Inositol 1,4,5-triphosphate and protein kinase C are involved in thrombin- and trap-induced vascular smooth muscle contraction

Thrombin activation of proteinase-activated receptor 1 potentiates the myofilament Ca2+ sensitivity and induces vasoconstriction in porcine pulmonary arteries

BACKGROUND AND PURPOSE

Thrombus formation is commonly associated with pulmonary arterial hypertension (PAH). Thrombin may thus play an important role in the pathogenesis and pathophysiology of PAH. Hence, we investigated the contractile effects of thrombin and its mechanism in pulmonary artery.

EXPERIMENTAL APPROACH

The cytosolic Ca(2+) concentrations ([Ca(2+)](i)), 20 kDa myosin light chain (MLC20) phosphorylation and tension development were evaluated using the isolated porcine pulmonary artery.

KEY RESULTS

Thrombin induced a sustained contraction in endothelium-denuded strips obtained from different sites of a pulmonary artery, ranging from the main pulmonary artery to the intrapulmonary artery. In the presence of endothelium, thrombin induced a transient relaxation. The contractile effect of thrombin was abolished by either a protease inhibitor or a proteinase-activated receptor 1 (PAR(1)) antagonist, while it was mimicked by PAR(1)-activating peptide (PAR(1)AP), but not PAR(4)AP. The thrombin-induced contraction was associated with a small elevation of [Ca(2+)](i) and an increase in MLC20 phosphorylation. Thrombin and PAR(1)AP induced a greater increase in tension for a given [Ca(2+)](i) elevation than that obtained with high K(+)-depolarization. They also induced a contraction at a fixed Ca(2+) concentration in alpha-toxin-permeabilized preparations.

CONCLUSIONS AND IMPLICATIONS

The present study revealed a unique property of the pulmonary artery. In contrast to normal arteries of the systemic circulation, thrombin induces a sustained contraction in the normal pulmonary artery, by activating PAR(1) and thereby increasing the sensitivity of the myofilament to Ca(2+). This responsiveness of the pulmonary artery to thrombin may therefore contribute to the pathogenesis and pathophysiology of PAH.

Enhanced contractile response to thrombin in the pregnant rat myometrium

Thrombin causes various cellular events by activating protease-activated receptors (PARs). Here, we showed, for the first time, that thrombin induced myometrial contraction. To determine the mechanism of thrombin-induced myometrial contraction, we simultaneously measured intracellular Ca(2+) concentration ([Ca(2+)](i)) and tension of fura-PE3-loaded rat myometrium using front-surface fluorimetry. The expression of thrombin receptor mRNA in the rat myometrium were determined by reverse transcription-polymerase chain reaction analysis (RT - PCR analysis). Thrombin (0.01 - 3 u ml(-1)) caused dose-dependent increase in [Ca(2+)](i) and tension in the rat myometrium, and this effect was greatly enhanced in the pregnant myometrium. PAR1-activating peptide mimicked the effects of thrombin. In Ca(2+)-free PSS, thrombin induced no increase in [Ca(2+)](i) and tension in the pregnant myometrium. Both diltiazem (10 microM) and SK-F 96365 (10 microM) significantly inhibited the thrombin-induced elevations of [Ca(2+)](i) and tension, and their effects were additive. RT - PCR analysis revealed an approximately 10 fold increase in the level of thrombin receptor mRNA in the pregnant myometrium compared to that obtained in the non-pregnant myometrium. In conclusion, the contractile response to thrombin was greatly enhanced in the pregnant myometrium, mainly due to the up-regulation of thrombin receptor. We propose that initiation of a post-parturitional myometrial contraction is one of the most important physiological roles of thrombin receptor.