The effects of diadenosine tetraphosphate (AP4A), diadenosine pentaphosphate (AP5A), and diadenosine hexaphosphate (AP6A) on the cytosolic free calcium concentration ([Ca2+]i) were evaluated in cultured rat vascular smooth muscle cells (VSMC) using the fluorescent dye technique. A concentration-dependent increase of [Ca2+]i by AP4A, AP5A, and AP6A was observed in VSMC. Additions of 10 micromol/l AP4A, AP5A, and AP6A significantly increased [Ca2+]i in VSMC by 224 +/- 98 nmol/l (n = 6; p < 0.01, 205 +/- 27 nmol/l (n = 14; p < 0.01), and 269 +/- 98 nmol/l (n = 5; p < 0.05), respectively. Additions of AP4A, AP5A, and AP6A only 120 s prior to angiotensin II (Ang II) administration significantly attenuated the Ang-II-induced changes of [Ca2+]i in VSMC from 1,053 +/- 174 nmol/l to 283 +/- 42 nmol/l, 591 +/- 112 nmol/l, and 477 +/- 79 nmol/l, respectively (each p<0.01) as compared to the control). The AP6A-induced changes of [Ca2+]i were inhibited by the blockers of P2 purinoceptors, suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid, but not by the inhibitor of P2y purinoceptors, reactive blue. Adenosine triphosphate (ATP) also increased [Ca2+]i in VSMC, whereas the purinoceptor P2x agonist, alpha,beta-methylene-ATP, had no effect on [Ca2+]i in VSMC. Therefore diadenosine polyphosphates may induce changes of [Ca2+]i by interacting with purinoceptors and may be involved in local regulation of vascular resistance evoked by the Ca(2+)-dependent contractile response of VSMC.
