A critical analysis of carbonic anhydrase function, respiratory gas exchange, and the acid-base control of secretion in the rectal gland of Squalus acanthias

We compared in vivo responses of rectal gland secretion to carbonic anhydrase (CA) inhibition (10^-4 mol l^-1 acetazolamide) in volume-loaded dogfish with in vitro responses in an isolated-perfused gland stimulated with 5×10^-6 mol l^-1 forskolin and removed from systemic influences. We also measured respiratory gas exchange in the perfused gland, described the acid-base status of the secreted fluid, and determined the relative importance of various extracellular and intracellular acid-base parameters in controlling rectal gland secretion in vitro. In vivo, acetazolamide inhibited Cl^- secretion and decreased pHi in the rectal gland, but interpretation was confounded by an accompanying systemic respiratory acidosis, which would also have contributed to the inhibition. In the perfused gland, Ṁ_CO2 and Ṁ_O2 increased in linear relation to increases in Cl^- secretion rate. CA inhibition (10^-4 mol l^-1 acetazolamide) had no effect on Cl ^- secretion rate or pHi in the perfused gland, in contrast to in vivo, but caused a transitory 30% inhibition of Ṁ_CO2 (relative to stable Ṁ_O2) and elevation in secretion P_CO2 effects, which peaked at 2 h and attenuated by 3.5-4 h. Secretion was inhibited by acidosis and stimulated by alkalosis; the relationship between relative Cl^- secretion rate and pHe was almost identical to that seen in vivo. Experimental manipulations of perfusate pH, P_CO2 and HCO ₃^- concentration, together with measurements of pHi, demonstrated that these responses were most strongly correlated with changes in pHe, and were not related to changes in P_CO2 extracellular HCO ₃^-, or intracellular HCO₃^- levels, though changes in pHi may also have played a role. The acid-base status of the secreted fluid varied with that of the perfusate, secretion pH remaining about 0.3-0.5 units lower, and changing in concert with pHe rather than pHi; secretion HCO₃^- concentrations remained low, even in the face of greatly elevated perfusate HCO₃^- concentrations. We conclude that pH effects on rectal gland secretion rate are adaptive, that CA functions to catalyze the hydration of CO₂, thereby maintaining a gradient for diffusive efflux of CO₂ from the working cells, and that differences in response to CA inhibition likely reflect the higher perfusion-to-secretion ratio in vitro than in vivo.