Ca2+-driven intestinal HCO(3)(-) 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 Cl(-)/HCO(3)(-) exchange fueled by metabolic CO(2); and 3) alkaline precipitation of Ca(2+) as insoluble CaCO(3), which aids H(2)O absorption). The latter two processes involve high rates of epithelial HCO(3)(-) secretion stimulated by intestinal Ca(2+) and can drive a major portion of water absorption. At higher salinities and ambient Ca(2+) concentrations the osmoregulatory role of intestinal HCO(3)(-) secretion is amplified, but this has repercussions for other physiological processes, in particular, respiratory gas transport (as it is fueled by metabolic CO(2)) and acid-base regulation (as intestinal cells must export H(+) into the blood to balance apical HCO(3)(-) secretion). The flounder intestine was perfused in vivo with salines containing 10, 40, or 90 mM Ca(2+). Increasing the luminal Ca(2+) concentration caused a large elevation in intestinal HCO(3)(-) production and excretion. Additionally, blood pH decreased (-0.13 pH units) and plasma partial pressure of CO(2) (Pco(2)) levels were elevated (+1.16 mmHg) at the highest Ca perfusate level after 3 days of perfusion. Increasing the perfusate [Ca(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 HCO(3)(-) 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.
Cooper
Christopher A
CA
University of Exeter, Exeter EX4 4PS, UK.
Whittamore
Jonathan M
JM
Wilson
Rod W
RW
D000136:Acid-Base Equilibrium; D000138:Acidosis / Q000378:metabolism; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002119:Calcium Carbonate / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism; D002256:Carbonic Anhydrases / Q000378:metabolism; D004777:Environment; D005432:Flounder / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D007408:Intestinal Absorption; D007422:Intestines / Q000502:physiology; D007700:Kinetics; D010477:Perfusion; D012623:Seawater
D001639:Bicarbonates; D002245:Carbon Dioxide; D002256:Carbonic Anhydrases; D002119:Calcium Carbonate; D002118:Calcium