Dual effects of mastoparan on intracellular free Ca²⁺ concentrations in human astrocytoma cells

The effect of mastoparan, a wasp venom toxin, on intracellular free Ca2+ concentration ([Ca2+]i) was examined in human astrocytoma cells. Mastoparan inhibited [Ca2+]i induced by carbachol (100 microM) in a concentration-dependent manner in the absence of extracellular Ca2+, consistent with our previous results showing that mastoparan inhibits phosphoinositide hydrolysis in human astrocytoma cells. 2. In contrast, mastoparan itself increased [Ca2+]i and augmented carbachol-induced increase in the [Ca2+]i in the presence of extracellular Ca2+, suggesting that mastoparan elicited Ca2+ influx from the extracellular medium. The increase appeared to be maximum at extracellular Ca2+ concentrations of 0.1-0.2 mM. The higher concentrations of extracellular Ca2+ depressed the influx. 3. Pertussis toxin did not affect mastoparan-induced inhibition of [Ca2+]i in the absence of extracellular Ca2+, consistent with the previous results that pertussis toxin did not affect mastoparan-induced inhibition of phosphoinositide hydrolysis. 4. Pertussis toxin augmented mastoparan-induced increase in [Ca2+]i in the presence of extracellular Ca2+, suggesting that pertussis toxin substrate(s) seems to be inhibitory for Ca2+ influx induced by mastoparan. 5. Verapamil, nifedipine and diltiazem (each 10 microM), L-type Ca2+ antagonists, did not affect mastoparan-induced Ca2+ influx. However, verapamil (10 microM) slightly inhibited the increase in [Ca2+]i induced by carbachol in the presence of mastoparan. 6. The results obtained in the present study indicate that mastoparan has two opposite effects on [Ca2+]i in human astrocytoma cells and possibly has at least two sites of action.