Coronary vasospastic action of thromboxane A2 in isolated, working guinea pig hearts

The effects of thromboxane A2 (TXA2) on isolated, working guinea pig heart and left ventricular papillary muscle preparations were investigated. TXA2 was generated by the enzymatic conversion of prostaglandin H2 (PGH2) with indomethacin-treated horse platelet microsomes (IPM). TXA2 caused dose-dependently a considerable decrease in the coronary flow, left ventricular systolic pressure and left ventricular dp/dt in the perfused heart, while the similar effects of PGH2 were transient and weaker than those of TXA2. These effects of TXA2 disappeared after the TXA2-generating system was left standing at 37 degree C for 3 min. When IPM in the TXA2-generating system was pretreated with a TXA2 synthetase inhibitor, L-8027, the enhanced cardiac responses induced by the mixture of IPM and PGH2 disappeared. TXA2 had no direct effect on the contractile force of the papillary muscle. These results suggest that TXA2 has a specific coronary vasopastic action without a direct inotropic effect.