Calcium-induced calcium release in neurosecretory insect neurons: fast andslow responses

The dynamics of intracellular free Ca2+ ([Ca2+](i)) changes were investigat ed in dorsal unpaired median (DUM) neurons of the cockroach Periplaneta ame ricana. Activation of voltage-gated Ca2+ channels caused a steep increase i n [Ca2+](i). Depolarizations lasting for greater than or equal to 100 ms le d to Ca2+ release from intracellular stores as is indicated by the finding that the rise of [Ca2+](i) was greatly reduced by the antagonists of ryanod ine receptors, ryanodine and ruthenium red. There is a resting Ca2+ current which is potentiated on application of a neuropeptide, Neurohormone D (NHD ), a member of the adipokinetic hormone family. Ca2+ influx enhanced in thi s way again caused a rise of [Ca2+](i) sensitive to ryanodine and ruthenium red. Such rises developed and relaxed much more slowly than the depolariza tion-induced signals. Ca2+ responses similar to those induced by NHD were o btained with the ryanodine receptor agonists caffeine (20 mM) and cADP-ribo se (cADPR, 100 nM). These Ca2+ responses, however, varied considerably in s ize and kinetics, and part of the cells did not respond at all to caffeine or cADPR. Such cells, however, produced Ca2+ rises after having been treate d with NHD. Thus, the variability of Call signals might be caused by differ ent filling states of Ca2+ stores, and the resting Ca2+ current seems to re present a source to fill empty Ca2+ stores. In line with this notion, block of the endoplasmic Ca2+ pump by thapsigargin (1 muM) produced either no or largely varying Ca2+ responses. The Ca2+ signals induced by caffeine and c ADPR displayed different sensitivity to ryanodine receptor blockers. cADPR failed to elicit any response when ryanodine or ruthenium red were present. By contrast, the response to caffeine, in the presence of ryanodine, was o nly reduced by about 50% and, in the presence of ruthenium red, it was not at all reduced. Thus, there may be different types of Ca2+ release channels . Block of mitochondrial Ca2+ uptake with carbonyl cyanide m-chlorophenylhy drazone (CCCP, 1 muM) completely abolished cADPR-induced Ca2+ signals, but it did not affect the caffeine-induced signals. Taken together our findings seem to indicate that there are different stores using different Ca2+ upta ke pathways and that some of these pathways involve mitochondria. (C) 2001 Harcourt Publishers Ltd.