Expression of recombinant calcium channels support secretion in a mouse pheochromocytoma cell line

We have characterized a recently established mouse pheochromocytoma cell line (MPC 9/3L) as a useful model for studying neurotransmitter release and neuroendocrine secretion. MPC 9/3L cells express many of the proteins involved in Ca2+-dependent neurotransmitter release but do not express functional endogenous Ca2+-influx pathways. When transfected with recombinant N-type Ca2+ channel subunits alpha1B,beta2a,alpha2delta (Cav2.2), the cells expressed robust Ca2+ currents that were blocked by omega-conotoxin GVIA. Activation of N-type Ca2+ currents caused rapid increases in membrane capacitance of the MPC 9/3L cells, indicating that the Ca2+ influx was linked to exocytosis of vesicles similar to that reported in chromaffin or PC12 cells. Synaptic protein interaction (synprint) sites, like those found on N-type Ca2+ channels, are thought to link voltage-dependent Ca2+ channels to SNARE proteins involved in synaptic transmission. Interestingly, MPC 9/3L cells transfected with either LC-type (alpha1C, beta2a, alpha2delta, Cav1.2) or T-type (alpha1G, beta2a, alpha2delta, Cav3.1) Ca2+ channel subunits, which do not express synprint sites, expressed appropriate Ca2+ currents that supported rapid exocytosis. Thus MPC 9/3L cells provide a unique model for the study of exocytosis in cells expressing specific Ca2+ channels of defined subunit composition without complicating contributions from endogenous channels. This model may help to distinguish the roles that different Ca2+ channels play in Ca2+-dependent secretion.