| Abstract: | The relationship between fMet-Leu-Phe-induced changes in the cytosolic free Ca2+ concentration ( Ca2+]i), plasma membrane potential depolarization, and metabolic responses was studied in human neutrophils. Receptor-activated depolarization occurred both at high and resting Ca2+]i, but was inhibited at very low Ca2+]i. Phorbol 12-myristate 13-acetate-induced plasma membrane depolarization, on the contrary, was independent of Ca2+]i. The threshold fMet-Leu-Phe concentration for plasma membrane depolarization (10(-8) M) was at least 1 log unit higher than that for Ca2+]i increases (5 X 10(-10) M) and coincident with that for NADPH oxidase activation. Nearly maximal Ca2+]i increases were elicited by 3 X 10(-9) fMet-Leu-Phe in the absence of any significant plasma membrane potential change. This observation allowed us to investigate the effects of artificially induced plasma membrane depolarization and hyperpolarization at low fMet-Leu-Phe concentrations (10(-9) to 3 X 10(-9) M) which did not perturb plasma membrane potential. Depolarizing (gramicidin D at 10(-7) to 10(-6) M or KCl at 50 mM) and hyperpolarizing (valinomycin at 4 microM) treatments had little influence on unstimulated Ca2+]i levels, whereas fMet-Leu-Phe-induced transients were significantly altered. Gramicidin D and KCl decreased the fMet-Leu-Phe-induced Ca2+]i increases in Ca2+-containing or in Ca2+-free media. Valinomycin, on the contrary, increased receptor-stimulated Ca2+]i increases, and the effect was larger in the presence of extracellular Ca2+. Valinomycin also strongly potentiated secretion. It is suggested that plasma membrane depolarization in human neutrophils is a physiological feedback mechanism inhibiting receptor-dependent Ca2+]i changes. |
