Hypoxia-induced mitogenic factor (HIMF), also known as "found in inflammatory zone 1" (FIZZ1) or resistin-like molecule-alpha (RELMalpha), is a profound vasoconstrictor of the pulmonary circulation and a strong mitogenic factor in pulmonary vascular smooth muscle. To further understand the mechanism of these contractile and mitogenic responses, we examined the effect of HIMF on intracellular Ca(2+) in human pulmonary artery smooth muscle cells (SMC). Ca(2+) imaging in fluo 4-loaded human pulmonary artery SMC revealed that recombinant murine HIMF increased intracellular Ca(2+) concentration ([Ca(2+)](i)) in a sustained and oscillatory manner. This increase occurred independent of extracellular Ca(2+) influx. Pretreatment of human pulmonary artery SMC with U-73122, a specific inhibitor of phosphatidylinositol-phospholipase C (PLC) completely prevented the HIMF-induced Ca(2+) signal. The [Ca(2+)](i) increase was also abolished by pretreatment with 2-aminoethoxydiphenyl borate (2-APB), an inositol 1,4,5-trisphosphate (IP(3)) receptor antagonist. Ryanodine pretreatment did not affect initiation of [Ca(2+)](i) activation or internal release but reduced [Ca(2+)](i) at the plateau phase. Pretreatment with the Galpha(i)-specific inhibitor pertussis toxin and the Galpha(s)-specific inhibitor NF-449 did not block the Ca(2+) signal. Knockdown of Galpha(q/11) expression did not prevent Ca(2+) release, but the pattern of Ca(2+) release changed from the sustained oscillatory transients with prolonged plateau to a series of short [Ca(2+)](i) transients that return to baseline. However, pretreatment with the tyrosine kinase inhibitor genistein completely inhibited the internal Ca(2+) release. These results demonstrate that HIMF can stimulate intracellular Ca(2+) release in human pulmonary artery SMC through the PLC signaling pathway in an IP(3)- and tyrosine phosphorylation-dependent manner and that Galpha(q/11) protein-coupled receptor and ryanodine receptor contribute to the increase of [Ca(2+)](i).