Control of rectal gland secretion by blood acid–base status in the intact dogfish shark (Squalus acanthias)

In order to address the possible role of blood acid–base status in controlling the rectal gland, dogfish were fitted with indwelling arterial catheters for blood sampling and rectal gland catheters for secretion collection. In intact, unanaesthetized animals, isosmotic volume loading with 500 mmol L−1 NaCl at a rate of 15 mL kg−1 h−1 produced a brisk, stable rectal gland secretion flow of about 4 mL kg−1 h−1. Secretion composition (500 mmol L−1 Na+ and Cl−; 5 mmol L−1 K+; <1 mmol L−1 Ca2+, Mg2+, SO42−, or phosphate) was almost identical to that of the infusate with a pH of about 7.2, HCO3− mmol L−1 < 1 mmol L−1 and a PCO2PCO2 (1 Torr) close to PaCO2PaCO2. Experimental treatments superimposed on the infusion caused the expected disturbances in systemic acid–base status: respiratory acidosis by exposure to high environmental PCO2PCO2, metabolic acidosis by infusion of HCl, and metabolic alkalosis by infusion of NaHCO3. Secretion flow decreased markedly with acidosis and increased with alkalosis, in a linear relationship with extracellular pH. Secretion composition did not change, apart from alterations in its acid–base status, and made negligible contribution to overall acid–base balance. An adaptive control of rectal gland secretion by systemic acid–base status is postulated—stimulation by the “alkaline tide” accompanying the volume load of feeding and inhibition by the metabolic acidosis accompanying the volume contraction of exercise.