RECEPTOR-RECOGNIZED ALPHA-2-MACROGLOBULIN METHYLAMINE ELEVATES INTRACELLULAR CALCIUM, INOSITOL PHOSPHATES AND CYCLIC-AMP IN MURINE PERITONEAL-MACROPHAGES

Human plasma alpha2-macroglobulin (alpha2M) is a tetrameric proteinase inhibitor, which undergoes a conformational change upon reaction with either a proteinase or methylamine. As a result, a receptor recognition site is exposed on each subunit of the molecule enabling it to bind to its receptors on macrophages. We have used Fura-2-loaded murine peritoneal macrophages and digital video fluorescence microscopy to examine the effects of receptor binding on second messenger levels. Alpha2M-methylamine caused a rapid 2-4-fold increase in intracellular Ca2+ concentration ([Ca2+]i) Within 5 s of binding to receptors. The agonists induced a focal increase in [Ca2+]i that spread out to other areas of the cell. The increase in [Ca2+]i was dependent on the alpha2M-methylamine concentration and on the extracellular [Ca2+]. Both sinusoidal and transitory oscillations were observed, which varied from cell to cell. Neither alpha2M nor boiled alpha2M-methylamine, forms that are not recognized by the receptor, affected [Ca2+]i in peritoneal macrophages under identical conditions of incubation. The alpha2M-methylamine-induced rise in [Ca2+]i was accompanied by a rapid and transient increase in macrophage inositol phosphates, including inositol tris- and tetrakis-phosphates. Native alpha2M did not stimulate a rise in inositol phosphates. Finally, binding of alpha2M-methylamine to macrophages increased cyclic AMP transiently. Thus receptor-recognized alpha-macroglobulins behave as agonists whose receptor binding causes stimulation of signal transduction pathways.