Abstract

After the thrombus formation in cardiac cavities or coronaries, the serine protease thrombin is produced and can therefore reach the myocardial tissue by the active process of extravasation and binds to the G protein-coupled protease-activated receptor-1 (PAR1) expressed in human myocardium. The role of PAR1 was investigated in the thrombin effect on sodium current (I_Na). I_Na was recorded in freshly isolated human atrial myocytes by the whole-cell patch-clamp method. Action potentials (AP) were recorded in guinea pig ventricular tissue by the conventional glass microelectrode technique. Thrombin-activated PAR1 induced a tetrodotoxin-blocked persistent sodium current, I_NaP, in a concentration-dependent manner with an apparent EC₅₀ of 28 U/ml. The PAR1 agonist peptide SFLLR-NH₂ (50 μM) was able to mimic PAR1-thrombin action, whereas PAR1 antagonists N³-cyclopropyl-7-((4-(1-methylethyl)-phenyl)methyl)-7H-pyrrolo(3,2-f)quinazoline-1,3-diamine (SCH 203099; 10 μM) and 1-(3,5-di-tert-butyl-4-hydroxy-phenyl)-2-[3-(3-ethyl-3-hydroxy-pentyl)-2-imino-2,3-dihydro-imidazol-1-yl]-ethanone (ER 112787) (1 μM), completely inhibited it. The activated PAR1 involves the calcium-independent phospholipase-A₂ signaling pathway because two inhibitors of this cascade, bromoenol lactone (50 μM) and haloenol lactone suicide substrate (50 μM), block PAR1-thrombin-induced I_NaP.Asa consequence of I_NaP activation, in guinea pig right ventricle papillary muscle, action potential duration (APD) were significantly increased by 20% and 15% under the respective action of 32 U/ml thrombin and 50 μM SFLLR-NH₂, and these increases in APD were prevented by 1 μM tetrodotoxin or markedly reduced by application of 1 μM SCH 203099 or ER 112787. Thrombin, through PAR1 activation, increases persistent component of the Na⁺ current resulting in an uncontrolled sodium influx into the cardiomyocyte, which can contribute to cellular injuries observed during cardiac ischemia.