Mitochondria and chromaffin cell function

Abstract

Chromaffin cells are an excellent model for stimulus– secretion coupling. Ca2+ entry through plasma membrane voltage-operated Ca2+ channels (VOCC) is the trigger for secretion, but the intracellular organelles contribute subtle nuances to the Ca2+ signal. The endoplasmic reticulum amplifies the cytosolic Ca2+ ([Ca2+]C) signal by Ca2+- induced Ca2+ release (CICR) and helps generation of microdomains with high [Ca2+]C (HCMD) at the subplasmalemmal region. These HCMD induce exocytosis of the docked secretory vesicles. Mitochondria close to VOCC take up large amounts of Ca2+ from HCMD and stop progression of the Ca2+ wave towards the cell core. On the other hand, the increase of [Ca2+] at the mitochondrial matrix stimulates respiration and tunes energy production to the increased needs of the exocytic activity. At the end of stimulation, [Ca2+]C decreases rapidly and mitochondria release the Ca2+ accumulated in the matrix through the Na+/Ca2+ exchanger. VOCC, CICR sites and nearby mitochondria form functional triads that co-localize at the subplasmalemmal area, where secretory vesicles wait ready for exocytosis. These triads optimize stimulus–secretion coupling while avoiding propagation of the Ca2+ signal to the cell core. Perturbation of their functioning in neurons may contribute to the genesis of excitotoxicity, ageing mental retardation and/or neurodegenerative disorders.