OPIATE-DEPENDENT CHANGES IN THE SENSITIVITY OF ADENYLATE-CYCLASE TO STIMULATORY AGONISTS AND 5'-GUANYLYLIMIDODIPHOSPHATE ARE INDEPENDENT OF G-PROTEIN ABUNDANCE AND EUKARYOTIC ADP-RIBOSYLTRANSFERASE ACTIVITY IN NG108-15 CELLS

NG108-15 cells were exposed in culture to 1-mu-M [D-Ala2,D-Leu5]enkaphalin (DADLE) for 17 h. This treatment increased the maximum iloprost- and 5'-(N-ethylcarboxamido)adenosine-dependent activation of adenylate cyclase, as well as basal enzyme activity. In addition, there was an increase in the capacity of 5'-guanylylimidodiphosphate [Gpp(NH)p] to inhibit adenylate cyclase activity by direct interaction with the alpha-subunit of the G(i) regulatory protein. A similar effect was observed if the cells were exposed to 10-mu-M carbachol. These treatments of NG108-15 cells did not alter the capacity of NaF to activate adenylate cyclase by direct interaction with G(s-alpha). Exposure of NG108-15 cells to DADLE alone or DADLE plus carbachol had no effect on the capacity of pertussis toxin to ADP-ribosylate membrane proteins in these cells; neither was there any change in the activity of eukaryotic ADP-ribosyltransferase expressed in these cells. Under these conditions, the endogenous enzyme did not label any protein with a molecular mass similar to G(i-alpha), 41 kDa. Treatment of the cells with DADLE or carbachol had no effect on the abundance of G(s-alpha), G(i-alpha), or G(beta). The underlying mechanism for the changes in agonist-dependent stimulatory responses or Gpp(NH)p-dependent inhibition of adenylate cyclase remains obscure, but appears not to be mediated by eukaryotic ADP-ribosyltransferase activity or a change in the abundance of G proteins known to regulate adenylate cyclase.