HOWARD M. LENHOFF 227 



ation of glutathione with the receptor is rapidly attained ( Table 5 ) ; 

 the affinity of the receptor for glutathione is high (Fig. 3). After 

 glutathione combines with the receptor, it takes about 0.5 minutes 

 for all the events necessary for mouth opening to occur ( Table 1 ) . 

 Once the reflex begins, it will continue for 25 to 35 minutes (Tables 

 1, 3, 4 and 6; Figs. 3 and 4). The response does not stop because of 

 any alteration in the glutathione molecule (Table 6), but rather 

 because of some inherent property of Hydra. The duration of the 

 response is probably directly related to the conversion of some lim- 

 iting substance X to its product Y (Tables 1, 3, 4 and 6). Lowered 

 temperatures increase the duration of the feeding reflex, probably 

 by decreasing the rate at which the supply of X is exhausted ( Table 

 7 ) . It takes about 24 hours for X to be resynthesized either from Y 

 or anew (Fig. 4). The response can be stimulated in the absence 

 of glutathione by certain proteases (12). The protease probably 

 acts before the step involving the consumption of X. Furthermore, 

 the presence of small amounts of calcium ion in the medium sur- 

 rounding Hydra are required in order that a response may occur 

 (10). The calcium appears to be involved in steps occurring between 

 the site of activation by proteases and the effector system. 



X Y 



V f ^^Feeding 



GSH4-Rec^[GSH-Rec] — >E,-^E„-^=^E, »E., <^j > 



I ^k- Reflex 



Protease 



Fig. 5. Schematic outline of the glutathione receptor-effector system. 

 Rec represents the receptor; E,„ E,,, E,, and E,„ enzymes; X, the limiting 

 substance; and Y, its metabolic product. 



These results are concerned with a single biological system in 

 which a specific excitatory compound combines with its receptor 

 to activate a coordinated response. Activations by an excitatory com- 

 pound comprise the common step in many basic biological phe- 

 nomena such as chemoreception and hormone action. Some of the 



