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, post erior surface of each kidney in skates and rays. In this study, the morphol ogy, transepithelial ion transport, fluid constituents, and histochemistry of the alkaline glands of the Atlantic stingray, Dasyatis sabina, were inve stigated. The duct from each gland joined the corresponding;vas deferens an d the resulting two common ducts emptied into the cloaca. Dark burgundy, aq ueous fluid (pH 8.0-8.2) was secreted into the sacs by a simple columnar ep ithelium with extensive rough endoplasmic reticulum and large secondary lys osomes containing lipofuscin and membrane fragments. Zonulae occludentes we re deep (similar to 22 fibrils), reflecting an electrically tight epitheliu m (732 ohms/cm(2)). Carbonic anhydrase activity was localized histochemical ly within the intercellular spaces and less intensely in the mid-basal cyto plasm. In vitro electrophysiology showed that baseline short-circuit current (Isc, 29.1 mu A/cm(2)) 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/cm(2)/h). Luminal exposure to the chloride-bicarbonate ex change 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 Ise 48%, whereas amiloride (sodium ion channel blocker) and acetazolamide (carbonic anhydrase inhibitor) had no st atistically significant effect on Isc or alkalinization rates. Taken togeth er the results suggest the presence of apical epithelial bicarbonate exchan gers that are chloride or sodium dependent, basal sodium and NCO, transport , and an Ise that is not totally dependent on Na+-K+ ATPase.