The neurokinin A receptor activates calcium and cAMP responses through distinct conformational states

G protein-coupled receptors are thought to mediate agonist-evoked signal tr ansduction by interconverting between discrete conformational states endowe d with different pharmacological and functional properties. In order to add ress the question of multiple receptor states, we monitored rapid kinetics of fluorescent neurokinin A (NKA) binding to tachykinin NK2 receptors, in p arallel with intracellular calcium, using rapid mixing equipment connected to real time fluorescence detection. Cyclic AMP accumulation responses were also monitored. The naturally truncated version of neurokinin A (NKA-(4-10 )) binds to the receptor with a single rapid phase and evokes only calcium responses. In contrast, full-length NKA binding exhibits both a rapid phase that correlates with calcium responses and a slow phase that correlates wi th cAMP accumulation. Furthermore, activators (phorbol esters and forskolin ) and inhibitors (Ro 31-8220 and H89) of protein kinase C or A, respectivel y, exhibit differential effects on NKA binding and associated responses; ac tivated protein kinase C facilitates a switch between calcium and cAMP resp onses, whereas activation of protein kinase A diminishes cAMP responses. NK 2 receptors thus adopt multiple activatable, active, and desensitized confo rmations with low, intermediate, or high affinities and with distinct signa ling specificities.