WATER DOES NOT FLOW ACROSS THE TIGHT JUNCTIONS OF MDCK CELL EPITHELIUM

Although it has been known for decades that the tight junctions of flu id-transporting epithelia are leaky to ions, it has not been possible to determine directly whether significant transjunctional water moveme nt also occurs. An optical microscopic technique was developed for the direct visualization of the flow velocity profiles within the lateral intercellular spaces of a fluid-absorptive, cultured renal epithelium (MDCK) and used to determine the velocity of the fluid flow across th e tight junction. The flow velocity within the lateral intercellular s paces fell to near zero adjacent to the tight junction, showing that s ignificant transjunctional flow did not occur, even when transepitheli al fluid movement was augmented by imposition of osmotic gradients.