100 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



circuit current, indicating that nitrate is also transported by the gastric anion 

 transport system (49). 



The isolated gastric mucosa also transports thiocyanate (49), though less 

 avidly than it does chloride. The competition between the two ions cannot be 

 simply described by a ratio between their nutrient to secretory rate constants. 

 If SCN and Cl~ are present in equal concentration, the major anion being 

 nitrate, the ratio of the nutrient to secretory flux over the secretory to nutrient 

 flux is the same for both ions, 1.6, though in a given direction Cl~ is transferred 

 about twice as fast as SCN~. When the spontaneous potential is augmented to 

 60 mv, serosa positive, the SCN~ fluxes are reversed to a much greater degree 

 than are those of Cl~. The intracellular concentration of both ions also change 

 with potential. A detailed analysis of the diff'erences in the transport of these 

 two anions will require consideration of four fluxes, two into the cell and two out 

 of the epithelial cell. The low SCN~ gastric/juice plasma ratios found in the dog 

 (12), with the range of plasma concentrations limited by toxicity of SCN~, may 

 possibly be due to plasma protein binding. 



In contrast to the active chloride transport system which exhibits such a 

 feeble specificity, the sodium pump of isolated frog skin difl'erentiates distinctly 

 between Na+ and K+. For cation transport, this specificity suggests mediation 

 by a carrier. Though univalent anion transport does not exhibit the same degree 

 of specificity, a carrier mechanism is suggested by its exchange diffusion charac- 

 teristics. 



RELATION OF CHLORIDE TRANSPORT TO HYDROCHLORIC ACID SECRETION 



In elucidation of the mechanism of HCl formation, considerable stress has 

 been placed on the composition of gastric juice. As the H+ concentration is a 

 function of secretion rate, many investigators sought to define a hypothetical 

 'primary secretion'. An ingenious experiment of Linde, Obrink & Teorell (65) 

 disclosed that under the special conditions obtained with glycine buffer, ap- 

 parent H+ and Cl~ concentrations may be greater than 300 mM/1. when calcu- 

 lated from the H"*" or Cl~ increase divided by the net volume change (5, 39). 

 The experiment has two implications. It is a strong argument in favor of the 

 thesis that in the formation of gastric juice, water flow is a passive consequence 

 of the osmotic gradient created by transport of H+ and Cl~. The 'parietal cell 

 secretion' does not necessarily have a unique composition. The Uppsala school 

 retain the term 'primary secretion' to describe the composition of gastric tubular 

 juice before it appears in the lumen and undergoes change by diffusion. 



Our present knowledge suggests that the flow of water during gastric secre- 

 tion is the consequence of the osmotic pressure of the secreted solutes (41). In 

 attempting to understand the mechanism of the solute secretion, it becomes a 

 matter of interest to inquire what determines the interdependence of simul- 

 taneous active transport of Cl~ and H"*". Though this problem is still unsolved. 



