Serotonin-induced intercellular calcium waves in salivary glands of the blowfly Calliphora erythrocephala.

1. Blowfly salivary glands have been used extensively as a model system for the analysis of inositol phosphate-dependent signal transduction. To detect and characterize changes in intracellular free calcium ([Ca2+]i) that might be expected to be triggered by stimulation with serotonin (5-HT), we have carried out digital calcium-imaging experiments on intact glands using the Ca2+-sensitive dye fura-2. 2. 5-HT (1-10 nM) induced repetitive transient increases in [Ca2+]i, i.e. Ca2+ spikes whose frequency was a function of agonist concentration (EC50 = 2.8 nM). 3. Pre-incubation in EGTA decreased the frequency but did not inhibit spiking. Thapsigargin abolished periodic spike activity indicating that the [Ca2+]i rise results from Ca2+ release. Neither caffeine (10 mM) nor ryanodine (10 and 50 microM) induced increases in [Ca2+]i. 4. Oscillatory activity in individual cells was synchronized by regenerative intercellular Ca2+ waves that propagated over distances greater than 400 microm. Colliding waves annihilated each other. 5. Desynchronization of the oscillation pattern by 100 microM 1-octanol suggests the involvement of gap junctions and an intracellular messenger in wave propagation. 6. Local stimulation of glands elicited [Ca2+]i elevations in the stimulated area, but not in adjacent cells, indicating that local increases in [Ca2+]i are not sufficient to trigger Ca2+ waves. However, local stimulation was capable of evoking propagating Ca2+ waves when combined with low-dose 5-HT stimulation of the whole gland. 7. The data are consistent with the hypothesis that: (1) Ca2+ acts as the intercellular messenger and modulates its own release via positive and negative feedback on the inosital 1,4,5-trisphosphate (InsP3) receptor, and (2) sensitization of the InsP3 receptor to Ca2+ by InsP3 is required for the propagation of intercellular Ca2+ waves, as proposed for intracellular Ca2+ waves in Xenopus oocytes.