Agonist-induced activation of a non-selective ion current in glomerular endothelial cells

Hydrogen sulfide inhibits Ca2+-induced mitochondrial permeability transition pore opening in type-1 diabetes

Hydrogen sulfide (H2S) attenuates N-methyl-d-aspartate receptor-R1 (NMDA-R1) and mitigates diabetic renal damage; however, the molecular mechanism is not well known. Whereas NMDA-R1 facilitates Ca2+ permeability, H2S is known to inhibit L-type Ca2+ channel. High Ca2+ activates cyclophilin D (CypD), a gatekeeper protein of mitochondrial permeability transition pore (MPTP), thus facilitating molecular exchange between matrix and cytoplasm causing oxidative outburst and cell death. We tested the hypothesis of whether NMDA-R1 mediates Ca2+ influx causing CypD activation and MPTP opening leading to oxidative stress and renal injury in diabetes. We also tested whether H2S treatment blocks Ca2+ channel and thus inhibits CypD and MPTP opening to prevent renal damage. C57BL/6J and Akita (C57BL/6J-Ins2Akita) mice were treated without or with H2S donor GYY4137 (0.25 mg·kg-1·day-1 ip) for 8 wk. In vitro studies were performed using mouse glomerular endothelial cells. Results indicated that low levels of H2S and increased expression of NMDA-R1 in diabetes induced Ca2+ permeability, which was ameliorated by H2S treatment. We observed cytosolic Ca2+ influx in hyperglycemic (HG) condition along with mitochondrial-CypD activation, increased MPTP opening, and oxidative outburst, which were mitigated with H2S treatment. Renal injury biomarker KIM-1 was upregulated in HG conditions and normalized following H2S treatment. Inhibition of NMDA-R1 by pharmacological blocker MK-801 revealed similar results. We conclude that NMDA-R1-mediated Ca2+ influx in diabetes induces MPTP opening via CypD activation leading to increased oxidative stress and renal injury, and H2S protects diabetic kidney from injury by blocking mitochondrial Ca2+ permeability through NMDA-R1 pathway.

Calcium-dependent synthesis of prostacyclin in ATP-stimulated venous endothelial cells

Prostacyclin is a powerful vasodilator that is released from vascular endothelial cells. Previous studies in our laboratory have indicated that arachidonic acid metabolites from venous endothelium play an important role in the dilation of adjacent arterioles during muscle stimulation. Furthermore, recent studies have suggested that ATP released from red blood cells during hypoxia stimulates dilation of arterioles. We tested the hypothesis that an ATP-induced increase in intracellular Ca(2+) in venous endothelium promotes prostacyclin synthesis. Small branches of femoral veins were isolated from male golden hamsters, placed in a 1 mL bath, and cannulated for perfusion with 3-(N-morpholino) propanesulfonic acid (MOPS)-buffered physiological salt solution at 37 degrees C. Prostacyclin synthesis was determined by enzyme immunoassay of bath solution. Perfusion of veins with ATP increased prostacyclin synthesis from 50 +/- 5 to 627 +/- 46 pg/mL (n=49). ATP-induced prostacyclin synthesis was inhibited by removal of extracellular Ca(2+), chelation of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) (10 micromol/L for 10 minutes), and preincubation with cytosolic phospholipase A(2) (PLA(2)) inhibitors, AACOCF(3), and bromoenol lactone. Changes in intracellular Ca(2+) in cultured human venous endothelial cells were assessed by fura-2 spectrofluorometry. ATP induced a transient Ca(2+) peak within seconds, and the subsequent Ca(2+) plateau was abolished by removal of extracellular Ca(2+). An increase in prostacyclin synthesis was detected in these cells 2 minutes after application of ATP. These findings suggest that the ATP-induced increase in intracellular Ca(2+) stimulates prostacyclin synthesis in venous endothelial cells.

