating hormone (MSH), is pro- 

 duced by the intermediate part (Fig. 22-8) 

 of the gland. This hormone helps to control 

 skin color, particularly in amphibians, al- 

 though the pigmentation of other verte- 

 brates, including man, may also be suscepti- 

 ble to its influence. MSH stimulates the 

 melanocytes of the amphibian, causing them 

 to disperse the pigment granules; and as the 

 pigment extends out into the extensively 

 ramifying branches of these cells, the skin 

 color of the whole animal grows darker (p. 

 444). The evidence indicates that MSH acts 

 in opposition to another hormone, mela- 

 tonin, produced by the pineal gland (see 

 below). Melatonin causes the skin to become 

 much lighter, because it stimulates the me- 

 lanocytes to mobilize the pigment into a 

 small area deeply hidden at the cell center 

 (Fig. 24-13). Both of these hormones have re- 

 cently been isolated and identified by A. B. 

 Lerner and co-workers at the University of 

 Oregon; and MSH was recently synthesized 

 by Schwyzer and Sieber. MSH, formerly 

 called intermedin , is a terminally acetvlated 

 polypeptide, consisting of 13 amino acid 

 units; whereas melatonin is a smaller mole- 

 cule of evelic structure. 



CH, 



H— 



J 



-CH-.-CHo-NH-CO-CH a 



Control of Pituitary Secretion. A number 

 of different cell types are observed in micro- 

 scopic sections of the pituitary, and presum- 

 ably each type secretes one or more of the 

 pituitary hormones. Each appears to be sepa- 

 rately susceptible to control. 



Control of the trophic hormones of the 

 pituitary represents a direct feed-back type. 



