[Ca.sup.2+]-driven intestinal HC[O.sup.-.sub.3] secretion and CaC[O.sub.3] 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 (HC[O.sup.-.sub.3]) secretion and [Cl.sup.-] absorption via [Cl.sup.-]/HC[O.sup.-.sub.3] exchange fueled by metabolic C[O.sub.2]; and 3) alkaline precipitation of [Ca.sup.2+] as insoluble CaC[O.sub.3], which aids [H.sub.2]O absorption). The latter two processes involve high rates of epithelial HC[O.sup.-.sub.3] secretion stimulated by intestinal [Ca.sup.2+] and can drive a major portion of water absorption. At higher salinities and ambient [Ca.sup.2+] concentrations the osmoregulatory role of intestinal HC[O.sup.-.sub.3] 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.sup.+] into the blood to balance apical HC[O.sup.-.sub.3] secretion). The flounder intestine was perfused in vivo with salines containing 10, 40, or 90 mM [Ca.sup.2+]. Increasing the luminal [Ca.sup.2+] concentration caused a large elevation in intestinal HC[O.sup.-.sub.3] production and excretion. Additionally, blood pH decreased (-0.13 pH units) and plasma partial pressure of C[O.sub.2] (PC[O.sub.2]) levels were elevated (+1.16 mmHg) at the highest Ca perfusate level after 3 days of perfusion. Increasing the perfusate [[Ca.sup.2+]] 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 HC[O.sup.-.sub.3] production and osmoregulation with acid-base balance and respiratory gas exchange and the specific controlling role of ingested [Ca.sup.2+] independent of any other ion or overall osmolality in marine teleost fish. carbon dioxide; oxygen; protons; gill; carbonic anhydrase doi: 10.1152/ajpregu.00513.2009.