Functional antagonism between ETB and ETA receptors in JNK1 activation of human cardiac fibroblasts

Endothelin (ET)-1 was reported to play a role in the myocardial fibroblasts proliferation and collagen synthesis. Preliminary clinical trials have been carried on with the use of ET-1 competitive antagonist but whether an advantage exists in the use of selective ETA inhibitor when compared to non-selective ETA/ETB inhibitors is still not clear. The aim of the study was to evaluate the expression of the subtypes ETA and ETB receptors for ET1 on human cardiac fibroblasts and their ability of activating an intracellular pathway related with the collagen deposition (JNKI). Left ventricular fibroblasts, isolated by enzymatic dissociation from explanted hearts excluded from transplantation because of non cardiac reasons, showed an high density of ET-1 receptors (680±110 fmol/mg protein) with high affinity (Kd = 0.45nM). The prevalent receptor subtype was ETB (>90%). The prevalent expression of the mRNA for the ETB subtype was confirmed at RT-PCR studies. Fibroblast stimulation with ET-1 (100nM) resulted in 35% increase of JNK1 activity. JNK activation was completely inhibited by preincubation with a selective ETA inhibitor (BQ123, 10 µM). Conversely, in the presence of an ETB receptor antagonist (BQ788 10 µM) the ET-1 stimulation of fibroblasts resulted in a marked increase of JNK1 activity (210%). In conclusion, 1) ETB receptor is the prevalent ET-1 receptor subtype expressed by human fibroblasts and 2) it seems to antagonize JNK activation induced by ETA receptor stimulation.