Extracellular nucleotides regulate cellular functions of podocytes in culture

Extracellular nucleotides are assumed to be important regulators of glomerular functions. This study characterizes purinergic receptors in podocytes. The effects of purinergic agonists on electrophysiological properties and the intracellular free Ca(2+) concentration of differentiated podocytes were examined with the patch-clamp and fura 2 fluorescence techniques. mRNA expression of purinergic receptors was investigated by RT-PCR. Purinergic agonists depolarized podocytes. Purinergic agonists similarly increased intracellular free Ca(2+) concentration of podocytes. The rank order of potency of various nucleotides on membrane voltage and free cytosolic calcium concentration was UTP approximately UDP > [adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S)] > ATP > 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP) > 2'- and 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP) > ADP-beta-S. alpha,beta-Me-ATP was without effect. In the presence of UTP, BzATP did not cause an additional depolarization of podocytes. Incubation of cells with ATP or BzATP did not induce lactate dehydrogenase release. In RT-PCR studies, mRNAs of the P2Y(1), P2Y(2), P2Y(6), and P2X(7) receptors were detected within glomeruli and podocytes. The data indicate that extracellular nucleotides modulate podocyte function mainly by an activation of both P2Y(2) and P2Y(6) receptors.

Cyclic GMP-dependent relaxation of isolated rat renal glomeruli induced by extracellular ATP

The relaxing effect of extracellular ATP on renal glomeruli has been investigated by applying ATP and its analogues to suspensions of angiotensin II-precontracted rat renal glomeruli. Based on changes of glomerular [3H]inulin space (GIS) the relaxation of glomeruli was analysed in the presence of agonists: ATP, ADP, AMP, UTP, 2-methylthio-ATP (P2Y agonist), beta,gamma-methylene-ATP (P2X agonist) and adenosine. ATP, 2-methylthio-ATP, ADP and UTP induced concentration-dependent relaxation whereas AMP, beta,gamma-methylene-ATP and adenosine had no effect. The rank order of relaxation potency was 2-methylthio-ATP > ATP > ADP > UTP. An inhibitor of constitutive nitric oxide synthase (NOS), Nomega-nitro-L-arginine (NNA) prevented the ATP-induced increased accumulation of L-citrulline and the relaxation effect of ATP. An inhibitor of the neuronal isoform of NOS, 7-nitroindazole, had no effect on the relaxation effect of ATP. The relaxing effect of ATP was prevented in the presence of inhibitors of cyclic guanylyl cyclase: methylene blue (MB) and the more specific inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ). ATP stimulated an accumulation of cGMP that was diminished in the presence of MB. We indicated that extracellular ATP may relax the glomeruli via activation of P2Y receptors with the subsequent activation of the endothelial isoform of nitric oxide synthase and soluble guanylyl cyclase. We suggest that, based on the described mechanism, extracellular ATP may increase the filtration surface which, in turn, may influence the glomerular filtration rate.

Ca2+ response of rat mesangial cells to ATP analogues

The aim of this investigation was to characterise the effects of ATP analogues and UTP on the single cell intracellular Ca2+ concentration ([Ca2+]i) in cultured rat mesangial cells. Typically, there were two phases in the Ca2+ response to the agonists, an initial fast transient peak and a subsequent slower decline, or plateau, phase. For the peak amplitude in [Ca2+]i the agonists had about equal effect. But when taking in consideration the percentage of responding cells and the integrated Ca2+ response over 1 min, the order of efficacy of nucleotide agonists (100 microM) was UTP = ATP > ATPgammaS > ADP = 2MeS-ATP (2-methylthio-ATP). Adenosine, AMP and beta,gamma-Me-ATP (100 microM) had no effect. Suramine (100 microM) and reactive blue (50 microM) decreased the number of responding cells. Removing Ca2+ from the bath diminished neither the peak in [Ca2+]i nor the percentage of responding cells, but the average [Ca2+]i increase in 1 min was significantly reduced. The results indicate that P2Y2 receptors are present in rat mesangial cells but it cannot be excluded that there are receptors distinct from P2Y2 which also mediate a rise in [Ca2+]i.