RELAXATION OF RAT THORACIC AORTA INDUCED BY THE CA2+-ATPASE INHIBITOR, CYCLOPIAZONIC ACID, POSSIBLY THROUGH NITRIC-OXIDE FORMATION

1 The effect of the Ca2+-ATPase inhibitor, cyclopiazonic acid (CPA), was studied on rat thoracic aortic ring preparations. 2 At concentrations above 0.31 mu M, CPA induced relaxation in the arteries precontracted with phenylephrine. Removal of the endothelium abolished CPA-induced relaxation. 3 The nitric oxide (NO) synthase inhibitor N-G-nitro L-arginine (3-300 mu M), the free radical scavenger haemoglobin (0.1-3 mu M), the soluble guanylate cyclase inhibitor, LY83583 (0.1-10 mu M), each inhibited the endothelium-dependent relaxation to CPA, The potassium channel blocker, glibenclamide (10 mu M) and cyclo-oxygenase inhibitor, indomethacin (100 mu M for 60 min and then washed out) did not alter the action of CPA. 4 The calmodulin inhibitors calmidazolium (3-10 mu M) and W-7 (100 mu M) also abolished CPA-induced relaxation. 5 CPA (10 mu M) increased guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels in arteries with an intact endothelium, without affecting adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. 6 The inhibitors of NO synthesis and actions, the calmodulin inhibitor and removal of the endothelium abolished the CPA-stimulated increase in the levels of cyclic GMP. 7 In Ca2+-free solution, CPA failed to induce relaxation or to stimulate cyclic GMP production. Relaxation to nitroprusside was not affected under these conditions. 8 These results suggest that CPA can stimulate NO synthesis, possibly by inhibiting a Ca2+-ATPase, which replenishes Ca2+ in the intracellular storage sites in endothelial cells. Depletion of the Ca2+ store in the endothelium may then trigger influx of extracellular Ca2+, contributing to an increase in free Ca2+ in the endothelial cells, which activates NO synthase and NO formation.