Maintained tubuloglomerular feedback responses during acute inhibition of P2 purinergic receptors in mice

Abstract

lar 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. In the present study, 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. In two different strains of wild-type mice (SWR/J and FVB), TGF responses were determined in vivo by measuring the stop flow pressure (PSF) change caused by a saturating increase in loop of Henle flow rate before and during the administration of the P2 receptor inhibitors PPADS (12 mg/kg ϩ 35 mg·kg Ϫ1 ·h Ϫ1 iv) or suramin (50 mg/kg ϩ 150 mg·kg Ϫ1 ·h Ϫ1 ). Both agents significantly reduced the blood pressure response to the P2X agonist ␣,␤-methylene ATP. In SWR/J and FVB mice, elevating flow to 30 nl/min reduced PSF by 16.4 Ϯ 2.2 and 17.1 Ϯ 1.8%. During infusion of PPADS, PSF fell by 18.8 Ϯ 2 (P ϭ 0.4) and 16.5 Ϯ 1.5% (P ϭ 0.82) in the two strains of mice. During suramin infusion, PSF decreased by 14.7 Ϯ 2.4 (P ϭ 0.62) and 15 Ϯ 1.3% (P ϭ 0.4) in SWR/J and FVB mice, respectively. Including PPADS (10 Ϫ4 M) in the loop perfusate did not significantly alter the PSF response (18.9 Ϯ 1.8%; P ϭ 0.54). Arterial blood pressure was not systematically affected by the P2 inhibitors. As measured by free-flow micropuncture, PPADS significantly reduced proximal tubular fluid reabsorption in both fractional and absolute terms. These results indicate that the direct activation of P2 purinergic receptors by ATP is not a major cause of TGF-induced vasoconstriction in vivo.PPADS; suramin; stop-flow pressure; proximal reabsorption; micropuncture THE TUBULOGLOMERULAR FEEDBACK (TGF) response is defined as the change in glomerular filtration rate or glomerular capillary pressure when the perfusion rate in the loop of Henle of individual nephrons is altered (26). There is general agreement that this response is for the most part the result of a direct relationship between the loop of Henle perfusion rate and afferent arteriolar resistance. In addition, experimental evidence favors the notion that TGF-dependent vasomotion is initiated by purinergic control mechanisms activated by the flow-dependent release of ATP from macula densa cells into the underlying extraglomerular mesangial interstitium (3). It has remained uncertain how any released ATP may affect the tone of the afferent arterioles.One proposal has been that ATP activates P2 receptors on extraglomerular mesangial cells and that the resulting increase in cytosolic Ca is transmitted to the afferent arteriole through gap junctional coupling (14,19). ATP may also directly activate P2 receptors on the afferent arteriole, with P2X1 receptors being the most likely receptor subtype since its presence in afferent arterioles has been well documented (8). Alternatively, ATP may be dephosphorylated to AMP and adenosine by ecto-ATPases and ecto-5= nucleotidase, a paradigm that has been shown to underlie the ...

Purinergic signalling in the kidney in health and disease

“…In vivo TGF responses are blunted in mice lacking either the adenosine A 1 receptor [222,356] or ecto-5′-nucleotidase, the enzyme catalysing the final stage of the degradation of ATP to adenosine [47]. This proposition is supported by a recent in vivo study in which the TGF response in mice (as assessed by changes in stop-flow pressure in the proximal tubule) was unaffected during intravenous infusion of PPADS or suramin [319]. Nevertheless, an anatomical consideration argues for involvement of the P2 receptor system in the TGF response: the ATP released from macula densa cells cannot activate directly P2 receptors in the afferent arteriole, being physically separated in most species by the extraglomerular mesangium.…”

ATP acting through P2Y receptors causes activation of podocyte TRPC6 channels: role of podocin and reactive oxygen species

“…Glomerular endothelial cells are able to secrete ATP through connexin and/or pannexin hemichannels (50), and we favor this as a mechanism for propagation of Ca 2ϩ waves to podocytes during TGF. The question of whether it is adenosine or ATP that carries the initial signals from the macula densa to the afferent arteriole (45,46) is immaterial as to how the Ca 2ϩ wave is subsequently propagated into the rest of the glomerulus. We know from Lucifer yellow dye-filling experiments that podocytes are not connected to other cells by gap junctions (unpublished data) and therefore orthograde signals must pass into podocytes, and possibly from one podocyte to another, by means of diffusible messengers.…”

“…A mechanism of particular importance is tubuloglomerular feedback (TGF), a process in which cells of the macula densa detect changes in distal tubule flow rate and luminal Na ϩ content and initiate signals in response that ultimately act on glomerular cells to control GFR (26,48). While a vast body of work on TGF has focused on regulation of vascular smooth muscle in the afferent and efferent arterioles (41,42,45,46), recent studies have provided evidence that TGF generates a Ca 2ϩ wave that propagates from smooth muscle through extra-and intraglomerular mesangial cells, into endothelial cells, and finally, some 20 -40 s later, into podocytes (34). Importantly, the propagation of this Ca 2ϩ wave was suppressed by extracellular ATP scavengers, and by blockade of P2 purinergic receptors (34).…”

Intravital imaging of podocyte calcium in glomerular injury and disease

“…The changes in glomerular tuft area were measured using Leica LAS AF software quantification tools. In some mice, Ang II (400 ng/kg BW; Phoenix Phamaceuticals), the purinergic receptor inhibitor suramin (50 mg/kg bolus, followed by 150 mg/kg/h maintenance infusion; Sigma-Aldrich) (44) and/or the AT1 receptor blocker losartan (9.9 mg/kg BW; Sigma-Aldrich) (45) were administered by i.v. infusion.…”