142 ERNST FLOREY AND HARRIET MERWIN 



depends not so much on the absolute level of potassium ion concentration 

 as on the direction of change so that a change from four times to two times 

 the normal potassium concentration has tiie same effect as a change from 

 normal potassium concentration to a fraction of this amount. It is also 

 striking that in Protothaca the effect of a change in potassium ion concentra- 

 tion is minimal if the direction of the change is toward the normal potassium 

 concentration. The fact that the direction of change is more important than 

 the absolute concentration (particularly when the change is toward a con- 

 centration other than the normal one) indicates that the internal potassium 

 concentration rapidly follows the outside concentration. If this were true, the 

 effects of altering the concentration of external potassium would be to set up 

 de novo a potassium potential which would add to or subtract from a resting 

 potential which is not normally dominated by potassium. 



In this regard it appears of interest to refer to the "paradox" observed in 

 other lamellibranch hearts, for instance that of Clinocardiwn nutalli: if the 

 potassium is removed from the medium bathing the heart there is little change 

 in amplitude and frequency of the heartbeat. When the heart is returned to the 

 normal medium after prolonged exposure to the potassium-free medium, it 

 stops in diastole for a period that is proportional to the time of exposure to 

 the potassium-free medium. The phenomenon might be explained by the 

 hypothesis that potassium is lost from the cells and that when they are 

 returned to a medium with normal potassium content, the external potassium 

 concentration is sufficiently high with regard to the internal potassium con- 

 centration, so as to set up a potential gradient which reduces the resting 

 potential. The obvious difficulty of this explanation is the fact that the heart 

 becomes inhibited rather than excited. 



Thus far it has not been possible successfully to utilize techniques of intra- 

 cellular recording with microelectrodes since the muscle fibers of lamelh- 

 branch hearts have a diameter of not more than 1 or 2 /x. Therefore this 

 analysis rests on deduction rather than direct experimental evidence. 



The hearts of molluscs undoubtedly pose intriguing problems which 

 deserve much more serious attention than they have attracted hitherto. If the 

 few observations which we have mentioned provoke interest, argument and 

 more experimentation, our paper has served its purpose. 



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