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A number of tight urinary epithelia, as exemplified by the turtle bladder, acidify the luminal solution by active transport of H* across the luminal cell membrane . The rate of active H + transport (JH) decreases as the electrochemical potential difference for H + [o~H = AH(lumen) -~H(serosa)] across the epithelium is increased . The luminal cell membrane has a low permeability for H' equivalents and a high electrical resistance compared with the basolateral cell membrane . Changes in j,4 thus reflect changes in active H+ transport across the luminal membrane . To examine the control of JH by A/

A B S T R A C T Aldosterone stimulates not only Na4 absorption but also urinary acidification. In this investigation the effects of aldosterone on H4 transport are examined in vitro in turtle bladder, a urinary membrane in which several of the factors controlling H4 transport have been defined. H+ transport was increased in bladder halves exposed to aldosterone compared to control halves. Stimulation of H4 secretion was observed as early as 1 h after addition of aldosterone and occurred before that of Na4 transport. In bladders depleted of endogenous substrate addition of glucose increased H+ transport more in aldosterone-treated halves (10.0±1.3 nmol/min) than in control halves (6.8±2.3). Addition of pyruvate failed to increase H+ transport (-0.3+0.7) in control halves but caused significant increments (2.4±0.5) in aldosterone-treated halves. In aldosteronetreated bladders glucose caused larger increments (16.5 ±2.7) in H+ transport than pyruvate (9.3±2.0) when halves of the same bladders were compared. Na+ transport, however, was equally increased by the two substrates. Despite the differences in time course and substrate requirements between the stimulation of H+ and Na+ transport, both increases were abolished by actinomycin-D.To examine the effect of aldosterone on the force of the H+ pump, protonmotive force, the pH gradient that would nullify the transport rate was determined with and without aldosterone. Aldosterone did not alter protonmotive force but significantly increased the slope of the H+ transport rate on the applied pH gradient. It is concluded that aldosterone stimulates H4 transport independently of Na+ transport. It increases the responsiveness of the transport rate to glucose and to a lesser extent pyruvate, an effect probably secondary to the increased transport rate. Equivalent circuit analysis indicates that aldosterone facilitates the flow of protons through the active transport pathway but does not increase the force of the pump. INTRODUCTION Several lines of evidence indicate that aldosterone not only stimulates the renal absorption of sodium but also the process of urinary acidification. To which extent the increased acidification is a function of accelerated sodium absorption remains to be determined. At least some studies suggest that acidification may be stimulated independently of sodium transport. Lifschitz et al.(1) provided some evidence in the dog that renal H4 secretion is stimulated by a pathway that does not involve DNA-dependent synthesis of RNA. Ludens and Fanestil (2) reported that aldosterone increased the rate of acidification in the urinary bladder of the Colombian toad, as judged from the reversed short-circuit current after inhibition of sodium transport; they did not attempt, however, to explore the transport step in acidification that was stimulated by the hormone.Since turtle urinary bladder responds to aldosterone (3) and is capable of urinary acidification under conditions that permit close examination of the transport components (4), this preparation was se...

The changes in cell structure produced during stimulation of proton secretion by CO2 in turtle bladder were examined using ultrastructural morphometric methods. One hour after CO2 addition, the area of the luminal membrane of the carbonic anhydrase-containing (CA) cell population was increased 2.5-fold and the volume percent of electron-lucent cytoplasmic vesicles in these CA cells was decreased by 61%. No changes were observed in the epithelial granular cells. These results suggest that during CO2 stimulation the vesicles fuse with the luminal membrane. CO2 stimulation of proton secretion is inhibited by the cytoskeleton-disrupting drugs colchicine and cytochalasin B and by 99% deuterium oxide as the Ringer solvent. Deuterium oxide also inhibits the decrease in cytoplasmic vesicles. Thus stimulation of proton secretion by turtle bladder CA cells depends to a large extent on vesicle fusion and the resultant increase in luminal surface area.

Abstract. The mechanism of acidification by the urinary bladder of the water turtle was studied in an in vitro system which permitted control and measurement of electrical and concentration driving forces. The rate of hydrogen ion secretion was measured by means of a pH stat technique in the absence of exogenous carbon dioxide and bicarbonate.Transport of hydrogen ion into the solution bathing the mucosal surface of the bladder was associated with the appearance of alkali in the serosal compartment. The mean rate of hydrogen ion secretion in the absence of electrical and concentration gradients across the bladder was 0.96 Mumole/hr. The secretion rate was only slightly greater in the presence of the spontaneous potential difference. The maximal hydrogen ion gradient that could be generated by the bladder was 3.33 pH units in the presence of the spontaneous voltage and 3.02 pH units in the short-circuited state.Hydrogen ion secretion was markedly reduced by acetazolamide and anaerobiosis, which indicated that under our experimental conditions acidification depended on the production and enzymatic hydration of metabolic carbon dioxide. On the basis of the stoichiometry of the pH changes across the membrane under different conditions, it is suggested that the active transport mechanism for hydrogen ion is located near the mucosal surface of the epithelial cell and that the alkali generated in back of the pump moves passively into the serosal fluid along an electrochemical gradient.Introduction a limiting factor in our understanding of hydroThe urinary bladder of the fresh water turtle