DIADENOSINE POLYPHOSPHATES INDUCE TRANSPLASMA MEMBRANE CALCIUM INFLUXIN CULTURED GLOMERULAR MESANGIAL CELLS

The effects of diadenosine tetraphosphate (AP(4)A) diadenosine pentaph osphate (AP(5)A) and diadenosine hexaphosphate (AP(6)A) on the cytosol ic-free calcium concentration ([Ca2+](i)) were evaluated in cultured r at glomerular mesangial cells (MCs) using the fluorescent dye techniqu e. The addition of 10 mu mol L(-1) AP(4)A, AP(5)A or AP(6)A significan tly increased [Ca2+](i) in MCs by 57 +/- 9 nmol L(-1) n = 17; P < 0.01 ), 76 +/- 27 nmol L(-1) (n = 9; P < 0.01) or 65 +/- 12 nmol L(-1) (n = 18; P < 0.01) respectively. In the absence of extracellular calcium, there was no significant change in [Ca2+](i) in MCs after administrati on of diadenosine polyphosphates, indicating that these agents induce transplasma membrane Ca2+ influx. AP(6)A significantly enhanced the an giotensin II-induced changes in [Ca2+](i) in MCs. The AP(5)A-induced t ransplasma membrane Ca2+ influx was inhibited by the P-2 purinoceptor blockers suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), but was not affected by the adenosine A(1) receptor blo cker 8-cyclopentyl-1.3-dipropylzanthine (CPDPX). Adenosine triphosphat e (ATP) and adenosine 5'-O-(3-thio)triphosphate (ATP-gamma S) increase d [Ca2+]i in MCs, whereas alpha,beta-methylene ATP had no effect on [C a2+](i) in MCs. Measurements of diacylglycerol and phosphatidic acid s howed that AP(5)A and AP(6)A also stimulated phospholipase C, but had no effect on phospholipase D. The inhibition of phosphatidylcholine-sp ecific phospholipase C significantly reduced the AP(5)A-induced [Ca2+] (i) increase. In summary, diadenosine polyphosphates induce Ca2+ influ x through P-2 purinoceptors and may be involved in the local regulatio n of vascular resistance evoked by the Ca2+-dependent contractile resp onse of mesangial cells.