VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 0/ 



correctly — what your light energy is doing is merely supplying activation 

 energy. So you can look at this in the same way as pyrophosphate splitting, 

 freeing a complex. What you are doing is freeing a complex which is separated 

 from its products by activation energy. 



Dr. MacNichol: No, in this case the products are, I think, in a higher 

 energy state. 



Dr. McElrov: It is shown from retinene, from vitamin A for example, the 

 equilibrium is way over toward the vitamin A and to get the vitamin A di- 

 rectly, you have to remove the retinene by a combination with protein which, 

 again, is an energy yielding reaction. 



Dr. MacNichol: I am wondering whether any of that system has anything 

 to do with the propagation of the impulse, getting back down to nerve im- 

 pulses, whether it isn't really that in splitting off the retinene from the protein 

 when light is absorbed that reactive sites on the protein are uncovered. These 

 could then combine with something that belonged to the neural part of the 

 system and cause stimulation. In fact, Wald has shown that in the bleaching of 

 rhodopsin, sulfhydryl groups are liberated. These are of course highly reactive. 

 The question is what do they react with and what happens then? 



Dr. Lawrence Stark (School of Medicine, Yale University) : I have a com- 

 ment to make about the analogy between the trapped muscle physiology and 

 trapped nerve physiology and that is, associating some chemical reaction with 

 the process just because of the sequential change. We are all aware of the pit- 

 falls of the lactic acid story about 25 years ago, and now the ATP ideas which 

 have been gathered into plausible hypotheses seem to be failing in some recent 

 work which has shown that ATP does not seem to change during the actual 

 muscle twitch; it may not be responsible for the supplying of energy. 



Dr. McElroy: I would like to challenge that statement, that latter one 

 particularly. For 25 years, I have thought that ATP reacts with muscle protein 

 to form a phosphorylated muscle protein and that if you could isolate this and 

 get away from ATP one could show a contraction for a short period of duration 

 of the liberation of phosphate without a break of ATP. I really do not see any- 

 thing from the experiments of Momaerts and Hill, wasn't it? and Krebs, that 

 negates the idea that ATP is the essential reacting agent in muscle contraction. 



Dr. Stark: But it may not be used up in supplying the energy. 



Dr. McElroy: I don't think a lot of people have seriously said that. I 

 think a lot of people have read that into early statements, that ATP was broken 

 down at the time of contraction. A lot of people never had that notion who 

 are working in muscle physiology. 



Dr. Stark: A lot of people did. 



Dr. McElroy: I think this is just a misstatement. I think this is a misinter- 

 pretation of the statements, really. 



