Rat renal arcade segment expresses vasopressin-regulated water channel and vasopressin V2 receptor.

The arcades are long, branched renal tubules which connect deep and mid-cortical nephrons to cortical collecting ducts in the renal cortex. Because they are inaccessible by standard physiological techniques, their functions are poorly understood. In this paper, we demonstrate that the arcades are a site of expression of two proteins, aquaporin-2 (the vasopressin-regulated water channel) and the V2 vasopressin receptor, that are important to regulated water transport in the kidney. Using a peptide-derived polyclonal antibody to aquaporin-2, quantitative ELISA in microdissected segments showed that aquaporin-2 is highly expressed in arcades and that the expression is increased in response to restriction of fluid intake. Immunocytochemistry revealed abundant aquaporin-2 labeling of structures in the cortical labyrinth in a pattern similar to that of the Na(+)-Ca2+ exchanger and kallikrein, marker proteins expressed in arcades but not in cortical collecting ducts. RT-PCR experiments demonstrated substantial aquaporin-2 and V2 receptor mRNA in microdissected arcades. In situ hybridization, using 35S-labeled antisense cRNA probes for the V2 receptor demonstrated strong labeling of both arcades and cortical collecting ducts. Thus, these results indicate that the arcades contain the specific proteins associated with vasopressin-regulated water transport, and may be a heretofore unrecognized site of free water absorption.

PEPCK mRNA localization in proximal tubule and gene regulation during metabolic acidosis.

To identify the nephron segments expressing PEPCK in control and acidotic conditions, PEPCK mRNA was localized in rat kidney using the technique of reverse transcription and polymerase chain reaction (RT-PCR) in individual microdissected S1 S2, and S3 segments of the rat proximal tubule. In controls, the number of tubules expressing PEPCK mRNA was greatest in the S3 segment, moderate in the S2 segment, and least in the S1 segment of the proximal tubule. After NH4Cl feeding, strong signals for PEPCK mRNA were detected in all three proximal tubule segments. In situ hybridization demonstrated expression of PEPCK mRNA only in the medullary rays in controls. After NH4Cl, PEPCK mRNA was expressed throughout the cortex, confirming the RT-PCR results. These data demonstrate the ability of the rat kidney cortex to modulate the expression of PEPCK mRNA during metabolic acidosis by recruitment of additional cells in the proximal nephrons. Studies with cultured LLC-PK1-F+ cells indicated that increased PEPCK gene transcription at acid pH required a cis-acting element (enhancer) in the more distal 5' flanking region of the promoter.

Dissociation of ammoniagenic enzyme adaptation in rat S1 proximal tubules and ammonium excretion response.

We measured ammonium production rates, phosphate-dependent glutaminase (PDG) activity, and glutamate dehydrogenase (GDH) activity in microdissected S1 proximal tubules of rats to investigate the role of adaptations of PDG activity and GDH activity in response to a step increase in acid intake. In vivo ammonium excretion increased much more rapidly than did single-tubule ammonium production in vitro or ammoniagenic enzyme activities measured in microdissected tubules, manifesting an 85-fold increase in the first 24 h. In vitro ammonium production rates in microdissected tubules rose only twofold in the first 24 h, fourfold by day 2, and fivefold by day 4 of acid loading. The adaptation of PDG activity paralleled the increase in single-tubule ammoniagenic capacity measured in vitro. GDH activity, on the other hand, did not change significantly even after 4 days of acid loading. From these observations, we conclude that 1) the adaptation of in vitro ammoniagenic capacity in S1 proximal tubules is temporally associated with an adaptation in PDG activity and not GDH activity, and 2) a major portion of the increased ammonium excretion seen in the first 24 h is due to factors other than an adaptive increase in ammoniagenic enzyme activity.

Cellular and subcellular immunolocalization of vasopressin-regulated water channel in rat kidney.

Vasopressin (antidiuretic hormone) regulates body water balance by controlling water permeability of the renal collecting ducts. The control mechanisms may involve alterations in the number or unit conductance of water channels in the apical plasma membrane of collecting-duct cells. How this occurs is unknown, but indirect evidence exists for the "shuttle" hypothesis, which states that vasopressin causes exocytic insertion of water channel-laden vesicles from the apical cytosol. To test key aspects of the shuttle hypothesis, we have prepared polyclonal antisera against the recently cloned collecting-duct water channel protein and used the antisera in immunolocalization studies (light and electron microscopic levels) in thin and ultrathin cryosections from rat kidney. Labeling was seen exclusively in collecting-duct principal cells and inner medullary collecting-duct cells. Apical membrane labeling was intense. There was heavy labeling of abundant small subapical vesicles and of membrane structures within multivesicular bodies. In addition, labeling of basolateral plasma membranes in inner medullary collecting ducts was present. Depriving rats of water for 24 or 48 hr markedly increased collecting-duct water-channel protein expression determined by immunoblotting and immunolabeling. These results are compatible with at least two complementary modes of water-channel regulation in collecting-duct cells: (i) control of channel distribution between the apical membrane and a reservoir in subapical vesicles (shuttle hypothesis) and (ii) regulation of the absolute level of expression of water-channel protein.

