Ca²⁺ channel activation by platelet-derived growth factor-induced tyrosine phosphorylation and Ras guanine triphosphate-binding proteins in rat glomerular mesangial cells

We investigated the signaling pathways mediating 1-pS Ca²⁺ channel activation by PDGF in cultured rat mesangial cells. In cell-attached patches, intrapipette PDGF-BB (PDGF B chain homodimer isoform) (50 ng/ml) dramatically stimulates channel activity (P < 0.003, n = 6). Tyrosine kinase inhibition (100 μM genistein or 10 (μM tryphostin 9) abolished PDGF-induced channel activation (P < 0.02, n = 6). In excised patches, the effect of tyrosine kinase inhibition could be reversed by 200 μM GTPγS (P < 0.02, n = 4). In contrast, 200 μM GDPβS inhibited PDGF-induced channel activity (P < 0.04, n = 6). Pertussis toxin (250 ng/ml) had no effect on PDGF-induced channel activity (P = 0.45, n = 6). When excised patches were exposed to anti-Ras antibody (5 μg/ ml), PDGF-induced channel activity was abolished (P < 0.002, n = 11). Western immunoblots revealed that PDGF-BB binding stimulates the formation of a membrane-bound complex consisting of growth factor receptor-binding protein 2, son of sevenless, and the PDGF-β receptor. Complex formation was abolished by genistein. In mesangial cells, the intrinsic tyrosine kinase activity of the PDGF-β receptor stimulates the formation of a membrane-bound growth factor receptor-binding protein 2/son of sevenless/PDGF-β receptor complex and activation of the pertussis toxin-insensitive GTP-binding protein, p21-Ras, which leads to the opening of 1-pS Ca²⁺ channels.