1 86 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



in an undissociated state and that the secretory to nutrient flux is predominantly 

 by exchange diffusion. This unexpected feature also interferes with a simple 

 estimate of the partial chloride conductivity. 



The concept of exchange diffusion is useful in understanding isotopic transfer 

 (97) but exchange diffusion of a strong ion through a living membrane had thus 

 far not been demonstrated. The exchange diffusion feature of the gastric 

 chloride pump is not observed in the sodium pump which has been most ex- 

 tensively studied, that of the isolated frog skin (100). While an apparent ex- 

 change diffusion is not proof that chloride moves through the mucosa in an 

 undissociated state, it does strongly suggest that chloride movement is mediated 

 by a carrier and the contemporary 'carrier' hypothesis for active transport 

 acquires further experimental support. 



Tentatively, Na+ and K+ account for only some 30% of the sum of partial 

 ionic conductances or the membrane conductance. Undoubtedly passive diffu- 

 sion of Cl~ and/or H"*" will account for a major share of the discrepancy. Though 

 it has been claimed that the gastric mucosa is anion impermeable (25), the 

 experimental results cited in support of this contention actually lead to the 

 contrary conclusion (27). 



Though a marked intracellular Cl~ concentration is not a prerequisite for 

 transcellular Cl~ transport, it would be desirable to know whether gastric epi- 

 thelial cells actually have an unusually high Cl~ content. The fundic mucosa 

 has a surprisingly high C\~ content compared to Na"^ (72, 74). The observed 

 values, need not necessarily represent a high intracellular Cl~ because there 

 might be appreciable inclusion of extracellular HCl. The observation that only 

 some 50% of the mucosal CI" rapidly equilibrates with plasma radiochloride 

 (73) does not permit the conclusion that an appreciable portion of the Cl~ ex- 

 changes slowly. It is quite possible that the low epithelial specific activity is 

 due to a secondary exchange with unlabeled chloride within the gastric lumen. 

 Comparison of the radiochloride space of the isolated frog gastric epithelium 

 with the two inulin spaces (one accessible from the nutrient surface and the 

 other from the secretory surface) indicates that the intracellular chloride con- 

 centration under these conditions may approach that of Ringer's solution (51). 

 If only one surface of the isolated epithelium is exposed to radiochloride, the 

 measured radiochloride space is lower and suggests that the apparent 'slow' 

 exchange of gastric chloride /;; vivo may be due to the secondary exchange with 

 unlabelled chloride in the lumen. 



Thus far we have little indication of the possible mechanism of active CI" 

 transport and none of its anatomical location. Presumably in the case of the 

 isolated frog epithelium, it occurs in one of the two cell types; the surface epi- 

 thelial (mucous) cell or the 'secretory' cell which lines the gastric tubule and 

 is said to have intracellular canaliculi like mammalian parietal cells. Most phys- 

 iologists consider that hydrochloric acid is secreted by the parietal cell, pri- 



