EFFECTS OF THROMBIN AND THROMBIN RECEPTOR ACTIVATING PEPTIDES ON RAT AORTIC VASCULAR SMOOTH-MUSCLE

The role of thrombin receptor activation in isolated rat aortic rings was examined. The human thrombin receptor activating peptides (TRAPs) SFLLRNPNDKYEPF (TRAP1-14), SFLLRNP (TRAP1-7) and rat TRAP1-7 (SFFLRNP) all caused concentration-related (0.1-100 muM) contractions of endoth elium-rubbed rat aortic rings. Reversal of the first two amino acids i n TRAP1-14 (''reverse TRAP1-14'') resulted in total loss of activity. The contractions caused by the TRAPs were reduced substantially in end othelium-intact rings due to endothelium-derived relaxing factor relea se because the reduced contractions were reversed by N(omega)-nitro-L- arginine or methylene blue. Contractions were significantly but only s lightly enhanced by alpha receptor blockade and were not affected by t hromboxane- or endothelin-receptor blockade or by cyclooxygenase inhib ition. TRAP1-7 had no effect on contractile responses to norepinephrin e, serotonin, angiotensin II or endothelin-1; however, pretreatment wi th nifedipine or removal of extracellular Ca++ markedly inhibited the contraction. Neither human nor rat a-thrombin had any contractile effe ct on rat aortic rings. In cultured rat aortic smooth muscle cells, al pha-thrombin (EC50 = 1.9 +/- 0.7 nM), TRAP1-14 (EC50 = 30 +/- 4 muM) a nd TRAP1-7 (EC50 = 20 +/- 9 muM) caused concentration-dependent increa ses in intracellular calcium [Ca++]i, whereas reverse TRAP1-14 was ine ffective. The effect of thrombin on [Ca++]i was abolished by the throm bin inhibitor MD-805, whereas the responses to TRAP were unaffected. T hese data indicate that rat aortic smooth muscle contains thrombin rec eptors which are activated specifically by noncatalytic thrombin recep tor stimulating peptide mimics which increase [Ca++]; and cause contra ction. However, the thrombin receptor on intact rat aorta seems to be already ''cleaved'' whereas cultured rat smooth muscle cells retain th e intact thrombin receptor.