| Abstract: | Despite the presence of a similar number of glucagon and VIP receptors in liver membranes, VIP induces a negligeable stimulation of adenylate cyclase when compared with glucagon effect. In order to elucidate these discrepancies, the effects of guanine nucleotides on the VIP and glucagon-responsive adenylate cyclase of liver were compared using pure ATP as substrate. 10?8 M VIP accounted for a 1.5-fold increase of basal activity. In the presence of GTP or Gpp(NH)p (10?9 to 10?5 M), the level of cAMP production induced by VIP was no more than additive. In contrast, Gpp(NH)p potentiated the effect of glucagon on liver adenylate cyclase. These discrepancies are not explained by a difference in the peptide binding process. These data suggest that, in liver membranes, a GTP-binding protein N2 is associated with the glucagon-sensitive adenylate cyclase, but is not detected for VIP. It is suggested that N2 appears to be specific for the peptidic receptor. |
