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 t o the proliferation of astrocytes remain unclear. We investigated ET-i nduced DNA synthesis in astrocytes using ETB receptor-deficient mutant rats (aganglionosis rats: sl/sl). Western blotting with anti-ET recep tor subtype-specific antibodies and Scatchard analysis of binding reve aled that ETB receptor expression in astrocytes depended on gene dosag e (+/+: sl/+: sl/sl = 2:1:0), whereas ETA receptor expression was unch anged among the three genotypes. ET-1 (10 nM) stimulated [H-3]thymidin e incorporation and mitogen-activated protein kinase (MAP kinase) acti vity 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 obser ved in +/+ astrocytes. Treatment with pertussis toxin (PTX) suppressed the ET-1-induced increases in the incorporation and MAP kinase activi ty in +/+, but not sl/sl astrocytes, indicating that the ETB receptor- , but not the ETA receptor-, mediated pathway to DNA synthesis involve s PTX-sensitive G proteins, e.g. G(i) and/or G(o) (G(i/o)). in +/+ ast rocytes, ET-1 (1 nM) stimulated cAMP accumulation, and the ETB recepto r-selective agonist IRL 1620 (1 nM) suppressed 10 mu M forskolin-induc ed cAMP accumulation, suggesting G(s) coupling to the ETA receptor and Oil, coupling to the ETB receptor. On the other hand, ET-1 did not in crease cAMP accumulation in sl/sl astrocytes, although ET-1 (1 nM) sup pressed the forskolin-induced response, suggesting G(i/o) coupling to the ETA receptor, Our results suggest the possibility that the selecti vity of G protein for ETA receptor is changed from G(s) to G(i/o) in E TB receptor-deficient astrocytes.