Thromboxane-induced contractile response of human coronary arterioles is diminished after cardioplegic arrest

Background: We investigated the contractile response of human coronary microvasculature to thromboxane A-2 (TXA-2), with and without the blockade of TXA-2 receptors or the inhibition of phospholipase-C (PLC) or of protein kinase C-α (PKC-α) in the human coronary microvasculature before and after cardioplegia, followed by reperfusion (CP/Rep). Protein/gene expression and localization of TXA-2 receptors, TXA-2 synthase, PLC, and other TXA-2related proteins was also examined. Methods: Right atrial tissue was harvested before and after cold blood cardioplegia, followed by about 10 minutes of reperfusion, from 28 patients undergoing cardiac operations. Coronary arterioles (90 to 170 μm in diameter) were dissected from the harvested tissue. Results: The post-CP/Rep contractile response of coronary arterioles to TXA-2 analog U-46619 was significantly impaired vs pre-CP/Rep (p < 0.05). The TXA-2 receptor antagonist SQ-29548 (10⁶ M) prevented the contractile response to U-46619 (p < 0.05). Pretreatment with the PLC inhibitor U73122 (10 ⁶ M) significantly inhibited the U-46619induced contractile response (p < 0.05). Administration of the PKC-α inhibitor safingol failed to affect U-46619induced contraction. Total protein levels and gene expression of TXA-2 receptors, TXA-2 synthase, PLC-β3, phosphoPLC-β3, PLC-γ1, and phosphoPLC-γ1 were not altered after CP/Rep. Confocal microscopy showed no significant differences in the expression of TXA-2 receptors or PLC-β3 in the microcirculation. TXA-2 receptors and PLC-β3 were both present in smooth muscle and endothelium. Conclusions: Cardioplegia/Rep decreases the contractile response of human coronary arterioles to TXA-2 soon after cardiac operations. The contractile response to the TXA-2 analog U-46619 is through activation of TXA-2 receptors and PLC.