Alpha adrenoceptor antagonists selectively reduce thrombin-stimulated contraction in rabbit arteries

Thrombin-induced contractions and the influence on them of alpha adrenoceptor antagonists were studied in the rabbit femoral and central ear artery using in vitro methods. Maximum contraction with thrombin represented between 50 and 66% of the maximum arterial response to norepinephrine (NE). The thrombin dose-response relationship was complex and did not have a classic sigmoidal shape, whether the endothelium was functional or not. In the femoral artery, the nonselective alpha adrenoceptor antagonist phentolamine (10(-6) M) and the selective alpha-1 adrenoceptor antagonist prazosin (10(-8) and 10(-7) M) significantly reduced the contractions induced with concentrations of thrombin greater than 6 U X ml-1, and increased the concentration of thrombin required to produce maximal contraction. The selective alpha-2 adrenoceptor antagonist rauwolscine (10(-7) M) did not alter the contraction initiated by thrombin or by NE. In the rabbit ear artery, contraction to thrombin and NE could also be reduced with prazosin but not rauwolscine. Reserpine pretreatment did not alter the magnitude of the thrombin-induced contraction in the femoral and central ear artery, indicating that the response and its sensitivity to alpha adrenoceptor blockade was not related to the release of NE from nerve stores. Neither thrombin (0.1-16 U X ml-1) nor NE (10(-9)-10(-5) M) produced any significant relaxation in partially contracted rabbit femoral arteries, whether or not the endothelium was functional, i.e., exhibiting a 50 to 100% maximum relaxation with methacholine or when either alpha-1, alpha-2 or beta adrenoceptor antagonists were present.(ABSTRACT TRUNCATED AT 250 WORDS)