VOL. 4 (1950) A CHALLENGE TO BIOCHEMISTS 9 



Nineteen years ago my colleagues and I found, (Hill and Kupalov'; Hill and 

 Parkinson^) in muscles stimulated to exhaustion in nitrogen, a lowering of vapour 

 pressure considerably too large to be accounted for by chemical changes known to occur, 

 if the precursors of the chemical substances produced were themselves osmotically 

 active. In normal muscles complete exhaustion led to a decrease of vapour pressure 

 corresponding to an increased concentration in the free water of a muscle of 0.12 M. 

 The production of 0.35% lactic acid dissolved in the free water, (taken as 0.77 g per g) 

 of the muscle, would lead to a concentration change of 0.050 M. The liberation of 

 creatine and phosphate by the complete breakdown of phosphagen in amounts equiva- 

 lent to 65 mg. P/ioo g would give 0.054 M. The production of phosphate and adenylic 

 acid from ATP in amounts equivalent to 30 mg P/ioo g would give 0.012 M. The total, 

 0.116 M, is not far from that (0.12 M) calculated from the observed change of vapour 

 pressure. We have assumed, however, that the phosphagen and the ATP were not 

 themselves osmotically active; if they had been the increase would have been 0.031 M 

 less, namely 0.085 M instead of 0.12 M. The vapour pressure measurements were cer- 

 tainly not that much wrong. 



Again, in muscles poisoned with iodoacetale complete exhaustion led to a mean 

 decrease of vapour pressure corresponding to an increased concentration of 0.050 M. 

 If phosphagen and ATP breakdown are assumed, as above, to be the only chemical 

 reactions involved, the corresponding change of concentration in the free water of the 

 muscle would be 0.066 M. It is impossible, however, in muscles adequately poisoned 

 to ensure that some preliminary breakdown of phosphagen has not occurred : and if the 

 poisoning is not quite sufficient, there is likely to be some formation of lactic acid. Either 

 cause would tend to make the observed change of vapour pressure smaller than that 

 calculated from the assumed breakdowns. Even so, had the phosphagen and ATP 

 originally been osmotically active, the change calculated from the constituents would 

 have been only 0.035 M, considerably less than the 0.050 M observed. 



Unless, therefore, some chemical reactions hitherto unknown occur in a muscle 

 stimulated to exhaustion in nitrogen, we are forced to conclude that phosphagen and 

 ATP are not themselves osmotically active in the normal muscle. This would be the 

 case if they were bound to other molecules and their constituents only became free 

 when they broke down. These older experiments are worth recalling now because they 

 are pertinent to the question of how phosphagen and ATP exist in the living muscle. 

 Looking back at them today I see no reason to question their results. If those are correct, 

 ATP and phosphagen exist in a combined form in muscle, exerting no osmotic pressure 

 on their own account until they are broken down. 



The work which an isolated muscle of frog or toad can perform under optimal 

 conditions may be as high as 40% of the total energy given out in the initial process, as 

 distinguished from recovery (Hill^). This high efficiency is obtained just the same at 

 0° C as at higher temperatures, and there are no grounds at all for supposing that the 

 nature of contraction is in any way altered, except in speed, by a change of temperature. 

 The muscle twitch is rather stronger at 0° C than at 25° C, and quite as efficient. If 

 theory predicts otherwise, so much the worse for the theory. The highest efficiency is 

 obtained with a comparatively large load and slow shortening ; under isotonic conditions, 

 with a load about half the maximum which the muscle can lift. In such a contraction 

 the work done is about twice the heat of shortening : two thirds of the total energy set 

 free, in excess of the heat of activation (or maintenance), is external mechanical work. 

 References p. 11. 



