VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 



47 



8 10 12 



TIME IN DARK (min. - ) 



Fig. 13. Effects of changes in external potassium and calcium concentration upon 

 the rate of dark adaptation of a Limulus photoreceptor unit. After equilibration in 

 each test solution the receptor was subjected to a bright light (0 log units on the curve) 

 for 10 minutes after which test flashes of 0.02 sec. duration were presented at progres- 

 sively decreasing intensities and the time of occurrence of the first impulse at each 

 intensity noted. (#) (O) Normal salt concentration. ( )) (I ) 20 mM potassium 

 added, other salts normal. (A) (A) No potassium in solution, other salts normal. 

 (□) No calcium in solution, other salts normal. 



open squares and the dotted curve relating to the run in which calcium was 

 decreased. Although before light adaptation the preparation was even more 

 sensitive than when potassium was added, in fact it was firing spontaneously, 

 it dark adapted less rapidly. Except in the initial stages the threshold followed 

 along the control curves rather than along the curve representing the effects 

 of increased potassium. Therefore it appears that potassium exerts a specific 

 effect on the rate of dark adaptation rather than just the general increase in 

 sensitivity that you would expect to get with either increased potassium or 

 decreased calcium. It has been suggested that potassium exerts its effect on 

 cells by partially depolarizing the membrane, while calcium affects its per- 

 meability. How such changes might account for differences in the rate of adap- 

 tation we do not know. We at first thought that potassium might have a specific 

 effect upon the bleaching and regeneration of the visual pigment. However, 

 from other work, it now seems that the bulk of the evidence indicates that 

 light and dark adaptation are exerted primarily by events subsequent to the 

 bleaching of the photopigment and not by the bleaching itself as was formerly 

 supposed. 



Hartline, Milne and Wagman (1947) have done some experiments that indi- 

 cate that the number of quanta required to stimulate Limulus receptors in- 

 creases with light adaptation. If adaptation was produced by bleaching, a con- 



