Segregation of discrete Gsα‐mediated responses that accompany homologous or heterologous desensitization in two related somatic hybrids

Abstract

1 Prostacyclin and adenosine A₂ receptors activate adenylate cyclase in the neuroblastoma hybrid cell lines NG108–15 and NCB-20. Prolonged exposure of NG108–15 cells to iloprost (a stable analogue of prostacyclin) results in a subsequent reduction in the capacity for adenylate cyclase activation by iloprost, the adenosine analogue 5′-(N-ethyl)-carboxamidoadenosine (NECA) or NaF. In contrast prolonged exposure of NCB-20 cells to iloprost results only in the loss of iloprost responsiveness. 2 Iloprost pretreatment of NG108–15 cells also magnified the morphine-dependent inhibition of iloprost-stimulated adenylate cyclase activity from 36 to 48%. This change was not due to lower iloprost stimulation following desensitization, since the % inhibition of adenylate cyclase activity by morphine in control cells was constant irrespective of enzyme activity. 3 These heterologous effects observed in NG108–15 cells following iloprost pretreatment may involve changes in the G_sα protein, since there was a reduction of about 30% in the cholera toxin-induced [³²P]-ADP-ribosylation of a 45 kDa protein from cell membranes (corresponding to the extent of loss of NECA or NaF responsiveness). A similar reduction was not observed in NCB-20 cells. 4 These results indicate that iloprost pretreatment induces different forms of desensitization in NG108–15 and NCB-20 cell lines. The heterologous desensitization in the former may, like the human platelet, involve a functional loss of G_sα from the cell membrane. Changes in the activity of G_sα may also account for the heterologous effects on receptors that mediate inhibition of adenylate cyclase.