Thromboxane A2 (TXA2) is a potent, labile vasoconstrictor which stimulates vessel contraction through vascular smooth muscle TXA2 receptors differing from those in platelets. We studied TXA2-stimulated events in cultured adult rat aortic smooth muscle cells. The stable TXA2 mimetic (15S)-hydroxy-11α,9α-(epoxymethano)prosta-5Z,13E-dienoic acid (U46619) competed for TXA2 agonist binding to vascular smooth muscle cells with an IC50 of 10 ± 1 nM. In fura-2-loaded cells, U46619 increased free cytosolic Ca++ concentration with an EC50 of 49 ± 14 nM. The increase in free cytosolic Ca++ was rapid, transient and independent of extracellular Ca++ or Ca++ antagonists and thus was due to release from intracellular stores. U46619-mediated Ca++ release was temporally associated with phosphorylation of myosin light-chains, increased accumulation of 1,4,5-inositol trisphosphate (EC50 = 32 ± 4 nM) and cytoplasmic acidification from pH 7.06 ± 0.01 to 7.00 ± 0.02 (P = .02). Ca++ release was 53% attenuated by the phospholipase C inhibitor, 1-[6-[[17β-3-methoxyestra-1,3,5(10)-trien- 17-yl]amino]hexyl]-1H-pyrrole-2,5-dione. In rat aortic rings U46619 caused TXA2 receptor-mediated contractions (EC50 of 28 ± 2 nM) which were not attenuated by removal of extracellular Ca++ from the superfusion buffer. Together, these results suggest that agonist occupation of TXA2 receptors produces vascular smooth muscle contraction through initial activation of phospholipase C with production of 1,4,5-inositol phosphate, release of intracellular calcium stores and phosphorylation of myosin light chains associated with cellular acidification, presumably via activation of Ca++ ATPase.
|Number of pages||10|
|Journal||Journal of Pharmacology and Experimental Therapeutics|
|State||Published - Jan 1 1993|