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

1 Prostacyclin and adenosine A 2 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 [ 32 P]‐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.