Biphasic, opposing modulation of cloned neuronal alpha 1E Ca channels by distinct signaling pathways coupled to M2 muscarinic acetylcholine receptors

Neuronal alpha 1E subunits are thought to form R-type Ca channels. When exp ressed in human embryonic kidney cells with M2 muscarinic acetylcholine rec eptors, Ca channels encoded by rabbit alpha 1E exhibit striking biphasic mo dulation. Receptor activation first produces rapid inhibition of current am plitude and activation rate. However, in the continued presence of agonist, alpha 1E currents subsequently increase. Kinetic slowing persists during t his secondary stimulation phase. After receptor deactivation, kinetic slowi ng is quickly relieved, and current amplitude over-recovers before returnin g toward control levels. These features indicate that inhibition and stimul ation of alpha 1E are separate processes, with stimulation superimposed on inhibition. Pertussis toxin eliminates inhibition without affecting stimula tion, demonstrating that inhibition and stimulation involve distinct signal ing pathways. Neither inhibition nor stimulation is altered by coexpression of Ca channel beta 2a or beta 3 subunits. Stimulation is abolished by stau rosporine and reduced by intracellular 5'-adenylylimidodiphosphate, suggest ing that phosphorylation is required. However, stimulation does not seem to involve cAMP-dependent protein kinase, protein kinase C, cGMP-dependent pr otein kinase, tyrosine kinases, or phosphoinositide 3-kinases. Stimulation does not require a Ca signal, because it is not specifically altered by var ying intracellular Ca buffering or by substituting Ba as the charge carrier . In contrast to those formed by alpha 1E, Ca channels formed by alpha 1A o r alpha 1B display only inhibition and no stimulation during prolonged acti vation of M2 receptors. The dual modulation of alpha 1E may confer unique p hysiological properties on native R-type Ca channels. As one possibility, R -type channels may continue to mediate Ca influx during steady inhibition o f N-type and P/Q-type channels by muscarinic or other receptors.