Effects of acidosis and carbonic anhydrase inhibition in the elasmobranch rectal gland.

We studied the effects of acidosis and carbonic anhydrase inhibition on rectal gland fluid secretion. In the anesthetized dogfish shark, Squalus acanthias, volume expanded by a constant infusion of Ringer solution, fluid formation was halved by severe systemic metabolic and respiratory acidosis (pH approximately 7.10) and carbonic anhydrase inhibition. Mild respiratory acidosis (pH approximately 7.6) equivalent to the measured systemic pH during carbonic anhydrase inhibition was without effect. Inhibition of rectal gland enzyme produced severe glandular acidosis, which is indicated by an increase in gland fluid PCO2 (from 7 to 26 mmHg) and HCO3- (from 1.2 to 2.4 mM) and a decrease in pH (from 6.7 to 6.4). Gland tissue total CO2 dropped from 18 to 11 mmol/kg. These changes occurred despite nearly 50% reduction in fluid formation, O2 consumption, and CO2 output. We propose that carbonic anhydrase facilitates CO2 transfer from sites of metabolism to capillary blood by its conversion to HCO3-. This maintains a tolerable intracellular acid-base milieu during stimulated fluid secretion.