Inhibition of AC-II activity following chronic agonist exposure is modulated by phosphorylation

Chronic exposure to opiate agonists (followed by agonist withdrawal) leads to a large increase in the activity of adenylyl cyclase (AC) isozymes I, V, VI, and VIII, a phenomenon defined as AC superactivation (or supersensitization). On the other hand, AC isozymes belonging to the AC-II family (AC-II, AC-IV, and AC-VII) show decreased activity, referred to as superinhibition. Using COS-7 cells transiently transfected with mu-opioid receptor and AC-II, we show here that inhibition of PKC and tyrosine kinase activities synergistically reduced the level of AC-II superinhibition. Moreover, inhibitor of Raf-1 kinase also led to a decrease in AC-II superinhibition. These data suggest that Raf-1, activated by PKC and tyrosine kinase, has a role in the regulation of AC-II superinhibition.