Preactivation permits subsequent stimulation of phospholipase C by G(i)-coupled receptors.

In the complex signal transduction networks involving G protein-coupled receptors there are numerous examples where G(i)-linked receptors augment G(q)-dependent signals. The mechanistic basis of such occurrences is thought to entail signal convergence at phospholipase Cbeta (PLCbeta) via the G protein betagamma-dimers. Herein, we explored the possibility that augmentation by betagamma-dimers requires preactivation of PLCbeta. COS-7 cells were transiently cotransfected with cDNAs encoding various combinations of receptors and G protein subunits. The G(i)-coupled delta- and kappa-opioid receptors could not stimulate PLCbeta unless they were coexpressed with Galpha(16). The opioid-induced response was dose-dependent and partially inhibited by pertussis toxin or coexpression with transducin, indicating the involvement of betagamma-subunits released from the G(i) proteins. When PLCbeta was preactivated by constitutively active mutants of Galpha(16), Galpha(q), or Galpha(14), opioids enhanced the activity by 80 to 300% and such responses were mostly pertussis toxin-sensitive. The opioid-induced enhancement was dose-dependent and could not be blocked by staurosporin, a protein kinase C inhibitor. Other G(i)-coupled receptors that were ineffective on their own also acquired the ability to stimulate PLCbeta in the presence of a constitutively active mutant of Galpha(q). Coactivation of endogenous or exogenous G(q)-coupled receptors with the delta-opioid receptor produced strong stimulations of PLCbeta and such responses could be partially blocked by pertussis toxin. These results show that enhancement of G(q)-dependent signals by G(i)-coupled receptors requires activated PLCbeta and is mediated via the betagamma-dimer.