Water transport in cultured cells from the rat inner medullary collecting duct

In this study, cells derived from rat inner medullary collecting duct formed a polarized monolayer when grown in wells on a membrane. When the membrane was sealed in an Ussing apparatus, the cells passed water from their apical to basolateral surfaces in the presence of an osmotic gradient. This was detected by the movement of the meniscus (1 mm = 1 microL) in a capillary tube connected to the basolateral chamber. The movements were measured by a travelling microscope, and the effects of vasopressin, oxytocin, and kappa opioids were explored at 37 +/- 1 degrees C. In the presence of vasopressin and oxytocin, water movement increased to between the threshold of 0.2 pM and the maxima of 100 pM (vasopressin) and 20 pM (oxytocin). Higher concentrations had a lesser effect (vasopressin) or no greater effect (oxytocin). It was possible to get a similar effect with each of two doses of 10 pM vasopressin at intervals as short as 3 min, but tachyphyllaxis lasting 70 min followed a dose of 20 pM oxytocin. The effects of vasopressin and oxytocin could be blocked completely by the cAMP antagonist adenosine-3',5'-cyclic monophosphorothioate Rp isomer. The benzenacetamide kappa agonist U-50488 had no effect on the response to 1 pM vasopressin but the kappa antagonist norbinaltorphimine significantly increased the effects of 1 pM vasopressin; this action was exerted earlier on in the initiation of water transport, as norbinaltorphimine did not affect the response to Sp-cAMPS, an activator of cAMP-dependent kinases.