Vascular smooth muscle thromboxane A2 receptors mediate arachidonic acid-induced sudden death in rabbits

We recently identified a subgroup of rabbits (called nonresponders) that were deficient in vascular thromboxane A2 receptors. Thromboxane A2-mediated platelet aggregation was not different between responders and nonresponders. In the present study, we utilized these nonresponders as a model to study the relative contribution of the platelet and vascular thromboxane A2 receptors to the observed hemodynamic responses associated with arachidonic acid-induced sudden death. Mean arterial pressure was slightly but not significantly lower in the nonresponders compared with the responders. However, nonresponders were protected from arachidonic acid-induced sudden death. While 100% of the responders died at the 2.0 mg dose of arachidonic acid, only 27% of nonresponders died at this same dose. Administration of the thromboxane A2 mimetic U46619 (5 micrograms/kg IV) decreased blood pressure by 41 +/- 6 mm Hg in responders but had no effect in the nonresponders. The affinity and density of thromboxane A2 receptors in cultured aortic vascular smooth muscle cells obtained from both responders and nonresponders were assessed using radioligand binding. The Kd values were not different (4.4 +/- 1.0 versus 2.4 +/- 0.6 nmol/L, responder versus nonresponder). However, there was a significant decrease in the density of receptors from vascular smooth muscle cells of nonresponders (Bmax = 397 +/- 59 versus 157 +/- 59 fmol/10(6) cells, responder versus nonresponder, P < .01). U46619 produced a concentration-dependent increase in [3H]-thymidine incorporation into responder vascular smooth muscle cells but had no effect in the nonresponder cells. Using an anti-thromboxane A2 receptor antibody, we compared the amount of receptor expressed in aortic tissue obtained from responders and nonresponders. Consistent with the results observed with [3H]-thymidine uptake and radioligand binding assays, the expression of thromboxane A2 receptor protein was decreased in nonresponder compared with responder vascular tissue. Platelet thromboxane A2 receptor expression was not different. These studies demonstrate that the vascular smooth muscle cells of nonresponder rabbits are deficient in the thromboxane A2 receptor. Furthermore, the reduction in arachidonic acid-induced sudden death in nonresponders indicates that the vascular smooth muscle thromboxane A2 receptor mediates this effect.