Selective inhibition of agonist‐induced but not shear stress‐dependent release of endothelial autacoids by thapsigargin

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Abstract
The effects of the Ca2+‐ATPase inhibitor, thapsigargin, on the shear stress‐dependent and on the agonist‐stimulated release of endothelium‐derived relaxing factor, i.e. nitric oxide (NO), and prostacyclin (PGI2) were studied in bovine and human cultured endothelial cells as well as in endothelium‐intact arterial segments of the rabbit. Preincubation with thapsigargin (1 μm for 10 min) had no effect on the shear stress‐dependent release of NO from bovine aortic endothelial cells grown on beads, but abolished the release of NO induced by ADP, bradykinin, ionomycin or poly‐l‐lysine. Similarly, thapsigargin completely abrogated the agonist‐stimulated PGI2 release from these cells, but had no effect on the shear stress‐dependent release of PGI2. The acetylcholine‐induced release of NO from the luminally perfused thoracic aorta and femoral artery of the rabbit was suppressed by pretreatment with thapsigargin (1 μm). In contrast, thapsigargin did not affect the shear stress‐dependent release of NO from the femoral artery. Administration of thapsigargin to these vascular preparations or to cultured endothelial cells alone produced a substantial release of both NO and PGI2. This release declined towards previous values after washout of thapsigargin. In human and bovine cultured endothelial cells, thapsigargin (1–1000 nm) caused a dose‐dependent sustained rise in [Ca2+]i, an effect that was abolished in the absence of extracellular Ca2+. Stimulation of these cells with bradykinin, histamine, ADP or ionomycin after previous exposure to thapsigargin (30–1000 nm) no longer caused an increase in [Ca2+]i. These findings demonstrate that by emptying intracellular Ca2+ stores, thapsigargin selectively blocks the agonist‐stimulated release of both NO and PGI2, suggesting a principal difference in the control by [Ca2+]i of the release of these endothelial autacoids caused by shear stress or receptor‐dependent and independent agonists.