Vascular Smooth Muscle Thromboxane A 2 Receptors Mediate Arachidonic Acid-Induced Sudden Death in Rabbits

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Abstract
We recently identified a subgroup of rabbits (called nonresponders) that were deficient in vascular thromboxane A 2 receptors. Thromboxane A 2 -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 A 2 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 A 2 mimetic U46619 (5 μg/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 A 2 receptors in cultured aortic vascular smooth muscle cells obtained from both responders and nonresponders were assessed using radioligand binding. The K d 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 (B max =397±59 versus 157±59 fmol/10 6 cells, responder versus nonresponder, P <.01). U46619 produced a concentration-dependent increase in [ 3 H]-thymidine incorporation into responder vascular smooth muscle cells but had no effect in the nonresponder cells. Using an anti-thromboxane A 2 receptor antibody, we compared the amount of receptor expressed in aortic tissue obtained from responders and nonresponders. Consistent with the results observed with [ 3 H]-thymidine uptake and radioligand binding assays, the expression of thromboxane A 2 receptor protein was decreased in nonresponder compared with responder vascular tissue. Platelet thromboxane A 2 receptor expression was not different. These studies demonstrate that the vascular smooth muscle cells of nonresponder rabbits are deficient in the thromboxane A 2 receptor. Furthermore, the reduction in arachidonic acid-induced sudden death in nonresponders indicates that the vascular smooth muscle thromboxane A 2 receptor mediates this effect.