Characterization of Ca2+ signaling pathways in mouse adrenal medullary chromaffin cells

In the present study, we characterized the Ca2+ responses and secretions induced by various secretagogues in mouse chromaffin cells. Activation of the acetylcholine receptor (AChR) by carbachol induced a transient intracellular Ca2+ concentration ([Ca2+](i)) increase followed by two phases of [Ca2+](i) decay and a burst of exocytic events. The contribution of the subtypes of AChRs to carbachol-induced responses was examined. Based on the results obtained by stimulating the cells with the nicotinic receptor (nAChR) agonist, 1,1-dimethyl-4-phenylpiperazinium iodide, high K(+) and the effects of thapsigargin, it appears that activation of nAChRs induces an extracellular Ca2+ influx, which in turn activate Ca(2+)-induced Ca2+ release via the ryanodine receptors. Muscarine, a muscarinic receptor (mAChRs) agonist, was found to induce [Ca2+](i) oscillation and sustained catecholamine release, possibly by activation of both the receptor- and store-operated Ca2+ entry pathways. The RT-PCR results showed that mouse chromaffin cells are equipped with messages for multiple subtypes of AChRs, ryanodine receptors and all known components of the receptor- and store-operated Ca2+ entry. Furthermore, results obtained by directly monitoring endoplasmic reticulum (ER) and mitochondrial Ca2+ concentration and by disabling mitochondrial Ca2+ uptake suggest that the ER acts as a Ca2+ source, while the mitochondria acts as a Ca2+ sink. Our results show that both nAChRs and mAChRs contribute to the initial carbachol-induced [Ca2+](i) increase which is further enhanced by the Ca2+ released from the ER mediated by Ca(2+)-induced Ca2+ release and mAChR activation. This information on the Ca2+ signaling pathways should lay a good foundation for future studies using mouse chromaffin cells as a model system.