Platelet-activating factor (PAF) and platelet-derived growth factor (PDGF) likely play important roles in glomerular processes. To identify mechanisms of PAF- and PDGF-induced mesangial cell activation, we characterized effects of both agents on cytosolic free [Ca2+] ([Ca2+]f) and phospholipase activation. When cultured rat renal mesangial cells were stimulated by PAF, [Ca2+]f increased (within 10 s) from 110 +/- 6 to 209 +/- 14 nM, due to release of Ca2+ from intracellular storage sites, as well as entry of Ca2+ from external milieu. PAF also rapidly increased free arachidonate, diacylglycerol, and inositol trisphosphate (IP3) levels. PDGF increased [Ca2+]f from 78 +/- 12 to 192 +/- 22 nM, but the time response was different from that seen with PAF. Peak [Ca2+]f increase occurred at approximately 1 min subsequent to stimulation. Like PAF, increased [Ca2+]f was due to release of Ca2+ from intracellular stores and entry from extracellular media. PDGF increased levels of free arachidonate prior to an increase in diacylglycerol levels. PDGF increased inositol bisphosphate and IP3 levels at 2 min but not at 15 s. Thus, PDGF may mediate a phospholipase C-independent activation of phospholipase A2. In permeabilized mesangial cells, IP3 released Ca2+ from nonmitochondrial storage sites. Thus, PAF- and PDGF-induced increases in [Ca2+]f were likely secondary to activation of phospholipase C, resulting in increased levels of IP3. Understanding differences in mechanisms of mesangial cell activation by PAF and PDGF will likely lead to a better understanding of signal transduction pathways and provide insight into the role of these activators in glomerular disease.