H. J. KOCH 



a well-aerated 8 mM external solution. If we take into account that such a gill con- 

 tains about 2 -5 mg. of nitrogen, corresponding at most to 15-6 mg. of protein, we 

 obtain 2 5 mg. NaCl/g. tissue/hour. This figure is quite impressive for the perform- 

 ance of such a gill, especially when we remember that the centre of the gill is not 

 active under such circumstances. 



Fast and steady absorption is recorded only on freshly prepared gills ; the speed of 

 absorption declines slowly (Figure 1 ) . After three or four hours a condition is usually 

 reached in which no net intake is observed. However this only means, in the termin- 

 ology used by Ussing (1948), that outflux counterbalances influx, as may be shown by 



Figure 1 . Time-course of the absorption of Ma, CI, 



and NaCl from an 8 mu JVaCl solution as determined 



on three different gills of a crab. (From Koch, Evans 



and Schicks, 1954c) 



means of 22 Na used as tracer. The outflux continues independently of the simultane- 

 ous influx, as will be seen from the following experiment, which also shows the influ- 

 ence of CO 2 (Figure 2) . A gill is allowed to absorb NaCl from a relatively strong radio- 

 active solution containing 22 Na, the total concentration of NaCl being 8 mM. After 

 thirty minutes or so it is transferred immediately, after careful washing, to a 

 non-radioactive solution of the same strength (concentration of NaCl, 8 mM; volume 

 of solution used, 12 cc). In the latter solution the absorption measured by conduc- 

 tivity continues. At a certain time the air is replaced by a mixture of air and CO a so 

 that the influx of NaCl stops. The outflux continues and can be measured by means 

 of the outwardly diffusing tracer. When pure air replaces C0 2 , a strong influx again 

 takes place. Taking into account the total NaCl content of the gill at the end of the 

 experiment, it becomes possible to calculate the absolute magnitude of the outflux. 



16 



