Regulation of macula densa Na:H exchange by angiotensin II

Regulation of macula densa Na:H exchange by angiotensin II.BackgroundAngiotensin II (Ang II) is a positive modulator of tubuloglomerular feedback (TGF). At the present time, the site(s) at which Ang II interacts with the signal transmission process remains unknown. In certain renal epithelia, Ang II is known to stimulate apical Na:H exchange. Since macula densa cells possess an apical Na:H exchanger and Ang II subtype I receptors (AT1-receptors), we tested the possibility that Ang II might stimulate exchanger activity in these cells.MethodsUsing the isolated perfused thick ascending limb with attached glomerulus preparation dissected from rabbit kidney, macula densa intracellular pH (pHi) was measured with fluorescence microscopy using BCECF.ResultsControl pHi, during perfusion with 25mm NaCl and 150mm NaCl in the bath, averaged 7.22 ± 0.02 (N = 24). Increasing luminal [NaCl] to 150mm elevated pHi by 0.54 ± 0.04 (N = 7, P < 0.01). Ang II (10-9 m), added to the bath in the same paired experiments, significantly elevated baseline pHi by 0.17 ± 0.04, increased the magnitude of change in pHi (Δ = 0.71 ± 0.05) and initial rate of alkalinization (by 69%) to increased luminal [NaCl]. Ang II produced similar effects when added exclusively to the luminal perfusate. In addition, low-dose Ang II (10-9 m) stimulated while high-dose Ang II (10-6 m) inhibited Na-dependent pH-recovery from an acid load. AT1 blockade prevented the stimulatory but not the inhibitory effects of Ang II.ConclusionThrough the AT1, Ang II may influence macula densa Na transport and regulate cell alkalinization via the apical Na:H exchanger. Thus, Ang II may modulate the TGF signal transmission process, at least in part, through a direct effect on macula densa cell function

A juxtaglomeruláris apparátus parakrin szabélyozása: Funkcionális és morfológiai összefüggések normál és kóros körülmények között = Paracrine Control of the Juxtaglomerular Apparatus: Functional and Morphological Correlations in Health and Disease

Speciális technika kifejlesztésével és a két foton lézermikroszkóp segítségével elsőként tettük láthatóvá in vivo a patkány vese afferens arteriola disztális szakaszán található endotheliális fenesztrációt. Kimértük a fenesztrált terület nagyságát és bizonyítottuk annak életkortól, illetve a szervezet aktuális állapotától függő változását. Real time formában nyomon követtük a renin granulumok ezen keresztüli keringésbe jutását. Igazoltuk, hogy a filtráció már a glomerulust megelőző disztális afferens arteriola szakaszban megkezdődik. Vizualizáltuk a juxtaglomeruláris apparátus intersticiális folyadék mozgását és kimutattuk, hogy a juxtaglomeruláris apparátusba folyadék filtrálódik az afferens arteriolából, a mesangiumon keresztül a glomerulusból, illetve a glomerulusból a macula densan keresztül a disztális tubulusba. Ezek az eredmények alapvetően megkérdőjelezik a juxtaglomeruláris apparátus működésére és jelentőségére vonatkozó eddigi elképzeléseinket. A fenesztráció kialakulását, illetve a hozzátartozó nanocsatornák morfológiáját atomerőmikroszkópiával analizáltuk sejttenyészetben. Az angiotenzin II és a vaszkuláris endotheliális növekedési faktor (VEGF) fokozta a fenesztrumok képződését az AT1, illetve a VEGFR-2 receptoron keresztül. E folyamat függ a p38 aktiválódásától. A fenesztráció fokozódása együtt járt a 40kD-os dextran áteresztőképesség növekedésével és a sejtréteg elektromos impedancia csökkenésével. | With a special technical development and the use of two photon laser microscopy, for the first time, in vivo, we managed to visualize the endothelial fenestration at the distal section of the afferent arteriole in rat kidney. The area of the fenestrated section was measured and its change depending on age and actual condition was demonstrated. The passage of the renin granules into the circulation via these fenestrations were followed in real time. We documented that filtration already starts at the distal part of the afferent arteriole before glomerulus. The interstitial fluid movement in the juxtaglomerular apparatus was visualized. It was also shown that in JGA, fluid is filtered via afferent arteriole, mesangium, glomerulus and macula densa into the distal tubule. These findings basically question our understanding about the function and importance of JGA. The development of fenestration and the morphology of the related nanochannels were analyzed in the cell culture with the help of AFM. Angiotensin II and Vascular Endothelial Growth Factor (VEGF) increased fenestrates via AT1 and VEGFR-2 receptors. This process is p38 dependent. Increase in fenestration was accompanied by increase in permeability to 40kD dextran and decrease in cell layer electrical impedance

A new view of macula densa cell microanatomy

Although macula densa (MD) cells are chief regulatory cells in the nephron with unique microanatomical features, they have been difficult to study in full detail due to their inaccessibility and limitations in earlier microscopy techniques. The present study used a new mouse model with a comprehensive imaging approach to visualize so far unexplored microanatomical features of MD cells, their regulation, and functional relevance. MD-GFP mice with conditional and partial induction of green fluorescent protein (GFP) expression, which specifically and intensely illuminated only single MD cells, were used with fluorescence microscopy of fixed tissue and live MD cells in vitro and in vivo with complementary electron microscopy of the rat, rabbit, and human kidney. An elaborate network of major and minor cell processes, here named maculapodia, were found at the cell base, projecting toward other MD cells and the glomerular vascular pole. The extent of maculapodia showed upregulation by low dietary salt intake and the female sex. Time-lapse imaging of maculapodia revealed highly dynamic features including rapid outgrowth and an extensive vesicular transport system. Electron microscopy of rat, rabbit, and human kidneys and three-dimensional volume reconstruction in optically cleared whole-mount MD-GFP mouse kidneys further confirmed the presence and projections of maculapodia into the extraglomerular mesangium and afferent and efferent arterioles. The newly identified dynamic and secretory features of MD cells suggest the presence of novel functional and molecular pathways of cell-to-cell communication in the juxtaglomerular apparatus between MD cells and between MD and other target cells.NEW & NOTEWORTHY This study illuminated a physiologically regulated dense network of basal cell major and minor processes (maculapodia) in macula densa (MD) cells. The newly identified dynamic and secretory features of these microanatomical structures suggest the presence of novel functional and molecular pathways of cell-to-cell communication in the juxtaglomerular apparatus between MD and other target cells. Detailed characterization of the function and molecular details of MD cell intercellular communications and their role in physiology and disease warrant further studies