Structure of tight junctions during Cl secretion in the perfused rectal gland of the dogfish shark

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Abstract
In epithelia that secrete sodium chloride, high-conductance tight junctions between cells have been proposed as the primary pathway for transepithelial sodium flux. We examined the properties of tight junctions in the perfused rectal gland of the dogfish shark during basal secretion and following adenosine 3',5'-cyclic monophosphate stimulation of sodium chloride secretion. Freeze-fracture electron microscopy revealed extensive interdigitation of adjacent cells with an associated amplification in the length of tight junctions per area of luminal surface, averaging 102 +/- 4.7 m/cm2 in outer regions of 80 +/- 6.7 in inner regions of the gland. Marked heterogeneity of junctional structure was present with junctional elements varying from single strands to three duplex elements and junctional depth varying from 15 to 60 nm. In glands perfused with lanthanum chloride, ionic lanthanum filled the intercellular space up to but not through the tight junctions. Characteristics of tight junctions were not different during basal and maximally stimulated sodium chloride secretion. These studies define tight junctions in the rectal gland as an anatomical barrier capable of restricting the passage of relatively small molecules such as urea while providing a greatly amplified junctional area for the passive diffusion of sodium and water.