Carbonate precipitates and bicarbonate secretion in the intestine of sea bass, Dicentrarchus labrax
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 Isc 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.