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Calcium wave of tubuloglomerular feedback

Department of Physiology and Biophysics and Department of Medicine, Zilkha Neurogenetic Institute, University of Southern California, Los Angeles, California

Submitted 25 October 2005 ; accepted in final form 20 February 2006

ATP release from macula densa (MD) cells into the interstitium of the juxtaglomerular (JG) apparatus (JGA) is an integral component of the tubuloglomerular feedback (TGF) mechanism that controls the glomerular filtration rate. Because the cells of the JGA express a number of calcium-coupled purinergic receptors, these studies tested the hypothesis that TGF activation triggers a calcium wave that spreads from the MD toward distant cells of the JGA and glomerulus. Ratiometric calcium imaging of in vitro microperfused isolated JGA-glomerulus complex dissected from rabbits was performed with fluo-4/fura red and confocal fluorescence microscopy. Activation of TGF by increasing tubular flow rate at the MD rapidly produced a significant elevation in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in extraglomerular mesangial cells (by 187.6 ± 45.1 nM) and JG renin granular cells (by 281.4 ± 66.6 nM). Subsequently, cell-to-cell propagation of the calcium signal at a rate of 12.6 ± 1.1 µm/s was observed upstream toward proximal segments of the afferent arteriole and adjacent glomeruli, as well as toward intraglomerular elements including the most distant podocytes (5.9 ± 0.4 µm/s). The same calcium wave was observed in nonperfusing glomeruli, causing vasoconstriction and contractions of the glomerular tuft. Gap junction uncoupling, an ATP scavenger enzyme cocktail, and pharmacological inhibition of P₂ purinergic receptors, but not adenosine A₁ receptor blockade, abolished the changes in [Ca²⁺]_i and propagation of the calcium wave. These studies provided evidence that both gap junctional communication and extracellular ATP are integral components of the TGF calcium wave.

ATP; adenosine; purinergic receptors; gap junction; fluo-4; fluorescence microscopy; podocyte

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