190 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



secretion is accompanied by a decrease in mucosal resistance (83). On the other 

 hand, 'non-secreting' isolated frog mucosae may increase their resistance when 

 stimulated to secrete (8). When the rate of H+ secretion of the spontaneously 

 secreting isolated frog mucosa is doubled by exogenous CO2 there is a definite 

 increase in mucosal resistance (46). 



Several different investigators have advanced a hypothesis that secretion of 

 hydrochloric acid occurs as a result of an oxidation-reduction reaction at the 

 surface of the parietal canalicular border, the so-called 'redox-pump' (5, 25, 82). 

 The elements of the redox pump are contained in the earlier model developed 

 by Lundegardh to explain salt uptake by plants (70, 71). It has been suggested 

 (25) that the only experimental finding particularly favorable to the oxidation- 

 reduction hypothesis, namely response of H+ secretion to an imposed potential, 

 is adventitious. 



METABOLIC ASPECTS OF HCl SECRETION 



An explanation of active transport must be sought in terms of the metabolism 

 of the gastric epithelium. Though we have a very incomplete understanding of 

 how oxidative energy is transferred to an ion attaining a higher electro-chemical 

 potential, ultimately there must be a union between physical biochemistry 

 and enzymatic biochemistry. Most of the metabolic studies of the gastric 

 mucosa have been developed in terms of H+ secretion rather than Cl~ secretion. 

 But as the two go hand in hand, these studies are pertinent to active Cl~ 

 transport. 



The first step has been taken by relating the rate of H+ secretion to oxygen 

 consumption of the isolated gastric mucosa. The in vitro stomach has an un- 

 usually active metabolism and the demands of increasing H+ secretion are met 

 by augmented oxygen consumption. Early studies (25) (26) implied that as 

 many as 12 moles of H+ were secreted for every additional mole of O2 con- 

 sumed, with a putative energetic efficiency of 100% or better. A better perspec- 

 tive is obtained from Davenport's studies (14, 17, 18). The isolated frog gastric 

 mucosa when supplied with oxygen at 610 mm Hg and glucose secretes 2 moles 

 of hydrogen ion for every mole of oxygen consumed. Such respiratory studies 

 prompt estimates of the energetic efficiency. Assuming that glucose is the sole 

 substrate with complete oxidation and that hydrochloric acid secretion is 

 actually o.i m/1. for the particular conditions of the study, in round figures i 

 mole of oxygen consumed might provide 100,000 calories and i mole of H+ 

 secreted requires expenditure of 10,000 calories. This points to an energetic 

 efficiency of about 20%, but this estimate should be accepted with reserv^ation 

 because of the several uncertainties implicit in the calculation. 



Interest in the respiration of the isolated gastric mucosa has led to a detailed 

 study of its lactate metabolism. Davenport (17) has advanced evidence for a 

 glycolytic contribution to the energy required for H+ secretion, pointing again 



