Scanning Microscopy


To identify the turtle urinary bladder cells involved in Na and Cl absorption and Hand HCO3 secretion cellular electrolyte concentrations and uptake of Br and Solutrast were determined using electron microprobe analysis. Whereas inhibition of transepithelial Na transport by ouabain (reversion of short circuit current) led to a pronounced K-Na exchange in granular, and most of the basal cells, surface CA-cells and some basal cells were ouabain insensitive. Surface CA-cells could be divided into a large Cl-rich and a small Cl-poor population. Since the ouabain-induced K-Na exchange could be completely prevented by blocking passive luminal Na entry by amiloride, granular and most of the basal cells seem to form a syncytial Na transport compartment. Luminal uptake of Br only occurred in Cl-poor surface CA-cells, indicating the sole responsibility of these cells for electrogenic and electroneutral Cl absorption and HCO3 secretion.

Serosal Br was taken up into all cell types. Whereas H secretion and serosal Br uptake into all cell types could be inhibited by 4-isothiocyano-4'-acetamido-2,2'-disulfonic stilbene (SITS), blockade of H secretion by lowering luminal pH to 4.5 diminished Br uptake only in Cl-rich surface CA-cells. Theses results indicate: a) Only Cl-rich surface CA-cells have a serosal anion exchanger involved in H secretion and b) granular and basal cells also possess a serosal anion exchanger, possibly responsible for cellular pH regulation.

Luminal endocytosis of the I-containing Solutrast was observed in apical regions of Cl-rich surface CA-cells after inhibition of H secretion, but not under steady-state conditions, indicating a transport related but not a constitutive endo-exocytosis.

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