H. A. S. VAX DEN BRENK 



Page^* have shown it to cause intense smooth muscle contraction, similar 

 to its hydroxyl derivative. 



Auxin {3-indole acetic acid and 5-hydroxyindole acetic ar/rf)— These substances 

 fail to give significant protection in vivo, and are pharmacologically inactive 

 in other respects. However indoles protect polymethacrylic acid in vitro^ 

 and since auxin is the natural growth hormone of plants, an investigation of 

 its protective properties in the latter, would seem indicated. It has been 

 reported that the tryptophan-indole acetic acid enzyme system in plants 

 is very sensitive to irradiation, 1 r causing 10 per cent inhibition of the 

 enzyme ^^. 



Histamine — For rats, this substance is a much weaker protective agent than 

 5-OHT, and only with huge doses (60 to 80 mg/kg) is protection obtained 

 with 20 to 30 per cent survivors at 30 days. 



Endogenous 5-OHT — Effects of release and depletion on radiation lethality in rats 



The mean body content of 5-OHT in the adult rat is approximately 

 125 iig/kg^^—i.e. approximately 2 x 10^ moles 5-OHT in a 150 g rat. By 

 comparison an exogenous dose of 0-5 x 10"^ moles 5-OHT is required for 

 substantial radio-protection, i.e. about 25 per cent of the total body content. 

 Also free 5-OHT is rapidly destroyed by amine oxidase present in many 

 tissues, an enzyme which is strongly inhibited by l-isopropyl-2-isonicotinyl- 

 hydrazine (iproniazid). 



Endogenous 5-OHT is bound to tissue proteins in an inactive state, and 

 not available to 5-OHT receptors in this state. It can be released in a 'free' 

 form together with catechol amines by reserpine, or together with histamine 

 by certain diamines {e.g. Compound 48/80). If its breakdown is blocked by 

 an amine oxidase inhibitor, a high concentration of the ' free ' form in tissues 

 can be induced. On the other hand reserpine alone maintains low levels of 

 'bound' and 'free' 5-OHT in the animal during the tranquilUzed state. 

 However, reserpinized tissues have reduced sensitivity to the pharmacological 

 action of 5-OHTi" — a feature which is demonstrated by the accompanying 

 kymograph recordings [Figure 2) of a rat stomach preparation, used for the 

 bioassay of 5-OHTi*. 



Radiation lethality in 5-OHT depleted rats — Rats reserpinized 16 hours before 

 irradiation, show the same lethality to irradiation as untreated rats. If 

 similar dosage of reserpine is given five minutes before irradiation, a slight 

 but variable protection results, partly attributable to an increased free 5-OHT 

 available to receptors, which are partially refractory to its action. If exo- 

 genous 5-OHT is given in the usual dosage to reserpinized rats, a substantial 

 protection results (40 to 80 per cent survivors at 30 days, 1000 r whole-body 

 irradiation) comparable to similar doses of 5-OHT in unreserpinized animals. 

 However, the reserpinized animal tolerates the 5-OHT much better than 

 control rats. These findings suggest that radiation protection with 5-OHT 

 does not depend per se on its effect on smooth muscle receptors, and vaso- 

 constriction. 



Protection due to release of endogenous 5-OHT — Iproniazid alone failed to 

 build up 'free' 5-OHT in the intact animal, sufficient to give protection, 

 but alternative metabolic pathways exist for the inactivation of 5-OHT, 

 which do not involve amine oxidases. Reserpine alone gave slight protection 



173 



