Both ETA and ETB receptors are involved in mitogen‐activated protein kinase activation and DNA synthesis of astrocytes: study using ETB receptor‐deficient rats (aganglionosis rats)

Endothelin (ET) is known to be a potent mitogen in astrocytes. However, the contribution and signalling pathway of ETA and/or ETB receptor to the proliferation of astrocytes remain unclear. We investigated ET-induced DNA synthesis in astrocytes using ETB receptor-deficient mutant rats (aganglionosis rats: sl/sl). Western blotting with anti-ET receptor subtype-specific antibodies and Scatchard analysis of binding revealed that ETB receptor expression in astrocytes depended on gene dosage (+/+: sl/+: sl/sl = 2: 1: 0), whereas ETA receptor expression was unchanged among the three genotypes. ET-1 (10 nm) stimulated [3H]thymidine incorporation and mitogen-activated protein kinase (MAP kinase) activity not only in +/+ via both ETA and ETB receptors, but also in sl/sl astrocytes via ETA receptor with about half the extent of those observed in +/+ astrocytes. Treatment with pertussis toxin (PTX) suppressed the ET-1-induced increases in the incorporation and MAP kinase activity in +/+, but not sl/sl astrocytes, indicating that the ETB receptor-, but not the ETA receptor-, mediated pathway to DNA synthesis involves PTX-sensitive G proteins, e.g. Gi and/or Go (Gi/o). In +/+ astrocytes, ET-1 (1 nm) stimulated cAMP accumulation, and the ETB receptor-selective agonist IRL 1620 (1 nm) suppressed 10 μm forskolin-induced cAMP accumulation, suggesting Gs coupling to the ETA receptor and Gi/o coupling to the ETB receptor. On the other hand, ET-1 did not increase cAMP accumulation in sl/sl astrocytes, although ET-1 (1 nm) suppressed the forskolin-induced response, suggesting Gi/o coupling to the ETA receptor. Our results suggest the possibility that the selectivity of G protein for ETA receptor is changed from Gs to Gi/o in ETB receptor-deficient astrocytes.