C. ADRIAN M. HOGBEN 1 83 



The gastric epithelium of the frog can be readily isolated, with a minimum 

 of connective tissue, as a thin film 0.6 to i.o mm thick by stripping it away from 

 the thicker muscular coat. Bathed by modified Ringer's solutions containing 

 glucose and aerated by 100% oxygen, it recovers from isolation within 30 

 minutes to i hour, and then remains relatively stable for better than 10 hours, 

 maintaining a potential difference of 30 to 40 mv, serosa positive to mucosa, 

 capable of continuously generating a short-circuit electrical current of some 

 0.1 ma cm~- and secreting H+ at 1-3 nJLq cm~- hr~^ Experimental design is 

 usually such that the mucosal-secretory surface is in contact with an ap- 

 preciable volume of solution so that the pH of the secretory solution does not 

 decrease much below 3, but undiluted juice collected from the isolated frog 

 stomach approaches a pH of i {^^). In spite of the fact that the undisturbed 

 intact frog may have neutral gastric contents, some investigators find that the 

 isolated stomach always spontaneously secretes acid though the rate may be 

 augmented by stimuli such as histamine (14, 44). Others report that an ap- 

 preciable number of stomachs fail to secrete spontaneously (8, 7,2), but un- 

 fortunately the conditions propitious for reproducing the 'resting' isolated 

 stomach have not been characterized. Apart from the observation that the 

 isolated mucosa sustains a high rate of continuous H+ secretion, the isolated 

 mucosa does in fact secrete hydrochloric acid formed from its nutrient-serosal 

 solution because the latter accumulates equivalent 0H~ for H+ appearing 

 in the secretory solution (95). 



The electrical potential across the epithelium can be measured by placing 

 agar-salt bridges on either side of the isolated mucosa. These bridges feed 

 through reversible calomel electrodes into a high impedance potentiometer. 

 If two other agar-salt bridges are then placed at some distance from the mucosa 

 and an external current sent through the latter bridges, the mucosal potential 

 can be varied as desired. In particular, if sufficient current from an external 

 battery flows through the 'current' bridges, the mucosal potential will be re- 

 duced to zero. The mucosa is then short-circuited and the current measured in 

 the external battery circuit is equal and opposite to the electrical current gene- 

 rated by the mucosa (loi). By varying the external current, it is possible to 

 plot external current against mucosal potential obtaining a straight line (over 

 the useful range of +90 to —30 mv) whose slope is mucosal conductance (7, 

 48). In practice, mucosal conductance is obtained from the ratio of short-circuit 

 current and spontaneous potential. Polarization phenomena indicated by over- 

 and under-shooting, when passing from one potential to another, are usually 

 negligible and complete within 2 to 3 minutes. 



With these electrical circuits it is possible to reconstruct a fairly complete 

 picture of the ionic balance across an isolated epithelium if the fluxes of the 

 several ions are measured with the membrane short-circuited and bathed by 

 two identical solutions. Passive ions will have symmetrical fluxes which can be 

 transformed into partial conductance and expressed as a fraction of the mucosal 



