The Pigmentary Effector System. 



325 



5. Relation of the Melanophore Stimulant to Histamine. 



The possibility that the melanophore stimulant present in infundibular 

 extracts is identical with the oxytocic principle led to experiments on the 

 effect of histamine, since Abel and Kubota (1919) have raised the question of 

 the identity of histamine and the uterine stimulant. Dudley has since 

 produced evidence that the effects of pituitary extract on plain muscle are 

 not due to the presence of traces of histamine as these workers suggested, 

 aud more recently Dale and Dudley (1921) have conclusively shown that 

 Abel and Kubota were wrong on this point. It can also be stated, with the 

 utmost confidence, that the melanophore stimulant is likewise not identical 

 with histamine ; doses of the latter as considerable as O0036 grm. do not 

 induce melanophore expansion in the frog. 



That the two substances are not identical is shown further by the action of 

 trypsin. Histamine is not acted upon by trypsin,* whereas Dale (1909) and 

 Dudley (1919) have shown that both the commonly recognised pituitary 

 autocoids are destroyed rapidly by trypsin. In this connection it may be 

 noted that Schafer and Herring (1906) claimed that pepsin destroys the 

 pressor activity of pituitary extract, leaving intact the diuretic principle ; they 

 denied that trypsin destroyed either. The following experiment indicates that 

 pepsin does not affect the melanophore stimulant, but that the latter is rapidly 

 destroyed by trypsin. 



Six solutions were made up as follows : — 



A. - 5 per cent, infundibular extract in 0'2 per cent. HC1 and 0*5 per cent, 

 pepsin. 



B. - 5 per cent, infundibular extract in - 2 per cent. HC1, without pepsin. 



C. 0"2 per cent. HC1 and 0*5 per cent, pepsin. 



D. 0*5 per cent, infundibular extract in 0'5 per cent, saline trypsin. 



E. 0*5 per cent, saline trypsin alone. 



F. 0'5 per cent, infundibular extract in - 5 per cent, boiled solution of 

 trypsin. 



At the conclusion of 2 hours' digestion at 34° C. A-C were neutralised, 

 boiled, and diluted to a concentration approximately isotonic with Singer. 

 D-F were boiled. After cooling, - 5 c.c. of each fluid was injected into a pair 

 of frogs ; and the 12 animals were examined 45 minutes afterwards, a strip 

 of skin being removed for microscopic examination. 



Of A-C both the A and B pairs showed macroscopically and microscopically 

 the characteristic pituitary reaction. Of D-F, F alone showed expansion of 

 the melanophores on being examined by macroscopic and microscopic methods. 



* Dudley {op. cit.). 



