Carbonate precipitates and bicarbonate secretion in the intestine of sea bass, <Emphasis Type="Italic">Dicentrarchus labrax</Emphasis>

The aim of this paper was to study the chemical composition of the precipitates found in the intestine of Dicentrarchus labrax and the source of HCO₃ ⁻ secreted into the intestinal lumen. The chemical analysis was performed by employing the potentiometric double titration method and by means of an electron microscope coupled with a spectrometer and X-ray powder diffraction. The results obtained suggest the presence of very insoluble intestinal precipitates, presumably formed by a mixture of CaCO₃ and MgCO₃, with a higher quantity of the former with respect to the latter. HCO₃ ⁻ secretion rate was investigated with the aid of the pH stat method in isolated tissues mounted in Ussing chamber, where the transepithelial electrical parameters were also measured. When the serosal surface of the intestinal mucosa was bathed in HCO₃ ⁻-Ringer bubbled with 1% CO₂ in O₂ while the serosal surface was bathed in HCO₃ ⁻ free Ringer solution bubbled with pure O₂, bicarbonate secretion proceeded at an almost stable rate of 0.9 ± 0.05 μeq cm⁻² h⁻¹ for about 3 h while I _sc maintained a constant value of 38 ± 1.5 μA cm⁻². The carbonic anhydrase inhibitor ethoxyzolamide elicited a progressive reduction of HCO₃ ⁻ secretion that was about 75% of the initial value after 80 min. When serosal HCO₃ ⁻–CO₂ saline was substituted with Hepes–O₂ saline base secretion progressively declined reaching a value of about 20% of the initial value. It was also strongly inhibited when Na⁺ was substituted with the impermeant cation choline and when either DIDS or ouabain were added to the basolateral side. These results suggest that most of the bicarbonate secreted is of extracellular source and is probably transported across the basolateral membrane by both Na⁺ independent mechanism and Na⁺ dependent transporter, presumably a NaHCO₃ cotransport.