304 HORMONES 



amino acid residues has not been determined. One physiological role 

 of oxytocin seems to be to expedite labor by increasing uterine contrac- 

 tions. Partially purified preparations have found chnical application 

 for this purpose and for control of hemorrhage after delivery. The 

 hormone is ineffective orally. Oxytocin also plays a role in milk secre- 

 tion in that it stimulates contraction of the smooth muscles in the walls 

 of the milk ducts. 



Vasopressin. The pressor hormone of the posterior lobe has been vari- 

 ously called vasopressin, pressin, or pitressin. Like oxytocin it is a basic 

 octapeptide, which has a molecular weight of about 1,000. It has been 

 obtained as a white, amorphous, water-soluble powder. On hydrolysis 

 it gives the same products as does oxytocin, except that arginine and 

 phenylalanine are present, but leucine and isoleucine are not. When 

 injected into animals vasopressin causes a pronounced but temporary 

 rise in blood pressure. Presumably it must function normally to help 

 regulate blood pressure, along with thyroxine, epinephrine, and other 

 substances. 



Abnormally high blood pressure (hypertension) is a common human 

 disease and a major cause of death. Attempts to learn its cause have 

 uncovered several other substances of natural occurrence which greatly 

 influence blood pressure. One of these is renin, a protein secreted into 

 the blood stream by the kidney. Renin is a proteolytic enzyme. It 

 acts on a particular protein, one of the globulins in the blood plasma, 

 which therefore is called renin substrate. Renin itself does not have 

 pressor activity, but the product of its action on the substrate, a peptide 

 called angiotonin or hypertensin, is highly active. The elevated pressure 

 is caused by contraction of small arteries combined with increased heart 

 action. The effect from a single dose lasts only a few minutes because 

 angiotonin is quickly destroyed in normal individuals by another enzyme, 

 angiotonase. Pathological hypertension could conceivably be produced 

 by overproduction of vasopressin or renin, or by lack of angiotonase, but 

 these possibilities have not been proved correct, and no really satisfac- 

 tory treatment for the disease has yet been found. 



Still another pressor substance normally present in the animal body 

 is serotonin. This substance was isolated from blood serum in the form 

 of a crystalline product which proved to be a mixed sulfate of serotonin 

 and creatinine. The effective component, serotonin itself, is 5-hydroxy 

 tryptamine : 



CH2CH2NH2 



