EFFECTS OF ATP AND BRADYKININ ON ENDOTHELIAL-CELL CA2+ HOMEOSTASIS AND FORMATION OF CGMP AND PROSTACYCLIN

ATP and bradykinin are known to activate Ca2+ release from intracellular Ca2+ pools as well as induce the influx of Ca2+ in many cell types. In adrenal medulla endothelial cells, we found that ATP and bradykinin could activate Ca2+ influx, although Ca2+ influx did not appear to be due to depletion of intracellular Ca2+ pools per se, since depletion of intracellular Ca2+ pools with thapsigargin reduced rather than enhanced both unidirectional and steady-state Ca-45(2+) uptake. In addition, Ca2+ influx, activated by ATP but not bradykinin, was mostly abolished after agonist removal in cells in which intracellular Ca2+ pools had not been allowed to refill, suggesting that continued receptor occupancy was necessary for ATP to activate Ca2+ influx. The role of Ca2+ in activating guanosine 3',5'-cyclic monophosphate (cGMP) formation [a marker for nitric oxide (NO) secretion] and prostacyclin (PGI2) secretion was also studied. Bradykinin-induced cGMP and PGI2 formation and ATP-induced PGI2 formation each required Ca2+ release from intracellular Ca2+ pools, since depletion of these pools with thapsigargin inhibited their formation. In contrast, ATP-induced cGMP formation, particularly at early time points, did not appear to require either Ca2+ release or Ca2+ influx. This suggests that ATP, but not bradykinin, either induces Ca2+-independent NO formation or that ATP stimulates the generation of cGMP independently of NO. The latter supposition is supported by our observation that NO synthase inhibitors inhibited ATP-induced cGMP formation by at