Morphology and Epithelial Ion Transport of the Alkaline Gland in the Atlantic Stingray (Dasyatis sabina).

The alkaline glands are two fluid-filled sacs that lie on the ventral, posterior surface of each kidney in skates and rays. In this study, the morphology, transepithelial ion transport, fluid constituents, and histochemistry of the alkaline glands of the Atlantic stingray, Dasyatis sabina, were investigated. The duct from each gland joined the corresponding vas deferens and the resulting two common ducts emptied into the cloaca. Dark burgundy, aqueous fluid (pH 8.0-8.2) was secreted into the sacs by a simple columnar epithelium with extensive rough endoplasmic reticulum and large secondary lysosomes containing lipofuscin and membrane fragments. Zonulae occludentes were deep (~22 fibrils), reflecting an electrically tight epithelium (732 ohms/cm2). Carbonic anhydrase activity was localized histochemically within the intercellular spaces and less intensely in the mid-basal cytoplasm. In vitro electrophysiology showed that baseline shortcircuit current (Isc, 29.1 {mu}A/cm2) was reduced 67.0% after Cl- removal from the medium. Cl- removal also completely abolished luminal alkalinization (baseline 4.5 +/- 0.7 {mu}Eq of acid/cm2/h). Luminal exposure to the chloridebicarbonate exchange inhibitor, DIDS, reduced Isc by 38%. Simultaneous administration of DIDS and bumetanide (Na+/K+/Cl- cotransport inhibitor) to the serosal side of the tissue caused the Isc to decrease >100%. Serosal exposure to ouabain (Na-K, ATPase inhibitor) decreased Isc 48%, whereas amiloride (sodium ion channel blocker) and acetazolamide (carbonic anhydrase inhibitor) had no statistically significant effect on Isc or alkalinization rates. Taken together the results suggest the presence of apical epithelial bicarbonate exchangers that are chloride or sodium dependent, basal sodium and HCO3- transport, and an Isc that is not totally dependent on Na+-K+ ATPase.