Oxytocin as an antidiuretic hormone. II. Role of V2 vasopressin receptor.

We conducted this study to determine what receptor mediates the effect of oxytocin to increase osmotic water permeability (Pf) in the rat inner medullary collecting duct (IMCD). Reverse transcription-polymerase chain reaction (RT-PCR) experiments demonstrated that mRNA for both the oxytocin receptor and the V2 receptor is present in the rat terminal IMCD. In isolated perfused IMCD segments, we found that the V2 vasopressin receptor antagonist [d(CH2)5(1),D-Ile2,Ile4,Arg8]vasopressin, but not oxytocin receptor antagonists, blocked the hydrosmotic response to 200 pM oxytocin. The selective oxytocin receptor agonist [Thr4,Gly7]oxytocin did not increase water permeability. Oxytocin also increased urea permeability in IMCD segments. Studies in IMCD suspensions showed that oxytocin increases adenosine 3',5'-cyclic monophosphate production in a dose-dependent fashion with a half-maximal (EC50) response at 5.2 nM. The dose-response curves were virtually identical for IMCD suspensions from Sprague-Dawley rats and Brattleboro rats. The oxytocin dose-response curve was displaced to the right of the vasopressin dose-response curve (EC50, 0.44 nM). From these results, we conclude that the V2 receptor mediates the hydrosmotic action of oxytocin in rat IMCD.

Oxytocin as an antidiuretic hormone. I. Concentration dependence of action.

Circulating concentrations of oxytocin increase to 10-40 pM in rats in response to osmotic stimuli, suggesting that oxytocin could play a role in regulation of water balance. The present studies tested whether oxytocin at such concentrations increases osmotic water permeability (Pf) in isolated perfused terminal inner medullary collecting ducts (IMCD). In IMCD segments from Sprague-Dawley rats, 20 pM oxytocin added to the peritubular bath caused a two- to threefold increase in Pf, whereas 200 pM oxytocin increased Pf by five- to sixfold (n = 8, P < 0.01). IMCD from Brattleboro rats, which manifest central diabetes insipidus, exhibited a 2.8-fold increase in Pf in response to 20 pM oxytocin and a 4.7-fold increase in response to 200 pM oxytocin. However, in Brattleboro rats, the response to 20 pM oxytocin was dependent on prior water restriction of the rats. Immunoblotting showed no change in the expression of the aquaporin-CD water channel in Brattleboro rats in response to water restriction. Nevertheless, immunofluorescence studies of inner medullary tissue from Brattleboro rats revealed a marked redistribution of the aquaporin-CD water channels to a predominantly apical and subapical localization in IMCD cells in response to water restriction, similar to the redistribution seen in response to vasopressin. Mathematical modeling studies revealed that the measured increase in Pf in response to oxytocin is sufficient to generate a concentrated urine. We conclude that oxytocin can function physiologically as an antidiuretic hormone, mimicking the short-term action of vasopressin on water permeability, albeit with somewhat lower potency.

Segmental localization of urea transporter mRNAs in rat kidney.

Renal epithelia express at least two distinct urea transporter mRNAs, termed UT1 and UT2, that are derived from a single UT gene by alternative splicing. Previous immunolocalization studies using a polyclonal antibody that does not distinguish between the protein products of these two transcripts revealed that expression of urea transporter protein is restricted to inner medullary collecting ducts and descending thin limbs of Henle's loop. To identify which transcripts account for protein expression in these two structures, we carried out reverse transcription-polymerase chain reaction studies in microdissected structures using UT1- and UT2-specific primers. UT1 mRNA was detected only in the inner medullary collecting duct, consistent with its identification as the vasopressin-regulated urea transporter. In contrast, UT2-mRNA was detected in the late part of descending thin limbs of short loops of Henle and in the inner medullary part of descending thin limbs of long loops of Henle. This localization is consistent with the predicted role of UT2 in medullary urea recycling. Thus, in conjunction with foregoing physiological studies, our data indicate that these transporters play central roles in the urinary concentrating mechanism.

Regulation of collecting duct water channel expression by vasopressin in Brattleboro rat.

