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Adjustments of renal vascular resistance in response to changes in blood pressure are mediated by an interplay between the myocyte-inherent myogenic and the kidney-specific tubuloglomerular feedback (TGF) mechanisms. Using mice with deletion of the A1 adenosine receptor (AIAR) gene, we tested the prediction that the absence of TGF, previously established to result from AIAR deficiency, is associated with a reduction in the efficiency of autoregulation. In anesthetized wild-type (A1AR+/+) and A1AR-deficient mice (A1AR-/-), glomerular filtration rate (GFR) and renal blood flow (RBF) were determined before and after reducing renal perfusion pressure through a suprarenal aortic clamp. In response to a blood pressure reduction by 15.9 ± 1.34 mmHg in A1AR-/- (n = 9) and by 14.2 ± 0.9 mmHg in A1AR+/+ mice (n = 8; P = 0.31), GFR fell by 187.9 ± 37 µl/min and by 72.3 ± 10 µl/min in A1AR-/- and A1AR+/+ mice, respectively (P = 0.013). Similarly, with pressure reductions of 14.8 ± 1.1 and 13.3 ± 1.5 mmHg in A1AR-/- (n = 9) and wild-type mice (n = 8), respectively (P = 0.43), RBF fell by 0.17 ± 0.02 ml/min in A1AR-/- mice and by only 0.08 ± 0.02 ml/min in wild-type animals (P = 0.0039). Autoregulatory indexes for both GFR and RBF were significantly higher in A1AR-/- compared with A1AR+/+ mice, indicating reduced regulatory responsiveness in the knockout animals. We conclude that autoregulation of renal vascular resistance is less complete in A1AR-deficient mice, an effect that is presumably related to absence of TGF regulation in these animals. [ABSTRACT FROM AUTHOR]

Aims: Adenosine causes vasoconstriction of afferent arterioles of the mouse kidney through activation of adenosine A1 receptors and Gi-mediated stimulation of phospholipase C. In the present study, we further explored the signalling pathways by which adenosine causes arteriolar vasoconstriction. Methods and results: Adenosine (10−7 m) significantly increased the intracellular calcium concentration in mouse isolated afferent arterioles measured by fura-2 fluorescence. Pre-treatment with thapsigargin (2 μm) blocked the vasoconstrictor action of adenosine (10−7 m) indicating that release of calcium from the sarcoplasmic reticulum (SR), stimulated presumably by IP3, is involved in the adenosine contraction mechanism of the afferent arteriole. In agreement with this notion is the observation that 2 aminoethoxydiphenyl borate (100 μm) blocked the adenosine-induced constriction whereas the protein kinase C inhibitor calphostin C had no effect. The calcium-activated chloride channel inhibitor IAA-94 (30 μm) inhibited the adenosine-mediated constriction. Patch clamp experiments showed that adenosine treatment induced a depolarizing current in preglomerular smooth muscle cells which was abolished by IAA-94. Furthermore, the vasoconstriction caused by adenosine was significantly inhibited by 5 μm nifedipine (control 8.3 ± 0.2 μm, ado 3.6 ± 0.6 μm, ado + nifedipine 6.8 ± 0.2 μm) suggesting involvement of voltage-dependent calcium channels. Conclusion: We conclude that adenosine mediates vasoconstriction of afferent arterioles through an increase in intracellular calcium concentration resulting from release of calcium from the SR followed by activation of Ca2+-activated chloride channels leading to depolarization and influx of calcium through voltage-dependent calcium channels. [ABSTRACT FROM AUTHOR]

By crossing aquaporin 1 (AQP1)−/− and adenosine 1 receptor (A1AR)−/− mice, we generated an animal model that combines a proximal tubular absorption defect with absence of tubuloglomerular feedback (TGF) regulation of glomerular filtration rate (GFR) . The aim of studies in these animals was to determine whether a TGF-induced reduction of GFR is a prerequisite for preventing potentially fatal fluid losses. In contrast to AQP1 deficient mice, AQP1/A1AR−/− mice were found to have a normal GFR. TGF responses were abolished in these animals, in contrast to AQP1−/− mice in which TGF responses of single nephron glomerular filtration rate (SNGFR) were left-shifted. Proximal tubule fluid absorption in AQP1/A1AR−/− mice was reduced to levels previously reported for AQP1−/− mice. However, SNGFR was significantly higher in AQP1/A1AR−/− than AQP1−/− mice (10.6 ± 0.8 nL min−1 vs. 5.9 ± 0.7 nL min−1). As a consequence of the normal GFR and the reduced proximal reabsorption distal fluid delivery was markedly higher in the double knockout compared with normal or AQP1−/− mice (5.5 ± 0.5 nL min−1 vs. 2.35 ± 0.3 nL min−1 in AQP1−/−). Despite the approximate doubling of distal fluid and Cl delivery, AQP1/A1AR−/− mice have a normal salt excretion, normal arterial blood pressure, and only a small increase in plasma renin concentration. The ability to compensate for proximal tubule malabsorption without a TGF-induced reduction of GFR attests to a remarkable adaptability of distal tubule transport mechanisms. [ABSTRACT FROM AUTHOR]

Objective: Absence of tubuloglomerular feedback responses (TGF) in adenosine 1 receptor knockout mice suggests a critical role of adenosine in the pathway that links the macula densa segment of the nephron with vascular smooth muscle cells in the afferent arteriole. The source and mode of generation of adenosine are still unclear. Inhibition of the TGF response during pharmacologic blockade of ecto-5'-nucleotidase (CD73) suggest a key role of this enzyme in supplying adenosine during the TGF response. However, the ability of the used drugs to access the JGA and their specificity are unclear. In the current study we have used a gene targeting approach to assess whether the TGF response magnitude is dependent upon an intact extracellular ATP breakdown pathway. Methods: CD73 deficient mice were generated by homologous recombination in embryonic stem cells. Micropuncture experiments were performed to address the TGF function. Results: Immunohistochemistry confirmed the lack of CD73 protein in the kidneys of CD73 knockout mice. CD73-/- mice showed no gross abnormalities; wildtype, heterozygous and homozygous mice were born in a close to Mendelian ratio. Blood pressure and blood chemistry appeared normal. Micropuncture experiments to address the TGF function in CD73-/- mice showed a significantly reduced change in stop flow pressure in the knockouts (-5.5 ± 0.8 mmHg, n = 24) compared to their wildtypes (-9.8 ± 0.9mmHg, n = 13, p = 0.002) during an increase of tubular perfusion flow from 0 to 30 nL/min. The TGF response did not change significantly during repeated changes in perfusion flow (up to 5 times) in CD73+/+; however, in the CD73-deficient mice, the magnitude of the TGF response was gradually diminished during consecutive changes in tubular perfusion flow, dropping from a stop flow pressure change of -5.5 ± 0.8mmHg (n = 24) during the first response to -1.3 ± 0.5mmHg during the fifth response (n = 10, p = 0.004 compared to first response; p < 0.0001 compared to wild type). The contractile response of isolated afferent arterioles to adenosine was normal in CD73-/- mice. Conclusions: These data indicate that signaling between MD cells and vascular smooth muscle cells involves ecto-5'-nucleotidasedependent formation of adenosine presumably from ATP released by macula densa cells. [ABSTRACT FROM AUTHOR]