Low-density (LDL) and high-density (HDL3) lipoproteins dose-dependently activate phosphoinositide turnover and elevate cytosolic free Ca2+ concentrations ([Ca2+]i) in cultured vascular smooth muscle cells (VSMCs) from either human (microarterioles and aorta) or rat (aorta) sources. High-performance liquid chromatography analysis of cell extracts revealed comparable spectra of inositol phosphate isomers generated in response to either LDL, HDL3, or angiotensin II (Ang II). Thus, lipoproteins and Ang II may use similar, if not identical, signal transduction pathways for the generation and metabolism of inositol phosphates and intracellular Ca2+ mobilization in VSMCs. When Ang II was added in combination with either LDL or HDL3, the phosphoinositide and [Ca2+]i responses of VSMCs were either equal to or even greater than the sum of the effects elicited by the agonists individually. This additivity/synergy between Ang II and the lipoproteins was not accompanied by alteration in the half-maximally effective dose requirements of VSMCs for either Ang II (approximately 2 nmol/L, with or without lipoproteins) or lipoproteins (approximately 50 micrograms/mL for LDL and HDL3, with or without Ang II). Neither short-term (up to 10 minutes) nor long-term (48 hours) exposure of VSMCs to lipoproteins caused desensitization of phospholipase C and intracellular Ca2+ mobilization responses to either Ang II or lipoproteins. Since constant exposure of VSMCs to lipoproteins is a physiological circumstance, and because elevation of [Ca2+]i and activation of phosphoinositide turnover are pivotal events for VSMC contraction and growth, we suggest that the low concentrations of lipoproteins in the vessel intima may play an important role in regulating the response of the vasculature to Ang II.