296 O. HUG AND H. J. SCHLIEP 



of the muscle are undisturbed under irradiation or if they are restored 

 after irradiation, and a contraction which is irreversible over many 

 hours under all conditions examined by us. This irreversible part of the 

 contraction can be derived from the nearly linear part of the contraction 

 curve following the initial S-shaped curve under continuous irradiation 

 in Fig. 3 and from the residual contraction in Fig. 7. Its dose response 

 is very much lower than that of the reversible contraction. 



The following discussion deals mainly w ith the immediate recoverable 

 reaction hitherto unobserved. Studying it quantitatively we would, 

 however, have to subtract the contraction curve from the irreversible 

 residual contraction. 



A particularly striking phenomenon is the marked dependence of the 

 radiation -induced reversible muscle contractions on the distribution 

 of dose with time. Not only the latent period and the slope of the con- 

 traction curve but also the final contraction under continuous or re- 

 peated irradiation is determined by the dose-rate. This holds true 

 especially in muscles with undisturbed metabolic properties. Here the 

 tendency of the muscle to relax counteracts the contraction-producing 

 effects of radiation. But also in muscles in which relaxation is inhibited 

 a dose-rate dependence exists as shown in Fig. 3 by the different levels 

 which the sigmoid parts of the curves tend to ap])roach. and in Fig. 5. 

 Therefore at least two different mechanisms must be responsible for the 

 dose-rate dependence: firstly an unknown process counteracting the 

 primary physico-chemical changes in the cell which produce contraction, 

 and secondly the relaxation tied to metabolic properties. As seen in 

 Fig. 8 under not too high dose-rates a "steady state" of contraction can 

 be reached, which must be considered as the result of the ratio of dose- 

 rate on the one hand and the two counteracting processes on the other. 



Thus in a two-fold sense the muscle acts as a sort of "biological 

 dose-rate meter", o])erating badly, however, because of the changes of 

 its characteristic properties with increasing dose, i.e. the increasing 

 residual contraction and the decrease of relaxation rate. 



In our current experiments we are attempting to elucidate the 

 mechanism of action of the radiation-induced muscle contractions. 



Treatment of muscles with glycerol destroys cell membranes but 

 leaves the contractile muscle elements intact and resjionsive to ATP. 

 In preliminary experiments muscles thus treated and then exposed to 

 X-rays failed to contract. This would exclude a direct effect of radiation 

 contractile proteins. 



In all further experiments additional evidence accumulated that 

 irradiation primarily affects the cell membranes. So, for example, it is 

 possible to imitate the radiation effect on muscle simply by increasing 



