No abstract

Homeostasis of body fluid is maintained by the kidneys, which contain two million glomeruli for blood filtration. A glomerulus is formed by growth of Bowman's capsule harmonized with a capillary during kidney development. The vascular endothelial growth factor (VEGF) is an essential angiogenic cytokine, and VEGF deficiency is known to be fatal in mice in early embryonic stages. As secretions of VEGF from cultured kidneys vary according to developmental stages, the role of VEGF in kidney development was studied in vivo by blocking the endogenous VEGF activity with antibody in newborn mice, in which most organs are already developed but kidneys are still developing. The antibody-treated animals showed normal growth but systemic edema. Vessel formation in the superficial renal cortex was disturbed, nephrogenic areas were diminished, and the number of developing nephrons decreased significantly.

Several factors interact to maintain precise control of electrolyte transport in the mammalian cortical collecting duct. We have studied the effects of deoxycorticosterone, arginine vasopressin, and bradykinin on net transepithelial sodium and potassium transport in isolated, perfused rat cortical collecting ducts. Chronic administration of deoxycorticosterone to rats increased both sodium absorption and potassium secretion above very low basal levels. Consequently, deoxycorticosterone-treated rats were used for all remaining studies. Arginine vasopressin (10-10 M in the bath) caused a sustained fourfold increase in net sodium absorption and a sustained threefold increase in net potassium secretion. Bradykinin (10-9 M in the bath) caused a reversible 40-50% inhibition of net sodium absorption without affecting net potassium transport or the transepithelial potential difference. In the perfusate, up to 10-6 M bradykinin had no effect. We conclude: (a) As in rabbits, chronic deoxycorticosterone administration to rats increases sodium absorption and potassium secretion in cortical collecting ducts perfused in vitro. (b) Arginine vasopressin causes a reversible increase in net potassium secretion and net sodium absorption. (c) Bradykinin in the peritubular bathing solution reversibly inhibits net sodium absorption, possibly by affecting an electroneutral sodium transport pathway.

The urate-anion exchanger URAT1 is a member of the organic anion transporter (OAT) family that regulates blood urate level in humans and is targeted by uricosuric and antiuricosuric agents (Enomoto, A., Kimura, H., Chairoungdua, A., Shigeta, Y., Jutabha, P., Cha, S. H., Hosoyamada, M., Takeda, M., Sekine, T., Igarashi, T., Matsuo, H., Kikuchi, Y., Oda, T., Ichida, K., Hosoya, T., Shimotaka, K., Niwa, T., Kanai, Y., and Endou, H. (2002) Nature 417, 447-452). URAT1 is expressed only in the kidney, where it is thought to participate in tubular urate reabsorption. We found that the multivalent PDZ (PSD-95, Drosophila discs-large protein, Zonula occludens protein 1) domain-containing protein, PDZK1 interacts with URAT1 in a yeast two-hybrid screen. Such an interaction requires the PDZ motif of URAT1 in its extreme intracellular C-terminal region and the first, second, and fourth PDZ domains of PDZK1 as identified by yeast two-hybrid assay, in vitro binding assay and surface plasmon resonance analysis (K D ‫؍‬ 1.97-514 nM). Coimmunoprecipitation studies revealed that the wildtype URAT1, but not its mutant lacking the PDZ-motif, directly interacts with PDZK1. Colocalization of URAT1 and PDZK1 was observed at the apical membrane of renal proximal tubular cells. The association of URAT1 with PDZK1 enhanced urate transport activities in HEK293 cells (1.4-fold), and the deletion of the URAT1 C-terminal PDZ motif abolished this effect. The augmentation of the transport activity was accompanied by a significant increase in the V max of urate transport via URAT1 and was associated with the increased surface expression level of URAT1 protein from HEK293 cells stably expressing URAT1 transfected with PDZK1. Taken together, the present study indicates the novel role of PDZK1 in regulating the functional activity of URAT1-mediated urate transport in the apical membrane of renal proximal tubules.Urate is the major inert end product of purine degradation in humans and higher primates in contrast to most other mammals because of the genetic silencing of hepatic oxidative enzyme uricase (1, 2). The kidney plays a dominant role in urate elimination; it excretes ϳ70% of the daily urate production. Urate exists primarily as a weak acid at physiological pH (pK a 5.75), and most of it is dissociated in blood and is freely filtered through the glomerulus. Thus, urate enters the proximal tubule in its anionic form, but it hardly permeates the tubular cells in the absence of facilitated mechanisms owing to its hydrophilicity. The transport mechanisms for urate are localized in the proximal tubule. In humans, urate is almost completely reabsorbed, which results in the excretion of ϳ10% of its filtered load. The absence of uricase and the presence of an effective renal urate reabsorption system contribute to higher blood urate levels in humans. Therefore, it was postulated that defects in tubular urate transport cause hypouricemia and decreased renal urate clearance leads to hyperuricemia in most hyperuricemic patients (3).Recently, we have id...

We investigated immunohistochemical localization of V2 vasopressin receptor along the nephron using a specific polyclonal antibody. Staining was observed in some of thick ascending limbs and all of principal and inner medullary collecting duct (IMCD) cells. Not only basolateral but also luminal membrane was stained in collecting ducts, especially in terminal IMCD (tIMCD).To learn the functional role of luminal V2 receptor in tIMCD, we studied the luminal effects of arginine vasopressin (AVP) on osmotic water permeability (Pf), urea permeability (Pu), and cAMP accumulation using isolated perfused rat tIMCD. In the absence of bath AVP, luminal AVP caused a small increase in cAMP accumulation, Pf and Pu, confirming the presence of V2 receptor in the lumen of tIMCD. In contrast, luminal AVP inhibited Pf and Pu by 30-65% in the presence of bath AVP by decreasing cAMP accumulation via Vla or oxytocin receptors and by an unknown mechanism via V2 receptors in the luminal membrane of tIMCD.These data show that V2 receptors are localized not only in the basolateral membrane but also in the luminal membrane of the distal nephron. Luminal AVP acts as a negative feedback system upon the basolateral action of AVP in tIMCD. (J. Clin. Invest. 1995. 96:1768-1778