146 



KINETIC HORMONES — II 



ACTH secretion by the usual type of "feed-back" reaction, 

 tending to maintain a steady level. If, therefore, an injection of 

 ACH precedes a histamine injection, it is able to block the secretion 

 of ACTH. There is then no reduction in ascorbic acid, as compared 

 with the control (Fig. 4-10). This result can be achieved, even 

 though the ACH injection precedes the histamine by as little as 



500 



400 



-e 300 



200 



Control 



Fig. 4-10. Ascorbic acid remaining in the adrenal cortex, of six 

 groups of rats, killed shortly after receiving injections. Control rats 

 were given saline injections ; the others had all been given similar 

 injections of histamine, sufficient to produce a noxious, but not 

 a fatal, stimulus. The marked lowering of ascorbic acid due to 

 histamine alone, which elicits full secretion of ACTH from the 

 hypophysis, is shown on the right. Additional injection of an 

 adrenal cortical extract, A.C.E. , 3 to 10 seconds after the histamine 

 has practically no effect upon this lowering of ascorbic acid. A 

 similar injection of A.C.E., given 2 seconds before the histamine, is 

 able to block ACTH secretion, so that the histamine injection 

 produces practically no more response than the saline given to the 

 controls. As in Fig. 4-9, the loss of ascorbic acid corresponds to 

 release of ACH from the adrenal cortex (from Munson and Briggs, 



1955). 



2 sec, or just sufficient time for the ACH to reach and affect the 

 source of ACTH in the adenohypophysis. 



By reversing the order of the intravenous injection of the 



