Spontaneous [Ca2+]i fluctuations in rat chromaffin cells do not require inositol 1,4,5-trisphosphate elevations but are generated by a caffeine- and ryanodine-sensitive intracellular Ca2+ store.
A considerable fraction (65%) of single rat chromaffin cells loaded with the fluorescent [Ca2+]i indicator fura-2 exhibited spontaneous rhythmic fluctuations with an average period of approximately 100 s. Parallel patch clamp experiments as well as fura-2 experiments carried out in Ca2(+)-free and other modified media in the presence of Ca2+ and Na+ channel blockers indicated an origin from intracellular stores. Appropriate concentrations of agonists (bradykinin and histamine) for receptors (B2 and H1) that trigger generation of inositol 1,4,5-trisphosphate induced increased fluctuation frequency, recruitment of silent cells, and large [Ca2+]i changes at high doses. These effects were blocked by cell pretreatment with neomycin, a drug that inhibits inositol 1,4,5-trisphosphate generation. In contrast, spontaneous fluctuations and the effects of another drug, caffeine, which also induced increased frequency and recruitment, were unaffected by neomycin. Ryanodine caused first a prolongation and then (approximately 10 min) a block of both spontaneous fluctuations and caffeine effects, where the single transients after bradykinin and histamine were maintained. Caffeine and ryanodine are known to affect selectively the process of calcium-induced Ca2+ release; this is the first demonstration of [Ca2+]i fluctuation activity arising from Ca2(+)-induced Ca2+ release in nonmuscle cells with no strict requirement for inositol 1,4,5-trisphosphate involvement.
Malgaroli
A
A
Department of Pharmacology, Consiglio, Nazionale delle Ricerche Center of Cytopharmacology, Milan, Italy.
Fesce
R
R
Meldolesi
J
J
D000313:Adrenal Medulla / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D000470:Alkaloids / Q000494:pharmacology*; D001920:Bradykinin / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D004533:Egtazic Acid / Q000494:pharmacology; D006632:Histamine / Q000494:pharmacology; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D009355:Neomycin / Q000494:pharmacology; D009568:Nitrendipine / Q000494:pharmacology; D012433:Ryanodine / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence
D000470:Alkaloids; D012433:Ryanodine; D002110:Caffeine; D004533:Egtazic Acid; D006632:Histamine; D015544:Inositol 1,4,5-Trisphosphate; D009568:Nitrendipine; D009355:Neomycin; D001920:Bradykinin; D002118:Calcium