Action of philanthotoxin on ion channels of arthropod muscle

Calcium ions play an important role in many signalling pathways involved in normal cell metabolism. Pertrebations of normal Ca++ signalling may also play a pivotal role in the initiation of cell death. In these studies I have examined the influx of 45Ca++ into the extensor tibiae muscle of the locust (Schistocerca gregaria ). 45Ca++ entry could be stimulated by the addition of glutamate receptor-agonists or by activation of voltage activated calcium channels. L-glutamate, L-quisqualate and NMDA stimulated the influx of 45Ca++ while L-aspartate had only a small effect. DL-ibotenate, kainate, AMPA and glycine had no effect on 45Ca++ uptake (all agonists were tested at concentrations up to (100μM). Glycine (1μM) enhanced the 45Ca++ entry induced by NMDA and L-glutamate. Only the glycine potentiation of L-glu-stimulated responses was abolished in the presence of Mg++ (2mM) or AP5 (10μM) whereas the NMDA-stimulated response was completely abolished by these agents. These finding suggests that in the presence of glycine, L-glutamate may activate NMDA receptors and that in the absence of glycine L-glu-stimulated 45Ca++ entry occurs via activation of the qGluR. Depolarisation of the extensor tibiae muscles (50mM KCl) stimulated 45Ca++ influx by activation of voltage-sensitive calcium channels. Philanthotoxin-343 (0.1μM) had no effect on depolarisation activated calcium entry, however, nifedipine (1μM) an L-type calcium channel antagonist inhibited this Ca++ influx. Nifedipine did not inhibit L-glu-stimulated Ca++ entry suggesting that in these muscles L-type Ca++ channels are not involved in the Ca++ influx pathway following G1uR activation. Philanthotoxin-433 (PhTX-433) and many of its synthetic analogues are potent inhibitors of locust GluR. In the future these analogues may prove as useful potential neuroprotective agents or as novel pesticides. Over 100 analogues of PhTX-433 have been synthesized with changes made in the four regions of the structure, the thermospermine