Na+ reabsorption from the distal renal tubule involves electroneutral and electrogenic pathways, with the latter promoting K+ excretion. The relative activities of these two pathways are tightly controlled, participating in the minute-to-minute regulation of systemic K+ balance. The pathways are interdependent: the activity of the NaCl cotransporter (NCC) in the distal convoluted tubule influences the activity of the epithelial Na+ channel (ENaC) downstream. This effect might be mediated by changes in distal Na+ delivery per se or by molecular and structural adaptations in the connecting tubule and collecting ducts. We hypothesized that acute inhibition of NCC activity would cause an immediate increase in Na+ flux through ENaC, with a concomitant increase in renal K+ excretion. We tested this using renal clearance methodology in anesthetized mice, by the administration of hydrochlorothiazide (HCTZ) and/or benzamil (BZM) to exert specific blockade of NCC and ENaC, respectively. Bolus HCTZ elicited a natriuresis that was sustained for up to 110 min; urinary K+ excretion was not affected. Furthermore, the magnitude of the natriuresis was no greater during concomitant BZM administration. This suggests that ENaC-mediated Na+ reabsorption was not normally limited by Na+ delivery, accounting for the absence of thiazide-induced kaliuresis. After dietary Na+ restriction, HCTZ elicited a kaliuresis, but the natiuretic effect of HCTZ was not enhanced by BZM. Our findings support a model in which inhibition of NCC activity does not increase Na+ reabsorption through ENaC solely by increasing distal Na+ delivery but rather by inducing a molecular and structural adaptation in downstream nephron segments.

Collagen type IV is the major structural component of the basement membrane and COL4A1 mutations cause adult small vessel disease, familial porencephaly and hereditary angiopathy with nephropathy aneurysm and cramps (HANAC) syndrome. Here, we show that animals with a Col4a1 missense mutation (Col4a1(+/Raw)) display focal detachment of the endothelium from the media and age-dependent defects in vascular function including a reduced response to nor-epinephrine. Age-dependent hypersensitivity to acetylcholine is abolished by inhibition of nitric oxide synthase (NOS) activity, indicating that Col4a1 mutations affect vasorelaxation mediated by endothelium-derived nitric oxide (NO). These defects are associated with a reduction in basal NOS activity and the development of heightened NO sensitivity of the smooth muscle. The vascular function defects are physiologically relevant as they maintain in part the hypotension in mutant animals, which is primarily associated with a reduced red blood cell volume due to a reduction in red blood cell number, rather than defects in kidney function. To understand the molecular mechanism underlying these vascular defects, we examined the deposition of collagen type IV in the basement membrane, and found it to be defective. Interestingly, this mutation also leads to activation of the unfolded protein response. In summary, our results indicate that mutations in COL4A1 result in a complex vascular phenotype encompassing defects in maintenance of vascular tone, endothelial cell function and blood pressure regulation.

Purinergic signaling within the kidney is becoming an important focus in the study of renal health and disease. The effectors of ATP signaling, the P2Y and P2X receptors, are expressed to varying extents in and along the nephron. There are many studies demonstrating the importance of the P2Y2 receptor on kidney function, and other P2 receptors are now emerging as participants in renal regulation. The P2X4 receptor has been linked to epithelial sodium transport in the nephron and expression levels of the P2X7 receptor are up-regulated in certain pathophysiological states. P2X7 antagonism has been shown to ameliorate rodent models of DOCA salt-induced hypertension and P2X4 null mice are hypertensive. Interestingly, polymorphisms in the genetic loci of P2X4 and P2X7 have been linked to blood pressure variation in human studies. In addition to the increasing evidence linking these two P2X receptors to renal function and health, a number of studies link the two receptors in terms of physical associations between their subunits, demonstrated both in vitro and in vivo. This review will analyze the current literature regarding interactions between P2X4 and P2X7 and assess the potential impact of these with respect to renal function.

Aldosterone binds to the mineralocorticoid receptor (MR) and increases renal Na+ reabsorption via up-regulation of the epithelial Na+ channel (ENaC) and the Na+-K+-ATPase in the collecting system (CS) and possibly also via the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). However, whether aldosterone directly regulates NCC via MR, or indirectly through systemic alterations remains controversial. We used mice with deletion of MR in 20% of renal tubule cells (MR/X mice), in which MR-positive (MRwt) and -negative (MRko) cells can be studied sideby-side in the same physiological context. Adult MR/X mice showed similar mRNA and protein levels of renal ion transport proteins to control mice. In MR/X mice, no differences in NCC abundance and phosphorylation was seen between MRwt and MRko cells and dietary Na+ restriction up-regulated NCC to similar extent in both groups of cells. In contrast, MRko cells in the CS did not show any detectable alpha-ENaC abundance or apical targeting of ENaC neither on control diet nor in response to dietary Na+ restriction. Furthermore, Na+-K+-ATPase expression was unaffected in MRko cells of the DCT, while it was lost in MRko cells of the CS. In conclusion, MR is crucial for ENaC and Na+-K+-ATPase regulation in the CS, but is dispensable for NCC and Na+-K+-ATPase regulation in the DCT. PAEJ-D-16-00005 -NCC regulation is MR independent 2 ACKNOWLEDGEMENTSThe authors would like to thank Günter Schütz and Stefan Berger, Burkhard Becher, Celso E. Gomez-Sanchez, and Eric Feraille for kindly providing us with the MRlox/lox mouse line, the cmv-cre mouse line, the anti-MR antibodies, and the anti-Na + -K + -ATPase antibody, respectively. The expert technical assistance by Monique Carrel and Michèle Heidemeyer is gratefully acknowledged. GRANTS DISCLOSURESThe authors declare no conflict of interest, financial or otherwise. PAEJ-D-16-00005 -NCC regulation is MR independent 3 ABSTRACTAldosterone binds to the mineralocorticoid receptor (MR) and increases renal Na + reabsorption via up-regulation of the epithelial Na + channel (ENaC) and the Na + -K + -ATPase in the collecting system (CS) and possibly also via the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). However, whether aldosterone directly regulates NCC via MR, or indirectly through systemic alterations remains controversial. We used mice with deletion of MR in ~20% of renal tubule cells (MR/X mice), in which MR-positive (MR wt ) and -negative (MR ko ) cells can be studied sideby-side in the same physiological context. Adult MR/X mice showed similar mRNA and protein levels of renal ion transport proteins to control mice. In MR/X mice, no differences in NCC abundance and phosphorylation was seen between MR wt and MR ko cells and dietary Na + restriction up-regulated NCC to similar extent in both groups of cells. In contrast, MR ko cells in the CS did not show any detectable alphaENaC abundance or apical targeting of ENaC neither on control diet nor in response to dietary Na + restriction. Furtherm...