The increase in intracellular free calcium concentration is an important step in signal transduction leading to phagocyte activation. The undecapeptide substance P can influence various functions of human polymorphonuclear leukocytes, including chemotaxis, phagocytosis, and respiratory burst. In this study we investigated the ability of low-concentration (that can occur in vivo) substance P (10(-7) M) and its precursor alpha-protachykinin (3 x 10(-7) M) to increase the intracellular free calcium concentration in human polymorphonuclear leukocytes. Cells isolated from ten healthy donors were incubated with substance P or alpha-protachykinin in 1 mM calcium medium for 5 min and the intracellular free calcium concentration was monitored using the fluorescent calcium indicator Fura-2am. Polymorphonuclear leukocytes from 40% of donors responded to both agonists. The substance P- and alpha-protachykinin-induced increase in intracellular free calcium concentration was 59 +/- 13 nM and 58 +/- 12 nM and the extracellular calcium influx contributed to 87 +/- 8% and 54 +/- 8% of the calcium response, respectively. alpha-Protachykinin released almost all the calcium from intracellular stores, while substance P mobilized only 24 +/- 5% of this calcium pool. Finally, cells that responded to a single challenge with substance P and alpha-protachykinin were able to increase their intracellular free calcium concentration in response to each of three consecutive stimulations with these agonists. This may be an additional mechanism by which substance P and its precursor modify the function of human polymorphonuclear leukocytes.