Neurohormone D increases the intracellular Ca2+ level in cockroach neurones through a Cd(2+)-sensitive Ca2+ influx

The modulating effect of the octapeptide neurohormone D (NHD) on the intracellular calcium level [Ca2+]i of neurones from the dorsal midline of the cockroach terminal ganglion was investigated with fluorescence measurements. [Ca2+]i of cells loaded with Fura 2 was determined by photon counting and imaging at wavelengths of 340 and 380 nm. After application of NHD, [Ca2+]i increased within 3 min from a value of 93 +/- 36 nM to 153 +/- 51 nM, corresponding to an enhancement to 164 +/- 35%. In Ca(2+)-free solution, [Ca2+]i was lowered (52 +/- 6 nM) and NHD no longer affected the intracellular calcium level. The presence of 0.1 mM Cd2+, in normal saline, prevented the NHD-induced increase of [Ca2+]i. The results were explained by postulating a Ca2+ resting current in these cells which is augmented by NHD.