Vasopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin-CD water channels to plasma membrane.

Abstract

Water excretion by the kidney is regulated by the peptide hormone vasopressin. Vasopressin increases the water permeability of the renal collecting duct cells, allowing more water to be reabsorbed from collecting duct urine to blood. Despite long-standing interest in this process, the mechanism of the water permeability increase has remained undetermined. Recently, a molecular water channel (AQP-CD) has been cloned whose expression appears to be limited to the collecting duct. Previously, we immunolocalized this water channel to the apical plasma membrane (APM) and to intracellular vesicles (IVs) of collecting duct cells. Here, we test the hypothesis that vasopressin increases cellular water permeability by inducing exocytosis of AQP-CD-laden vesicles, transferring water channels from IVs to APM. Rat collecting ducts were perfused in vitro to determine water permeability and subcellular distribution of AQP-CD in the same tubules. The collecting ducts were fixed for immunoelectron microscopy before, during, and after exposure to vasopressin. Vasopressin exposure induced increases in water permeability and the absolute labeling density of AQP-CD in the APM. In parallel, the APM:IV labeling ratio increased. Furthermore, in response to vasopressin withdrawal, AQP-CD labeling density in the APM and the APM:IV labeling ratio decreased in parallel to a measured decrease in osmotic water permeability. We conclude that vasopressin increases the water permeability of collecting duct cells by inducing a reversible translocation of AQP-CD water channels from IVs to the APM.Vasopressin (the antidiuretic hormone) is a 9-amino acid peptide hormone, secreted by the neurohypophysis, which acts on the kidney via the adenylyl cyclase-coupled vasopressin receptor (V2 receptor) to regulate water excretion. Vasopressin reduces urinary water excretion in part by increasing the water permeability of the renal collecting duct, thereby accelerating the osmotically driven absorption of water from the collecting duct lumen to the blood. Although the mechanism by which the water permeability increases is controversial, it presumably involves an increase in the number or unit conductance of water channels in the apical plasma membrane (APM), the rate-limiting barrier for net transepithelial water transport (1, 2). Several molecular water channels of the aquaporin (AQP) family are expressed in the kidney (3-5). One of these, AQP-CD (also called WCH-CD or AQP-2), appears to be expressed exclusively in the renal collecting duct (6, 7) and has been documented to be the major pathway responsible for vasopressin-regulated water transport across collecting duct cells (8,9). Several years ago, it was hypothesized (10) that vasopressin induces translocation of water channels from intracellular vesicles (IVs) to the APM by exocytosis (the "shuttle hypothesis"). However, direct experimental evidence for translocation ofwater channels is lacking. We recently demonstrated that the AQP-CD water channel is present in the APM and IVs of collecting d...

Identification of a novel protein containing two C2 domains selectively expressed in the rat brain and kidney

“…In the kidney, vasopressin regulates the permeability of the collecting duct (CD) to (i) water by the fusion of subapical cytoplasmic vesicles containing the water channel proteins, aquaporin-CD, into the apical membrane [20], and (ii) urea presumably by an increase in the urea transporter (UT2). The 3% agarose-formaldehyde gels as described in section 2.…”

Short-term regulation of bile acid uptake by microfilament-dependent translocation of rat ntcp to the plasma membrane

“…21 electron microscopy studies have further shown that AQP-2 translocation is associated with a decrease in AQP-2 staining of cytoplasmic vesicles and a concomitant increase in plasma membrane staining. 22 Similarly, in biliary epithelia, AQP-1 is translocated to the membrane in response to secretin, which increases intracellular cAMP, and this effect is inhibited by colchicine. 23,24 Together, these findings suggest that insertion of AQP water channels is regulated by cAMP, but in a fashion that depends on microtubules.…”

Phosphorylation of Soybean Nodulin 26 on Serine 262 Enhances Water Permeability and Is Regulated Developmentally and by Osmotic Signals

“…Numerous MIP proteins are subject to phosphorylation, and the effects on activity are varied, ranging from the stimulation (e.g., the plant MIPs ␣ TIP [Maurel et al, 1995] and PM28A [Johansson et al, 1998]) to the inhibition of water transport (e.g., mammalian AQP4 [Han et al, 1998;Zelenina et al, 2002]) and alterations in water permeability by affecting membrane trafficking and subcellular localization (e.g., AQP4 [Madrid et al, 2001] and AQP2 [Nielsen et al, 1995;Fushimi et al, 1997]). The present study shows that phosphorylation of nodulin 26 enhances its water permeability both upon expression in Xenopus oocytes and in symbiosome membrane vesicles.…”