l8o ELECTROLYTES IN BIOLOGICAL SYSTEMS 



(57) 5^)- He found that the salt-depleted animal will absorb chloride from NaCl 

 solutions as dilute as o.oi mM/'L, indicating that a Na"*" or Cl~ transport system 

 may attain a transport-potential of 240 mv. He elegantly demonstrated that 

 Cl~ absorption is not necessarily a passive concomitant of active Na+ absorp- 

 tion, because Cl~ is absorbed from dilute solutions of KCl, CaCU and NH4CI 

 without significant uptake of these cations. Absorbed Cl~ is replaced by HCOs" 

 accumulating in the external solution. The several possibilities for associated 

 ionic movement demanded by CI" uptake cannot be distinguished at this junc- 

 ture. There may be passive HCOs" exchange or active H+, HCOs", or 0H~ 

 transport, not unlike the situation encountered in the gastric mucosa. It is con- 

 ceivable that active transport of one of these ions could generate a potential 

 sufficient for passive entrance of chloride against a concentration gradient. 

 Krogh showed at this early date that when Br~ was substituted for Cl~ it was 

 also absorbed against a concentration gradient and again independent of cation 

 absorption. Both NOs" and SCN~ penetrate rapidly but not necessarily against 

 a gradient, while entry of I~ or 804"" is negligible. 



In a recent paper J^rgenson measured the potential difference across the skin 

 of intact living frogs absorbing Cl~ from dilute KCl solutions (53). As the po- 

 tential difiference does not account for the flux ratio nor the net flux there is in 

 fact active transport of CI" itself. This ability to absorb Cl~ from a Cl~ im- 

 poverished environment is apparently dormant when the salt supply is ade- 

 quate. In contrast to the active uptake of Cl~ from the outside, adrenaline 

 stimulates the isolated frog skin to actively transport Cl~ in the opposite di- 

 rection; from inside to out (55). While the locus of this adrenaline stimulated 

 Cl~ transport has not been positively identified, its association with the secre- 

 tion of mucus suggests that the Cl~ secretion is located in the skin glands rather 

 than the skin epithelium. 



The ability to extract Cl~ is widely distributed in fresh water animals. In the 

 case of the crab Eriocheir, Krogh found that the gill Cl~ absorbing mechanism 

 will concentrate SCN~ and CNO~ in addition to Br~. Freshwater fishes, in 

 varying degrees, take up Cl~ through their gills from dilute KCl solutions while 

 rejecting K"*" (59). This uptake of Cl"~ from outside to in is the reverse of the 

 secretion of CI" in the opposite direction by gills of marine fish. Salt secretion 

 to the outside by fish gills is usually thought to be a primary Cl~ secretion 

 though it is not known whether it is the result of a Cl~ or a Na+ transport 

 system. Other orders and classes able to take up Cl~ include Diptera, Annelida 

 (Haemopsis) and Mollusca (Limnea) (60). 



In contrast to the rather sparse documentation of active chloride transport, 

 there is a considerably larger body of literature favoring the view that in many 

 cases chloride moves passively through cellular barriers. Investigation of ion 

 distribution across the walls of muscle and nerve has developed evidence that 

 chloride diffuses passively across the plasma membranes of these cells (4, 43, 



