Diadenosine polyphosphate-induced increase in cytosolic free calcium in vascular smooth muscle cells

Objective: To evaluate the effects of two new endogenous vasoconstricting substances, diadenosine pentaphosphate (AP(5)A) and diadenosine hexaphosphate (AP(6)A) on the cytosolic free calcium concentration in vascular smooth muscle cells. Methods: Spectrofluorophotometric measurements of cytosolic Ca2+ were conducted in monolayers of cultured rat vascular smooth muscle cells using the calcium-sensitive fluorescent dye fura-2. Results: The resting Ca2+ concentration in vascular smooth muscle cells was 80 +/- 5 nmol/l (mean +/- SEM; n = 39). The addition of 10 mu mol/l AP(5)A or AP(6)A significantly increased Ca2+ in vascular smooth muscle cells to 248 +/- 55 and 358 +/- 124 nmol/l, respectively. The sustained increase in Ca2+ after administration of AP(5)A or AP(6)A was 143 +/- 40 and 148 +/- 57 nmol/l, respectively. Diadenosine polyphosphates induce a transplasmamembrane calcium influx as detected by experiments in the absence of external calcium or by using the manganese quenching technique to report unidirectional calcium fluxes. The effects of diadenosine polyphosphates were compared to those of the well known vasoconstrictor angiotensin II, which increased Ca2+ in vascular smooth muscle cells by 1053 +/- 174 nmol/l. Conclusions: Diadenosine polyphosphates increase Ca2+ in vascular smooth muscle cells, thereby regulating the contractility of vascular smooth muscle and subsequently blood pressure.