THROMBOXANE-A2 ANALOG, U-46619, POTENTIATES CALCIUM-ACTIVATED FORCE IN HUMAN UMBILICAL ARTERY

It has previously been shown that human umbilical artery (HUA) smooth muscle produces thromboxane A2 in response to increasing oxygen levels and that this thromboxane promotes contraction. To investigate the in tracellular action of thromboxane A2, strips of HUA longitudinal smoot h muscle were permeabilized using alpha-toxin from the bacterium Staph ylococcus aureus. This treatment rendered the surface membrane permeab le to low-molecular-weight substances but left functional thromboxane A2 receptors. Tension measurements were used to investigate the effect of the stable thromboxane A2 analogue, U-46619, on the Ca2+ sensitivi ty of smooth muscle contractile proteins. U-46619 (1 nM to 1 muM) pote ntiated submaximal Ca2+-activated force (generated by [Ca2+], 50 nM to 3 muM) but not maximal Ca2+-activated force (generated by [Ca2+], 110 -100 muM). The specific thromboxane A2 receptor antagonist, GR-32191B (1 muM), inhibited the action of U-46619 (0.1 muM). The potentiation o f submaximal Ca2+-activated force produced by the muscle in response t o U-46619 (0.1 muM) was antagonized by guanosine 5'-O-(2-thiodiphospha te) (1 mM), the nonhydrolyzable analogue of GDP, and mimicked by guano sine 5'-O-(3-thiotriphosphate) (100 muM), the nonhydrolyzable analogue of GTP. These results suggest that U-46619 acts via the previously id entified thromboxane A2 receptor to promote Ca2+ sensitivity of tensio n production in HUA smooth muscle. Furthermore, this effect appears to be mediated via a G protein.