Stimulation of lymphocyte receptor capping by the ionophore monensin

The carboxylic ionophore monensin has a biphasic effect on antibody-induced Thy-1 cap formation. At higher concentrations, 5 X 10(-6)-5 X 10(-5) M monesin causes a significant inhibition of receptor capping similar to that previously found with the Ca2+ selective ionophore A23187. At lower concentrations, 5 X 10(-8)-5 X 10(-7) M capping is stimulated. It is concluded that capping at lower ionophore concentrations is a specific response to the ability of monensin to induce a rise in intracellular Na+, which indirectly elevates intracellular Ca2+ activity. This in turn activates the contractile machinery required for the aggregation of surface receptors into capped structures. At higher concentrations monensin acts as a nonspecific detergent, which causes detrimental structural alterations in some of the membrane components involved in the capping process.