We characterized the enzymic properties of ADP-ribosyl cyclase in rat parotid acinar cells by using a fluorescence technique. ADP-ribosyl cyclase is capable of synthesizing the Ca2+-mobilizing nucleotide cADP-ribose (cADPR) from NAD+ and has previously been shown to be regulated by cGMP via a cGMP-dependent protein kinase (G kinase). We therefore investigated whether NO/cGMP-activated pathways are present in rat parotid acinar cells and whether NO/cGMP signalling exerts control over cellular Ca2+ signalling processes. Our results showed that stimulation of acinar cells with adrenaline, isoproterenol, substance P and NO resulted in a rise in the [cGMP]. In addition, NO induced a release of Ca2+ from intracellular ryanodine-sensitive stores via a cGMP/G-kinase-mediated process. Thus our data reveal that a rise in [cGMP], caused by either neurotransmitter or NO activation, activates a G kinase, which in turn controls Ca2+ release from ryanodine-sensitive stores. Since parotid acinar cells possess ADP-ribosyl cyclase activity, we propose a model in which cADPR is the link between NO/cGMP signalling pathways and release of Ca2+ from ryanodine-sensitive stores.

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