C. ADRIAN M. HOGBEN 197 



In figure 2a, b, and c several possible models are portrayed based on the 

 premise of paired ion transport. In these diagrams the events within the mu- 

 cosae are separated into a) forced transport, mediated by a hypothetical carrier 

 X, indicated by heavy lines, and b) net passive diffusion, along a concentration 

 gradient, designated by thin broken lines. The necessary secondary ionic inter- 

 actions are pictured in the nutrient (serosal) and secretory (mucosal) solutions. 

 Figure 2a is identical with Lundegardh's model (70). In contrast to figure 2a, 

 the diffusion potential in figures 2b and 2c is pictured to be a result of HCOa" 

 rather than of H+ diffusion. 



Though H+ certainly diffuses across the stomach as a whole, if there is to 

 be a reasonable degree of efficiency it is not unreasonable to expect that the 

 membrane is semipermeable at the actual site of H"*" formation, say the canalicu- 

 lar border. Pursuing this diffusion potential model a step further, attention 

 might be directed to the fact that the mucosal short-circuit currents of the 

 'resting' and secreting stomachs are not appreciably different and the current 

 is relatively independent of the rate of H"*" secretion. Transition from rest to 

 secretion would imply an appreciable change of intracanalicular pn, which, if 

 the diffusion potential were due to H+, should be reflected in an increased back 

 diffusion of H+ and thus an increased mucosal current. However, if the diffusion 

 pathway were saturated, adsorption semi-permeability might be advanced to 

 explain the constant mucosal current. 



If the mucosal current arose from HCOs" diffusion, the current would be 

 dependent on the intracellular HCOs". If the intracellular HCOs" closely 

 paralleled the plasma or nutrient HCOs" concentration, the current would be 

 expected to be relatively constant and independent of the H+ secretion. In 

 figure 2C, forced anion exchange is presented as an alternative to an oxidation- 

 reduction formation of hydrogen ion. 



There is little evidence in favor of accepting or rejecting a hypothesis of a 

 diffusion potential. One line of investigation that is favorable is the response of 

 the isolated frog mucosa to bicarbonate. When a solution containing bicarbonate 

 and buffered by carbon dioxide is placed in contact with the secretory surface 

 and if bicarbonate is omitted from the solution bathing the nutrient solution, 

 the spontaneous potential and short-circuit current are depressed (45). This 

 has not been confirmed for the 'resting' dog stomach (28); however, circum- 

 stances are perhaps unfavorable for diffusion into the gastric tubule and limit 

 interpretation of the negative result. 



Two other considerations might be mentioned with respect to the model of 

 a diffusion potential. In the earlier discussion of the metabolism of gastric 

 secretion it was mentioned that the energetic efficiency is probably about 20%, 

 considering only the work required to raise H"*" across a large electro-chemical 

 potential gradient and chloride against a smaller gradient. If the mucosal cur- 

 rent were due to simple passive back diffusion of either H"*" or HCOa", one of 



