Ca2+ resting current and Ca2+-induced Ca2+ release in insect neurosecretory neurons

The effect of a Ca2+ resting current on spiking and intracellular Ca2+ concentration [Ca2+]i of isolated cockroach dorsal unpaired median neurons was investigated by combining microfluometry (Fura2) with patchclamping. The resting current was reduced by 10 microM 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and 10 microM mefenamic acid, which caused slower spiking of cells. Up-modulation of the resting current by neurohormone D (NHD, 100 fM to 10 pM) transiently increased [Ca2+]i. This Ca2+ transient was largely reduced by the ryanodine receptor antagonist dantrolene (10 microM) but it was not reduced by the IP3-receptor antagonist heparin which indicates Ca2+-induced Ca2+ release. The NHD-effect on [Ca2+]i was strongly attenuated by NPPB and mefenamic acid. Thus, NHD caused Ca2+-induced Ca2+ release by enhancing the Ca2+ resting current.