C. ADRIAN M. HOGBEN 195 



pear in ilie nutrient solution as bicarbonate. However there is ample evidence 

 that the isolated frog epithelium can continuously secrete H^ for many hours 

 without exogeneous C0> (14, 46). In Davenport's careful study (14), without 

 exogenous CO2, two moles of H"^ are secreted for every mole of oxygen con- 

 sumed and presumably for every mole of CO2 formed. In these experiments 

 endogenous C0> was absorbed by alkali in a separate compartment of the 

 respiration vessel. In the other study, which lacked respiratory data but other- 

 wise yielded comparable values, mucosae secreted H+ at a fairly constant rate 

 for 12 hours or more and any exogenous COo escaping into the bathing solu- 

 tions was bubbled away by 100% 0>. In these experiments there would not 

 appear to be enough endogenous CO2 to allow for excretion of 0H~ by the 

 simple passive diffusion of HCOs". It is then necessary to postulate that there 

 is some other mechanism for elimination of 0H~. If such is the case, it is not 

 apparent that either the carbonic acid system or carbonic anhydrase would be 

 essential. The role of carbonic anhydrase is still an enigma. One may entertain 

 a question, in this particular problem, as to whether its function is necessarily 

 one of CO2-H2CO3 hydra t ion-dehydration or, without any favorable evidence, 

 whether it may not be implicated directly in anion exchange. 



Obrink (77) has drawn attention to the difficulty of demonstrating HCOs" 

 in the parietal cell secretion even when the rate of secretion is very slow. He 

 suggests that carbonic anhydrase facilitates the maintenance of a H+ barrier 

 to movement of HCOs" from the serosal to mucosal aspect. Any HCO3" diffus- 

 ing across the canalicular border would encounter H+ and, provided carbonic 

 anhydrase were active at the border, would be converted to CO2 which could 

 readily diffuse away. This has much in common with the concept of a diffusion 

 potential considered below. 



In considering the problem of the carbonic acid system a better understand- 

 ing of the action of SCN~ would be desirable. The suggestion that its effect is 

 primarily upon Cl~ transport rather than H+ secretion (23) is not consistent 

 with the maintenance, if not enhancement, of the spontaneous potential and 

 mucosal short-circuit current when the H+ secretion is virtually abolished by 

 SCN~. The suggestion (77) that SCN~ increases the permeability of the mucosa 

 to HCOs" requires further study. 



If the extensions of the 'carrier' concept for ion transport are pursued, one 

 is confronted with two alternatives. It may be postulated that a carrier is 

 capable of accepting a single ion, transferring it across an interface, and the 

 carrier itself returning in the charged state (6). The alternative view, par- 

 ticularly prevalent in renal physiology, postulates carriers that have to transport 

 ion pairs, either two of the same sign in opposite directions, or two of opposite 

 sign in the same direction, such that the carrier does not move as a charged 

 particle in an electrical field. 



When active transport of an ion yields a membrane current the second al- 