AQP-CD is a vasopressin-regulated water channel expressed exclusively in the renal collecting duct. We have previously shown that AQP-CD is present in the apical plasma membrane and subapical vesicles of collecting duct cells, consistent with membrane-shuttling mechanisms that have been proposed to explain the short-term action of [Arg8] vasopressin (AVP) to regulate apical water permeability. We propose here that AVP may also have long-term actions on the collecting duct to regulate the expression of the AQP-CD water channel. We used immunoblotting, immunohistochemistry, and in vitro perfusion of renal tubules to investigate water channel regulation in collecting ducts of diabetes insipidus (Brattleboro) rats treated with a 5-day infusion of AVP or vehicle. Immunoblotting and immunohistochemistry demonstrated that collecting ducts of vehicle-infused Brattleboro rats had markedly reduced expression of AQP-CD relative to normal rats. In response to AVP infusion there was a nearly 3-fold increase in AQP-CD expression as detected by immunoblotting. Immunocytochemistry demonstrated that the increased expression was predominantly in the apical plasma membrane and subapical vesicles of collecting duct cells. Inner medullary collecting ducts of AVP-infused Brattleboro rats displayed a 3-fold increase in osmotic water permeability relative to vehicle-infused controls, in parallel with the change in AQP-CD expression. Based on these findings, we conclude that (i) long-term infusion of AVP, acting either directly or indirectly, regulates expression of the AQP-CD water channel and (ii) AQP-CD is the predominant AVP-regulated water channel.

Evidence for dual signaling pathways for V2 vasopressin receptor in rat inner medullary collecting duct.

Previous studies have demonstrated that both the V2-receptor agonist, 1-desamino-8-D-arginine vasopressin (dDAVP), and the V1a-receptor agonist, [Phe2, Orn8]vasotocin (PO-VT), increase intracellular calcium concentration ([Ca2+]i) in the rat inner medullary collecting duct (IMCD). The present studies were done to clarify the receptor subtype(s) responsible for calcium mobilization. Measurements of [Ca2+]i, using fura 2 in microdissected IMCD segments, confirmed that arginine vasopressin (AVP), dDAVP, and PO-VT stimulate an increase in [Ca2+]i and that the response to all three agents could be blocked by the specific V2-receptor antagonist, [d(CH2)5(1),D-Ile2, Ile4, Arg8]vasopressin (II-VP). These results would suggest that all three agents acted through the V2 receptor. Furthermore, we showed that PO-VT increased cAMP production in IMCD suspensions and water permeability in isolated perfused tubules. These responses were also blocked by II-VP, indicating that PO-VT is also a V2 agonist in the IMCD. Finally, we utilized the quantitative reverse transcription-polymerase chain reaction technique of Wiesner (Nucleic Acids Res. 20: 5863-5864, 1992) to evaluate V1a and V2 mRNA levels in rat collecting duct. In terminal IMCD, we estimated > 30 copies/cell for V2 receptor mRNA but less than 1 copy/cell of V1a receptor mRNA, thus there is littler or no V1a mRNA expression in the terminal IMCD. These results suggest that calcium mobilization in response to vasopressin analogues is associated with the V2 receptor and that the V2 receptor is linked to both adenylyl cyclase and calcium mobilization in the rat IMCD.

Escape from vasopressin-induced antidiuresis: role of vasopressin resistance of the collecting duct.

Previously, we demonstrated that escape from vasopressin-induced antidiuresis ("vasopressin escape") in rats is associated with a large, selective decrease in whole kidney expression of aquaporin-2, the vasopressin-regulated water channel. Here, we show that isolated perfused inner medullary collecting ducts (IMCDs) from vasopressin-escape rats desamino-[D-arginine]vasopressin (DDAVP)/water-loaded have dramatically reduced vasopressin-dependent osmotic water permeabilities [46% of control rats (DDAVP alone)], which coincides with a fall in inner medullary aquaporin-2 protein abundance as measured by immunoblotting in the opposite kidney. Furthermore, we demonstrate in IMCD suspensions that cAMP accumulation in response to DDAVP is substantially reduced in the vasopressin-escape rats both in the presence and absence of the phosphodiesterase inhibitor IBMX. By immunoblotting, we show that the abundance of two proteins important in cAMP generation: the stimulatory heterotrimeric G protein subunit Gs and adenylyl cyclase type VI, do not change. We conclude that vasopressin escape is associated with relative vasopressin resistance of the collecting duct cells manifested by decreased intracellular cAMP levels. The decreased cAMP levels can contribute to the demonstrated decrease in collecting duct water permeability in two ways: 1) by causing a decrease in aquaporin-2 expression and 2) by limiting the acute action of vasopressin to increase collecting duct water permeability.