iqS electrolytes in biological systems 



these ions would have to be Hfted against a large electro-chemical gradient 

 and in an amount equivalent to the rate of current generation. The short- 

 circuit current of the isolated frog mucosa expressed in /xEq cm~-hr.~^ is more 

 than twice that of the spontaneous H+ secretion. The inefficiency of simple 

 back diffusion would thus entail a much larger expenditure of energy than that 

 reflected simply in the requirements for the end products of gastric secretion. 

 The efficiency of ion transport would be of the order of 60% and could be prop- 

 erly considered unreasonable. 



In the case of forced anion exchange, there is a further unsatisfactory feature. 

 If the mechanism is to produce HCl from carbon dioxide and be capable of vary- 

 ing the rate of secretion from 'rest' to copious secretion, it becomes necessary 

 to postulate either regulated entry of carbon dioxide into the canaliculus or 

 another rate limiting step which is subject to control. The former alternative 

 is not compatible with our present knowledge of the ease with which carbon 

 dioxide diffuses across cell membranes. It may be suggested that perhaps the 

 dehydration of carbon dioxide is the limiting step and can be accelerated, say 

 by carbonic anhydrase, but at the present juncture this only increases the 

 element of speculation. 



It is difficult to distinguish by experiment between these three models. Po- 

 tentially, of the three possibilities, forced anion exchange would be distinctive. 

 In the secreting stomach the secretory to nutrient flux of carbon dioxide- 

 bicarbonate would be greater than the nutrient to secretory flux. Such a dif- 

 ference between the two opposing fluxes of carbon dioxide-bicarbonate is 

 actually observed (45). However, the difference between the two fluxes is not 

 sufficient for accepting forced anion exchange. 



Osterhout is responsible for developing a model which explains the movement 

 of weak electrolytes and their distribution across cell membranes separating 

 solutions of different pH (78). The distribution of hydrogen sulfide between the 

 cell sap of Valonia and its exterior could be explained by assuming that the 

 cell membrane is freely permeable to the unionized moiety, H2S, and imper- 

 meable to the ionized moiety, HS~. If the two solutions on either side of the 

 membrane have different pn's, as depicted in figure 3a, the steady state con- 

 centration of the unionized moiety will be equal on either side. In each solution 

 the ionized moiety will be in equilibrium with the unionized moiety and the 

 hydrogen ion concentration. Hence the total concentration in each solution 

 will be different and determined by pn. If the pH of the two solutions differed 

 and if by experimental design the total concentrations were maintained equal, 

 the opposing fluxes of the combined unionized and ionized moieties would 

 differ and be determined by pH. 



Without further qualification this model does not serve to explain the net 

 flux of carbon dioxide-bicarbonate across the gastric mucosa because the fluxes 

 are different when the pH of the two bathing solutions is equal (48). 



