Ca2+-driven intestinal HCO3−secretion and CaCO3precipitation in the European flounder in vivo: influences on acid-base regulation and blood gas transport

Abstract

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 (HCO3−) secretion and Cl−absorption via Cl−/HCO3−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 Ca2+independent of any other ion or overall osmolality in marine teleost fish.