l82 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



ance because less than 2 % trapped serum would account for all of the observed 

 conductance. Analysis of the resistance of blood based on the theory of sus- 

 pended spheres is consistent with an 'infinite' membrane resistance (36). While 

 it cannot be concluded that the erythrocyte membrane conductance is less 

 than that demanded for anion exchange by simple passive diffusion, it is pos- 

 sible that exchange diffusion plays a role in the chloride shift. It should suffice 

 to summarise by stating that an extremely rapid rate of penetration is not by 

 itself sufficient reason for assuming passive diffusion. 



There are other instances where it would be desirable to have a fuller knowl- 

 edge of factors governing anion movement. The absorption of saline by the 

 colon is of interest. When isotonic saline is instilled, without an appreciable 

 change in total cation concentration, chloride concentration is significantly 

 reduced along with a compensatory accumulation of bicarbonate in the lumen 

 (10). Anion transfer by the small intestine (3, 102), pancreas (92) and salivary 

 gland (34) cannot be usefully interpreted at this stage. The rumen of Ruminata 

 are able to absorb chloride against a concentration gradient (79, 93). The recent 

 abstract of Dobson and Phillipson (31) reports that there is a potential dif- 

 ference of 25 to 44 mv with serosa (presumably) positive to mucosa and that 

 the net movement of chloride is in the direction of the electro-chemical gradient. 

 In the context of weak electrolyte movement which will be discussed later, it 

 is interesting to note that rumina absorb weak acids against a concentration 

 gradient in the same direction as that of net chloride movement. 



Anion movement across the gastric epithelium has been a matter of consider- 

 able interest because of attempts to explain the mechanism of hydrochloric 

 acid secretion. Before the era of isotopic tracers, Davenport and Fisher (20) 

 attempted to characterize chloride movement through the secreting gastric 

 epithelium utilizing a working premise that if the epithelium failed to dis- 

 criminate between bromide and chloride, chloride moved by passive dififusion. 

 Failing to find appreciable discrimination, they concluded that chloride diflfuses 

 passively. This interpretation has been generally accepted (5, 25), though Rehm 

 pointed out that it is incompatible with our knowledge of the electro-physiology 

 of the stomach (82). 



Later work indicates that bromide is actually transferred at a faster rate 

 than chloride (40). Based on their analysis of bromide secretion into gastric 

 juice, Heinz, Obrink, and Ulfendahl calculated that the ratio of the outward 

 rate constants for bromide and chloride movement is 1.5. 



ACTIVE CHLORIDE TRANSPORT SYSTEM OF THE GASTRIC MUCOSA 



Definitive study of chloride movement through the gastric epithelium was 

 made possible by the availability of isotopic tracers and the vitality of the 

 isolated frog mucosa. Because it is particularly suitable, relatively uncompli- 

 cated by other factors, for quantitative study of an active transport system for 

 chloride, it will receive more detailed attention. 



