Activation of the nitric oxide/cGMP pathway is required for refilling intracellular Ca2+ stores in a sympathetic neuron cell line

Fura-2 fluorescence imaging was used to measure changes in intracellular Ca2+ concentration in individual N1E-115 neuroblastoma cells during repeated activation of M1 muscarinic receptors with carbachol. Ca2+ transients could be elicited repeatedly at 4 min intervals with little decrement as long as external Ca2+ was present. When the cells were bathed in Ca(2+)-free saline, however, the response amplitude decreased rapidly in a use-dependent fashion, indicating that external Ca2+, and presumably Ca2+ influx, is required for refilling Ca2+ stores during the interval between trials. The response amplitude also decreased during repeated stimulation in cells treated with the NO-synthase inhibitor L-NMMA or with the guanylyl cyclase inhibitor LY-83583 even when Ca2+ was present. Application of the membrane permeable cGMP analog 8-Br-cGMP reversed the effect of L-NMMA and promoted refilling in the continued presence of NO-synthase inhibitor. These results indicate that activation of the NO/cGMP pathway is necessary for refilling Ca2+ stores during muscarinic signaling. Evidence is also presented suggesting that the NO/cGMP pathway is involved in long term modulation of the content of Ca2+ stores.