Ca²⁺-driven intestinal HCO₃^- secretion and CaCO₃ precipitation in the European flounder in vivo: Influences on acid-base regulation and blood gas transport

Marine teleost fish continuously ingest seawater to prevent dehydration and their intestines absorb fluid by mechanisms linked to three separate driving forces: 1) cotransport of NaCl from the gut fluid; 2) bicarbonate (HCO ₃^-)secretion and Cl^- absorption via HCO ₃^- exchange fueled by metabolic CO₂; and 3) alkaline precipitation of Ca²⁺ as insoluble CaCO₃, which aids H₂O absorption). The latter two processes involve high rates of epithelial HCO₃^- secretion stimulated by intestinal Ca²⁺ and can drive a major portion of water absorption. At higher salinities and ambient Ca²⁺ concentrations the osmoregulatory role of intestinal HCO₃^- secretion is amplified, but this has repercussions for other physiological processes, in particular, respiratory gas transport (as it is fueled by metabolic CO₂) and acid-base regulation (as intestinal cells must export H⁺ into the blood to balance apical HCO₃^- secretion). The flounder intestine was perfused in vivo with salines containing 10, 40, or 90 mM Ca²⁺. Increasing the luminal Ca²⁺ concentration caused a large elevation in intestinal HCO₃^- production and excretion. Additionally, blood pH decreased (-0.13 pH units) and plasma partial pressure of CO2 (PCO2) levels were elevated (+1.16 mmHg) at the highest Ca perfusate level after 3 days of perfusion. Increasing the perfusate [Ca²⁺] also produced proportional increases in net acid excretion via the gills. When the net intestinal flux of all ions across the intestine was calculated, there was a greater absorption of anions than cations. This missing cation flux was assumed to be protons, which vary with an almost 1:1 relationship with net acid excretion via the gill. This study illustrates the intimate link between intestinal HCO₃ ^- production and osmoregulation with acid-base balance and respiratory gas exchange and the specific controlling role of ingested Ca ²⁺ independent of any other ion or overall osmolality in marine teleost fish.