Phosphotidylinositol 4,5-Bisphosphate Signals Underlie Receptor-Specific Gq/11-Mediated Modulation of N-Type Ca2+Channels

Abstract

Modulation of voltage-gated Ca²⁺ channels via G-protein-coupled receptors is a prime mechanism regulating neurotransmitter release and synaptic plasticity. Despite extensive studies, the molecular mechanism underlying G_q/11-mediated modulation remains unclear. We found cloned and native N-type Ca²⁺ channels to be regulated by phosphotidylinositol 4,5-bisphosphate (PIP₂). In inside-out oocyte patches, PIP₂ greatly attenuated or reversed the observed rundown of expressed channels. In sympathetic neurons, muscarinic M₁ ACh receptor suppression of the Ca²⁺ current (I_Ca) was temporally correlated with PIP₂ hydrolysis, blunted by PIP₂ in whole-cell pipettes, attenuated by expression of PIP₂-sequestering proteins, and became irreversible when PIP₂ synthesis was blocked. We also probed mechanisms of receptor specificity. Although bradykinin also induced PIP₂ hydrolysis, it did not inhibit I_Ca. However, bradykinin receptors became nearly as effective as M₁ receptors when PIP₂ synthesis, IP₃ receptors, or the activity of neuronal Ca²⁺ sensor-1 were blocked, suggesting that bradykinin receptor-induced intracellular Ca²⁺ increases stimulate PIP₂ synthesis, compensating for PIP₂ hydrolysis. We suggest that differential use of PIP₂ signals underlies specificity of G_q/11-coupled receptor actions on the channels.