Abstract 1547: Thrombin Potentiates Myofilament Ca2+ Sensitivity and Induces Contraction by Activating Proteinase-Activated Receptor-1 in Pulmonary Artery
Thrombotic pulmonary arteriopathy accounts for ~50 % of primary pulmonary hypertension. Thrombin could thus play a critical role in pathogenesis of pulmonary hypertension. However, the role of thrombin in the regulation of the pulmonary artery tone remains to be determined. The present study thus demonstrates, for the first time, thrombin to elevate [Ca2+]i and enhance the myofilament Ca2+ sensitivity by activating proteinase-activated receptor 1 (PAR1), thereby contracting the normal pulmonary artery. We investigated the contractile effect of thrombin using both intact and α-toxin permeabilized preparations of the porcine pulmonary artery. In the intact strips, we simultaneously monitored the changes in [Ca2+]i and contraction by using front-surface fura-2 fluorometry. In the absence of an endothelium, thrombin concentration-dependently induced an increase in [Ca2+]i and tension, with a maximal effect obtained at 1 U/ml. The extent of an increase in [Ca2+]i and tension obtained with 1 U/ml thrombin was approximately 20 % and 60 % of that seen with 80 mM K+ -depolarization, respectively. TFLLR-NH2, a PAR1-activating peptide, also concentration-dependently induced an increase in [Ca2+]i and tension, while a PAR4-activating peptide, GYPGKF-NH2, induced no contraction. In the α-toxin permeabilized rings, thrombin and TFLLR-NH2 concentration-dependently caused a further contraction during steady state contraction induced by 300 nM Ca2+, thus indicating the increase in the myofilament Ca2+ sensitivity. SCH79797, an antagonist of PAR1, concentration-dependently inhibited the thrombin-induced Ca2+ sensitization, with the complete inhibition at 30 μM. We thus conclude that thrombin increased [Ca2+]i sensitivity by activating PAR1, and thereby induced a contraction and myofilament Ca2+ in pulmonary artery. The contractile effect of thrombin is rarely observed under physiological conditions in other types of artery, while the enhanced contractile responses were observed in vascular regions. The responsiveness of the normal pulmonary artery to thrombin is thus unique. Such unique property of the pulmonary artery may contribute to the development and exacerbation of pulmonary hypertension, especially when associated with thrombosis.