Adaptation to Respiratory Acidosis in the Turtle Bladder

Abstract

Abstract

The effect of in vivo respiratory acidosis for 4 and 48 hr was examined in the turtle bladder by placing turtles in hypercapnic chambers. Blood pH was significantly lowered and pCO₂ was significantly elevated over control values both at 4 and 48 hr, while blood bicarbonate was only increased after 48 hr. In vitro rates for H⁺ secretion determined by the reverse short-circuit current were significantly greater in bladders from 48 hr of respiratory acidosis than those of controls (27.3 ± 2.7 vs 20.6 ± 1.7 μA, P < 0.05). In vitro rates for HCO₃ ^- secretion determined by pH stat were not altered. Fluorescence microscopy was used to study cell morphology. The number of carbonic anhydrase cells (corrected for the total number of cells) as determined by four different fluorescence stains (6-carboxyfluorescein, rhodamine 123, acridine orange, and 3,3′-diethyloxacarbocyaninine iodide) was increased both after 4 and 48 hr of respiratory acidosis. However, the number of HCO₃ ^--secreting (β subtype) carbonic anhydrase cells, determined by a probe for the anion exchanger, NBD-taurine, was not increased. In vitro 1% CO₂ for 4 hr also resulted in an increase in H⁺ secretion and in the number of 6-carboxyfluorescein-positive cells, both of which could be blocked with SITS pretreatment. We conclude that CO₂ changes the mucosal cells more toward the carbonic anhydrase phenotype, and that if NBD-taurine accurately identifies the β cells, that the adaptation produces or recruits more α-carbonic anhydrase cells.