1. Tubuloglomerular feedback (TGF), the change of afferent arteriolar resistance initiated by changes of luminal NaCl concentration, is thought to be related to NaCl-dependent release of ATP by macula densa cells. We have explored the possibility that the released ATP may directly interact with vasoconstrictor P2 purinergic receptors in the vicinity of the glomerular vascular pole by determining the stop flow pressure (PSF) response to a saturating increase in loop of Henle flow rate before and during the i.v. administration of the P2 receptor inhibitors PPADS (12 mg/kg + 35 mg/kg hr) or suramin (50 mg/kg + 150 mg/kg hr) in two different strains of wild type mice (SWR/J and FVB). Both agents significantly reduced the blood pressure response to the P2X agonist z-methylene ATP indicating effective blockade of vascular P2 receptors. In both strains of mice, elevating flow to 30 nl/min reduced PSF by 16.4 2.2 % and 17.1 1.8 %. During infusion of PPADS PSF fell by 18.8 2 % (p=0.4) and 16.5 1.5 % (p=0.82) in the two strains of mice. During suramin infusion PSF decreased by 14.7 2.4 % (p=0.62) and 15 1.3 % (p=0.4) in SWR/J and FVB mice respectively. Including PPADS (10-4 M) in the loop perfusate did not significantly alter the PSF response (18.9 1.8 %;p=0.54). Arterial blood pressure was not systematically affected by the P2 inhibitors. Interestingly, PPADS significantly reduced proximal tubular fluid reabsorption both in fractional and absolute terms as measured by free flow micropuncture. These results indicate that the direct activation of P2 purinergic receptors by ATP is not a major cause of TGF-induced vasoconstriction in vivo. Furthermore, ATP appears to mediate stimulation of proximal tubular fluid reabsorption through P2 receptor activation. 2. Inhibition of glucose reabsorption along the renal proximal tubule is actively being considered as a treatment modality for the reduction of blood glucose in diabetic conditions. However, the physiologic consequences of inhibition of tubular glucose reabsorption are unclear. We have used a novel mouse model generated by N-ethyl-Nnitrosurea (ENU) mutagenesis that carries a nonsense mutation in the Na-linked glucose transporter gene Slc5a2 resulting in loss of transporter protein function. The phenotype of SP mutants is characterized by higher urinary excretion of calcium, magnesium and protein with retarded growth. Studies of kidney function revealed constant glomerular filtration rates and well maintained proximal tubular fluid reabsorption. The prominent diuresis without enhanced natriuresis appears to be generated along the distal nephron by osmotic overload. Following the administration of streptozotocin to induce diabetes, mutants exhibited better overall glycemic control than wild type mice. However, there was a somewhat higher infection risk and an exaggerated mortality rate in mutant animals. Nevertheless, the phenotype of SGLT2 mutants is markedly similar to patients who carry mutations in the Scl5a2 gene making these mice a useful new tool to examine the long-term benefits and risks associated with inhibition of SGLT2 for the management of diabetes. 3. Defects in excretion of salt and water by the kidney appear to be the final common pathway in the pathogenesis of arterial hypertension, but this issue has remained controversial in that recent studies have suggested critical contributions of other systems including the vasculature and central nervous system. We have tested whether selective inhibition of proximal tubular NaCl reabsorption can cause hypertension. Since reabsorption of NaCl and water in the proximal tubule is under the tonic stimulatory control of angiotensin II, we have examined whether disruption of angiotensin II actions has a critical and non-redundant role in determining the level of blood pressure. Disruption of AT1 angiotensin receptor signaling in the proximal tubule was produced by deleting floxed angiotensin receptors with cre recombinase under control of the PEPCK promoter. Mutant mice had a significant lowering of arterial blood pressure, despite intact vascular responses. Micropuncture studies showed that elimination of this pathway reduced proximal fluid reabsorption and altered the expression of key sodium transporters. Our data confirm that selective reduction of salt transport in the proximal tubule is sufficient to affect blood pressure suggesting that targeting of reabsorptive functions of the proximal tubule of the kidney should be a useful therapeutic strategy in hypertension.
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